Fabrication of Solution Processed Semiconducting Films
based Devices
By
Muhammad Yasin
2011-NUST-DirPhD-EE-42
Supervisor
Dr. Tauseef Tauqeer
Co-Supervisor
Prof. Dr. Khasan S Karimov
THIS DISSERTATION IS SUBMITTED TO NUST, IN PARTIAL FULFILLMENT OF
DOCTOR OF PHILOSOPHY IN ELECTRICAL ENGINEERING
Department of Electrical Engineering, School of Electrical Engineering and
Computer Science, National University of Sciences and Technology, Islamabad
2016
II
III
Certificate by All GEC Members
It is certified that PhD thesis in respect of PhD student Reg No. 2011-NUST-DirPhD-EE-42
Name: Muhammad Yasin has been vetted and approved by the following GEC:
Names Signatures
Dr. Tauseef Tauqeer (Supervisor)
Prof. Dr. Khasan S. Karimov (Co-Supervisor)
Prof. Dr. Sait Eren SAN (External GEC Member)
Prof. Dr. Syed Muhammad Hassan Zaidi (GEC Member)
Dr. Hamood Ur Rahman (GEC Member)
Dr. Muhammad Umar Khan (GEC Member)
IV
This work is dedicated to my beloved mother, wife and daughters.
V
Acknowledgements
All honors and glories are for Almighty Allah, The Most Beneficent and Merciful Who enabled
me to complete this dissertation within the given time frame. May Almighty Allah shower His
countless blessings on Muhammad (PBUH), The Last Prophet, who is not only the continuous
source of knowledge, wisdom and torch of guidance for entire mankind but also messenger of
peace for all creatures of Almighty Allah.
I am highly thankful to my supervisor Dr. Tauseef Tauqeer, Co-Supervisor Prof. Dr. Khasan
Sanginovich Karimov and GEC members Prof. Dr. Sait Eren, Prof. Dr. S. M. Hassan Zaidi, Dr.
Hamood Ur Rahman and Dr Muhammad Umar Khan, for their valuable support, supervision,
devoted guidance and constructive criticism. I am unable to express my entire heart feelings in
words but I can pray to Almighty Allah (SWTA) to reward them for their kindness. I feel myself
honored to be the student of such renowned scholars.
I am thankful to Turkish Research and Technological Council (TUBITAK) for providing me
funded opportunity to carry out a part of PhD research work at Polymer Electronics Research
Laboratory, GEBZE Technical University (GTU) Turkey. Special thanks to Prof. Dr. Sait Eren
SAN who not only supervised my research but also organized my research visit at GTU. I am
greatly inspired by his personality and professional skills, and sincerely appreciate his
consideration that enabled me to experience fruitful research in a friendly and relaxed
environment.
Besides Prof. SAN, precious assistance, guidance and moral support of the faculty members,
graduate students and staff of the Department of Physics at GTU, Turkey, must also be
acknowledged. In particular, I am greatly thankful to Prof. Dr. Yusuf Yerli, Prof. Dr. Mustafa
Okutan, Arif Kosemen, Zuhal Alpaslan Kosemen, Cigdem Cakarlar, Dr. Ayesha Akpinar, Noor-
ul-Hassan, Betul canimkurbey, Gokhan Casdaman, Dilek Taskin, Hasan Turkan, Dr. Saadullah
Ozturk, Dr. Hakki Acarlar, Abuzer Yaman, Abdullah HAKAN, Ihlas Cicek, Adem Chen and
Ahmet Nazim for helping me during the fabrication & characterization of electronic devices and
interpretation of results.
VI
I am highly grateful to all the faculty members, graduate students, staff and every one of my
parent institution, National University of Sciences and Technology, School of Electrical
Engineering and Computer Science (NUST-SEECS) for their moral support and guidance. I am
also thankful to Prof. Dr S. M. Hassan Zaidi, Principal & Dean NUST-SEECS who not only
managed and administered my research activities but also encouraged me at every foot step of
my PhD study.
I am grateful to my employer, NESCOM for allowing me to pursue PhD studies. I am also
thankful to my close office colleagues who helped and prayed for the success of this work.
Finally, I highly appreciate my family for their love, kindness and support for the materialization
of this work. They have always been a source of motivation and inspiration for me. In particular,
I am indebted to my wife, Asma and beloved daughters Maryam, Ayesha & Hamna for their
understanding, patience, encouragement and continual support.
VII
Declaration
I hereby declare that the dissertation entitled “Fabrication of Solution Processed
Semiconducting Films based Devices” is my own work. It does not contain any previously
published/ submitted material or work accepted for the award of any degree or diploma at
NUST-SEECS or any other institute, except where due acknowledgement is made in the
dissertation. Any contribution made to this research work by any local or foreigner researcher
with whom I worked, is explicitly acknowledged. It is also certified that the intellectual content
of this dissertation is the outcome of my own work. Approval must be taken prior to use the
work presented in or derived from this dissertation.
Author Name: Muhammad Yasin
Signature:
VIII
Journal Publications
1. M. Yasin, T. Tauqeer, K.S. Karimov, S.E. San, A. Kösemen, Y. Yerli, A.V. Tunc,
P3HT:PCBM blend based photo organic field effect transistor, Microelectronic
Engineering, 130 (2014) 13-17.
2. M. Yasin, T. Tauqeer, H. Rahman, K.S. Karimov, S. San, A. Tunc, Polymer–Fullerene
Bulk Heterojunction-Based Strain-Sensitive Flexible Organic Field-Effect Transistor,
Arab J Sci Eng, 40 (2015) 257-262.
3. M. Yasin; T. Tauqeer; S.M.H. Zaidi; S.E. San; A. Mahmood; M.E. Köse; B.
Canimkurbey; M. Okutan; Synthesis and electrical characterization of Graphene Oxide
films, Thin Solid Films, 590 (2015) 118-123.
4. A. Mahmood; A. Naeem; M. Y. Kaya; M. Yasin; E. Mensur-Alkoy; S. Alkoy; Effect of
Co/Mg ratio on the phase, microstructure, dielectric and impedance properties of lead
zirconate titanate; Accepted for publication in Journal of Materials Science: Materials in
Electronics; DOI: 10.1007/s10854-015-3697-5
5. A. Asimov; M. Ahmetoglu; A. Kirsoy; M. Özer; M. Yasin; The electrical properties of
AU/P3HT/N-TYPE SI schottky barrier diode, Accepted for Publication in Journal of
Nanoelectronics and Optoelectronics
6. Tauseef Tauqeer; Muhammad Yasin; Sait E San; Hamood Ur Rahman; Khasan
Karimov, Fabrication and characterization of P3HT:MR:PCBM blend based organic
phototransistor, Journal of Nanoelectronics and Optoelectronics (Accepted for
Publication)
7. Hidayatullah Khan, Muhammad Amin, Muhammad Ali, Muhammad Iqbal, Muhammad
Yasin, Turkish Journal of Electrical Engg and Computer. Sc. (Accepted for Publication)
IX
Conference Publications
1. M. Yasin, T. Tauqeer, S.E. San, H. Ur Rahman, K.S. Karimov, Fabrication and
characterization of organic bulk heterojunction based displacement and bend sensitive field
effect transistors, in: 12th International Bhurban Conference on Applied Sciences and
Technology (IEEE-IBCAST), 2015, pp. 1-5.
DOI: 10.1109/IBCAST.2015.7058470
2. Muhammad Yasin; T. Tauqeer; Sait Eren San; Zuhal A. Kösemen; Kh. S. Karimov; Asad
Mahmood; Investigation of the effect of gate voltage on the performance of organic bulk
hetero-junction based phototransistor; IEEE 17th
International Multi topic Conference (IEEE-
INMIC), 2014, pp. 445-449.
DOI: 10.1109/INMIC.2014.7097381
3. Erdinc Doganci, Cigdem Cakirlar, Sumeyra Bayir, Sait Eren San, Muhammad Yasin,
Mustafa Okutan, Cavit Uyanik, Faruk Yilmaz, 1st International Conference on Organic
Electronic Material Technologies, Turkey, (25-28 March 2015)
4. Muhammad Yasin; T. Tauqeer; Sait Eren San; Abuzer Yaman; Influence of Polymer
Intrinsic Properties on the Performance of Organic Bulk Heterojunction Solar Cells;
(Submitted)
X
Abstract
In recent years, solution processed semiconducting layers based devices, such as Solar Cells
(SCs), Field Effect Transistors (FETs) and Light Emitting Diodes (LEDs) have attracted much
interest as potential, inexpensive and flexible alternatives to inorganic devices. Despite
considerable understanding of device physics, investigation of well-known solution processed
semiconductors based devices having simpler configurations and optimal performance is crucial
to further developments in this area. This research work mainly focuses on the fabrication and
characterization of solution processed semiconducting films based electronic devices having
simpler architectures.
Firstly, Organic Solar Cells (OSCs) were developed to investigate the effect of polymer intrinsic
properties on the performance of the devices. Secondly, Organic Field Effect Transistors
(OFETs) were investigated with a motivation to develop low cost sensors for photo, strain, bend
and displacement sensing applications. Lastly, parallel plate capacitor type structures were
developed to study the electrical properties of Graphene Oxide (GO), Cobalt & Magnesium
doped Lead Zirconate Titanate (PbTi0.5Zr0.3(Co1-xMgx)0.2O3) Ceramics and Methacrylate-based
Side Chain Liquid Crystalline Polymers (SCLCP) with an aim to explore their potential for
electronic device applications.
Polymer-Fullerene BHJ based Solar Cells (OSCs) were fabricated using different batches of
poly[2-methoxy,5-(30,70-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) in order to
investigate the effect of polymer intrinsic properties i.e. molecular weights, polydispersity (PDI)
values, charge carrier densities (NA) and band gap energies (Eg) on the performance of the
devices. Devices were under dark as well as UV-Vis illuminations. Results showed that polymer
intrinsic properties significantly influenced on the structural properties of active layers.
Efficiency of the devices was found highest for polymer having higher PDI and NA. It was
attributed to the increased photon absorption capability and favorable nano morphology of active
layer for MDMO-PPV batch with higher PDI and NA values.
Organic Bulk Hetero-Junction (BHJ) based transistors were fabricated in MESFET configuration
with top gate and bottom drain source contacts on glass and flexible PET substrates for sensing
XI
applications. Semiconducting layers of the devices were based on P3HT:PCBM blends which
were processed using spin coating technique. Gate and drain/source electrodes of the transistors
were made using Aluminum and Silver, respectively, using physical vapor deposition technique.
Electrical performance of the devices was explained with the help of energy band diagrams.
Organic Transistors developed on glass substrates using P3HT:PCBM (1:1 & 1:0.8 wt/wt ratios)
and P3HT:MR:PCBM blends (1:0.2:1 wt/wt ratio) were investigated as photo sensors. Results
showed that MESFET based photo sensitive devices followed the behavior of typical low voltage
phototransistors with weak saturation trend, in particular under illumination. Photo responsivity
of the devices was found to increase with illumination intensity and to decrease with negative
gate voltage. Further, phototransistors developed using the blend of P3HT:MR:PCBM were
found to have higher photo sensitivity values than P3HT:PCBM blends based photo detectors.
Organic MESFET based devices developed on glass and flexible substrates with 1:1 wt/wt ratio
of P3HT and PCBM blends were also investigated for strain and displacement sensing
applications. Increase in displacement and compressive bending results in increase of drain to
source current. Further, drain to source resistance was reduced from 15.40 MΩ to 13.15 MΩ
when displacement was varied from 0 to 250μm.
Electrical and dielectric properties of Graphene Oxide (GO), PbTi0.5Zr0.3(Co1-xMgx)0.2O3
Ceramics and SCLCPs with varying lengths of aliphatic spacer were studied as Metal-Insulator-
Metal (MIM) capacitors using temperature dependent parallel plate impedance spectroscopic
technique. Analyses showed that GO film possses Direct Current (DC) and Correlated Barrier
Hopping (CBH) mechanisms of conductivity at low and high frequency ranges, respectively.
Analysis of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 Ceramics with varying values of composition exhibited
high magnitude of dielectric (ɛr = 4261) over a wide range of temperature at 100 kHz. Dielectric
constants of the SCLCP films were found to increase with the decrease of flexible spacer length.
Detailed analysis revealed that the conductivity of SCLCP films followed Quantum Mechanical
Tunneling (QMT) and CBH conductivity mechanisms at low frequency regime and, Super
Linear Power Law (SLPL) and DC conductivity mechanisms at high frequency region.
Impedance spectroscopic results suggest that, solution processed films being investigated using
impedance spectroscopy can be proposed as model systems for applications in large area flexible
arrays and other advanced microelectronic devices.
XII
Table of contents
Certificate by All GEC Members ................................................................................................. III
Acknowledgements ........................................................................................................................ V
Declaration ................................................................................................................................... VII
Journal Publications ................................................................................................................... VIII
Conference Publications ............................................................................................................... IX
Abstract .......................................................................................................................................... X
Table of contents .......................................................................................................................... XII
List of Figures ............................................................................................................................ XVI
List of Tables ............................................................................................................................. XXI
List of Abbreviations ................................................................................................................ XXII
Chapter-1: Introduction ................................................................................................................... 1
1.1 Preamble ........................................................................................................................... 1
1.2 Aims and objectives ......................................................................................................... 2
1.3 Thesis Organization.......................................................................................................... 3
Chapter-2: Theory and Background................................................................................................ 5
2.1 Organic Semiconductors .................................................................................................. 5
2.1.1 General Aspects ........................................................................................................ 5
2.1.2 Charge Transport in Organic Semiconductors .......................................................... 7
2.1.3 Classification of Organic Semiconductors ................................................................ 7
2.1.4 Organic Semiconductor Sensors ............................................................................. 10
2.2 Organic Solar Cells ........................................................................................................ 11
2.2.1 Introduction ............................................................................................................. 11
XIII
2.2.2 Energy Conversion Steps ........................................................................................ 12
2.2.3 Solar Cell Performance Parameters ........................................................................ 13
2.2.4 Different Architectures of Solar Cells .................................................................... 14
2.3 Organic Thin Film Transistors (OTFTs) ........................................................................ 15
2.3.1 Device Theory ......................................................................................................... 15
2.3.2 OTFTs based Sensors ............................................................................................. 22
2.4 AC Impedance and Dielectric Spectroscopic Studies .................................................... 22
Chapter-3: Materials and Experimental Details ............................................................................ 28
3.1 Materials ......................................................................................................................... 28
3.1.1 Semiconducting Materials for Solar Cells and Transistors..................................... 28
3.1.2 Semiconducting Materials for MIM Capacitors ..................................................... 31
3.2 Device Fabrication ......................................................................................................... 35
3.2.1 Vacuum Evaporation .............................................................................................. 35
3.2.2 Spin Coating............................................................................................................ 36
3.2.3 Drop Casting Process .............................................................................................. 37
3.3 Characterization Setup for Films and Devices ............................................................... 38
Chapter-4: Polymer-Fullerene BHJ Solar Cells and Phototransistors .......................................... 41
4.1 MDMO-PPV:PCBM blend based Organic Solar Cells ................................................. 41
4.1.1 Introduction ............................................................................................................. 41
4.1.2 Experimental Methods ............................................................................................ 42
4.1.3 Results and Discussion ........................................................................................... 44
4.2 P3HT:PCBM blend based Organic Phototransistors ..................................................... 48
4.2.1 Introduction ............................................................................................................. 49
4.2.2 Fabrication and Characterization ............................................................................ 50
4.2.3 Results and Discussion ........................................................................................... 51
XIV
Chapter-5: Strain and Displacement Sensitive OFETs ................................................................. 68
5.1 Organic Bulk Hetero-junction based Strain/ bend Sensitive Flexible Organic Field
effect Transistors ....................................................................................................................... 69
5.1.1 Introduction ............................................................................................................. 69
5.1.2 Experimental Methods ............................................................................................ 71
5.1.3 Results and Discussion ........................................................................................... 72
5.2 Polymer-Fullerene BHJ Based Displacement Sensitive OFET ..................................... 79
5.2.1 Introduction ............................................................................................................. 79
5.2.2 Experimental Details ............................................................................................... 80
5.2.3 Results and Discussion ........................................................................................... 82
Chapter-6: Electrical Characterization of Graphene Oxide, Lead Zirconate Titanate and Liquid
Crystalline Polymer Films ............................................................................................................ 86
6.1 Electrical Characterization of Graphene Oxide (GO) Films .......................................... 88
6.1.1 Introduction ............................................................................................................. 88
6.1.2 Experimental Procedure .......................................................................................... 89
6.1.3 Results and Discussion ........................................................................................... 90
6.2 Effect of Co/Mg ratio on the structural and electrical properties of lead zirconate
titanate films .............................................................................................................................. 96
6.2.1 Introduction ............................................................................................................. 96
6.2.2 Experimental Part.................................................................................................... 97
6.2.3 Results and Discussion ........................................................................................... 97
6.3 Electrical Conductivity Analysis of Methacrylate-based Liquid Crystal Polymers with
Pendant Cholesterol Groups .................................................................................................... 104
6.3.1 Introduction ........................................................................................................... 104
6.3.2 Experimental ......................................................................................................... 105
6.3.3 Results and discussion .......................................................................................... 106
XV
Chapter-7: Conclusions and Future Work .................................................................................. 113
7.1 Conclusions .................................................................................................................. 113
7.2 Future Work ................................................................................................................. 116
References ................................................................................................................................... 118
XVI
List of Figures
Figure 2-1: Schematic description of sp2 hybridization of Carbon atom [8] .................................. 5
Figure 2-2: Diagram of a HOMO and LUMO level of a molecule [10]......................................... 6
Figure 2-3: Commonly used p-type organic semiconductors (a) P3HT (b) Pentacene (c) CuPc (d)
Rubrene (e) MDMO-PPV (f) MEH-PPV (g) F8BT (h) Alpha-Sexithiophene ............................... 9
Figure 2-4: Commonly used n-type organic semiconductors (a) Fullerene-C60 (b) Fullerene-C70
(c) PCBM (d) Perylene (e) TCNQ (f) BBL (g) 4FPEPTC (h) DBP ............................................... 9
Figure 2-5: Current Voltage (I-V) Characteristics of a typical Solar Cell under dark and
illumination conditions ................................................................................................................. 15
Figure 2-6: Typical top gate top contacts FET ............................................................................. 16
Figure 2-7: Idealized energy band diagram of an OTFT with p-channel operation under given
biasing values [64] ........................................................................................................................ 18
Figure 2-8: Idealized energy band diagram of an OTFT with n-channel operation under given
biasing values [64] ........................................................................................................................ 18
Figure 2-9: (a) Output and (b) Transfer I-V characteristics of a typical OFET [65] .................... 20
Figure 2-10: 3D schematics of OFET in different operating regimes [61] (a) VD<< VG - Vth,
Linear regime (b) VD, sat = VG - Vth, Pinch off point formation near drain contact (c) VD > VD, sat,
Beyond Pinch off point ................................................................................................................. 21
Figure 3-1: Molecular Formula of P3HT ...................................................................................... 29
Figure 3-2: Molecular Structure of MDMO-PPV ......................................................................... 30
Figure 3-3: Chemical Formula of PCBM ..................................................................................... 30
Figure 3-4: Molecular Architecture of Graphene Oxide ............................................................... 31
Figure 3-5: Position and Orientation of molecules when matter is in (a) Crystalline (b) Liquid
Crystalline and ( c) Liquid state .................................................................................................... 33
XVII
Figure 3-6: Schematic illustration of (a) Nematic, (b) Cholesteric and (c) Smectic phase of liquid
crystals .......................................................................................................................................... 34
Figure 3-7: Leybold Univex 450 vacuum evaporator ................................................................... 36
Figure 3-8: (a) Semiconductor in powder form as received from Manufacturer (b) Solvent (c)
Semiconductor solution (d) Stirring Station with temperature control and monitoring ............... 37
Figure 3-9: Glove Box System ..................................................................................................... 38
Figure 3-10: DEKTAK-8 Surface Profiler ................................................................................... 38
Figure 3-11: UV-Vis Spectrophotometer interfaced with computer ............................................ 39
Figure 3-12: Impedance Analyzer HP 4194 and Novotherm Heating Station ............................. 40
Figure 3-13: Semiconductor Characterization System: Keithley 4200 SCS ................................ 40
Figure 4-1: (a) Schematic of the device architecture, and molecular structures of (b) PEDOT:PSS
(c) MDMO-PPV (d) PC71BM ...................................................................................................... 44
Figure 4-2: 10×10 µm2 surface and high resolution 3D AFM Profiles of Polymer: Fullerene Bulk
heterojunction based semiconducting layers for three different batches of MDMO-PPV : A[(a,
b)], B[c, d], and C[(e, f)] ............................................................................................................... 45
Figure 4-3: Absorption Spectra of MDMO-PPV:PCBM nanolayers (with 1:4 w/w ratio) prepared
using three different batches of MDMO-PPV Polymer ................................................................ 46
Figure 4-4: Current Density-Voltage (J-V) Curves of BHJ solar cells developed using three
different batches of MDMO-PPV Polymers under an illumination intensity of 100W/cm2 ........ 47
Figure 4-5: (a) Design schematic of organic phototransistors, and molecular structures of (b)
P3HT (c) MR (d) PCBM............................................................................................................... 51
Figure 4-6: (a) Absorption of P3HT:PCBM blend based active layer (with 1:1 wt/wt ratio) as a
function of wavelength, inset: its SEM based side view and (b) its (10µm × 10µm) surface AFM
view ............................................................................................................................................... 52
Figure 4-7: (a) S-D I-V curve (b) G-S I-V curve when VDS = 0 V (c) Energy Band Diagram of
the phototransistor, showing exciton generation, its dissociation, and transportation of holes and
electrons towards electrodes [93].................................................................................................. 53
XVIII
Figure 4-8: Current Voltage (I-V) Curves (output) of the Phototransistor without being
illuminated, inset: gate leakage current vs drain current at VGS = - 3V as a function of drain
voltage ........................................................................................................................................... 54
Figure 4-9: Transfer I-V Characteristics of the photo-OFET under dark ..................................... 54
Figure 4-10: Specific capacitance as a function frequency between gate and source/ drain
electrodes ...................................................................................................................................... 56
Figure 4-11: Output I-V Characteristics of the Photo-OFET (a) under fixed UV-Vis illumination
intensity of 100mW/cm2 and (b) under given UV-Vis illumination intensities compared with
dark ............................................................................................................................................... 58
Figure 4-12: Absorption spectrum of 350 nm thick P3HT:PCBM blend based active layer ....... 59
Figure 4-13: AFM 3D profiles (5µm×5µm) [(a) & (b)] and SEM images of the active layer (c) 60
Figure 4-14: Phototransistor output I-V characteristics under dark.............................................. 61
Figure 4-15: Phototransistor output I-V characteristics under UV-Vis illumination ................... 62
Figure 4-16: (a) Photo sensitivity and (b) responsivity as a function of Gate Voltage ................ 63
Figure 4-17: (a) Surface AFM image (10µm × 10µm) of P3HT:MR:PCBM blend based active
layer, inset: its high resolution 3D AFM profile, (b) Absorption Spectra, (c) Capacitance of
active layer as a function of frequency between Source/Drain and Gate electrodes and (d)
Capacitance of active layer as a function of DC bias voltage between Source/Drain and Gate
electrodes at specified frequency values. ...................................................................................... 65
Figure 4-18 I-V Characteristics of the Phototransistor under dark: Gate leakage current and drain
current as a function of drain to source voltage at VGS = - 4V. ................................................... 66
Figure 4-19 Output Current -Voltage (I-V) Characteristics of the Phototransistor under dark and
given UV-Vis illumination intensities when (a) VGS = - 3 V (b) VGS = - 4 V. ......................... 67
Figure 5-1: Design schematic of the flexible device .................................................................... 71
Figure 5-2: Energy Band Diagram ................................................................................................ 73
Figure 5-3: (a) 5×5 µm AFM image and (b) Cross section SEM view of 300 nm thick active
layer developed on substrate for sample-1 .................................................................................... 73
XIX
Figure 5-4: Section Roughness of active layer developed on flexible substrate for sample-2 ..... 74
Figure 5-5: (a) Out Put and (b) Transfer I-V Characteristics of sample-1 OFET under no bending
state ............................................................................................................................................... 75
Figure 5-6: Out Put I-V Characteristics of sample-2 OFET under no bending state .................... 75
Figure 5-7: Photographs of the flexible devices under bending state ........................................... 77
Figure 5-8: Schematic of the devices when bent at (a) 0º and (b) 90º w.r.t. drain current ........... 77
Figure 5-9: Drain to source current (IDS) as a function of drain to source voltage (VDS) under
suspended gate and the given bending conditions for strains applied at (a) 0o
and (b) 90o
with
respect to the direction of current for sample-1 ............................................................................ 78
Figure 5-10: Drain to source current (IDS) as a function of drain to source voltage (VDS) under
suspended gate and the given bending conditions for strains applied at (a) 0o and (b) 90o with
respect to the direction of current for sample-2. ........................................................................... 78
Figure 5-11: Design schematic of Displacement Sensor based on Organic Field Effect Transistor
(OFET) .......................................................................................................................................... 81
Figure 5-12: Section roughness of P3HT PCBM blend based semiconducting layer .................. 83
Figure 5-13: Output I-V Characteristics of the OFET .................................................................. 83
Figure 5-14: Drain to source current (IDS) as a function of drain to source voltage (VDS) under
given displacements when gate was suspended, inset1: Schematic of the displacement
measurement setup, inset2: Variation of the drain to source resistance (RDS) with displacement at
VDS = -8V. ..................................................................................................................................... 84
Figure 6-1: (a) Schematic View of parallel plate structures and (b) Chemical Formula of GO ... 90
Figure 6-2: Experimental set up for temperature dependent electrical analysis of GO Films. ..... 91
Figure 6-3: (a) Surface and (b) 3D AFM profiles, (c) Absorption and (d) Transmittance Spectra
of GO Films .................................................................................................................................. 91
Figure 6-4: (a) Real and (b) Imaginary Dielectric Constants, and (c) Loss Tangent Plots of GO
film as a function of frequency at given temperatures. ................................................................. 93
Figure 6-5: AC Conductivity of GO film as a function of frequency at given temperatures ....... 94
XX
Figure 6-6: Complex impedance spectrum (Nyquist Plot) for the GO at various temperatures,
inset: Equivalent RC network ....................................................................................................... 95
Figure 6-7: SEM micrographs of the thermally itched surface of PbTi0.5Zr0.3(Co1-xMgx)0.2O3
ceramics. ....................................................................................................................................... 98
Figure 6-8: Variation of dielectric properties of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
frequency....................................................................................................................................... 98
Figure 6-9: Variation of dielectric properties of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
temperature ................................................................................................................................... 99
Figure 6-10: Variation of AC conductivity of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
temperature. ................................................................................................................................ 100
Figure 6-11: Variation of AC conductivity of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
frequency at various temperatures. ............................................................................................. 102
Figure 6-12: Complex impedance plots for PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics at 300 oC .... 103
Figure 6-13: Design Schematic of Parallel Plate Structure used for Impedance Spectroscopic
Analysis of Co-polymers and Homo-polymers .......................................................................... 106
Figure 6-14: Angular frequency evolution of (a) the real (εʹ), and (b) imaginary (εʺ) dielectric
constants of the polymers............................................................................................................ 107
Figure 6-15: Cole–Cole plots of the dielectric constant (ε"– ε') ................................................. 110
Figure 6-16: Specific conductivity of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA) (n=
3, 7, and 10); inset: High resolution plots of specific conductivity of samples in high frequency
regime ......................................................................................................................................... 111
Figure 6-17: Specific conductivity versus frequency plot of Poly(Chol-n-MMA) and poly(Chol-
n-MMA-co-MMA) (n= 3, 7, and 10). ......................................................................................... 112
XXI
List of Tables
Table 2-1: Pressure Sensors based on OTFTs .............................................................................. 23
Table 2-2: Strain Sensors based on OTFTs .................................................................................. 24
Table 2-3: Environment Monitoring based on OTFT ................................................................... 24
Table 4-1: Various Parameters such as Fill Factor (FF), Short Circuit Current Density (Jsc), Open
Circuit Voltage (Voc) and efficiency (η) of polymer fullerene BHJ based solar cells realized using
three different batches of MDMO-PPV Polymers ........................................................................ 48
Table 6-1: Absorption coefficient relaxation time (o), dielectric parameters (εs, ε∞, ε''max,
and Δε) and critical frequencies (fc) of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA)
(n= 3, 7, and 10). ......................................................................................................................... 108
XXII
List of Abbreviations
P3HT Poly(3-hexylthiophene)
PCBM [6,6]-phenyl C61-butyric acid methylester
MDMO-PPV Poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene]
CuPc Copper Phthalocyanine
DC Direct current
EA Electron affinity
Eg Band Gap Energy
GaAs Gallium arsenide
HOMO Highest occupied molecular orbital
LUMO Lowest unoccupied molecular orbital
LVDT Linear variable differential transformer
OFETs Organic field effect transistors
OLEDs Organic light emitting diodes
VDS Drain to Source Voltage
VGS Gate to Source Voltage
Vth Threshold Voltage
Ci Capacitance of Active Layer per unit Area
OSCs Organic Solar Cells
Voc Open Circuit Voltage
FF Fill Factor
XXIII
Isc Short Circuit Current
𝜂 Efficiency
QE Quantum Efficiency
OTFTs Organic Thin Film Transistors
Pc Phthalocyanine
PMMA Poly (methyl methacrylate)
SEM Scanning electron microscopy
UV-Vis Ultraviolet Visible
rpm Rotation Per Minute
GUI Graphical User Interface
AMOLED Active Matrix Organic Light Emitting Diode
Co Cobalt
Mg Magnesium
SCLCPs Side Chain Liquid Crystal Polymers
LCs Liquid Crystals
AC Alternating Current
1
Chapter-1: Introduction 1 1
1.1 Preamble
During the last few decades, extensive research has been done on the organic semiconducting
materials for their applications in electronic devices due to their low temperature processing, low
cost and promising features. Further ease of their fabrication on flexible substrates makes them
potential candidates for future electronic applications. Furthermore, properties of organic
semiconductors can be easily tuned for specific applications by modifying their molecular
architecture using doping, a simple chemical synthesis technique [1]. Liu et al. [2] have recently
shown the improvement in the dielectric properties of organic Donor-Acceptor blend by doping
it with salt.
Inorganic semiconductors such as silicon and gallium arsenide have been the backbone of
conventional microelectronic area for the last five decades. History of organic microelectronics
dates back in 1950s, when drift mobility and photoconductivity analysis of Anthracene, a low
molecular weight organic material, was carried out for the first time to investigate its potential as
semiconducting material for electric device applications [3, 4]. Around three decades later,
doped polyacetylene [5], was demonstrated as a promising polymeric organic semiconductor for
electronic industry for the first time in 1977. Discovery of semiconducting properties in
anthracene and polyacetylene triggered an active research for the investigation of
semiconductive properties in a variety of low molecular weight and conjugated polymeric
organic materials. Initially performance, reliability, stability and reproducibility of the devices
developed using organic semiconductors were very poor. However with gradual advancements in
synthesis process and understanding of the architecture of these conjugated materials along with
processing of new materials such as polythiophenes and fullerenes during last two decades, the
demands for utilization of organic semiconductors in Organic Light Emitting Diodes (OLEDs),
Organic Field Effect Transistors (OFETs) and Organic Solar Cells (OSCs) are now greater than
ever due to promising commercial applications [6, 7]. Corporations such as Philips, Kodak,
Samsung and Sony have introduced commercial devices based on organic semiconductors due to
simplicity in processing and low cost. Active research is going on to process new organic
2
semiconductors and to optimize existing device architectures for drastic improvements in organic
microelectronic area.
1.2 Aims and objectives
The main objective of this dissertation is to fabricate and investigate solution processed
semiconducting films based Organic Solar Cells, Organic Field Effect Transistors and parallel
plate type capacitors. Fabrication of the devices was carried out using low cost materials
(commercially available & locally synthesized) having promising features and simpler
technology to introduce low cost devices for various industrial, environmental and medical
applications. By considering the demand of industry for sensors and new materials with adequate
performance, solution processing, low cost, flexibility and simplicity of structure, the objectives
of the current research work were as follows:
I. Realization of MDMO-PPV and [6,6]-phenyl C71-butyric acid methylester (PC71BM)
blend based Organic Solar Cells (OSCs) on Indium Tin Oxide (ITO) coated glass
substrates in order to investigate the effect of polymer intrinsic properties on the
performance of the devices
II. Fabrication and characterization of Organic Bulk Hetero-junction based MESFETs
for the first time, having one of the simplest transistor architecture
III. Investigation of thin film transistors based low cost photo sensors with adequate
sensitivity using commercially available solution processed organic materials
IV. Investigation of Organic BHJ based transistors for strain, bend and displacement
sensing applications
V. Development of capacitors type micro structures having solution processed
active/semiconducting layers sandwiched between two conductive electrodes, in
order to investigate and explore structural, electrical and dielectric properties of
locally synthesized materials with promising features and applications
3
1.3 Thesis Organization
The thesis consists of four major sections, which includes Theory and Literature survey (chapter-
2) and brief overview of Materials and Experimental Details (chapter-3) along with Results &
Discussion (chapter-4 to chapter-6) followed by Conclusion & Future Work (chapter-7). The
chapter wise detail of the thesis is as follows:
Chapter-2: This chapter comprises of three major sections. In the first part (section 2.1), basic
theory and literature related to organic semiconductors is presented which includes description
regarding general aspects, transport properties, types and utilization for the sensing applications.
Section 2.2 briefly describes theory of Organic Solar Cells, whereas section 2.3 is dedicated to
explain basic operation, parameters and potential of OTFTs for sensing applications. The
introduction and literature survey of parallel plate impedance spectroscopic technique, utilized to
investigate electrical properties of locally synthesized solution processed semiconducting
materials, is given in the last part (section 2.4) of the chapter.
Chapter-3: Detailed information about the semiconducting materials used in this work for
different investigations is given in this chapter. Furthermore, fabrication techniques and the
experimental setups used for the fabrication & characterization of various electronic devices
have also been described.
Chapter-4: Fabrication and characterization of Organic bulk hetero-junction based solar cells
and photo-MESFETs are presented in this chapter. The Chapter is divided into two main parts.
First part (section 4.1) presents experimental results of an investigation carried out to study the
effect of polymer intrinsic properties such as molecular weights, polydispersity (PDI) values,
charge carrier densities (NA) and band gap energies (Eg) on the performance of MDMO-
PPV:PCBM blend based Organic Solar Cells. Second part (section 4.2) presents the fabrication
and investigation of organic BHJ based phototransistors. Three different types of
phototransistors, having solution processed semiconducting layers made of P3HT:PCBM (with
1:1 and 1:0.8 wt/wt ratios) and P3HT:MR:PCBM (with 1:0.2:1 wt/wt ratio) blends were
developed and investigated. Electrical behavior of the phototransistors was discussed with the
help of energy band diagram and related literature.
4
Chapter-5: This chapter comprises of two main parts. First part describes fabrication &
characterization of organic bulk hetero-junction based strain/ bend sensitive OFETs, developed
on flexible PET substrates, whereas second part presents displacement sensing analysis of
OFETs realized on glass substrates. Semiconducting layers of the investigated devices were
polymer-fullerene blends having 1:1 wt/wt ratio.
Chapter-6: Development and electrical characterization of electronic microstructures, having
solution processed semiconducting films sandwiched between two parallel conductive plates, is
presented in this chapter. Characterization was carried using temperature dependent impedance
spectroscopic technique, in order to investigate structural, electrical conduction and dielectric
properties along with relaxation behaviors of locally synthesized solution processed materials i.e.
Graphene Oxide, Cobalt & Magnesium doped Lead Zirconate Titanate (PbTi0.5Zr0.3(Co1-
xMgx)0.2O3) ceramics and Methacrylate based side chain liquid crystal polymers as a function of
frequency and temperature.
Chapter-7: This chapter highlights conclusion and few directions for future investigation of the
current work.
5
Chapter-2: Theory and Background 2 1
2.1 Organic Semiconductors
2.1.1 General Aspects
Organic semiconductors are fascinating kind of materials having electrons conducting properties
in addition to other promising features. The interest in organic semiconductors is motivated by
increasing demands of materials that offer easier and inexpensive processing methods. Similar to
all organic compounds, organic semiconductors are based on carbon atoms which in case of
polymers form the main chain. Other elements or functional groups which are attached to the
backbone significantly influence its chemical and electrical properties. Each carbon atom in an
organic semiconductor makes covalent bond with other carbon atoms and partially ionic
interactions to the atoms of other elements. Each carbon atom has four valence electrons.
Carbon atoms form sp2- hybridization with other atoms as shown in Figure 2-1 [8]. Among four
valence electrons of each carbon atom, three are located in three different sp2 orbitals (lie on the
same plane) whereas fourth electron is located in pZ orbital (which lies on a plane perpendicular
to the plane of sp2 orbitals). Bonds formed due to the overlap of sp
2-sp
2 orbitals of two carbon
atoms are called as covalent/ strong bonds or ζ-bond and overlapping of pZ and pZ orbitals
results in weaker bonds called as π bonds. Both these types of bonds (ζ bond and π bond) exist in
a carbon molecule.
Figure 2-1: Schematic description of sp2 hybridization of Carbon atom [8]
6
Further, carbon atoms make single, double or triple bonds. Single and double bonds are regularly
alternated in organic semiconducting material in addition to delocalized out of plane π electrons.
Energy levels in organic semiconductors are discrete which are termed as molecular orbitals. The
two important orbitals are Highest Occupied Molecular Orbital (HOMO) and Lowest
Unoccupied Molecular Orbital (LUMO). These levels in organic materials are equivalent to
valence and conduction bands in inorganic materials, respectively. Compared to energy bands in
inorganic materials, delocalization of electrons across many π-orbitals results in splitting of wide
spread molecular orbital energy levels in organic materials. Energy required for the
delocalization of an electron from HOMO to LUMO level in organic materials is called as band
gap energy (Eg), similar to inorganic material‘s theory. The magnitude of Eg indicates electronic
and optical properties as well as possible applications for effective utilization of materials for
optimal performance of the devices [9]. Figure 2-2 presents diagram of the HOMO and LUMO
of an organic semiconductor molecule. Each circle shows an electron in an orbital; when light of
a high enough frequency and energy is absorbed, electron jumps from HOMO to the LUMO.
Figure 2-2: Diagram of a HOMO and LUMO level of a molecule [10]
7
2.1.2 Charge Transport in Organic Semiconductors
Transportation of charge carriers in crystalline/ inorganic semiconductors occurs in delocalized
states, and can be described by band theory/model. This band model is not applicable to organic
semiconductors having disordered architectures. Further, conduction phenomenon in organic
semiconductors is not entirely uncovered and understood. Weaker intermolecular wander wall
bonds exist in semiconducting polymers as compared to strong covalent bonds in inorganic
semiconductors. Organic materials have lower melting temperature, less hardness and smaller
delocalization due to weaker bonds. Transport properties are strong function of delocalization in
material [11]. Transport in organic materials is due to hopping of charges between localized
states. It is known that in organic materials, mean free path for charge carriers is smaller than the
mean atomic distance [12]. Polymers which are originally insulators are made conductive by the
inclusion of charge carriers in side chains by the process of chemical doping, light exposure or
injection from electrodes. Mobility of charge carriers in organic semiconductors is low compared
to inorganic semiconductors due to disordered transport [13].
It has been shown that mobility of charge carriers in organic materials is the function of
inhomogeneous density of hoping sites, film morphology, molecular weight, molecular
orientation, surface passivation, molecular conformation, packing and environmental
conditions/treatment. In addition, mobility of the charge carriers and other electrical properties
are also sensitive to side chains of the soluble polymers. Charge transport is generally faster
along the polymer chain as compared to bulk. Mobility of charge carriers (holes) along one
dimensional polymer chains of MDMO-PPV is reported to be equal to 0.2-0.5cm2/VS, Whereas
mobility of charge carriers in thin films of MDMO-PPV is found to be five times smaller as
compared to charge carrier‘s mobility in side chains [14]. It is also known that transport
properties of polymers are the function of polymer chain conformation and nano-scale structural
ordering [15]. Thus performance of an organic device can be tuned by varying the properties of
polymer films using different solvents, annealing/thermal curing and doping/ nano-manipulation.
2.1.3 Classification of Organic Semiconductors
Electronic devices based on solution processed thin films of organic semiconductors have
attracted much attention of the researchers in recent years due to their low cost, ease of
processing and variety of promising application. Organic semiconducting materials can be
8
broadly classified as conjugated polymers and low molecular weight materials. Low molecular
weight materials are also called as small molecules. Polymeric materials which constitute
repetition of basic fundamentals units/monomers show solubility in many organic solvents. Low
molecular organic materials are further classified as pigments and dyes. Pigments are in-soluble
and dyes are soluble low molecular weight organic semiconductors. Solution processing
techniques such as spin coating, drop casting, dip coating, spraying, etc, are generally used to
make films of polymeric materials and thermal evaporation/sublimation techniques are utilized
for coating thin films of small molecules. Single crystal films are normally grown using low
molecular weight organic materials. Poly(3-hexlthiophene) and pentacene are the representative
organic semiconductors for polymeric and small molecule categories, respectively.
Based on the electrical conduction, organic semiconductors can be p or n-type. Figure 2-3 and
Figure 2-4 depicts molecular architectures of some commonly known p and n-type organic
semiconductors, respectively. As stated before, alternating single and double bonds are clearly
seen in both types of organic semiconductors, indicating a conjugated π electron system. In the
inorganic electronics area, p-type and n-type semiconductors are developed using the process of
doping. For example addition of small amount of group III and V materials in Silicon (group IV
semiconductor) make it p-type and n-type semiconductor, respectively. However in the organic
electronics area formation of p-type and n-type organic semiconductors does not cover the same
concept, as their realization does not necessarily requires the injection of electron donor and
acceptor species. An organic semiconductor is said to be p-type if mobility of holes is greater
than electrons and n-type if mobility of electrons is higher than holes. Thus organic
semiconductors are classified as p-type and n-type based on the preference transport of holes and
electrons, respectively. Further, injection of electrons from electrodes and interfaces is easy in n-
type organic semiconductors and holes in p-type organic semiconductors. Conductivity of
specific charge carriers (holes and electrons) in organic semiconductors is the function of work
functions of electrodes in comparison to conventional organic semiconductors, where
conductivity of holes and electrons is mainly the function of doping type and its concentration.
9
Figure 2-3: Commonly used p-type organic semiconductors (a) P3HT (b) Pentacene (c) CuPc (d)
Rubrene (e) MDMO-PPV (f) MEH-PPV (g) F8BT (h) Alpha-Sexithiophene
Figure 2-4: Commonly used n-type organic semiconductors (a) Fullerene-C60 (b) Fullerene-C70 (c)
PCBM (d) Perylene (e) TCNQ (f) BBL (g) 4FPEPTC (h) DBP
10
2.1.4 Organic Semiconductor Sensors
A sensor is a device that measures physical, biological, chemical, thermal and environmental
quantities. Sensor converts these quantities into an equivalent electrical signal. Needs for sensors
are ever increasing as these are the vital part of modern measurement and control systems.
Renewable energy harvesting, food safety, environmental monitoring, medical diagnostics, and
security are the possible areas that would be benefitted from the exploitation of sensors. Sensors
are conventionally classified according to the quantity to be measured or specific technique used
for such measurement. For example, some of the organic species based sensors, utilized for
biomedical applications are listed as under [16]:
Displacement Sensors
Flow Sensors
Strain Sensors
Gas Sensors
Temperature Sensors
Speed Sensors
Pressure Sensors
Optical Sensors
Hearing Sensors
Piezoelectric Sensors
Capacity Senors
Oflaction Sensors
Harsanyi Sensors
Humidity Sensors
Enzyme Sensors
Micro-organization Sensors
Immunity Sensors
Tissue Sensors
DNA Sensors
11
Materials having semiconducting properties, also called as semiconductors, are mostly used for
the fabrication of sensors. Materials can be organic, inorganic or composite. Organic materials
based sensors and devices are preferred over inorganic ones due their ease of processing, low
cost, simple architectures and flexibility which make them suitable for large area applications.
Although conventional in-organic semiconductors based sensors are robust but are rigid and
expensive. During the last decade, organic microelectronic researchers have well investigated the
potential of organic semiconductors for low cost sensors development. Recently, pressure [17,
18], strain [19], humidity [20-22] and temperature [23, 24] sensors based on simpler
architectures were realized using organic and organic-in-organic composite materials which
indicates their potential for similar applications.
2.2 Organic Solar Cells
2.2.1 Introduction
Solar cells are used to convert light energy into electrical energy. Initially most of the solar cells
were based on inorganic materials such as crystalline silicon [25]. Maximum efficiency of
inorganic solar cells based on crystalline silicon is 24.5% [26] and 42.3% for various tandem and
multi-junction solar cells [27]. However development of these solar cells involves complex
fabrication processes which makes solar panels costly. Silicon wafer processing technology
limits the development of solar cells in large sizes. Solar cells based on the organic materials are
cheap as these are easily processed in an ordinary clean room. Focused research is required to
enhance the performance of organic solar cells in order to compete with inorganic photoelectric
energy conversion devices and to catch industrial applications.
Organic solar cells are based on the thin films of small molecules or polymer materials. These
can be fabricated utilizing cost effective processing techniques such as thermal evaporation, spin
coating, dip coating, screen printing and spray deposition. There is a huge difference between
organic and conventional solar cells as far as the pattern of carrier generation is concerned, in
both these devices. In conventional/ inorganic solar cells, electrons and holes are generated in the
bulk of the cell and these are not tightly bound to each other [28]. Separation of the carriers is
governed by the built in potential or electric field. After separation these reach at their respective
electrodes and are transported out from the active cell material. Whereas in organic solar cells
12
generated carriers (electron and hole pairs) are tightly bound to each other and, are called as
excitons in organic devices. Further, dissociation of these excitons mainly occurs at the
electrodes or at the interface between donor and acceptor organic materials. Diffusion length of
these excitons is relatively small [29-37]. Current flow in conventional solar cells is due the
diffusion of minority carriers. In contrast to these solar cells, current in organic solar cells is
because of the flow of majority carriers.
Efficiency of the first generation of the cost effective organic solar cells was very low in the
range of 10-3
to 10-2
%. Research and fabrication of these devices gained pace in past years to
achieve the efficiency of 4% for the case of evaporated bilayer devices [38, 39]. More focused
research on the various aspects has further improved their efficiency and current reported
efficiency of ultrathin organic solar cell [40] developed by Stephen et al.‘s cell is 8%. Reduction
in loss during photo excitons generation process to collection of carriers at the electrodes process
would increase efficiency up to 10% [41].
2.2.2 Energy Conversion Steps
Generation of electrical energy from sunlight (solar) energy using organic solar cells consists of
four steps:
2.2.2.1 Exciton Formation
Optical absorption bandwidth of the organic material defines the amount of light absorbed/
excitons formed in any organic solar cell. Only absorbed photons participate in the generation of
excitons. Unfortunately, organic materials normally comprises of short absorption bands and
able to absorb only 40% of the sunlight spectrum. Absorption and exciton formation capability
of the cell can be enhanced by increasing the thickness of the organic layers. But increase in the
thickness of organic layers reduces mobility of the carriers is reduced, which is already low in
the organic materials. Decreasing the band gap of the organic material is one way to increase
light absorption and to improve the efficiency of the device [42, 43]. Donor and acceptors made
up of properly tuned conjugated materials show better harvesting of solar spectrum [44]. Doping
of semiconducting films with dye is another way of increasing the absorption capability of the
device [45]. Kymakis et al. [46] have demonstrated increase in the absorption capability of an
13
organic solar cell due to inclusion of an organic dye at the interface of carbon nanotube and
polymer.
2.2.2.2 Diffusion of Exciton to the Donor and Acceptor (D-A) interface
As there are more excitons near the surface of the cell as compared to the outer surface of the
cell. Therefore excitons start moving towards the D-A interface by the process of diffusion.
Excitons with higher diffusion lengths have more probability to reach at the interface as
compared to those with lower diffusion lengths. Normally triplet type excitons have higher
diffusion length because of their higher lifetime. Thus diffusion length of an exciton is increased
by converting a singlet exciton into triplet exciton [47]. Peumans et al. [29] have developed
efficient double hetero-junction organic solar cell using materials having extended diffusion
lengths. Another way of increasing the chances of exciton diffusion to D-A interface is by
decreasing the distance between donor and acceptor [48-51].
2.2.2.3 Dissociation of excitons
Due to strong bond (Columbic interaction) of electron and hole pair in an exciton, a strong field
or force is normally required at D-A interface for the dissociation of the excitons. Difference in
LUMO and HOMO of donor and acceptor materials mainly provides this field in case of
multilayer and BHJ organic solar cells. However, in single layer organic solar cells dissociation
occurs because of the difference of the work functions of two electrodes.
2.2.2.4 Transportation of electron and hole towards the electrodes
After dissociation of the excitons into electrons and holes, electrons are accepted by the acceptor
material and holes by the donor material where these are transported towards their respective
electrodes. Better efficiency is achieved by the use of materials having good morphology. There
are more chances of the carriers to reach at the electrodes when these are transporting through
the smooth films. Smaller grain size of donor and acceptor favours dissociation and
transportation. Morphology can be adjusted by the annealing of the film [52] and solvent mixing.
2.2.3 Solar Cell Performance Parameters
Basic characterization parameters of an organic solar cells are Open Circuit Voltage (Voc), Short
Circuit Current (Isc), Maximum Power Point (MPP), Fill Factor (FF), Power Conversion
14
Efficiency (η) and Quantum Efficiency (QE). First three of these parameters are highlighted in
current voltage Curve of a typical solar cell shown in Figure 2-5. Fill factor (FF) can be
calculated using formula given below:
𝐹𝑖𝑙𝑙 𝐹𝑎𝑐𝑡𝑜𝑟 = 𝐹𝐹 = 𝐼𝑚𝑉𝑚
𝐼𝑠𝑐𝑉𝑜𝑐 2-1
Where Im and Vm are the values of the current and voltage at the maximum power point
mentioned in the above graph. Power conversion efficiency can be calculated using the following
relation:
𝑃𝑜𝑤𝑒𝑟 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝜂 = 𝐼𝑠𝑐𝑉𝑜𝑐𝐹𝐹
𝑃𝑖𝑛 2-2
Where Pin is the input power to the device i.e. Power of the solar incident radiations. Normally
its value is fixed (100W/cm2) for solar cell characterization purposes. Thus power conversion
efficiency is ratio of the output power to the input power of a solar cell. Output power of device
is also the function of the wavelength or energy of the incident photons. Performance of the
device under varying wavelengths of the incident solar radiations lies under the definition of
Quantum Efficiency (QE). Thus quantum efficiency is a measure of the amount of electrons and
holes collected at the electrodes per incident photon.
2.2.4 Different Architectures of Solar Cells
Basic structure of an organic solar cell consists of one or multiple layers of organic materials
sandwiched between two conductive electrodes. Solar cells with only one organic layer are
called single layer organic solar cells. Tang and Albrecht et al. [53] developed first single layer
organic solar cell in 1975 using chlorophyll-a as the semiconducting material but efficiency of
this cell was very low. Solar Cells in which two organic layers are stacked between the
electrodes, making a planar interface are called bilayer solar cells [54]. Anode is normally made
up of glass coated with a transparent conductive oxide (TCO) such as ITO. Rowel et al. [55]
have shown that a layer of Carbon Nano Tubes (CNTs) can also serve as TCO. Surface of the
active layer which is intact with the anode is degraded due to the diffusion of electrodes‘
particles [56]. PEDOT: PSS layer is used in between active layer and cathode of the cell to get
rid of this issue and to smooth the surface of ITO.
15
Figure 2-5: Current Voltage (I-V) Characteristics of a typical Solar Cell under dark and
illumination conditions
In bilayer device configurations, excitons generated far from the D-A interface have to travel a
lot to reach at the interface, therefore only a few of these excitons can reach at the interface by
the process of diffusion. This limits power conversion efficiency of these devices a lot [57]. In
bulk hetero-junction organic solar cells, this issue is addressed by reducing the distance between
donor and acceptor domains by adopting a specific design pattern [58]. Efficiency of the bulk
hetero-junction devices is much higher as compared to bilayer hetero-junction devices [59].
Tandem organic solar cells consist of two or more than two organic subcells in which one covers
lower range while other covers higher range of wavelengths, separated by an inverter.
2.3 Organic Thin Film Transistors (OTFTs)
2.3.1 Device Theory
Transistors which are processed using organic semiconductors are termed as Organic Thin Film
Transistors (OTFTs). Transistors are the most used electronic devices in integrated electronics
area where these are aimed to perform switching operations [60]. Thin Film Transistors are
widely fabricated as Field Effect Transistors (FETs) with three terminals. These terminals are
16
named as Source, Drain and Gate. Every transistor has a semiconducting layer between its drain
and source terminals. Transistors are classified in various ways depending upon the position of
conductive terminals/electrodes, architectures, semiconductor, substrate and dielectric type. All
basic types of transistors also have a dielectric layer between gate electrodes and semiconducting
layers. A top gate top (drain-source) contacts type transistor is shown in Figure 2-6. L and W
represent channel length and width, respectively. The current between drain source electrodes is
the function of field (voltage) applied at gate terminals in addition to the biasing applied at drain
terminals of the devices. Based on the different biasing conditions different operating regimes of
transistor can be identified. In the enhancement mode FET devices, conductance of the channel
is very low when zero or no gate voltage is applied, and the device is said to be in the off state.
The drain and gate voltages are applied with respect to source and referred as VDS and VGS,
respectively. Due to this reason, sometimes source terminal is called as ground terminal.
Although both organic and inorganic transistors show similar behavior and operation modes,
however electrical and structural properties of organic semiconductors are quite different from
inorganic semiconductors. Like inorganic semiconductor, organic semiconducting materials can
be p-type or n-type.
Figure 2-6: Typical top gate top contacts FET
17
Positions of LUMO and HOMO level in organic semiconductors are the function of applied gate
voltage. Whereas positions of the Fermi levels of electrodes remain fixed until drain voltage is
applied. Figure 2-7 schematically depicts change in the positions of LUMO and HOMO levels of
p-type organic semiconductor of a common source OTFT with change in drain and gate voltage
values of the device. When gate voltage is equal to zero, the current will not flow between drain
source contacts even if a small value of drain to source voltage is applied. This is due to
unavailability of mobile carriers and large difference between the Fermi level (work function) of
drain-source contacts and HOMO level (which carries holes) of organic semiconductor as shown
in Figure 2-7 (a). As the negative gate voltage is applied LUMO and HOMO levels of the
semiconductor shifts up and HOMO level comes in-line with Fermi Level of drain – source
contacts and electrons will come/spill out from semiconductor leaving behind mobile holes as
shown in Figure 2-7 (b). In other words, accumulation positive charges in semiconducting layer;
a few nanometers above dielectric layer is due to polarization in the dielectric layer on
application of negative gate voltage. The number of accumulated charges is proportional to the
applied gate to source voltage [61-63]. The current will flow between drain source contacts in
response to the applied drain to source voltage. This current will be due to the flow of holes as
depicted in Figure 2-7(c), indicating p-type conductivity for the case of p-type organic
semiconductors.
Figure 2-8 schematically depicts idealized energy band diagram of a common source OTFT with
n-channel operation under given biasing values. When gate voltage is equal to zero, the current
will not flow between drain source contacts even if a small value of drain to source voltage is
applied. This is due to unavailability of mobile carriers and large difference between the Fermi
level of drain-source contacts and LUMO level (where electrons reside) of organic
semiconductor as shown in Figure 2-8 (a). As the positive gate voltage is applied, a large electric
field at the semiconductor and dielectric interface is established. Further, LUMO and HOMO
levels of the semiconductor shifts downward and LUMO level becomes resonant with Fermi
Level of drain – source contacts. Now electrons can easily move from the metal contacts to
semiconductor and vice versa as shown in Figure 2-8 (b). The current will flow between drain
source contacts in response to the applied drain to source voltage. This current will be mainly
due to the flow of electrons as depicted in Figure 2-8(c), indicating n-type conductivity as
already known for the case of n-type organic semiconductors. It is pertinent to mention that
18
source is always the charge-injecting electrode irrespective of the polarity of gate voltage, as
shown in Figure 2-7(c), when transistor is operating in hole accumulation mode and Figure
2-8(c), when transistor is operating in electron accumulation mode.
Figure 2-7: Idealized energy band diagram of
an OTFT with p-channel operation under
given biasing values [64]
Figure 2-8: Idealized energy band diagram of
an OTFT with n-channel operation under
given biasing values [64]
19
Current-Voltage Curves of a typical low voltage operatable Organic Thin Film Transistor [65]
are shown in Figure 2-9. Drain current remains almost zero and shows no variation when gate
voltage is less than the threshold voltage (Vth) of the device. However when gate voltage is set
higher than the threshold voltage value then initially drain current increases linearly for lower
values of drain voltage and channel resistance stays constant. The device is said to be in the
Ohmic or linear operation mode. When VDS is further increased, the average cross section of the
current flow between drain source contacts starts reducing and channel resistance increases due
to increase in the width of depletion at higher values of drain voltage. At this state VDS = VGS -
Vth. A depletion region is established near drain contact and device is said to be in the pinched
off state. With further increase in drain voltage, higher electric field is produced and space
charge limited current flows in the channel. In other words, electric field forces charges to move
to drain contact through narrow depletion region near drain contact. The potential difference
between source and depletion region stays constant even if drain voltage is increased further.
Thus drain current saturates at a certain value depending upon the gate voltage and shows a
constant value. The device is said to be in the saturation mode. Thus for every fixed value of gate
voltage (higher than the threshold voltage of the device), variation of the drain current with drain
voltage is linear at lower values of drain voltage, and almost no variation in current at higher
values of drain voltage. Device architecture in different operating regimes can be visualized as
depicted in Figure 2-10. Different electrical parameters of an OTFT which are used for its
characterization are field effect mobility (μ), threshold voltage (Vth), insulator layer capacitance
per unit area (Ci) and switching frequency (fs). Field effect mobility in various regimes of
operation can be calculated from transfer characteristics of the transistor. The mobility value
under linear regime can be calculated using the following equations [61, 66]:
𝐼𝐷𝑆 = 𝜇𝑙𝑖𝑛𝐶𝑖
𝑊
𝐿[ 𝑉𝐺𝑆 − 𝑉𝑡 𝑉𝐷𝑆 −
1
2(𝑉𝐷𝑆)2]
2-3
Field Effect Mobility in the saturation regime (μsat) can be calculated using the following:
𝜇𝑙𝑖𝑛
= 1
𝑤𝐶𝑖𝑉𝐷𝑆
𝜕𝐼𝐷𝑆
𝜕𝑉𝐺𝑆
2-4
Where W and L are the channel width and length, respectively, Ci is the specific capacitance
(capacitance of the active layer per unit area) and Vth is the threshold voltage.
20
Figure 2-9: (a) Output and (b) Transfer I-V characteristics of a typical OFET [65]
𝐼𝐷𝑆 = 𝜇𝑠𝑎𝑡𝐶𝑖
𝑊
2𝐿[ 𝑉𝐺𝑆 − 𝑉𝑡
2] 2-5
𝜇𝑠𝑎𝑡
=2𝐿
𝑊𝐶𝑖
(𝜕(𝐼
𝐷𝑆,𝑠𝑎𝑡)
12
𝜕𝑉𝐺𝑆
)2 2-6
Switching speed is also an important parameter of an OFET which can be estimated using the
following relation:
𝑓𝑠
= 𝜇𝑜
𝐿2 1
(𝑉𝐺𝑆 − 𝑉𝑡) 2-7
Above relation shows that switching speed is not only the function of device mobility but also on
its channel length, switching speed increases when mobility is increased and channel length is
reduced. Thus reducing channel length is a way to enhance the switching speed. However
transistor shows deviation from its ideal saturation behavior when channel length is decreased so
much. Such deviation of the device from its ideal behavior is sometimes called as short channel
behavior [67, 68] of the device.
21
Figure 2-10: 3D schematics of OFET in different operating regimes [61] (a) VD<< VG - Vth, Linear
regime (b) VD, sat = VG - Vth, Pinch off point formation near drain contact (c) VD > VD, sat, Beyond
Pinch off point
22
2.3.2 OTFTs based Sensors
Active research in the area of organic electronics during the last decade has suggested that
OTFTs being developed using solution processing techniques are ideal candidates for the
realization of various kinds of low cost and flexible sensors. OTFTs perform both switching and
sensing functions in these devices. It has been observed, due to physical and environmental
changes, channel current and threshold voltage of these devices varies significantly. These
variations in the electrical parameters of the devices are used to measure the external interacting
world. In this section review of various types of sensors developed using OTFT architecture is
presented. Two recently published articles [69, 70] present review of OTFTs based pressure,
vapour, chemical and bio sensors. Table 2-1 and Table 2-2 describe basic parameters of some
OTFTs realized for pressure and strain sensing applications, respectively. Environment
monitoring includes measuring ambient/environmental parameters such as light, temperature,
humidity, gases, chemicals, bio, etc. OTFTs were also investigated for sensing these
environmental parameters during the last decade.
Table 2-3 lists basic parameters of some OTFTs realized for the purpose of environment
monitoring in recent years.
2.4 AC Impedance and Dielectric Spectroscopic Studies
Impedance or dielectric spectroscopy is a powerful technique used to characterize bulk and
interface electrical properties of materials with conducting electrodes [71]. Dynamics of the
charge carriers of a wide variety of materials such as solid, liquid, semiconducting or insulator
can be studied using this technique. Furthermore, motions and relaxation behavior of the
molecules are also estimated as a function of frequency having a wide frequency range of 10-5
to
1011
Hz. When an electric field is applied across the terminals having a dielectric material in
between, the material polarizes i.e. atoms and molecules of the material are displaced from their
equilibrium positions. Polarization is said to be electronic if only displacement of electrons
within the atoms of a material occurs due to applied field and ionic if atoms of a molecule (which
are bonded with each other due opposite polarities) are displaced from their equilibrium
positions. Polarization in polymeric materials mainly occurs due to charge migration and re-
orientation of permanent dipoles. Impedance spectroscopy determines type as well as strength of
polarization which shows its power and importance in characterizing the material properties.
23
Table 2-1: Pressure Sensors based on OTFTs
Semiconducting
Layer Material
Dielectric
Layer
Material
Substrate
Response
Time/
Sensitivity
Possible
Applications Year References
PDPP3T,
NDI3HU-
DTYM2
PMMA,
CYTOP,
PS
Glass, PI,
PET 192 kPa−1
Acoustic
Wave
Sensing,
Wearable
Electronics,
Robotics
2015 [72]
Pentacene polyimide
precursors PEN 30 kPa−1
Artificial
Skin 2004 [73]
P3HT Rubber PET 22ms Artificial
Skin 2009 [74]
Pentacene
Plastic
Foil
(Mylar)
Plastic
Foil
(Mylar)
Tens to
hundreds of
ms
Physical
sensing 2007 [75]
Ruberene Single
Crystals
Micro
structured
rubber
PET Millisecond
s
Electronic
Skin 2010 [76]
Pentacene PVP Glass 20 s force sensing 2005 [77]
Poly iso indigo
bithiophene-
siloxane
PDMS PET 8.4 kPa
−1,
<10 ms
Electric Skin
and Health
Monitoring
2013 [78]
Pentacene PVP PEN n.a. Hydrophone 2007 [79]
CuPc
No
Dielectric
Layer
Glass n.a. Pressure
Sensing 2010 [80]
CuPc
No
Dielectric
Layer
Glass n.a. Telemetry 2012 [81]
24
Table 2-2: Strain Sensors based on OTFTs
Semiconducting
Layer Material
Dielectric
Layer
Material
Flexible
Substrate
Type
Response
Time/
Sensitivity
Possible
Applications Year References
P3HT:PCBM
Blend
No
Dielectric
Layer
PET
0.18 μA/%
and
0.65μA/%
Bend
Sensing 2015 [82]
P3HT,
Pentacene PET PET
Between
100-150 ms
Strain
sensing 2012 [83]
Pentacene PVA PET n.a. Strain
sensing 2013 [84]
Pentacene Polyimide PEN n.a. Bend
Sensing 2005 [85]
Pentacene PVP PEN
1.6 nA/%,
7.2 nA/%,
4.1 nA/%
Bend/Strain
Sensing 2007 [86]
Pentacene
Ta2O5
and PVP
bilayer
PEN 20 s Strain
Sensing 2005 [87]
Pentacene Parylene
C PVDF 0.182 μA/%
Piezoelectric
Conversion 2012 [88]
Table 2-3: Environment Monitoring based on OTFT
Semiconducting
Layer Material
Dielectric
Layer
Material
Flexible
Substrate
Type
Response
Time/
Sensitivity
Possible
Applications Year References
Pentacene PVPy Glass n.a. Temperature
Sensing 2012 [89]
P3HT SiO2 Si Wafer See ref. Gas sensing 2015 [90]
CuPc No
Dielectric Glass n.a.
Humidity
Sensing 2010 [91]
Pentacene
SiO2-
PMMA
Bilayer
Si n.a. Amonia
Sensing 2012 [92]
P3HT:PCBM
Blend
No
Dielectric Glass 3
Light
Sensing 2014 [93]
Biotinylated
F8T2 SiO2 Si n.a. Bio Sensor 2012 [94]
isoindigo-based
polymer SiO2 Si n.a
Detection in
the marine
environment
2014 [95]
Gain phase Impedance analyzers and LCR meters are usually used for Impedance Spectroscopic
Analysis. Electrical characterization using this technique involves following measurements:
25
a. Variation of the real part of complex impedance and dielectric constant with frequency,
signal amplitude and temperature
b. Variation of the imaginary part of complex impedance and dielectric constant with
frequency, signal amplitude and temperature
c. Variation of the complex impedance and dielectric constant modulus with frequency,
signal amplitude and temperature
d. Cole-Cole plots ( variation of the imaginary part of dielectric constant as a function of
real part of dielectric constant at certain frequency, signal amplitude and temperature)
e. Nyquist plots ( variation of the imaginary part complex impedance as a function of real
part of complex impedance at certain frequency, signal amplitude and temperature)
f. Dielectric Loss variation as a function of frequency
g. Dielectric Strength
h. Relaxation Type/Phenomenon
i. Relaxation time
j. Absorption Coefficient
k. Critical Frequency
The Complex Dielectric Constant (ɛ*) of the investigated films/materials can be mathematically
described as follows [96]:
휀∗ = 휀 + 𝑖휀ʺ 2-8
Here ɛ′ is real part of dielectric constant and ɛʺ is imaginary part of dielectric constant. Cole-Cole
form of the above relation can be written as shown below [97]:
휀∗ = 휀∞ +
(휀𝑠 − 휀∞)1 + (𝑖𝜔𝜏)1−𝛼 2-9
Here ɛs and ɛ∞ parameters specify dielectric constant values at lowest and highest frequency in
the measured frequency range, respectively. ω is the angular frequency which is equal to 2π
times the frequency, η is the relaxation time and α is the absorption coefficient. Capacitance can
be computed using the following relation:
26
𝑪 = 휀𝒐휀𝐴/𝑑 2-10
C is the capacitance, ɛo is the dielectric constant of free space, A is the surface area and d is the
thickness of the cell. Real dielectric constant (ɛ′) value can also computed from the above
relation if capacitance is known.
Imaginary dielectric constant (ɛʺ) of material is actually the measure of dielectric loss as shown
in the following relation:
εʺ = ε 𝑡𝑎𝑛 δ 2-11
Thus imaginary part of the dielectric constant is dielectric loss (tan δ) times the real part of
dielectric constant. Phase angle (θ) and δ are linked with each other as follows:
𝛿 = 90 − 𝜑 2-12
The complex impedance (Z*) of a material can be written as:
𝑍∗ = 𝑍ˊ + 𝑖𝑍ˊˊ 2-13
Where Zʹ and Zʺ are the real and imaginary parts of complex impedance, respectively. Zʹ can
also be written as follows [98]:
𝑍ˊ = 𝑅𝑠 + 𝑅𝑝/1 + (𝜔
𝜔𝑜)2 2-14
RS, RP and ωo in the above equation represent series resistance, parallel resistance and natural
angular frequency of the system, respectively. The dielectric strength (Δɛ) of the investigated
material can be calculated using the following relation:
∆휀 = 휀𝑠 − 휀∞ 2-15
27
The real part of dielectric constant of a material in cole-cole form can written as follows [99]:
2-16
The AC conductivity dependence of the angular frequency can be expressed by the following
relation, as the empirical Jonscher's universal law [100, 101]:
𝜎𝐴𝐶 𝜔 = 𝜎𝐷𝐶 + 𝐴𝜔𝑠 2-17
Where, A is a constant and s is the frequency exponent parameter which identifies AC
conduction mechanisms. ζDC specify DC conductivity of the material. The magnitude of
exponent s, which represents main body interactions of electrons and impurities, is usually less
than unity. The magnitude of parameter s (angular frequency exponent) is calculated from the
slopes of ln(ζʹ)-ln(ω) plots.
In fact Impedance Spectroscopy (IS) is a powerful tool, utilized to investigate important
parameters of materials in order to study their potential for various devices. Based on the values
of various parameters (stated above), relaxation and conduction mechanisms of the materials are
determined. Relaxation can be Debye, nearly-Debye or non-Debye type. The s parameter values
can be interpreted by conductivity mechanisms for frequency dependent conductivity given by
s≈0 [101] for DC conductivity mechanism, 0<s<0.7 [102] for Correlated Barrier Hoping (CBH)
conductivity mechanism, 0.7≤s<1 [103] for Quantum Mechanical Tunneling (QMT)
conductivity mechanism and 1<s<2 [104] for Super Linear Power Law (SLPL). A wide variety
of materials were investigated using this technique in past. These materials include polymers
[105-110] , Liquid Crystals [96, 99, 111-114], Dielelectrics/ Insulators [115, 116], Ceramics
[117-121] and others [122-128].
)1(2)(
2
1sin1)(21
2
1sin1)(1
)()('
oo
o
s
28
Chapter-3: Materials and Experimental
Details 3 1
In this chapter, the materials, techniques and setup used for the fabrication & characterization of
the devices will be introduced briefly. In section 3.1 general properties and applications of the
active layer‘s materials are presented. These materials are Poly(3-hexylthiophene)(P3HT),
Poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), [6,6]-
phenyl C61-butyric acid methylester (PCBM), Polymer-Fullerene blends, Graphene Oxide (GO),
Side Chain Liquid Crystalline based homo-polymers: Poly (chol- n-MMA) & copolymers: Poly
(chol-n-MMA-co-MMA), and Lead Zirconate Titanate Ceramics. In section 3.2, brief overview
of the techniques and equipment used for the fabrication of various films of the devices is
presented. Contact films of the devices were processed using Vacuum Evaporation. Whereas
active/semiconducting layers of the devices were solution processed using spin coating and drop
casting approaches. At the end, brief introduction of the characterization equipment is presented
in section 3.3. Characterization equipment includes AFM, SEM, DEKTAK-8 profiler,
Impedance Analyzer HP-4194, UV-Vis absorption spectrophotometer (S-3100) and Lamp (Lot
Oriel), Semiconductor Characterization System (Keitheley SCS-4200) and special test fixtures.
3.1 Materials
3.1.1 Semiconducting Materials for Solar Cells and Transistors
3.1.1.1 Poly(3-hexylthiophene)
Poly(3-hexylthiophene) which is commonly known as P3HT, is a well investigated p-type
organic semiconductor in the area of organic electronics, in particular for photo sensing
applications. It shows high solubility in many organic solvents as it contains alkyl-groups.
Further, it belongs to the category of conjugated polymers and is based on sp2 hybridized carbon
atoms which forms thiophene rings. P3HT is synthesized by the addition of 3-hexyl groups as
side chains in the thiophene rings of insoluble polymer Polythiophene following a chemical
process [129]. Based on the arrangement of side chains, P3HT can be divided into two
categories i.e. regiorandom and regioregular. P3HT is called as regiorandom P3HT if side chains
29
are arranged from head-to-head and tail-to-tail, and regioregular P3HT if arrangement of the side
chains is from head-to-tail all along the chain. Regioregular P3HT (rr-P3HT) is used in this work
which is generally preferred over regiorandom P3HT due to its flat and crystalline structure
which helps to achieve high mobility [130]. Baeg et al. [131] have recently reported rr-P3HT
based organic field effect transistor with a relatively high mobility of 0.4cm2/VS. P3HT is
soluble in many solvents such as chloroform, 1,2 dichlorobenzene, toluene etc. Slight variations
in the optical and structural properties of P3HT films are reported due to different solvents [132].
Band gap energy (Eg) of P3HT is around 1.9 eV. Figure 3-1 below shows molecular architecture
of P3HT.
Figure 3-1: Molecular Formula of P3HT
3.1.1.2 MDMO-PPV
Like P3HT Conjugated polymer Poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-
phenylenevinylene] (MDMO-PPV) is soluble in many organic solvents and has been used in the
realization of OLEDs [133, 134], OSCs [135-138], OTFTs [139] and AMOLEDs [140] . The
band gap of MDMO-PPV is about 2.2 eV. Figure 3-2 is presenting chemical formula of MDMO-
PPV conjugated polymer. MDMO-PPV and other conjugated polymers are potential candidates
for low cost and large area electronics due to solution processing and promising features.
MDMO-PPV serve as donor in bulk hetero-junction based systems. It is known that degradation
rate of MDMO-PPV is reduced when blended with fullerens [141]. Three different batches of
MDMO-PPV polymers with varying intrinsic properties were used to investigate their effect on
the performance of organic BHJ solar cells.
30
Figure 3-2: Molecular Structure of MDMO-PPV
3.1.1.3 PCBM
PCBM is soluble derivative of n-type low molecular weight organic semiconductor, Buckminster
Fullerene C60. It has been widely investigated as electron acceptor in polymer organic solar cells
[142] and n-type semiconductor for organic thin film transistors [143]. Its good acceptor
characteristics are probably due its spherical shape and suitable electron affinity value. Higher
cost and low absorption in the visible spectrum are the major disadvantages of PCBM. Band Gap
Energy (Eg) of PCBM is around 2.3 eV. Chemical Formula of PCBM is presented in Figure 3-3
below.
Figure 3-3: Chemical Formula of PCBM
31
3.1.1.4 Polymer Fullerene Blends
In recent years, polymer fullerene blends in which polymer serve as electron donor and fullerene
as electron acceptor were well investigated for photonic applications, in particular for Organic
Solar Cells [144]. Under illumination, photons having energy higher than the band gap energy of
polymers are absorbed and excitons (electron and hole pairs) are produced. These excitons move
towards donor and acceptor interface due to diffusion, dissociated into charge carriers and are
collected at the electrodes of the devices due to internally developed or applied electrical field.
P3HT:PCBM blends were used to realize OTFTs and MDMO-PPV:PCBM blends were utilized
to develop solar cells.
3.1.2 Semiconducting Materials for MIM Capacitors
3.1.2.1 Graphene Oxide
Graphene oxide (GO) has sp2 and sp
3 hybridized carbon atoms and can be considered as
insulating material compared to graphene. GO exhibits various carboxyl and hydroxyl functional
groups and is soluble in water. The optical and electronic properties of GO can be tuned by
varying the content of oxygen [145]. Recently, GO was demonstrated as channel and dielectric
material for field effect transistors [146], and efficient hole transport layer for organic solar cells
[147]. Due to the presence of oxygen based functional networks on the basal plane of GO, it can
make bonds with variety of organic and inorganic materials [148-150]. The optical and electrical
properties of GO can be enhanced using composites with conductive polymers. In this study,
Dielectric and Electrical properties of GO were investigated using parallel plate impedance
spectroscopic technique. Molecular structure of GO is shown in Figure 3-4.
Figure 3-4: Molecular Architecture of Graphene Oxide
32
3.1.2.2 Lead Zirconate Titanate (PbZrTiO3) Ceramics
Lead zirconate titanate (PbZrTiO3 or PZT) based materials/ceramics with perovskite ABO3 ((A =
Mono –or divalent; B = Tri- hexavalent ion)) structure has attained much focus of the researchers in
recent years because of the simplicity of its crystal structure which helps to understand the
relation between structural changes and physical properties. Perovskite based piezoelectric
materials show high electromechanical transformation, due to which these are commonly utilized
in electro-ceramic applications in industry. Furthermore PZT, due its excellent piezoelectric
properties is commonly utilized during the development of underwater/ultrasonic transducers,
such as hydrophones, actuators and underwater transducers. In industry, materials showing
thermally stable dielectric as well as piezoelectric properties around transition temperature are
required [151-155]. The dielectric properties of PZT can be tuned by using various isovalent and
aliovalent dopants. Under certain conditions when PZT crystals possess tetragonal or
rhombohedral symmetry, each crystal has a dipole moment.
PZT materials, exhibit a unique range of properties which make them suitable for the
development of sensors and actuators. In a basic sense, if a piezoelectric material is deformed, an
electrical signal proportional to bending/deformation due to piezoelectric effect. Also if electrical
signal is applied deformation or bending is observed in these materials due to inverse
piezoelectric effect. In order to improve the electrical properties of PZT, various attempts have
been reported in recent years. Doped PZT are mostly synthesized by using one of the following
methods:
a. conventional solid state sintering route,
b. peroxohydroxide method,
c. citrate precursor method, precipitation method,
d. hydrothermal method and
e. solgel method.
In order to investigate the effect Cobalt/Magnesium ratio on the electrical and structural
properties of PZT films, Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65,
0.85) ceramics films were solution processed using sol-gel route and characterized using parallel
plate impedance spectroscopy. Mg doped PZT ceramics possess activation energy (Ea 1.05
33
eV) which is comparable with the Ea values (11.1 eV) associated with doubly ionized oxygen
vacancies (VÖ) common in perovskite oxide piezoelectric materials.
3.1.2.3 Side Chain Liquid Crystalline Polymers
PMMA derivatives of side chain liquid crystalline homo-polymers (Poly (chol- n-MMA)) and
copolymers Poly (chol-n-MMA-co-MMA) were investigated using parallel plate dielectric and
impedance spectroscopic technique as an alternative to conventional solution processed PMMA
dielectric layer for OTFTs. These polymers were having a broken focal-conic fan texture with
smectic phase. Side chain liquid crystals demonstrate both properties of polymer and liquid
crystalline, which are formed by attaching polymer as side chain. Properties of the materials
were found to be the function of their backbone, aliphatic spacer length and mesogen. Liquid
crystals are intermediate state of matter, in between the crystalline (solid) and isotropic (liquid).
It must have some features of a liquid (e. g. fluidity, inability to support shear, formation and
coalescence of droplets) as well as solids (anisotropy in optical, electrical, and magnetic
properties, periodic arrangement of molecules in one spatial direction, etc.). Molecules are highly
order when material is in solid (crystalline) state and show long range of periodicity in three
dimensions. Molecules of a liquid have no specific intrinsic order. Although molecules in liquid
crystal do not exhibit any positional order similar to the case of liquids, but they do possess a
certain degree of orientational ordering of molecules. Above stated difference between solid,
liquid crystal and liquid states of a matter is schematically explained with the help of Figure 3-5.
Figure 3-5: Position and Orientation of molecules when matter is in (a) Crystalline (b) Liquid
Crystalline and ( c) Liquid state
34
Liquid crystals are generally anisotropic materials and their physical properties are dependent on
the average alignment of the molecules. Depending upon the amount of order in the material,
liquid crystals can have Nematic, Smectic and Cholesteric phases.
Nematic liquid crystals
In this type of liquid crystals, molecules can be freely positioned but oriented parallel to each
other. As can be seen in Figure 3-6 (a), the molecules point vertically but with no particular
order.
Cholesteric liquid crystals
This type of liquid crystal is based on a twisted structure or in other words the director rotates
about an axis as you move through the material. Further this phase consists of nematic layers
with different direction vector. Thus Cholesteric phase (shown in Figure 3-6 (b)) can also be
referred as helically twisted nematic phase. Cholesterics has no long-range orientation order and
no long-range order in positions of the centers of mass of molecules.
Smectic liquid crystals
As shown in Figure 3-6 (c), both positional and orientational ordering exist in this phase of liquid
crystals. Further this phase has three different types called as A, B and C.
Figure 3-6: Schematic illustration of (a) Nematic, (b) Cholesteric and (c) Smectic phase of liquid
crystals
.
35
3.2 Device Fabrication
In this part, brief description of the equipment and techniques used for the fabrication of devices,
is presented. Electronic devices are generally based on semiconducting, dielectric and metallic
films. Organic microelectronic devices are preferred over in-organic devices due to their low cost
and ease of processing. Organic semiconductors can be low molecular weight or conjugated
polymers. Low molecular weight organic films are generally processed using vacuum
evaporation whereas polymeric organic films are developed using drop casting, dip coating,
spraying and spin coating. In the conventional/in-organic semiconductors based electronic
devices different techniques are used for the development of dielectric films such as thermal
oxidation or plasma assisted CVD for the deposition of SiO2 dielectric film on Si substrate. In
organic microelectronic and in particular flexible electronics area, dielectric films are solution
processed using spin coating, drop casting and dip casting. Physical Vapor Deposition techniques
such as thermal oxidation and sputtering are used for the development of contacts for the
devices. The film‘s morphology and the properties of devices are dependent upon the fabrication
technology [156, 157]. Spin coating technique was used for the processing of polymer
semiconducting and dielectric films, drop casting for the development of GO films, sol-gel
process for lead zirconate titanate ceramics and Vacuum/Thermal evaporation for making
metallic contact films.
3.2.1 Vacuum Evaporation
Vacuum or thermal evaporation is one of the common and oldest techniques used for depositing thin
films [158, 159] and widely used for the deposition of metals, metal composites and semiconductors.
Film material is evaporated in vacuum which condensed into solid state after reaching at the substrate
surface
The following sequential steps to take place during film formation using vacuum evaporation
process:
i) Cleaning of the chamber, source material and Crucibles/Coils
ii) Fixing of crucibles/boats to electrodes
iii) Placing of suitable amount of source material in crucibles or coils
iv) Placement of sample/substrate on the sample holder station and adjust the distance
between samples holder and source material.
v) Establishment of required vacuum level with the help of vacuum pumps
36
vi) Heating of source boat (Coils or crucibles) to evaporate metal in the form of vapors
In this research work, Leybold Univex 450 Vacuum Evaporator has been used for metallic
contact deposition. The Univex Leybold 450 is multi-station vacuum coater, specifically
designed for the deposition of various metal films under high vacuum. The vacuum pumping
system for the Evaporator comprises a TRIVAC D 40 B two-stage rotary vane pump backed by a
TURBOVAC 1000 turbo molecular pump. Figure 3-7 shows photograph of vacuum evaporator.
3.2.2 Spin Coating
Spin coating is a technique used to develop uniform thin films on flat substrates. Polymer
semiconducting films of the investigated devices were processed using spin coating process after
preparing homogenous solution using magnetic stirring station shown in Figure 3-8. A specified
small amount of semiconducting solution is applied on the samples after placing samples on the
spin coating station. Then spinning of the samples is undertaken in steps, initially with low speed
of around 800 rpm for 20 seconds followed by high speed of around 2000 rpm for 50 seconds.
Films deposited using spin coating processes are generally uniform and reproducible. Further
thicknesses and structural properties of the films are the function of spin speed and time.
Figure 3-7: Leybold Univex 450 vacuum evaporator
37
Figure 3-8: (a) Semiconductor in powder form as received from Manufacturer (b) Solvent (c)
Semiconductor solution (d) Stirring Station with temperature control and monitoring
After formation of semiconducting films, samples were annealed. Annealing process was carried
out by using hot plate system placed in the nitrogen filled glove box. Photograph of the Glove
Box is shown in Figure 3-9. Glove box is generally used to store processed samples in order to
prevent oxidation of films.
3.2.3 Drop Casting Process
Drop casting involves dropping solution on the substrate and evaporation of solvent. This
deposition technique was used for the development of graphene oxide films. In order to
evaporate solvent, thermal curing of the samples was done in industrial oven. This is a simple
process of film formation. Unlike spin coating process, no wastage of material occurs. However
films deposited using this process are less uniform. Further, it is hard to control the thickness of
the films deposited using this method.
38
Figure 3-9: Glove Box System
3.3 Characterization Setup for Films and Devices
Structural properties of the films which involve surface roughness, film morphology, uniformity,
size of the film constituents along with inter-particle distance and overall film thicknesses were
studied using Multimode Atomic Force Microscope and Scanning Electron Microscope (XL30,
FEi Co., Hillsboro). Film thicknesses were measured using DEKTAK-8 profiler shown in Figure
3-10. Optical properties of the films were investigated using UV–Vis Spectrophotometer, S-3100
(Shown in Figure 3-11).
Figure 3-10: DEKTAK-8 Surface Profiler
39
Figure 3-11: UV-Vis Spectrophotometer interfaced with computer
Dielectric and electrical conduction properties of the films were studied using Spectrum
Analyzer HP 4194. Spectrum Analyzer was interfaced with a computer, Novotherm heating
system and a temperature controller. Process parameters can be easily adjusted using Labview
based GUI. Figure 3-12 shows photograph of the spectrum analyzer and Novotherm heating
station used for the electrical characterization of the investigated films. Current Voltage (I-V)
characteristics of the films were studied using Keithely Semiconductor Characterization System
SCS-4200. Precise measurement in the Pico Amperes range is possible using Keithley 4200
measurement system shown in Figure 3-13.
40
Figure 3-12: Impedance Analyzer HP 4194 and Novotherm Heating Station
Figure 3-13: Semiconductor Characterization System: Keithley 4200 SCS
41
Chapter-4: Polymer-Fullerene BHJ Solar
Cells and Phototransistors 4 3
4.1 MDMO-PPV:PCBM blend based Organic Solar Cells
Investigation of organic bulk heterojunction solar cells (OSCs) realized using three different
batches of poly[2-methoxy,5-(30,70-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV)
with varying intrinsic properties such as molecular weights, polydispersity (PDI) values, charge
carrier densities (NA), band gap energies (Eg), etc. is presented in this section. Blend of MDMO-
PPV and [6,6]-phenyl C71-butyric acid methylester (PC71BM) as photoactive layer of the devices.
Properties of the active layers were initially studied using UV-Vis Absorption Spectroscopic and
Atomic Force Microscopic techniques. Polymer intrinsic properties have significantly influenced
on the structural properties of active layers. Devices were characterized under dark and
illumination conditions, and their results were compared. Efficiency of the device was found
highest for polymer having higher PDI and NA values and vice versa. It was attributed to the
increased photon absorption capability and favourable nano morphology of active layer for
MDMO-PPV batch with higher PDI and NA values. These results showed that intrinsic properties
of polymer play a vital role on the performance parameters of Polymer-Fullerene bulk hetero-
junction based OSCs.
4.1.1 Introduction
During the past decade, investigation of solar cells using organic semiconductors have attracted
much attention of the optoelectronics researchers due to their potential applications for the
realization of flexible, light weight and cost effective energy conversion platforms [160].
Organic bulk heterojunction solar cells, in which photoactive layer is based on the blends of p-
type and n-type organic semiconductors, have consistently been playing a leading role in this
emerging area of photovoltaics. P-type and n-type organic semiconductors which are also called
as Donor and Acceptor, respectively, form an interpenetrating network with a nanoscale phase
separation in bulk volume of the organic layer. Processing of semiconducting layer is mostly
carried out using vapour deposition approach for the case of low molecular weight organic
materials whereas solution processing is adopted for the case of polymer based systems [161].
42
Polymer fullerene blends are among the well-studied combinations for bulk hetero-junction solar
cells due to their ease of processing and promising features [51, 162]. Thompson et al. [163]
have reviewed various polymer fullerene bulk hetero-junction (BHJ) solar cells and discussed the
effect of BHJ film‘s morphology and polymer-fullerene interactions on the performance of these
devices.
MDMO-PPV:PCBM blend is among the well investigated polymer fullerene combination for
BHJ solar cells [164, 165]. Earlier studies on this typical polymer fullerene blend include the
investigation of the effect of solvent type [166, 167], blend ratio [168, 169], temperature-
degradation [170] and electric field variation [171] on the their properties. It has been shown that
these variations significantly influence upon the morphology of these films, and short-circuit
current (Jsc), being the strong function of organic film morphology [172], changes as a result.
Thus performance of a solar cell can be modified by tuning the morphology and transport
properties of its BHJ photoactive layer.
Performance of polymer solar cells is known to be the function of polymer intrinsic properties, in
particular, weights of the molecules and their orientations. Liu et al. [173] have investigated the
effect of PTB7 polymer‘s molecular weight on the performance of polymer fullerene BHJ solar
cells and reported 1.6 times increase in its efficiency with the increase of polymer molecular
weight from 18 to 128 kg/mol. Recently, Tumbleston et al. [174] have shown that the
performance of BHJ solar cell is also function of the orientation of the molecules in the organic
Donor-Acceptor blend based films. In this work (which is continuation of the work undertaken
by Yaman et al. [175]), combined effect of polymer intrinsic properties such as Molecular
Weight, Polydispersity, Charge carrier density and band gap energy have been studied on the
performance of MDMO-PPV:PCBM blend based Solar Cells which is not previously
investigated to our best knowledge. Three different batches of MDMO-PPV polymer with
varying intrinsic properties were used for this study.
4.1.2 Experimental Methods
Effect of polymer intrinsic properties on the performance of BHJ solar cells is investigated in
this study using three different batches of MDMO-PPV polymer denoted as polymer batch A, B
and C. Recently, Tunc et al. [176] have investigated intrinsic properties of these polymer batches
43
using different methods and investigated the effect of polymer intrinsic properties on the I-V
characteristics of Organic Field Effect Transistors (OFETs), in particular on the short channel
effect of the OFETs. Polymer batch-B has the highest polydispersity (PDI) and Charge Carrier
Density (NA) values, whereas batch-C carries the highest Mn and Mw values among all the
polymer batches. Mn and Mw are the number and weight averages of the molecular weights,
respectively, and polydispersity is the ratio of Mn and Mw. Polymer batch-A has the lowest Mn,
Mw and PDI values among all the investigated polymer batches. Varying NA values of different
polymer batches indicates differences in their impurity levels. Among all other properties of
polymers, Eg is of particular importance for photovoltaic applications because photon absorption
is the direct function of it. Band gap energy (Eg) values of polymer batch A, B and C are equal to
2.12, 2.08 and 2.06 eV, respectively. It is pertinent to mention that polymer batch-A which has
the lowest molecular weights, carries the highest energy gap. It is expected that these parameters
will influence on the electrical properties of the organic films by affecting on the inter-chain
ordering of the molecules.
Fabrication of the devices was started with the cleaning of Indium Tin Oxide (ITO) coated
patterned glass substrates in an ultrasonic bath using detergent, de-ionized water, acetone and
isopropyl alcohol. Poly(3,4-ethylenedioxylenethiophene)-polystyrenesulfonate (PEDOT:PSS)
film was coated on ITO substrates using spin coating technique. This hole conductive layer not
only smoothes ITO surface but also used to increase the work of bottom electrode so that it can
accept holes, and to decrease the work function of top electrode so that it can accept electrons
from the active layer [177]. PEDOT:PSS coated samples were annealed at 110 ºC for 30 minutes.
MDMO-PPV:PCBM blends with 1:4 w/w ratio, were prepared in dichlorobenzene by the process
of stirring at 50 ºC for 24 hours, using three different batches of polymer (MDMO-PPV). In
order to attain similar thicknesses of all the films i.e. ≈ 100nm, varying concentrations [12, 6 and
7 mg/mL] of polymer batch A, B and C were used. Optimal performance of MDMO-PPV:PCBM
blends based BHJ solar cells has been demonstrated with 1:4 blends ratio [178], which is the
reason of selecting this specific ratio of these blends for this study. Spin coating technique was
used to coat organic semiconducting layers on the samples using these MDMO-PPV:PCBM
blends. Then samples were thermally cured in the Glove Box at 50 ºC for 10 minutes. Finally,
around 50 nm thick top contact was made on the organic films by the evaporation of Aluminium
in the evaporation chamber. Figure 4-1 schematically presents the structure of devices along with
44
the molecular formulas of PEDOT:PSS, MDMO-PPV and PCBM. Current-voltage (J–V)
characteristics of the devices were studied under dark and UV-Vis illumination of 100mW/cm2,
using Semiconductor Characterization System, SCS-Keithley4200, and 150W Oriel Solar
Simulator with AM1.5 filter. Solar Simulator was calibrated by a reference solar cell during the
measurements. Structural profiles of the semiconducting layers were studied using NanoScope
Multimode Atomic Force Microscope (AFM). Absorption spectra were extracted using UV-Vis
Spectrophotometer (S-3100). Thicknesses of the organic and electrode films were measured
using DEKTAK-8 profiler. All the test and measurements were performed in the ambient
conditions.
Figure 4-1: (a) Schematic of the device architecture, and molecular structures of (b) PEDOT:PSS
(c) MDMO-PPV (d) PC71BM
4.1.3 Results and Discussion
Figure 4-2 shows 10×10 µm2 surface and high resolution 3D AFM Profiles of Polymer:Fullerene
bulk hetero-junction based semiconducting layers developed using three different batches of
MDMO-PPV polymers. Thin film casted from polymer batch-A exhibits the highest height
variations. Root Mean Square (RMS) roughness values of the blend films cast from polymer
45
batch A, B and C were found to be 3.104, 0.909 and 1.647 nm, respectively. Lowest surface
roughness and highest PDI value was found for the polymer batch with moderate molecular
weights i.e. batch-B. Further, phase separation of the composite film developed using polymer
batch-B was found significantly larger as compared to the films prepared using other polymer
batches.
Figure 4-2: 10×10 µm2 surface and high resolution 3D AFM Profiles of Polymer: Fullerene Bulk
heterojunction based semiconducting layers for three different batches of MDMO-PPV : A[(a, b)],
B[c, d], and C[(e, f)]
Figure 4-3 shows UV-Vis absorption spectra of Polymer:Fullerene bulk heterojunction based
nanolayers prepared from three different batches of MDMO-PPV polymers. Thin films, prepared
using polymer batch-B showed highest UV-Vis absorption with maximum absorption peak (λmax)
at 497 nm, and two shoulder peaks at 416 and 598 nm. Thin films prepared from polymer batch-
46
A demonstrated the lowest absorption of photons. Further, no significant peaks were observed
for polymer-fullerene blend based films prepared from batch-A and batch-C.
Figure 4-3: Absorption Spectra of MDMO-PPV:PCBM nanolayers (with 1:4 w/w ratio) prepared
using three different batches of MDMO-PPV Polymer
Formation of excitons, their diffusion towards Donor and Acceptor (D-A) interface and
dissociation into free carriers (electrons and holes) at D-A interface, and transportation of these
carriers towards electrodes are among the basic steps which are essentially followed in BHJ solar
cells during the process of solar energy conversion into electrical power [179]. Therefore, role of
the organic film morphology and D-A interface is of primary importance in these devices. The
ideal film architecture is the one which supports the excessive generation of excitons, minimizes
the chances of exciton and carrier recombination simultaneously enhancing charge carrier
generation and their transportation towards electrodes.
The performance of the devices developed using three different batches of MDMO-PPV were
compared using their Current Density-Voltage (J-V) curves, shown in Figure 4-4. Current
Density (Jsc), Open Circuit Voltage (Voc), Fill Factor (FF) and Efficiency (η) values of devices,
under an illumination intensity of 100 mW/cm2, are summarized in Table 4-1. Results showed
that polymer intrinsic properties have significantly influenced on the Jsc and FF values of the
devices. In earlier investigations, these parameters of BHJ solar cells have been shown to be
47
sensitive to the type of solvents [180, 181], annealing conditions [182, 183] evaporation rate
[184], doping species [185], molecular weights of polymers [173] etc. In most of these reports,
variation in the FF and Jsc parameters of the devices was suggested due to differences in the
morphology of their organic films. BHJ solar cell developed using polymer batch-B with
moderate molecular weights and highest PDI and NA values, among the investigated polymer
batches, has shown highest efficiency which was attributed due to higher photon absorption
capability (Figure 4-3) and favourable morphology of its photoactive layer. Furthermore, proper
separation of Donor and Acceptor phases for the case of polymer batch-B (Figure 4-2), may also
be the reason of its enlarged efficiency. Thus based on the earlier investigations [173, 186] and
current study, it is suggested that the performance of a polymer fullerene BHJ solar cell can be
enhanced by using polymer having higher PDI values. In short, in this work an effort was made
to highligt the importance of polymer intrinsic properties for MDMO-PPV:PC71BM blend based
solar cells.
Figure 4-4: Current Density-Voltage (J-V) Curves of BHJ solar cells developed using three
different batches of MDMO-PPV Polymers under an illumination intensity of 100W/cm2
48
Table 4-1: Various Parameters such as Fill Factor (FF), Short Circuit Current Density (Jsc), Open
Circuit Voltage (Voc) and efficiency (η) of polymer fullerene BHJ based solar cells realized using
three different batches of MDMO-PPV Polymers
Polymer Batch JSC (mA/cm2) Voc(V) FF %η
A 2.78 0.84 0.3 0.7
B 5.16 0.8 0.51 2.1
C 4.61 0.78 0.49 1.76
In conclusion, Polymer-Fullerene BHJ based organic solar cells were fabricated using three
different batches of MDMO-PPV polymer with a motivation to investigate the effect of polymer
intrinsic properties i.e. Molecular Weights, Polydispersity values, Charge Carrier Densities and
HOMO/LUMO Levels, on the performance of these devices. Polymer batches with significant
varying intrinsic properties were used for this investigation. Structural and optical properties of
MDMO-PPV:PCBM blends based nanolayers developed using three different batches of
polymer, were studied using Atomic Force Microscopic and UV-Vis absorption spectroscopic
techniques, respectively. Current Density-Voltage (J-V) characteristics of the devices were
investigated under an UV-Vis illumination intensity of 100mW/cm2 and their results were
compared. Differences in the nano morphology of organic films, casted using three different
batches of polymers were observed which have significantly influenced on Jsc and FF parameters
of the devices. Efficiency of the solar cell, realized using polymer with moderate Molecular
Weights (Mn= 85 kg/mol and Mw= 1400 kg/mol), and highest Polydispersity (16.8) and Charge
Carrier Density (3.2×10-17
cm-3
) values, was found maximum, which was attributed to higher
photon absorption capability and favorable morphology of its photoactive layer.
4.2 P3HT:PCBM blend based Organic Phototransistors
This section describes fabrication and characterization of BHJ based photo organic Metal
Semiconductor Field Effect Transistors (MESFETs). Drain/ Source and Gate electrodes of the
devices were made using Silver and Aluminum, respectively. Development of the electrodes was
carried out using Physical Vapor Deposition technique, named as Vacuum or Thermal
evaporation. Gate electrode which was made on the semiconducting layer of the devices has
shown Schottky type contact properties whereas Drain/Source contacts which were made on the
49
glass substrates prior deposition of semiconducting layers, have shown Ohmic type contact
behavior with the semiconducting/active layers of the devices. Surface profiles of the active
layers were studied using AFM. Photon absorption capability of the semiconducting layers were
studied a function of wavelength using UV-Vis absorption spectroscopy. Current-Voltage (I-V)
characteristics of the devices were investigated under dark and UV-Vis illumination using SCS,
4200 (made of Keithley intruments) and Lamp (made of LOT Oriel). Devices have shown low
voltage operation and weak saturation trend. Fabrication & characterization of phototransistors,
results and conclusions are presented in sequence to uncover the details of this work.
4.2.1 Introduction
Organic Semiconducting layers based devices have gained much attention of the microelectronic
researchers in recent years due to their various promising features such as low cost, room
temperature processing in ordinary lab environment, flexibility and variety of advanced
applications. Some examples of these devices are Organic Transistors [187], Solar Cells [160],
Light Emitting Devices [188], Photodiodes and Photo-transistors [189].
In particular, optoelectronic researchers have well investigated organic phototransistors with an
aim to replace conventional inorganic photo-detectors with these low cost devices in recent
years. Output current of a phototransistor, not only depends upon the electric field which is
applied at its gate terminal but also on the intensity of the light being illuminated over it. In order
to optimize the performance of OPTs, much effort has been made during the last decade.
Researchers have used films of low molecular weight organic semiconductors (which are
normally processed using thermal sublimation techniques) [190-194], polymers (which are
solution processed) [195-199], and blends of polymers & low molecular weight semiconductors
to investigate the effect of low molecular weight and polymer type semiconductor‘s properties
on the performance of OPTs [200].
Phototransistors can be MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or
MESFET (Metal Semiconductor Field Effect Transistor) type. The investigated devices were
developed in MESFET architecture. MESFETs are preferred over MOSFETs because their low
voltage operation and fewer processing steps [201]. Before this work, MESFET architecture
based phototransistors were known but with only one organic semiconductors [202, 203]. The
50
devices were fabricated using the blends of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl
C61-butyric acid methylester (PCBM) with 1:1 & 1:0.8 wt/wt ratio and blends of poly(3-
hexylthiophene)(P3HT), [6,6]-phenyl C61-butyric acid methylester (PCBM) and Methyl Red
(MR) with 1:1:0.2 wt/wt ratio.
4.2.2 Fabrication and Characterization
MESFET type OFETs were fabricated on the glass substrate. Figure 4-5 shows schematic
diagram of the devices and molecular structures of active layer‘s materials. Devices fabrication
initiated with the deposition of 50 nm thick Silver Drain/ Source contacts on the cleaned glass
slides. Drain/ source contacts were made by PVD process at 10-6
mbar pressure in the
sublimation chamber (Leybold-Univex-450). Three different types of semiconducting solutions
were prepared using overnight magnetic stirring process/ station. Solution-1 was prepared using
P3HT (Aldrich) and PCBM (Aldrich) with 1:1 wt/wt ratio in 1,2 dichlorobenzene (Alfa Aesar),
solution-2 was prepared using P3HT (Merck) and PCBM (Ossila) with 1:0.8 wt/wt ratio 1,2
dichlorobenzene (Merck) and Solution-3 was prepared using P3HT(Aldrich), Methyl
Red(Aldrich) and PCBM (Aldrich) with 1:0.2:1 wt/wt ratio 1,2 dichlorobenzene (Alfa Aesar).
Semiconducting layers were spin coated on Drain Source coated glass substrates using these
semiconducting. Channel length (L) and width (W) were found equal to 30µm and 2mm,
respectively, for the case of P3HT:PCBM blends with 1:1 wt/wt ratio and P3HT:MR:PCBM
blends. Whereas channel length (L) and width (W) of the devices realized using P3HT:PCBM
with 1:0.8 wt/wt ratio were found to be equal to 50µm and 2mm, respectively. Band gaps of
P3HT and PCBM are known to be equal to 1.9 eV and 2.3 eV, respectively. After formation of
active layers, samples were placed in the glovebox and annealed in the nitrogen environment at
110 ºC for 30 minutes. At the end, gate contacts were made by depositing Aluminium on the BHJ
solution processed films in the vacuum chamber.
Following equipment was used to characterize the devices:
Electrical Characterization System, Keithley-4200
UV-Vis Lamp Source
Photodiode (as reference)
Spectro-photometer, S-3100
51
AFM
SEM
Impedance/ Network Analyzer, HP-4194.
Surface Profiler, Dektak-8.
Figure 4-5: (a) Design schematic of organic phototransistors, and molecular structures of (b) P3HT
(c) MR (d) PCBM
4.2.3 Results and Discussion
4.2.3.1 Phototransistors based on P3HT:PCBM blends with 1:1 wt/wt ratio
Results of the phototransistor fabricated on glass substrate using P3HT:PCBM blends with 1:1
wt/wt ratio are presented and discussed in this section. The absorption spectrum of the 230 nm
thick active layer of the device is shown in Figure 4-6 (a), inset presents cross section SEM cross
section view of the active layer on glass substrate. The AFM image (10µm × 10µm) of the active
layer is shown in Figure 4-6 (b). Root mean square (RMS) roughness of the BHJ polymer-
fullerene film was observed to be equal to 4.89 nm. It is known that Aluminium (Al) shows
Schottky type rectifying contact whereas Silver (Ag) makes Ohmic type linear contact with
organic semiconducting materials, respectively [204]. Electrical results (I-V curves shown in
Figure 4-7 (a) and (b)) indicated Ohmic type contact of the semiconducting layer with Silver
52
drain-source electrodes and Schottky type (rectifying) contact with Aluminum gate electrode
which is in accordance with earlier investigations [205]. Figure 4-7 (c) presents energy band
diagram of the organic phototransistor. Energy bands are drawn as straight lines as band bending
which happens due to gate field, has not been taken into consideration. HOMO level of P3HT is
-4.9 eV. Work function of Silver is -4.7 eV [206, 207] and LUMO energy level of PCBM is -3.7
eV. In this typical configuration, holes can easily enter into the drain source electrodes from the
semiconducting film or from drain source electrodes to semiconducting film than electrons,
indicating limited injection of electrons. Rectifying type contact of the semiconducting BHJ film
with gate electrode of the device was assumed due to the energy difference of 0.6 eV between
the work function of gate electrode and HOMO level of P3HT & LUMO level of PCBM.
Figure 4-6: (a) Absorption of P3HT:PCBM blend based active layer (with 1:1 wt/wt ratio) as a
function of wavelength, inset: its SEM based side view and (b) its (10µm × 10µm) surface AFM view
53
Figure 4-7: (a) S-D I-V curve (b) G-S I-V curve when VDS = 0 V (c) Energy Band Diagram of the
phototransistor, showing exciton generation, its dissociation, and transportation of holes and
electrons towards electrodes [93]
The output Current-Voltage (I-V) curves of the phototransistor without being illuminated, are
depicted in Figure 4-8, inset compares gate leakage current with drain current at VGS = - 3 V.
Figure 4-9 shows drain current (IDS) as a function of gate voltage (VGS) without illumination at
VDS = -5 V. I-V curves of the device were found similar to p-type mode characteristics of a
typical ambipolar FET [208, 209]. Weak kink in the output characteristics (at 0 gate to source
voltage) confirmed ambipolar nature [210, 211] of the device‘s semiconducting layer. Drain
current was found to increase with negative gate voltage as can be seen in Figure 4-8 which is an
indication of p-type behavior of ambipolar MESFET. It was also observed that this device can
54
operate as transistor 0 to -5 V gate voltages because gate current increases rapidly if gate voltage
is further increased. Further, ON-OFF current ratio of the device was also observed to be low
similar to earlier investigations [212].
Figure 4-8: Current Voltage (I-V) Curves (output) of the Phototransistor without being illuminated,
inset: gate leakage current vs drain current at VGS = - 3V as a function of drain voltage
Figure 4-9: Transfer I-V Characteristics of the photo-OFET under dark
55
Drain/ channel current (IDS) can be found mathematically following the relation as under [213]:
𝐼𝐷𝑆 =𝜇𝑊𝐶𝑖
2𝐿(𝑉𝐺𝑆 − 𝑉𝑡)2 4-1
Where
μ = field effect mobility,
Ci = Dielectric/ gate capacitance per unit area,
W = channel width,
L = channel length,
VGS = gate to source voltage and
Vth = threshold voltage.
In the saturation regime, μ can be determined using following relation:
𝜇 = 𝑚2 2𝐿
𝑊𝐶𝑖 4-2
Where ‗m‘ in the above relation is the slope of (IDS)1/2
- VGS curve. Figure 4-10 is showing gate
specific capacitance variation as a function of frequency, indicating Ci value to be equal to
0.21µF/cm2 at 100 Hz. Field effect mobility value of the device was determined as 1.6×10
-4
cm2V
-1s
-1 which is much higher as compared to the mobility values of earlier investigated
devices, having similar configurations [91, 214]. However device mobility was observed lower
as compared to MOSFET architecture based organic transistors which may be attributed due to
thicker semiconducting films in MESFET based devices [215].
The output I-V characteristics of the photo-OFET at fixed UV-Vis illumination of 100mW/cm2
are shown in Figure 4-11(a). Pronounced kinks were observed in the output I-V characteristics of
the device at 0 and -1 gate at higher drain voltage (greater than -6 V) indicating increase in the
ambipolarity of active layer under illumination. The Current-Voltage Curves of the device at VGS
= -5 V under dark and varying fixed UV-Vis illuminations are described in Figure 4-11 (b). It
was observed that drain current increases with illumination intensity. It was attributed that
56
Photovoltaic (generation of voltage) and Photoconductive (change in the conductivity of the
semiconducting layer due to illumination) phenomena were responsible for the photosensitive
properties of these devices as discussed in [189]. Photo-responsivity (R) of the phototransistor
can be determined using the following relation [216]:
𝑅 =𝐼𝑝
𝑃=
(𝐼𝑑𝑙 − 𝐼𝑑𝑑 )
𝐸𝐴 4-3
Where
Iph = Drain/ Channel Current generated due to illumination,
Idl = Drain current under illumination,
Idd = Drain current under dark and
P = Incident Optical Power (which can be estimated by multiplying device channel area with
intensity of the incident light).
Figure 4-10: Specific capacitance as a function frequency between gate and source/ drain electrodes
57
Another parameter which is sometime used to determine the performance of a photo-sensor is
ratio of drain current illumination to drain current under dark (r) as under [21]:
𝑟 = 𝐼𝑑𝑙𝐼𝑑𝑑
4-4
Photo responsivity and ‗r‘ values of the device were calculated at VGS = 0 V and VDS = -8 V, and
were determined to be equal to 0.047 A/W and 3, respectively, under an illumination intensity of
100mW/cm2 UV-Vis. Photo-responsivity value of the device was found higher as compared to
reported single semiconductor based photo organic MESFETs [202, 203], comparable with the
photo-responsivity values of glass substrate based photo organic MOSFETs [217, 218] and lower
than Si/SiO2 substrate based photo-OFETs [200, 219]. Although photo-responsivity values of
Si/SiO2 substrate based phototransistors are superior but these are rigid and comparatively costly
which spurs the interest in organic semiconductor based low cost photo-detectors, having option
of their processing on flexible substrates in addition to other promising features.
4.2.3.2 Phototransistors based on P3HT:PCBM blends with 1:0.8 wt/wt ratio
Figure 4-12 presents absorption spectrum of 350 nm thick active layer based on P3HT:PCBM
blends with 1:0.8 wt/wt ratio. Highest peak was found at 534 nm with two subsequent peaks at
556 nm and 600 nm which were originated from the crystalline domains of P3HT. Further,
absorption in the range of 370 nm to 450 nm is attributable to PCBM. Figure 4-13 (a) and (b)
shows 5µm×5µm AFM 3D profiles of P3HT:PCBM blend based active layer. The surface
topography of the blended 350 nm thick film showed a rough surface morphology, with Root
Mean Square (RMS) and Average roughness values equal to 26.9 nm and 21.3 nm, respectively.
The nano-scale phase separation of P3HT:PCBM blend is observable in the 3D AFM image. No
considerable structural defects were found in ther semiconducting film surface. Figure 4-13 (c)
shows SEM surface image of the active layer. The SEM micrograph of the active layer showed a
crack-free surface at a given resolution of 20 m. The inset of Figure 4-13 (c) which presents
SEM image at high resolution (2 m) showed no considerable porosity. Further, no aggregates
were found in the SEM images of the active layer. Aluminium electrodes make Rectifying type
contacts whereas Silver electrodes show Ohmic contacts with semiconducting layers [80].
58
Figure 4-11: Output I-V Characteristics of the Photo-OFET (a) under fixed UV-Vis illumination
intensity of 100mW/cm2 and (b) under given UV-Vis illumination intensities compared with dark
59
Through I-V measurements, Schottky type contact of semiconducting layer with Aluminium gate
electrode and Ohmic type contact with Silver drain-source electrodes was noted. Rectification
ratio was found equal to 10 at ±2 V gate to source voltage when drain to source voltage was
equal to zero. Previously, contact properties of P3HT:PCBM blend based organic film have been
elaborated with the help of the energy band diagram in this specific device configuration by
Yasin et al.[93]. Figure 4-14 shows output I-V characteristics of device under dark. Drain current
was found to increase with negative gate voltage which evidenced p-type conductivity of the
semiconducting layer. P-type behavior of the semiconducting layer was associated to Silver
based drain-source electrodes as already reported for the case of P3HT:PCBM blends [217].
Further, Current- Voltage (I-V) characteristics of the device were found similar to typical p-type
organic semiconductors based field effect transistors [197, 220]. Gate leakage current (IGS)
increases rapidly and becomes comparable with drain current when positive or higher negative
gate voltage (higher than -5 V) is applied. Therefore, the proposed device can only work for
lower negative gate voltages. As already known for the case of organic MESFETs [221], device
has shown low voltage operation, with threshold voltage Vth equal to 0.1 V.
Figure 4-12: Absorption spectrum of 350 nm thick P3HT:PCBM blend based active layer
60
Drain to source current (IDS) of the device in the saturation regime can be mathematically found
using following relation [222]:
𝐼𝐷𝑆 = 𝐾 (𝑉𝐺𝑆 − 𝑉𝑡)2 4-5
Where VGS is the voltage between gate and source electrodes, Vth is the threshold voltage and k
is the conduction parameter which can be determined using following relation:
𝐾 = 𝜇ɛ𝑊
2𝑎𝐿 4-6
Here, µ is the field effect mobility, ε and a are the parameters which specify permittivity and
thickness of the semiconducting layer, respectively. L and W represent channel length and
width, respectively.
Figure 4-13: AFM 3D profiles (5µm×5µm) [(a) & (b)] and SEM images of the active layer (c)
61
Figure 4-14: Phototransistor output I-V characteristics under dark
Specific capacitance (at 100 Hz) and permittivity values were found equal to 0.2µF/cm2 and
0.84×10-11
F/cm, respectively. Field effect mobility value of the device was found to be 7.7×10-3
cm2V
-1S
-1 at VGS = -3 V and VDS = -8 V. Figure 4-15 shows output I-V characteristics of the
phototransistor under UV-Vis illumination of 90mW/cm2. Enhancement in the drain to source
current of the device is due to photo generated carriers which were produced due to the
absorption of photons having energy higher than the band gap energy of the polymer i.e.
P3HT. Pronounced kinks in the I-V characteristics of the device at lower gate to source voltages
(0 to -1 V) when drain to source voltage becomes higher than -6 V, specifies ambipolar nature
of the active layer as already known [211, 223]. This nonlinear behavior of the device is due to
injection of both types of carriers from drain source electrodes under these biasing conditions.
Further, Current-Voltage (I-V) characteristics of the device under illumination were found in
good agreement to p-type mode characteristics of typical ambipolar FETs [209].
Performance of a photo detector can be indicated by its photo sensitivity (P) and responsivity (R)
parameter values which can be found using the following relations, respectively [224]:
62
Figure 4-15: Phototransistor output I-V characteristics under UV-Vis illumination
𝑃 = 𝐼𝐿𝑖𝑔𝑡 − 𝐼𝐷𝑎𝑟𝑘
𝐼𝐷𝑎𝑟𝑘 4-7
𝑅 = 𝐼𝐿𝑖𝑔𝑡 − 𝐼𝐷𝑎𝑟𝑘
𝑃𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 4-8
Where ILight and IDark are the values of drain to source current under illumination and dark,
respectively, and PIncident is the optical power incident on the channel of the device which was
determined by multiplying channel area of the device with incident light intensity. In order to
investigate the effect of gate voltage on the performance of phototransistor, both its performance
parameters i.e. photo sensitivity and responsivity, were plotted as a function of gate voltage
when drain to source voltage was equal to -8 V, as can be seen in Figure 4-16.
Photo sensitivity and responsivity values were found maximum at zero gate to source voltage.
Further, both these values were found to decrease exponentially with the increase of negative
gate voltage. This is due to reason that at lower gate voltages both types of carriers are injected
in drain and source electrodes. Photo sensitivity of the device was analyzed at two different
illumination intensities i.e. 80 and 90mW/cm2 and its value is found higher for 90mW/cm
2
illumination intensity throughout the investigated gate voltage range as expected. Thus, based on
63
Figure 4-16: (a) Photo sensitivity and (b) responsivity as a function of Gate Voltage
the experimental results, significant effect of gate voltage on the performance of investigated
MESFET type phototransistor is being reported.
4.2.3.3 Phototransistors based on P3HT:MR:PCBM blends
Results of the devices fabricated using P3HT:MR:PCBM blends with 1:0.2:1 ratio are described
in this section. Methyl red which is reported to be rich in π-electrons, is a PH indicator azo dye
64
and is soluble in acidic solutions [225]. Methyl Red, having conjugated structure with molecular
formula NC6H4COOH is not well investigated as compared to P3HT and PCBM. During the
recent years, it has been investigated as dopant in Liquid Crystals by various research groups
around the world [114, 226-228]. Further, solution processed Methyl Red films based organic
devices such as diodes [229], solar cells [230] and humidity sensors [22] with promising features
highlight its potential and future applications. Figure 4-17(a) presents 10µm × 10µm AFM image
of P3HT:MR:PCBM blends based 350 nm thick semiconducting/active layer; inset shows its
high resolution (2µm × 2µm) 3D AFM profile. Nano-scale phase separation of donor-acceptor
blends is observable in high resolution AFM image. Root mean square and average roughness
values were found to be equal to 13.95 nm and 10.96 nm, respectively. As can be seen from the
AFM profiles, organic film presents homogenous distribution of polymer: fullerene and methyl
red particles and is free from any major structural disorders which are generally associated with
solution processed films. UV-Vis absorption spectra of P3HT:PCBM and P3HT:MR:PCBM
blends based semiconducting layers is shown in Figure 4-17(b). Methyl Red is the most
nonlinear dye ever known which experiences Trans-Cis photoisomerization under light [231]
which is a reversible process. As can be seen, active layer with Methyl Red absorbs more
photons when wavelength is in the range of 450 nm to 600 nm which is known to be the main
absorption range of Methyl Red [232]. Aluminum based gate electrode of the device made
Schottky type contact with P3HT:MR:PCBM blends based semiconducting layer. Investigation
in hand is the continuation of our previous work [93], in which P3HT:PCBM blend based
semiconducting layer has also showed Schottky type contact with gate electrode. This behavior
of P3HT:PCBM blends between Aluminum and Silver electrodes were explained with the help
of energy band diagram. However, rectification ratio was found higher for the case of
P3HT:MR:PCBM blends, indicating comparatively better Schottky contact properties. Figure
4-17(c) presents variation in the capacitance of active layer between gate-source/ drain electrodes
as a function of frequency. As can be seen, at low frequency ranges (1 KHz – 100 KHz), the
capacitance of the film was found to be equal to 0.7nF, indicating no significant effect of
frequency variation. However capacitance decreases with increase in frequency above 100 KHz
following the behavior of typical Schottky diodes and dielectric films. The effect of DC voltage
on the capacitance of active layer between gate and source/drain electrodes was also studied as
shown in Figure 4-17(d), at some specified frequency values (1 KHz, 10 KHz, and 100 KHz).
65
The capacitance was found to increase significantly with increase in applied DC voltage when
Schottky diode was reversed biased. This is due to increase in the width of depletion layer with
applied DC voltage.
Figure 4-17: (a) Surface AFM image (10µm × 10µm) of P3HT:MR:PCBM blend based active layer,
inset: its high resolution 3D AFM profile, (b) Absorption Spectra, (c) Capacitance of active layer as
a function of frequency between Source/Drain and Gate electrodes and (d) Capacitance of active
layer as a function of DC bias voltage between Source/Drain and Gate electrodes at specified
frequency values.
Figure 4-18 shows Current - Voltage (I-V) characteristics of device under dark conditions. Drain
to source current (IDS) and gate to source current (IGS) as a function of drain to source voltage
(VDS), were measured for comparison. IDS and IGS values were found to be equal to 22nA and
0.94pA at VDS = -5V and VGS = -4V, respectively. Output Current - Voltage (I-V) characteristics
of the device under given UV-Vis illuminations are shown in Figure 4-19 (a) and (b) when gate
to source voltage values were equal to - 3 V and - 4 V, respectively. As can be seen, illumination
66
Figure 4-18 I-V Characteristics of the Phototransistor under dark: Gate leakage current and
drain current as a function of drain to source voltage at VGS = - 4V.
significantly enhances drain to source (IDS) of the device. Photons absorbed under illumination
move towards donor-acceptor interface due to diffusion where these are dissociated into
electrons and holes. Conversion of excitons into electrons and holes is the function of film
microstructure, diffusion length and life time of excitons etc. Electrons and holes are then
collected at source drain electrodes due to applied biasing, to enhance the channel current. Drain
current was found to increase with illumination intensity and negative gate voltage following the
behavior of p-type organic semiconductors based phototransistors [197, 220], with weak
saturation trend and low voltage operation. Increase in the drain to source current (IDS) with gate
voltage was attributed due to variation in the depletion width of active layer as shown in Figure
4-17 (d). Weak saturation behavior of the device was attributed due to presence of both p-type
and n-type organic species. Gate leakage current increases rapidly and becomes comparable with
drain current when positive gate voltage is applied. Therefore, device can only work for negative
gate voltages. Capacitance of the active layer was found to be equal to 0.7nF at 1KHz. Field
effect mobility value of the device was estimated using equations 4-5 and 4-6 and found to be
equal to 5.8 × 10-3
cm2/VS at VGS = - 4V and VDS = -5V, which is similar to field effect mobility
values of polymer-fullerene BHJ based OFETs, recently investigated for strain and displacement
sensing applications [82, 233]. The values of photo responsivity (R) and light to dark current
67
ratio (r) of the investigated device at VGS = - 4 V and VDS = - 5 V were found to be equal to
0.3mA/W and 27, respectively. Photo responsivity (R) of the investigated device is lower
whereas ratio of drain current under light to drain current under dark (r) is higher as compared to
photo sensitive MESFET recently investigated using the blend of P3HT and PCBM as active
semiconducting layer [93]. Higher value of r for the case of P3HT:MR:PCBM blends may be
due to lower value of gate leakage current and higher absorption of photons in UV-Vis regime.
Whereas lower value of R is attributable to higher surface roughness of photo sensitive layer
with more charge carrier trap sites. Investigation of the effect of Methyl Red concentration on the
performance of BHJ based photo detector is left as a future work.
Figure 4-19 Output Current -Voltage (I-V) Characteristics of the Phototransistor under dark and
given UV-Vis illumination intensities when (a) VGS = - 3 V (b) VGS = - 4 V.
68
Chapter-5: Strain and Displacement
Sensitive OFETs 5 4
In this chapter solution processed Polymer (P3HT)-Fullerene (PCBM) BHJ based Organic Field
Effect Transistors (OFETs) were investigated for strain/ bend and displacement sensing
applications, respectively. P3HT is a well-known p-type organic semiconductor, soluble in many
solvents. PCBM is soluble derivative of n-type low molecular weight organic semiconductor.
OFETs were processed on flexible and glass substrates following low cost fabrication procedure.
Metal contacts of the devices were made using Physical Vapor Deposition (PVD) Technique,
Thermal Evaporation whereas semiconducting layers of the devices which were developed using
spin coating technique made Ohmic type contacts with its Silver based Drain & Source contacts
and Schottky type contacts with its Aluminum based Gate electrodes, and demonstrated
ambipolar properties. This particular behavior of the BHJ based semiconducting layers of the
devices was explained with the help of Energy Band Diagram. Surface roughness and structural
properties of the semiconducting layers were analyzed using Atomic Force Microscope (AFM)
and Scanning Electron Microscope (SEM). Devices have shown low voltage operation and
threshold voltages. Current Voltage (I-V) characteristics of the devices were measured using
Semiconductor Characterization System (SCS), Keithley 4200 and found similar to P-type mode
I-V characteristics of typical ambipolar OFETs.
In order to study strain sensing properties, devices developed on flexible PET substrates were
bent around the cylinders of varying radii and corresponding variation in the drain to source
current of the devices was noted. Furthermore, bending was applied at 0º and 90º w.r.t. drain to
source current. It was found that drain current increases with increase in bending from both sides
of the devices. Strain sensitivity of the devices was found much higher than recently investigated
OFETs for strain sensing applications. It was also concluded that strain sensitivity is not only
dependent on the geometrical parameters, applied biasing conditions and substrate sizes of the
devices but also on the bending directions.
Displacement sensing properties were investigated using a special test fixture after placing thin
plastic film and thick rubber in sequence on the gate films of the devices. Upper terminal of the
69
test fixture was moved downward until it touches top surface of rubber. This position of the
upper arm was considered as zero displacement position and then upper arm is moved downward
in 50μm steps and corresponding variation in drain current was measured. Current was found to
increase with increase in the displacement. Drain to source resistance was reduced to 13.15 MΩ
from 15.40 MΩ when displacement was varied from 0 to 250 μm.
Thus significant variation in the drain current of the devices was noticed due to bending and
displacement variation. The changes in the structural and morphological properties of
semiconducting layer due to applied tensile strain were attributed as the reason of the variation in
the electrical properties of these devices.
5.1 Organic Bulk Hetero-junction based Strain/ bend Sensitive Flexible
Organic Field effect Transistors
5.1.1 Introduction
During the past decade, electronic devices based on organic semiconductors such as conjugated
polymers, oligomers and low molecular weight materials have attracted much attention due to
their low cost, flexibility, ease of processing, large area capability and variety of applications.
Prominent examples of organic microelectronic devices are Organic Field Effect Transistors
(OFETs) [187], Organic Solar Cells (OSCs) [160], Organic Light Emitting Diodes (OLEDs)
[188], Organic Photodiodes (OPDs) and Phototransistors (OPTs) [234].
Investigation of OFETs for sensing applications has gained much attention in recent years [235,
236]. OFETs are responsible for performing both switching and sensing functions in these
devices [237]. Due to this, strain sensors based on OFETs were also well investigated using
various types of semiconducting and dielectric materials since the demonstration of first organic
semiconductor based strain sensor by Soyoun Jung and Tom Jackson [238]. Cosseddu P. et al.
[83] have demonstrated P3HT and Penatcene based strain sensors on a flexible substrate and
reported the strong influence of mechanical stress on the electrical behavior of these devices.
Scenev et al. [84], Yang et al. [239] and Sekitani et al. [240] have correlated change in the
electrical properties of pentancene based flexible strain sensors with change in the structural and
morphological properties of the pentacene thin films due to bending. Flexible organic strain
70
sensors based on transistor like Wheatstone bridge configuration were also investigated and
sensitivity of these devices was reported to be dependent on the direction of applied strain [241-
243].
OFETs can be based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or
MESFET (MEtal Semiconductor Field Effect Transistor) type configurations. MOSFETs show
higher field effect mobility and ION/IOFF ratio, whereas MESFETs are preferred over
MOSFETs due to their low voltage operation and fewer processing steps [201]. Pressure
sensitive OFETs using organic MESFET architecture in which gate makes Schottky type contact
with the active semiconducting layer, have been reported in the past [80, 244], but with only one
organic semiconducting material and on rigid glass substrates. However surprisingly, organic
bulk hetero-junction based flexible organic MESFETs and their sensing properties are not
investigated until now.
In this work, bend or strain sensing properties of organic bulk hetero-junction based MESFETs is
being reported for the first time. Two devices having different sizes of its flexible PET substrates
were fabricated and investigated using the blend of P3HT (soluble p-type polymer) and PCBM
(soluble derivative of n-type fullerene), making bulk hetero-junction with each other, as active
layer. Active layer made Schottky type (rectifying) contact with gate electrode and, Ohmic
contact with drain and source electrodes. Operation of the devices was found similar to the p-
type mode operation of typical ambipolar field effect transistors. Further, devices have
demonstrated low voltage operation and threshold voltage. In order to study strain sensing
properties of the device, varying bending strain parallel and perpendicular to the direction of
drain to source current (IDS) was applied. Significant influence of the bending strain on the drain
to source current of the device was observed. However variation in the current (IDS) under
applied bending was found function of bending direction. It was also observed that increase in
the drain current is sensitive to force applied by the bending arm in addition to bending direction.
That is why maximum increase in the drain current was noticed for the case of device developed
on small size substrate when bending was applied perpendicular to the drain current direction.
71
5.1.2 Experimental Methods
Figure 5-1 shows design schematic of the flexible devices, being developed for strain sensing
applications. Two different types of OFETs were developed on flexible PET substrates for this
investigation. The device realized on comparatively small substrate was named as sample-1
whereas the one prepared on large substrate was named as sample-2. Fabrication of the devices
was started with the cleaning of PET substrates in an ultrasonic bath by using acetone, isopropyl
alcohol and DI water. Drying of substrates was accomplished using nitrogen blow. Silver (Ag)
drain and source contacts with a thickness of around 50 nm were made on the flexible cleaned
substrates by physical vapor deposition process at 10-6
mbar using Vacuum Evaporator (Leybold
Univex 450). Active layer was prepared by mixing P3HT (Aldrich) and PCBM (Aldrich) with
1:1 (wt/wt ratio) in 1,2 dichlorobenzene (Alfa Aesar) and spin coated on the flexible substrate
with previously deposited drain and source electrodes. P3HT:PCBM blend based films, in which
P3HT serve as donor and PCBM as acceptor are well investigated for polymer bulk hetero-
junction solar cells [142].
Figure 5-1: Design schematic of the flexible device
Band gaps of P3HT and PCBM are around 1.9eV and 2.3eV, respectively. After formation of
active layer, devices were annealed at 100 oC for 20 minutes. At the end, 40 nm thick gate
contact was made by depositing Aluminium directly on the semiconducting layer through the
shadow mask in the vacuum chamber at 10-5
mbar pressure. Hot metal vapors can easily pass
through the organic film, and can make a short contact with the drain source electrodes. This was
prevented by making a thin gate film with a low evaporation speed.
72
Current–Voltage (I-V) characteristics of the devices were measured with Semiconductor
Characterization System (Keithley 4200SCS). In order to investigate the surface properties of the
active layers, Nanoscope Multimode Atomic Force Microscope (AFM) was used. Capacitance
measurements of the samples were carried out using impedance analyzer (HP-4194). A signal of
1V peak to peak was applied to the samples during capacitance measurements. Thicknesses of
active layers and various electrode films were measured using Scanning Electron Microscope
(SEM). All the tests and measurements were performed at ambient air and room temperature.
5.1.3 Results and Discussion
5.1.3.1 Electrical Contact Properties of Polymer Organic Film
Earlier investigations showed that aluminum (Al) makes Schottky type contact, whereas silver
(Ag) makes Ohmic type contact with organic semiconducting materials, respectively [204].
Through I-V measurements, Schottky type (rectifying) contact between gate electrode (Al) and
active layer, and Ohmic contact between source drain electrodes (Ag) and the active layer was
noted. Such behavior of the active layer was discussed with the help of Figure 5-2(a), which
presents energy band diagram of organic semiconductors (used in this study) with respect to the
work functions of source/drain and gate electrodes. Energy bands are shown as straight lines as
band bending which occurs due to gate voltage was not taken into consideration. HOMO level of
P3HT (-4.9 eV) is quite close to the work function of Ag (-4.7 eV) [207, 245] but there is an
energy difference of around 1eV between the work function of Ag and LUMO level of
PCBM (-3.7 eV). Therefore holes can easily be injected into the active layer from drain source
electrodes as compared to electrons. Due to this close matching of the work functions of drain
source electrodes with the HOMO level of P3HT, holes of active layer are making Ohmic
contact with drain source electrodes. However electrons cannot be easily injected in the active
layer due to the presence of an injection barrier of about 1eV between the work function of drain
source electrodes and LUMO level of PCBM. Therefore injection of the electrons was limited in
our experiments. Schottky type contact of the active layer with gate electrode of the device is due
to the energy difference of 0.6eV between the work function of gate electrode (-4.3 eV) and
HOMO level of P3HT & LUMO level of PCBM.
73
Figure 5-2: Energy Band Diagram
5.1.3.2 Structural Properties of Solution Processed P3HT:PCBM Blend based Organic
Films
Figure 5-3(a) and (b) show 5µm × 5µm AFM image and cross section SEM view of 300 nm
thick active layer, respectively. Root mean square (RMS) roughness value of the active layer was
found to be equal to 4.34 nm. Figure 5-4 presents section roughness of active layer developed on
flexible PET substrate for sample-2. The RMS and average roughness values were found to be
equal to 14.70 nm and 11.25 nm, respectively.
Figure 5-3: (a) 5×5 µm AFM image and (b) Cross section SEM view of 300 nm thick active layer
developed on substrate for sample-1
74
Figure 5-4: Section Roughness of active layer developed on flexible substrate for sample-2
5.1.3.3 Current Voltage (I-V) characteristics of the devices under no bending conditions
Figure 5-5 describes output & transfer I-V characteristics of the sample-1 flexible OFET,
whereas Figure 5-6 shows output I-V characteristics of sample-2 OFET under no bending
condition. I-V characteristics of the devices were found similar to p-type mode characteristics of
typical ambipolar field effect transistors [208, 209]. Weak kink in the output characteristics of
the device (at 0 gate to source voltage) was observed which confirms ambipolar nature of its
active layer [246]. Non linear increase in the drain to source current at higher drain to source
voltage (higher than -4 V) and 0 gate to source voltage, is due to injection of both types of
carriers in the the channel. Meric I. et al. [223] have investigated ambipolar graphene based
field effect transistors and discussed the reasons of kinks in the I-V characteristics (at lower gate
to source voltages) with the help of schematic drawings. Gate voltage in organic MESFETs
controls the distribution of injected charge carriers and bulk conductivity of organic
semiconducting layer [247]. In our case, with the increase of negative gate voltage, holes are
drawn towards gate electrode which increases the bulk conductivity of the active layer, and
current between drain source electrodes increases as a result. As already known for the case of
organic MESFETs [221], devices have shown low voltage operation and threshold voltage (Vth )
values.
75
Figure 5-5: (a) Out Put and (b) Transfer I-V Characteristics of sample-1 OFET under no bending
state
Figure 5-6: Out Put I-V Characteristics of sample-2 OFET under no bending state
In the saturation regime, drain to source current (IDS) can be determined by using following
equation [222]:
𝐼𝐷𝑆 = 𝑘(𝑉𝐺𝑆 − 𝑉𝑡)2 5-1
76
Where VGS is the voltage between gate and source, Vth is the threshold voltage and k is the
conduction parameter, which can be determined by using a following relation:
𝑘 = 𝜇휀𝑠𝑊 2𝑎𝐿 5-2
Where, L, W and a, are the channel length, width and thickness respectively. ɛs is the permittivity
of the semiconducting material, and µ is the field effect mobility. The effective capacitance (Ci at
100 HZ) and permittivity (ɛs) was found to be 1.2×10-5
F/cm2 and 3.6×10
-10 F/cm, respectively.
Field effect mobility values of the devices were determined at VDS = -5 V and VGS = -3 V using
equation (5.1) and (5.2), and its values were found to be equal to 3.53×10-4
cm2V
-1s
-1 and
2.05×10-4
cm2V
-1S
-1 for sample-1 and Sample-2 respectively. Field effect mobility values of the
devices were higher as compared to the mobility values of MESFET based humidity sensitive
organic field effect transistor investigated by Murtaza et al. [91] and lower than usually reported
for organic MOSFET based devices. Structures of organic MESFETs are thicker as compared to
organic MOSFETs which may be the reason of their lower field effect mobility values [215].
5.1.3.4 Current Voltage (I-V) Characteristics of the devices under bending conditions
In order to investigate strain sensing properties of the devices, flexible devices (shown in Figure
5-7) bent around cylinders with radii (R) of 15, 10 and 5mm with corresponding strain values of
1, 1.6 and 3.2%, respectively. Bending was applied parallel and perpendicular to the drain to
source current (IDS) as indicated with the help of device schematics shown in Figure 5-8 and
corresponding strain was calculated using the following relation [248]:
Ɛ = 𝑍 𝑅 5-3
Where Ɛ is the strain generated in the device, Z is the thickness of the PET substrate and R is the
bending radius. Figure 5-9 and Figure 5-10 presents variations in drain to source current as a
function of drain to source voltage under shown bending conditions for sample-1 and sample-2,
respectively, when gate terminal was suspended. Proportional increase in drain current for both
parallel and perpendicular bending with respect to the current axis was observed. The bend or
strain sensitivity of the devices is significantly higher as compared to those reported for the case
of organic MOSFET based strain sensors [243]. Further, strain sensitivity of the transistors is the
77
function of induced geometric asymmetry as already reported [242, 249]. Although in both
samples drain current was found to increase due to applied bending from both sides however its
sensitivity was found strong function of bending direction, device geometrical parameters and
induced forces strength of the bending arm. Variation of current with applied tensile strain was
assumed due to changes in the transport and structural properties (such as inter molecular
distance) of polymer-fullerene bulk hetero-junction based semiconducting layer. The goal of this
work was to develop and demonstrate the results of an organic bulk hetero-junction based strain
sensitive organic MESFET.
Figure 5-7: Photographs of the flexible devices under bending state
Figure 5-8: Schematic of the devices when bent at (a) 0º and (b) 90º w.r.t. drain current
78
Figure 5-9: Drain to source current (IDS) as a function of drain to source voltage (VDS) under
suspended gate and the given bending conditions for strains applied at (a) 0o
and (b) 90o
with
respect to the direction of current for sample-1
Polymer-Fullerene bulk heterojunction based strain sensitive flexible OFETs were successfully
fabricated and demonstrated using MESFET configuration for the first time. Results showed that
active layer which was the blend of p-type and n-type organic semiconducting materials, made
Ohmic contact with drain/source electrodes and Schottky contact with gate electrode, and
adopted the behavior of an ambipolar organic semiconductor. Current-voltage (I-V)
characteristics of the devices were found similar to the p-type operation mode characteristics of a
Figure 5-10: Drain to source current (IDS) as a function of drain to source voltage (VDS) under
suspended gate and the given bending conditions for strains applied at (a) 0o and (b) 90o with
respect to the direction of current for sample-2.
79
typical low voltage ambipolar field effect transistor. Strain sensing properties of the devices were
studied by bending it around cylinders of varying radii, at 0o and 90
o with respect to drain to
source current direction. Strain from both sides of the device has significantly influenced drain to
source current of the device. However strain sensitivity of the devices was found function of the
bending direction, device physical parameters and induced forces of bending arms. Realization
of such low voltage devices will provide potential for the future development of low cost
portable large area flexible sensor arrays and other low power electronic devices.
5.2 Polymer-Fullerene BHJ Based Displacement Sensitive OFET
5.2.1 Introduction
In recent decades, conjugated polymers, oligomers and low molecular weight organic
semiconductors based microelectronic devices have attracted much attention of the researchers
due to their low cost, flexibility, ease of processing, large area capability and variety of
applications. Prominent organic microelectronic devices include Organic Field Effect Transistors
(OFETs) [250], Organic Solar Cells (OSCs) [251], Organic Light Emitting Diodes (OLEDs)
[252], Organic Photodiodes (OPDs) [253] and Phototransistors (OPTs) [254].
Investigation of organic field effect transistors (OFETs) for sensing applications has seen
progress in recent years [95, 255]. OFETs, which are responsible for performing both sensing
and switching functions in sensors, were well investigated for various sensing applications
during the last decade which includes bio-sensing [256-259], humidity sensing [91, 260, 261],
pressure sensing [74, 262, 263], strain sensing [84, 85, 264], photo sensing [265, 266] etc.
Transistors in these sensors were realized in various configurations, utilizing different
semiconducting and dielectric materials on flexible and rigid substrates, in order to investigate its
effect on the performance of these devices.
OFETs can be based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or
MESFET (MEtal Semiconductor Field Effect Transistor) architectures. Transistors which are
based on MOSFET architecture contain a dielectric layer between its semiconducting layer and
gate contact. However MESFETs in which gate makes Schottky type contact with the
semiconducting layer, do not contain dielectric layer. Further MOSFET based devices show
higher field effect mobility and ION/IOFF ratio, whereas MESFETs are preferred over MOSFETs
80
due to their low voltage operation, fewer processing steps and low cost [267], thus most of the
reported OFET based sensors were developed using MOSFET type configuration. Someya et. al.
[268] have realized pressure sensitive flexible OFETs using pentacene based semiconducting and
polyimide based dielectric layers, respectively. The drain to source current of these sensors was
found to increase from 15nA to 6.7µA under the pressure of 30 kPa. Stefan S. C. B. et. al. [269]
have recently demonstrated flexible organic MOSFETs based pressure sensing array using
rubbery dielectric layers. Displacement sensitive OFETs using organic MESFET architecture
have also been reported, but with only one organic semiconducting material. Karimov et. al. [81,
270] have fabricated Copper Pthalocyanine (CuPc) based displacement sensitive MESFETs and
reported exponential decrease of drain to source resistance with the increase of displacement.
However organic bulk hetero-junction based displacement and pressure sensitive MESFETs are
not investigated until now. Displacement sensors were realized in the past with capacitive,
resistive, linear variable differential transformers and optical instruments [271, 272]. The
displacement range of these sensors was in the orders of 10-6
to 10-3
meters.
In this work, displacement sensing properties of polymer fullerene bulk hetero-junction based
MESFETs were investigated for the first time. The transistor was fabricated on glass substrate
using the blend of P3HT (soluble p-type polymer) and PCBM (soluble derivative of n-type
fullerene), making bulk hetero-junction with each other, as active layer.
5.2.2 Experimental Details
Figure 5-11 shows design schematic of displacement sensitive Organic Field Effect Transistor
(OFET). Transistor was fabricated in MESFET type configuration on glass substrate. Fabrication
of the devices was started with the sequential cleaning of glass substrates in an ultrasonic bath by
using acetone, methanol, ethanol, isopropyl alcohol and DI water. Drying of the substrates was
carried out using nitrogen blow and thermal oven. Silver (Ag) drain and source contacts with a
thickness of around 60 nm were made on the cleaned substrates by physical vapor deposition
(PVD) process at 10-5
mbar using Vacuum Evaporator (Leybold Univex 450). Channel length (L)
and width (W) were found equal to 50µm and 2mm, respectively. P3HT (Sigma Aldrich) and
PCBM (Sigma Adrich) with 1:0.8 (wt/wt ratio) were added in 1,2 dichlorobenzene (Sigma
Aldrich) and solution was prepared by stirring it for 24 hour. The temperature was maintained at
50oC during the process of stirring. Semiconducting layer was coated on drain source electrodes
81
deposited glass substrates using spin coating technique. The thickness of the active layer was
estimated to be equal to 280 nm by SEM measurements. P3HT:PCBM blend based nanolayers,
in which P3HT serve as donor and PCBM as acceptor, are well investigated for polymer bulk
heterojunction solar cells in recent years [142]. Band gaps of P3HT and PCBM are around 1.9
eV and 2.3 eV, respectively. Then samples were annealed in the glove box at 110oC for 30
minutes. At the end, 50 nm thick gate contact was made on the semiconducting layer by
evaporation of Aluminium through the shadow mask in the vacuum chamber at 10-5
mbar
pressure. Penetration of hot metal vapours through the organic film, to make a short contact with
the drain source electrodes, was prevented by making a thin Aluminium gate film with a low
evaporation speed. Five devices with similar aforementioned geometrical dimensions and
various film thicknesses were fabricated.
Figure 5-11: Design schematic of Displacement Sensor based on Organic Field Effect Transistor
(OFET)
Current–Voltage (I-V) characteristics of the devices were measured with Semiconductor
Characterization System (Keithley 4200SCS). Surface properties of the active layer, were
studied using Nanoscope Multimode Atomic Force Microscope (AFM). Capacitance
measurements of the samples were carried out using impedance analyser (HP-4194). A signal of
1V peak to peak was applied to the samples during capacitance measurements. Displacement
sensing properties of the samples were investigated using a special test fixture after placing a
thin plastic sheet and a thick rubber in sequence on the gate films of the devices. Range of the
vertical displacement depends upon the elastic properties and thickness of the rubber film.
Thicknesses of active layers and various electrode films were estimated using Scanning Electron
Microscope (SEM) and DEKTAK profilometer. All the tests and measurements were performed
at ambient air and room temperature.
82
5.2.3 Results and Discussion
Earlier investigations showed that aluminum (Al) makes Schottky type contact, whereas silver
(Ag) makes Ohmic type contact with organic bulk hetero-junction based semiconducting layers,
respectively [93]. Through I-V measurements, Schottky type (Rectifying) contact between gate
electrode (Al) and active layer, and Ohmic contact between source-drain electrodes (Ag) and the
active layer was observed. Such behavior of P3HT:PCBM blend based active layer has been
previously demonstrated [273] with the help of schematic energy level diagram of organic
semiconductors with respect to the work functions of source/drain and gate electrodes. Figure
5-12 shows section roughness of active layer. The RMS and Average roughness values of the
active layer were found equal to 11.297nm and 8.862 nm, respectively. No considerable
structural defects exist in the AFM profile, which are generally associated with solution
processed films.
Figure 5-13 describes output I-V characteristics of the OFET. I-V characteristics of the device
are similar to p-type mode I-V characteristics of typical ambipolar organic field effect transistor.
Weak kink in the output I-V characteristics of the device at lower gate voltages (0 to -1V) with
the increase of drain to source voltage, confirms ambipolar nature of its active layer [211]. In our
case, with the increase of negative gate voltage, holes are drawn towards gate electrode which
increases the bulk conductivity of the active layer, and current between drain source electrodes
increases as a result. Further, gate leakage current becomes comparable with drain to source
current when positive or higher negative gate voltage (above -5 V) is applied. Thus this device
can work for only lower negative gate voltages. Furthermore, our device, similar to reported
ambipolar field effect transistors [223], has shown weak saturation properties due to ambipolar
nature of its active layer. Field effect mobility of the device was calculated using Equation 5-2
and its value was found to be equal to 1.6×10-3
cm2V
-1S
-1. Displacement was varied from 0 to
250 µm and its effect on the drain to source current (IDS) of the device was studied.
Figure 5-14 presents drain to source current as a function of drain to source voltage under given
displacements when gate was suspended. It was found that drain to source current increases with
the increase of displacement. Behavior of the device under displacement is similar to CuPc based
displacement and pressure sensors [80, 81, 270] which were based on organic MESFET
83
architectures. Significant increase in drain to source current (was observed when displacement
was increased from 0 to 250µm.
Figure 5-12: Section roughness of P3HT PCBM blend based semiconducting layer
Figure 5-13: Output I-V Characteristics of the OFET
84
Figure 5-14: Drain to source current (IDS) as a function of drain to source voltage (VDS) under given
displacements when gate was suspended, inset1: Schematic of the displacement measurement setup,
inset2: Variation of the drain to source resistance (RDS) with displacement at VDS = -8V.
The drain-source resistance (RDS) of the device can be calculated using following relation [274]:
𝑅𝐷𝑆 =𝜌𝐴
𝑑=
𝑉𝐷𝑆
𝐼𝐷𝑆 5-4
Where ρ is the resistivity, A is the effective cross-sectional area of the device, d is the inter-
electrode distance which is equal to channel length, VDS and IDS are the voltage and current
between drain source electrodes, respectively. The value of RDS at 0 displacement was found
equal to 15.40 MΩ which reduces to 13.15 MΩ when displacement was increased to 250µm.
Variation of the current with displacement was assumed due to changes in the transport and
structural properties (such as inter molecular distance) of polymer-fullerene bulk hetero-junction
based semiconducting layer. As drain to source current is also function of the thickness of
semiconducting layer (Equation 5.2), therefore decrease in the thickness and increase in the
conductivity of semiconducting layer with displacement may also be the reason of the increase of
current due to displacement. Flexible strain sensors based on transistor like Wheatstone bridge
configuration and pentacene nanolayer were recently investigated [86, 87]. Significant influence
of mechanical stress on the electrical properties of these strain sensors was reported. The changes
85
in the structural and morphological properties of semiconducting layer due to applied tensile
strain were suggested as the reason of the variation of electrical properties of these devices. The
goal of this work was to develop and demonstrate the results of an organic bulk heterojunction
based displacement sensitive organic MESFET.
Solution processed Polymer-Fullerene bulk hetero-junction nanolayer based displacement
sensitive OFET was successfully fabricated and demonstrated using MESFET configuration for
the first time. Active layer which was the blend of p-type and n-type organic semiconducting
materials, demonstrated Ohmic contact with drain/source electrodes and Schottky contact with
gate electrode, and adopted the behavior of p-type organic semiconductor. The operation of the
device was found similar to a typical p-type low voltage organic field effect transistor.
Significant influence of the displacement variation on the electrical properties of the device was
observed. It was found that drain current increases with the increase of displacement. Mainly
ease of the processing of these devices motivated us for this work, and our results constitute a
concept basis for possible flexible devices and applications, because the fiction and assembly
would be the same in flexibles. In addition, one can expect that these devices will be ideal
candidates for future large area sensing applications.
86
Chapter-6: Electrical Characterization of
Graphene Oxide, Lead Zirconate Titanate
and Liquid Crystalline Polymer Films 6 6
In this chapter electrical characterization of solution processed Graphene Oxide (GO), Lead
Zirconate Titanate (PbTi0.5Zr0.3(Co1-xMgx)0.2O3) ceramics and methacrylate based Side Chain
Liquid Crystal Polymers (SLCP) films is presented. Parallel plate impedance/ dielectric
spectroscopic technique was used for this investigation. The chapter is divided into three parts.
Electrical characterization of GO, ceramic and SLCP films is described and presented in
sequence.Organic Electronic Devices are preferred over conventional in-organic semiconducting
materials based devices due to their low cost, room temperature solution processing and other
promising features which include flexibility, potential to develop large area sensing arrays, etc.
However, efficiency of organic microelectronic devices is comparatively low. To enhance the
performance of organic semiconducting devices, one way is to synthesize new organic materials
with desired features and to develop the devices using these newly developed materials.
Graphene Oxide (GO) which is reported to be highly sensitive to humidity, has been recently
investigated/ demonstrated as dielectric [275, 276] & semiconducting [277] material for Organic
Transistors and efficient hole transport material for Organic Solar Cells [147]. Variation in the
electrical properties (in particular transport and dielectric properties) of GO films with
temperature is being investigated in detail in this work. GO films were drop casted on Indium
Tin Oxide (ITO) coated glass substrates. AFM was used to study the films‘ structural and surface
profile. Electrical measurements of the films were taken using Impedance Analyzer in the
frequency range of 100 Hz to 10 MHz at different temperatures. It was noted that Analog
Current conductivity (ζac) of the films varies with angular frequency (ω). The electrical
parameters of GO were found to be dependent on both frequency and temperature. Results
indicated that GO film possesses direct current (DC) and Correlated Barrier Hopping (CBH) type
of conduction mechanisms at low and high frequency ranges, respectively. Excellent photon
absorption as well as transmittance capability in the visible range suggests suitability of the films
for the realization of flexible organic solar cells and large area photo sensing arrays.
87
Experimental results of Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65,
0.85) ceramics, synthesized by sol-gel route at 1150 oC for 2 h in air are presented and discussed
in the subsequent section. The final ceramics were characterized by using SEM and LCR-meter.
The dielectric constant (ɛr) was observed to increase with temperature in the investigated
temperature range of 30 oC to 300
oC). The x = 0.25 composition exhibited high magnitude of ɛr
(4261) and dielectric loss (tan = 0.05) over a wide range of temperature at 100 kHz. The
magnitude of tan of sample x = 0.65 was recorded to be 0.009 around room temperature at 100
kHz. The resistance of the ceramic samples was found to decrease increase in temperature
indicating a non-Debye type relaxation phenomenon. The AC conductivity of PbTi0.5Zr0.3(Co1-
xMgx)0.2O3 samples increased gradually with an increase in the both temperature and frequency.
Electrical characterization of a series of methacrylate based side chain liquid crystalline
copolymers (SCLCPs) containing cholesteryl mesogen (poly(Chol-n-MMA-co-MMA)) with
various lengths of aliphatic spacer (n = 3, 7, 10) by the chemical binding of cholesterol
molecules with side chains of comb like polymers, prepared via free radical polymerization is
presented in the last section of the chapter. The dielectric properties of homopolymers i.e.
poly(Chol-n-MMA) and copolymers i.e. poly(Chol-n-MMA-co-MMA) with various lengths of
aliphatic spacer (n = 3, 7, 10), were analyzed by impedance spectroscopic technique in the
frequency range of 100 Hz to 15 MHz at room temperature. Dielectric constants of the liquid
crystalline films were found to increase with the decrease of flexible spacer length. Cole-Cole
plots investigation indicated non-Debye type relaxation behavior of the samples except for co-
polymer with chain length 7 and homo-polymer with chain length 3 which showed nearly Debye
type relaxation properties. AC conductivity (ζac) of the crystalline polymer films was observed to
vary with angular frequency, ω as ωS with S<2. Detailed conductivity analysis revealed that the
conductivity of polymer films followed QMT (Quantum Mechanical Tunneling) and CBH
(Correlated Barrier Hoping) conductivity mechanisms at low frequency regime and, the SLPL
(Super Linear Power Law) and DC (Direct Current) conductivity mechanisms at high frequency
region. In the light of current study, films based on the polymers being investigated are proposed
as model systems for applications in large area microelectronic devices.
88
6.1 Electrical Characterization of Graphene Oxide (GO) Films
6.1.1 Introduction
During the past decade, materials such as fullerenes, nanotubes and graphene were demonstrated
as promising candidates for potential applications in electronic industry due to superior electrical
and optical properties. Graphene is a two dimensional monolayer of hexagonally arranged sp2
hybridized carbon atoms [278]. Due to excellent physiochemical, electrical, thermal and
mechanical properties, graphene is effectively used in many technological applications such as
hydrogen storage, field effect transistor, ultra-capacitors and organic solar cells [279-281] .
Graphene can be produced from water soluble graphene oxide using reduction process [282-
284].
Graphene oxide (GO) has sp2 and sp
3 hybridized carbon atoms and can be considered as
insulating material compared to graphene. GO exhibits various carboxyl and hydroxyl functional
groups and is soluble in water. The optical and electronic properties of GO can be changed by
varying the content of oxygen and water molecules. Recently, GO was demonstrated as channel
and dielectric material for field effect transistors [146], and efficient hole transport layer for
organic solar cells [147]. Due to the presence of oxygen based functional networks on the basal
plane of GO, it can make bonds with variety of organic and inorganic materials [148-150]. The
optical and electrical properties of GO can be enhanced using composites with conductive
polymers.
Purpose of this work is to synthesize and investigate temperature dependent transport properties
of solution processed Graphene Oxide thin films in order to explore the possibility of their usage
for flexible organic electronic devices. Transport properties of GO were studied using impedance
spectroscopy, a well-known technique used for similar investigations. Temperature dependent
AC conductivity analysis of the films indicated its semi-conductive behavior. Further, GO films
have shown DC and CBH conductivity mechanisms at lower and higher frequency ranges,
respectively.
89
6.1.2 Experimental Procedure
6.1.2.1 Synthesis of Graphene Oxide
Graphene Oxide (GO) was synthesized by researchers at TUBITAK Marmara Research Center,
Turkey adopting the following procedure. 0.5 g of graphite (Aldrich) and 25 mL of sulfuric acid
(98%) was mixed at room temperature for 2 hours. Then, 3 g of KMnO4 (Aldrich) was added
into this solution. The mixture was stirred at 50° C for 24 hours and left to cool down to room
temperature. Distilled water of about 40 mL was gradually added to above mixture. Then, 100
mL of distilled water was added and then 3 mL of H2O2 (35 wt % aqueous solution, Aldrich) was
added drop wise to the above solution until the reaction solution turned yellow. The yellow
graphene oxide dispersion was centrifuged at 7000 rpm for 2 hours to precipitate the GO. The
supernatant was removed after centrifugation. 30 mL of distilled water was further added to the
dispersion. The resulting dispersion was ultrasonically treated for 15 minutes to make a uniform
dispersion and remove any agglomeration. The dispersion was centrifuged again at 5,000 rpm for
30 minutes. This centrifugation process was repeated several times and supernatant was removed
at each cycle until the excess of acid was removed from the GO solution. Acid removal was
monitored by measuring the changes in the pH of supernatant. The neutralized GO solution was
then homogenized ultrasonically for 2 h to prepare 5mg/mL graphene oxide solution.
6.1.2.2 Structure Formation
ITO coated glass substrates were ultrasonically cleaned using methanol, ethanol, isopropyl
alcohol and de-ionized water, and dried using nitrogen blow. 3µm thick GO films were coated on
the substrates using drop casting technique and samples were baked in oven at 55° C for 3 hours.
Finally, around 50 nm thick conductive contacts were made on the GO films by evaporating
silver in the vacuum chamber at a pressure of 10-5
mbar using Univex Leybold Vacuum
Evaporator. The active area of the investigated samples was 0.06 cm2. The schematic view of the
fabricated parallel plate structures and, the chemical formula of graphene oxide are presented in
Figure 6-1.
90
Figure 6-1: (a) Schematic View of parallel plate structures and (b) Chemical Formula of GO
6.1.2.3 Characterization
Surface profiles of the active layers (GO Films) were studied using Nanoscope Multimode AFM.
Absorption and transmittance spectra of the films were taken using Cary5000 UV-Vis-NIR.
Electrical parameters of the samples were measured using the setup presented in Figure 6-2. This
setup consists of an impedance analyzer, HP 4194A and NOVOTHERM Temperature Control
System interfaced with computer. Electrical parameters such as real and imaginary dielectric
constants, dielectric loss and AC conductivity of the sample were measured as a function of
frequency at different temperatures. An AC signal with amplitude of 1V (RMS) was applied to
the samples during these measurements. The temperature was monitored with the help of a PT
100 resistor which was in direct contact with the sample.
6.1.3 Results and Discussion
AFM images and absorption & transmittance spectrums of GO film having 10µm×10µm
dimension are shown in Figure 6-4. The Root Mean Square and Average roughness values of the
films were found equal to 18.18 nm and 14.38 nm, respectively. It is clear from the AFM profiles
that films are free of any major structural disorder. Furthermore, surface and 3D AFM profiles of
the GO film presents a homogenous distribution of GO particles. The absorption peak found at
around 355 nm, can be associated with the * transition of carbon bonds (C-C) in aromatic
ring. Maximum transmittance of the films was found equal to 74% at 487nm.
91
Figure 6-2: Experimental set up for temperature dependent electrical analysis of GO Films.
Figure 6-3: (a) Surface and (b) 3D AFM profiles, (c) Absorption and (d) Transmittance Spectra of
GO Films
92
The variation in real & imaginary dielectric constants (ε and ε) and dielectric loss of GO film
as a function of frequency at different temperatures is shown in Figure 6-4. The frequency of the
applied AC signal was varied from 100 Hz to 10MHz. Figure 6-4(a) shows the real dielectric
constant - frequency plots of the films in the frequency range of 100 Hz to 10 MHz. At 100 Hz,
the magnitude of εwas observed to be decreased from 2.81 at 30 oC to 1.12 at 100
oC.
Furthermore, magnitude of ε was found to decrease with increase in frequency from 100 Hz to 2
KHz. However, no significant effect of frequency and temperature variation was observed as
frequency becomes higher than 2 KHz. The magnitude of ε' was found to be equal to around 0.5
within the frequency range of 2 KHz to 10 MHz at all the investigated temperature values.
Capacitance (C) of the film can be calculated using the following equation [106]:
𝐶 = 휀𝑜휀𝐴/𝑑 6-1
Where εo is the dielectric constant of free space, A is the surface area and d is the thickness of
active layer. Figure 6-4 presents the ɛ - frequency plots of the films in the frequency range of
100 Hz to 10 MHz at selected temperatures (30, 40, 50, 60, 70, 80, 90 and 100ºC).The behavior
of ɛ as a function of frequency at different temperatures was found similar to εʹ-frequency trend.
The magnitude of εʺ is given as [106]:
휀ʺ = 휀 𝑡𝑎𝑛 𝛿 6-2
Where tan δ, is called as loss tangent or dielectric loss. Thus ɛ is actually measurement of the
tan δ of the investigated films. The variation of the tan δ as a function of frequency and
temperature is shown in Figure 6-4. The magnitude of tan δ was observed to decrease with an
increase in both frequency and temperature.
The graph of lnζAC - lnω of the GO film at different temperatures is shown in Figure 6-5. It
shows strong dependency of AC conductivity (ζAC) on temperature at low frequencies.
Furthermore at low frequencies, a plateau was observed which characterizes the direct current
conductivity (DC), while at high frequency; the conductivity was increased gradually with an
increase in frequency. AC conductivity was observed to decrease with an increase in temperature
at low frequency, however, at high frequency, no significant effect of temperature on AC
93
conductivity of the film was observed. The dissipation of water molecules and impurities, out
from sample, with increase in temperature at low frequency might be the reason of decrease in
conductivity ofthe sample [285]. The sample has demonstrated semi-conductive behavior within
the investigated temperature range (30-100 ºC) as reported by Huang et al. [286] for the case of
GO paper. The AC conductivity dependence of the angular frequency can be expressed by the
following relation, as the empirical Jonscher's universal law [101, 287]:
𝜎𝐴𝐶 𝜔 = 𝜎𝐷𝐶 + 𝐴𝜔𝑠 6-3
Where, A is a constant, ω is the angular frequency and s is the frequency exponent parameter
which identifies AC conduction mechanisms. The magnitude of exponent s, which represents
main body interactions of electrons and impurities, is usually less than unity [112].
Figure 6-4: (a) Real and (b) Imaginary Dielectric Constants, and (c) Loss Tangent Plots of GO
film as a function of frequency at given temperatures.
94
Figure 6-5: AC Conductivity of GO film as a function of frequency at given temperatures
The magnitude of parameter s (angular frequency exponent) was calculated from the slopes of
Figure 6-5. In this work, the value of parameter s was found equal to 0 at low frequency region,
showing DC conductivity behavior [101] of GO film and very close to unity at high frequency
region. Furthermore its magnitude was found to vary slightly with increase in temperature. So
based on the magnitude of parameter s (which is close to unity at high frequency region) and its
temperature dependent behavior, it can be concluded that the results are consistent with the
correlated barrier hopping (CBH) model [288-290]. This model proposes temperature activated
hopping of charge carriers between localized sites [291]. Thus GO film possesses DC and CBH
mechanisms of conduction at low and high frequency regimes, respectively. The transition
region from DC to CBH conductivity shifted to low frequency with increase in temperature.
The Complex Impedance Spectra of Graphene Oxide (GO), obtained by plotting ZS Vs ZS at
different temperatures is shown in Figure 6-6. This pattern not only informs about the electrical
processes occurring within the sample but also presents their correlation with the microstructure
of the sample when modeled in terms of an electrical equivalent circuit [292] as presented in the
inset of the Figure 6-5. R2 in the equivalent circuit represents contact resistance whereas R1 in
parallel with ZC1 (impedance of C1) represents polarization resistance. The impedance spectrum
of the GO is based on semicircular arcs whose pattern is found to change with increase in
95
temperature. As can be seen the Nyquist plots of GO sample, complete semicircles does not form
at higher temperatures. This is due to very high impedance of GO sample at higher temperatures.
Figure 6-6: Complex impedance spectrum (Nyquist Plot) for the GO at various temperatures, inset:
Equivalent RC network
In this work, dielectric and transport properties of GO (synthesized using modified Hummers
Method) as a function of frequency and temperature Impedance spectroscopic technique.
Graphene Oxide films were drop casted on Indium Tin Oxide (ITO) coated glass slides. Atomic
force microscopy (AFM) was used to characterize the films structural and surface profiles.
Electrical measurements were taken using impedance analyzer in frequency regime varying from
100 Hz to 10 MHz) at different specified temperatures. Results indicated that AC conductivity,
ζac, of the films‘ varies with ω, the angular frequency. Temperature and frequency variation has
significantly influenced on the electrical properties of GO film, in particular at low frequency
regions. Based on the results it was attributed that film bears DC and CBH conductivity
mechanisms at low and high frequency ranges, respectively. Realization and understanding the
transport properties of similar solution processed films will provide potential and foundation for
the development of advanced microelectronic devices in future.
96
6.2 Effect of Co/Mg ratio on the structural and electrical properties of lead
zirconate titanate films
6.2.1 Introduction
Lead zirconate titanate (PbZrTiO3 or PZT) based materials/ceramics with perovskite ABO3 ((A =
Mono –or divalent; B = Tri- hexavalent ion)) structure has attained much focus of the researchers in
recent years because of the simplicity of its crystal structure which helps to understand the
relation between structural changes and physical properties. Perovskite based piezoelectric
materials show high electromechanical transformation, due to which these are commonly utilized
in electro-ceramic applications in industry. Furthermore PZT, due its excellent piezoelectric
properties is commonly utilized during the development of underwater/ultrasonic transducers,
such as hydrophones, actuators and underwater transducers. In industry, materials showing
thermally stable dielectric as well as piezoelectric properties around transition temperature are
required [151-155]. The dielectric properties of PZT can be tuned by using various isovalent and
aliovalent dopants. Under certain conditions when PZT crystals possess tetragonal or
rhombohedral symmetry, each crystal has a dipole moment.
PZT materials, exhibit a unique range of properties which make them suitable for the
development of sensors and actuators [293]. In a basic sense, if a piezoelectric material is
deformed, an electrical signal proportional to bending/deformation due to piezoelectric effect.
Also if electrical signal is applied deformation or bending is observed in these materials due to
inverse piezoelectric effect. In order to improve the electrical properties of PZT, various attempts
have been reported in recent years. Doped PZT are mostly synthesized by using one of the
following methods [293]:
f. conventional solid state sintering route,
g. peroxohydroxide method,
h. citrate precursor method, precipitation method,
i. hydrothermal method and
j. solgel method.
97
In order to investigate the effect Cobalt/Magnesium ratio on the electrical and structural
properties of PZT films, Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65,
0.85) ceramics films were solution processed using sol-gel route and characterized using parallel
plate impedance spectroscopy. Mg doped PZT ceramics possess activation energy (Ea 1.05
eV) which is comparable with the Ea values (11.1 eV) associated with doubly ionized oxygen
vacancies (VÖ) common in perovskite oxide piezoelectric materials.
6.2.2 Experimental Part
Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65, 0.85) powders,
synthesized by researchers at the Department of Materials Engineering, GEBZE Technical
University, Turkey, were electrically characterized. In order to prepare ceramics, PVB binder
was added in 5 wt% to the powders and mix-milled in agate and mortar for 15 minutes. The
powders were then pressed into pellets and binder was burned out at 600 oC for 30 minutes with
a heating step size of 5 oC/min and subsequently sintered at 1150
oC for 2 h to get dense
ceramics. The ceramic samples were used for further characterization.
Scanning Electron Microscopy (SEM) was used in order to investigate the microstructure of
sintered ceramics using a XL30, FEi Co., Hillsboro. The electrical measurements were
performed on parallel-plate capacitor configuration using an Agilent, HP 4194A LCR meter. In
order to make contacts of the samples, the opposite polished surfaces of the sintered pellets were
coated with Ag-paste and cured at 600 oC for 30 min. The impedance of the samples was
monitored between 30 oC to 300
oC using NOVOTHERM Temperature Control System.
Complete electrical characterization system (shown in Figure 6-2) includes impedance analyzer,
computer station, Novotherm temperature heating and control system.
6.2.3 Results and Discussion
Figure 6-7 presents the SEM views of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics surfaces at high
resolution. The SEM micrographs of surface showed well connected grain microstructures with
limited porosity, which is sometime observed for the case of PbO based ceramics. Figure 6-8
shows the variation of dielectric constant (ɛr) and Dielectric Loss (Tan δ) of PbTi0.5Zr0.3(Co1-
xMgx)0.2O3 ceramics as a function of frequency, ranging from 100Hz to 1MHz. It was observed
that the magnitude of ɛr for the x = 0 composition decreases with an increase in frequency, which
98
Figure 6-7: SEM micrographs of the thermally itched surface of PbTi0.5Zr0.3(Co1-xMgx)0.2O3
ceramics.
Figure 6-8: Variation of dielectric properties of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
frequency
99
is attributed due to the typical relaxation phenomena in the ferroelectric Perovskites [294].
Further, the magnitude of ɛr for the x > 0.00 compositions was almost constant with frequency.
This behavior confirmed that the Mg-doped samples exhibited non-relaxor characteristics. The
magnitude of ɛr for the x = 0.25 composition was highest among the investigated samples/
compositions and decreased with an increase in Mg content in the given frequency regime (100
Hz1MHz).
Figure 6-9 (a) and (b) shows the variation of ɛr and tan with temperature measured at 100 kHz,
respectively. The ɛr value was found to increase gradually with increase in the temperature. The
phase transition temperature was not observed in the current experimental setup and investigated
temperature range i.e. 30 oC to 300
oC. The tan value was constant over the wide range of
temperature. The magnitude of tan was observed to decrease with an increase in Mg content.
The conduction mechanism of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples were studied by
the measurement AC conductivity at 100 kHz in the temperature regime from room temperature
to 300 oC. The results are depicted in the Figure 6-10. The AC conductivity of the x = 0
composition was independent of temperature in the low temperature regime up to 250 oC which
then gradually increased with further increase in temperature. The x = 0.25 composition
exhibited high magnitude of conductivity in the investigated temperature regime.
Figure 6-9: Variation of dielectric properties of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
temperature
100
Figure 6-10: Variation of AC conductivity of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
temperature.
Variation of AC conductivity of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples with frequency at
various temperatures is presented in Figure 6-11. No significant difference in the conductivity of
the x = 0.00 to 0.85 compositions in the low frequency regime (up to 105 Hz) was observed,
which increased instantly/ abruptly with further increase in frequency for all the ceramic
samples.
The frequency dependent properties of a material can be described as complex permittivity (ɛ*),
complex impedance (Z*), complex admittance (Y*), complex electric modulus (M*) and
dielectric loss can be related by the following equations:
ɛ ∗ = ɛ + 𝑗ɛ 6-4
ɛ ∗ = ɛ + 𝑗ɛ 6-5
𝑍∗ = 𝑍 + 𝑗𝑍 = 1/𝑗𝐶𝑜ɛ∗ 6-5
𝑌∗ = 𝑌 + 𝑗𝑌 = 𝑗𝐶𝑜ɛ∗ 6-6
101
𝑀∗ = 𝑀 + 𝑗𝑀 = 1/ɛ∗ = 𝑗𝐶𝑜𝑍∗ 6-7
𝑡𝑎𝑛 = ɛ/ɛ = 𝑀/𝑀 = 𝑌/𝑌 = 𝑍/𝑍 6-8
where, = 2 is the angular frequency, Co is the free geometrical capacitance. The
microstructure of ceramics comprises of grains (bulk) and grain boundaries, exhibiting different
resistivities (ρ) and dielectric permittivities (ɛ). Impedance spectroscopy can be used to interpret
various complex parameters such as the real (ɛ , Z , Y , M) and imaginary (ɛ , Z , Y , M)
components of various parameters, in order to understand the electrical structure of materials.
Figure 6-12 shows the Nyquist plots, obtained by plotting Z vs. Z at various temperatures for
the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples. The plot showed single semicircular arcs
throughout the investigated frequency range (100 Hz to 1 MHz) for all the ceramic samples (at
300 oC). A complete semicircle was not obtained for the x = 0.85 composition at 300
oC. The
semicircles are depressed to certain degrees which showed deviation from ideal behavior. Such
behavior can be associated with the overlapping of two or more semicircles which could not be
resolved in the current experimental setup. Bulk capacitances (Cb) of the samples were calculated
from the peaks of semicircles using relation:
𝜔𝑚𝑎𝑥 𝐶𝑏𝑅𝑏 = 1 6-6
In summary, Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics were electrically characterized
using parallel plate impedance spectroscopic technique. The microstructure of ceramics was
found to be dependent on Mg doping content. The x = 0.00 composition showed a relaxor
behavior in the frequency regime from 100 Hz to 1 MHz. The magnitude of tan and ɛr of
PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples were decreased with an increase in Mg doping
content. The x = 0.25 composition was found to exhibit a high ɛr (4261) and tan (0.05) around
room temperature at 100 kHz frequency.
102
Figure 6-11: Variation of AC conductivity of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with
frequency at various temperatures.
103
Figure 6-12: Complex impedance plots for PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics at 300 oC
104
6.3 Electrical Conductivity Analysis of Methacrylate-based Liquid Crystal
Polymers with Pendant Cholesterol Groups
6.3.1 Introduction
Liquid Crystal Polymers (LCPs) are unique materials which combine the features of liquid
crystals and polymers. In order to exhibit liquid crystal characteristics for polymers, mesogenic
groups (rod-like or disk-like units) must be attached to polymer on the main or side chain. If they
are the part of the main chain of polymer, polymer is called main chain liquid crystal polymer
(MCLCP). Otherwise, side chain liquid crystal polymer (SCLCP) is formed when mesogens are
incorporated into polymer as side groups. MCLCPs cannot show mesogenic property in large
temperature ranges, however, mesogenic behavior of SCLCPs are in a large range [295-297].
SCLCPs can be varied by altering their components, which are the polymer backbone, the
mesogenic group, and the flexible spacer between the polymer backbone and the mesogen [298-
300].
The literature suggests that SCLCPs can be synthesized mostly with chiral compounds such as
cholesterol (Chol), isosorbide, and menthol [301-304]. Chol which plays a crucial role in the
field of self-organising organic systems is a rigid component of many natural lipid bilayers
[305]. In Chol containing polymers, the Chol groups produce mesomorphous structures [306]. 80
LCP with pendant Chol side groups have significant utility due to their optical properties [307].
They form chiral nematic and smectic phases and can be usually employed in electro- and
thermooptical applications [308] By changing the polymer backbone (i.e., acrylate, methacrylate,
and siloxane) and flexible spacers (i.e., methylene and siloxane), Chol containing SCLCPs have
been prepared [309, 310]. The synthesis of block copolymers [(Poly(methyl
methacrylatecholesterol-b-styrene) and poly(cholesterylmethacrylate-b-2-hydroxyethyl
methacrylate)] have been also reported [311, 312].
In semiconductor industry, liquid crystals are one of the most promising materials especially for
microelectronic devices [313-315]. In the last decades, organic field effect transistors (OFETs)
attract a great deal of attention as an important type of electronic devices for their use in
lightweight, elastic, and low-cost electronic instruments [316-319]. They can be implemented in
wide range of applications such as flat-panel displays, electronic paper, and chemical sensors
105
[320]. Potentially, SCLCPs could be used as a key component in the field of organic
microelectronics [321].
Poly(methyl methacrylate) (PMMA) is one of the well-known polymers for dielectric materials.
It is a hard, hydrophobic, less-polar, brittle, and easily prepared material. An advance property of
PMMA relies on its high resistance and non-tracking characteristics in high voltage applications.
It has a dielectric constant that ranges from about 3.5 (at 5 kHz) to 2.6 (at 1 MHz) and can be
used as a gate dielectric layer in OFETs [322, 323].
In this work, electrical and dielectric properties of methyl methacrylate-based SCLC
homopolymers (poly (chol-n-MMA)) and copolymers (poly (chol-n-MMA-co-MMA))
containing Chol pendant groups with different lengths of flexible aliphatic spacer (n= 3, 7, and
10 methylene units) were studied using parallel impedance spectroscopic technique in the
frequency range of 100 Hz to 15 MHz at room temperature, to fully exploit their possible
applications for flexible OFETs and organic photo voltaic (OPV) assemblies. The polymers
were synthesized by researchers at Department of Chemistry, Gebze Technical University
(GTU), and used as received. The dielectric constant values of the polymers were found to
increase with decreasing aliphatic methylene spacer length. Investigation of the polymers via
Cole-Cole plots technique demonstrated the fact that most of the polymers obey non- Debye type
relaxation behavior except homopolymer and copolymer with n = 3, which showed nearly Debye
type relaxation. Alternating current (AC) conductivity (ζac) of the liquid crystalline polymeric
films was observed to vary with angular frequency, ω as ωS with S<2. Detailed conductivity
analysis revealed that the conductivity of the polymeric films follow quantum mechanical
tunneling (QMT) and correlated barrier hoping (CBH) conductivity mechanisms at low
frequency regime, whereas it obeys super linear power law (SLPL) and direct current (DC)
conductivity mechanisms at high frequency region. In the light of the current study, the
investigation of the polymeric films helped to propose a model system for their possible potential
applications such as large area flexible arrays or as other advanced microelectronic devices.
6.3.2 Experimental
Electrical properties of the polymers were analyzed using Parallel Plate Impedance
Spectroscopic Technique. Parallel plate type structures were developed on glass substrates as
106
shown in Figure 6-13. Fabrication of these structures were started with the cleaning of Indium
Tin Oxide (ITO) coated glass substrates using Soap, Acetone, Methanol, Ethanol and Isopropyl
Alcohol in an ultrasonic bath. Drying of the substrates was carried out using Argon blow and
Thermal Oven. Solutions of the polymer samples were prepared in Toluene (Merck) by the
process of magnetic stirring for 24 hours at room temperature. Polymer films were coated on
cleaned ITO coated glass substrates by the process of spin coating at 2000 rpm using filtered
polymer solution. The size of the filter was 0.45µm. Then samples were kept in the nitrogen
environment for 12 hours in order to evaporate solvent. Finally, 50 nm thick top contact was
made by evaporating Aluminum on the polymer films through the shadow mask using Leybold
Univex 450, Vacuum Evaporator at 10-5
mbar pressure. Thicknesses of various polymer films
were measured using DEKTAK-8 profiler. Impedance Analyzer, HP-4194 was used to determine
the electrical and dielectric properties of the samples within the frequency range of 100Hz to
15MHz. A signal of 1 V peak to peak was applied to the samples during these measurements. All
the tests and measurements were performed in the ambient conditions.
6.3.3 Results and discussion
Electrical characterization of solution processed methacrylate based co-polymer (chol-n-MMA-
co-MMA) and homo-polymer (poly (chol-n-MMA)) is presented in this section, Where n = 3, 7,
10. The real εʹ(ω) and the imaginary εʺ(ω) parts of the complex dielectric constant are described
as ε*(ω) = εʹ(ω)-iεʺ(ω) [324]. Here, ε
*(ω) is the complex dielectric constant.
Figure 6-13: Design Schematic of Parallel Plate Structure used for Impedance Spectroscopic
Analysis of Co-polymers and Homo-polymers
107
Figure 6-14 shows the frequency dependence of the real εʹ(ω) and the imaginary εʺ(ω) parts of
the complex dielectric constant on a semi-log scale at room temperatures in the parallel plate
device under test, respectively. As can be seen, all the samples have shown almost constant value
of εʹ at low frequencies up to 10 kHz, indicating a good and stable polarization over a wide
frequency band. Among the investigated samples, polymers with chain length 3 i.e. Poly(Chol-
MMA-3) and Poly(Chol-MMA-3-co-MMA) have shown the highest dielectric values. Whereas
polymers with chain length 7 i.e. Poly(Chol-MMA-7) and Poly(Chol-MMA-7-co-MMA) have
demonstrated lowest values of dielectric constant. Real dielectric constant values of polymers
with chain length 10 i.e. Poly(Chol-MMA-10) and Poly(Chol-MMA-10-co-MMA) were found
higher than the polymers with chain length 7 and lower than those with chain length 3
throughout the investigated frequency range.
The real ε'(ω) part of the dielectric constant is described as [99]:
6-7
Figure 6-14: Angular frequency evolution of (a) the real (εʹ), and (b) imaginary (εʺ) dielectric
constants of the polymers
)1(2)(
2
1sin1)(21
2
1sin1)(1
)()('
oo
o
s
108
Here, ε∞ and εs are high and low angular frequency dielectric constant, ω=2π times the
frequency, η0 is a generalized relaxation time, and α is the absorption coefficient. The absorption
coefficient α parameter changes from zero to one (0 < α ≤1). If α =0, it corresponds to standard
Debye type relation, nearly-Debye type when α value is close to zero and non-Debye type for 0 <
α ≤1 regions. η0, relaxation time and α, absorption coefficient was calculated from equation (1)
and fitted by the results given in Figure 6-14 (a). The calculated values of η 0 and α are given in
Table 6-1. The values of parameter α are higher than zero which indicates a non-Debye type
relation [325]. The absorption coefficient values of the samples suggested that the investigated
polymers obeyed non-Debye type and nearly-Debye type relaxation processes. Table 6-1 also
presents the values of parameters εs, ε∞ and Δε which were estimated from Figure 6-14 (a). These
parameters change due to chain length of different samples. Equivalent dielectric constants vary
according to the chain length for both homopolymers and copolymers. Critical frequency of all
polymers is determined from Figure 6-14 (b). This frequency value corresponds to the maximum
peak frequency of imaginary dielectric constant. Significant variation in the critical frequency of
the samples is observed, which is in agreement with the variation of other parameters.
Table 6-1: Absorption coefficient relaxation time (o), dielectric parameters (εs, ε∞, ε''max, and
Δε) and critical frequencies (fc) of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA) (n= 3, 7,
and 10).
Samples (Adj. R-Square 0.99)
α 0 εs ε∞ ε''max. Δε fc[MHz]
Poly(Chol-3-MMA-co-
MMA) 0.39 7.07×10
-8 3.82 -0.102 1.96 3.902 1.31
Poly(Chol-7-MMA-co-
MMA) 0.04 5.80×10
-8 2.39 -0.135 1.43 2.425 3.26
Poly(Chol-10-MMA-co-
MA) 0.27 6.86×10
-8 3.25 -0.117 1.66 3.367 2.26
Poly(Chol-3-MMA) 0.064 5.22×10-8
4.02 -0.301 2.36 4.321 3.91
Poly(Chol-7-MMA) 0.28 6.64×10-8
2.66 -0.100 1.3 2.76 2.72
Poly(Chol-10-MMA) 0.74 8.01×10-8
2.85 0.029 1.3 2.821 0.53
Cole-Cole plots of the samples are shown in Figure 6-15. These Cole–Cole plots were obtained
from the imaginary versus real values of dielectric constants (ε"–ε'). Equivalent circuit model
109
analysis of the Cole–Cole plots show parallel Resistance-Capacitance (RC) and series Resistance
(R) regimes for the investigated samples.Plots have shown non–Debye and nearly-Debye type
relaxation phenomena. The non-Debye type plot are most suitable for the description of the
interface molecular relaxation mechanisms. εʺ– εʹ curves (Cole-Cole plots) of the samples show
depressed semicircles. Poly(Chol-MMA-7-co-MMA) and Poly(Chol-3-MMA) samples are close
to x-axis as they are exhibited nearly-Debye type properties. Angular frequency evolutions of the
ac conductivity (ζʹ) of samples are shown in Figure 6-16. To understand the conductivity
mechanism of the samples, lnζʹ-lnω graphs of the samples were drawn and investigated. As
indicated in Figure 6-17, some of these samples show frequency linear regions on their plots.
The values of parameter s (angular frequency exponent) were calculated from the slopes of
Figure 6-17. Linear increase in the Figure 6-17 with frequency is fitted and ac conductivity is
acquired from this fit. During the fit slope, linear equation, y=a+b*x, expressing b=ln/ln with
R-Square: 0.99 was used. The variations of lnAC with angular frequency have been given in
Figure 6-17 for Poly(Chol-n-MMA) and Poly(Chol-n-MMA-co-MMA) samples. Since the
angular frequency exponent s is used to determine the electrical conduction mechanism for
Poly(Chol-n-MMA) and Poly(Chol-n-MMA-co-MMA), its values have been calculated from the
slopes of Figure 6-17 for the low and high frequency regions. The s parameter values can be
interpreted by conductivity mechanisms for frequency dependent conductivity given by s≈0
[101] for DC conductivity mechanism, 0<s<0.7 [102] for Correlated Barrier Hoping (CBH)
conductivity mechanism, 0.7≤s<1 [103] for Quantum Mechanical Tunneling (QMT)
conductivity mechanism and 1<s<2 [104] for Super Linear Power Law (SLPL) high frequency
regions are given in inset tables in Figure 6-17.These samples contain four conductivity
mechanisms in the low and high frequency regions. These mechanisms are the QMT and CBH
conductivity mechanisms at low frequency regime and the SLPL and DC conductivity
mechanisms at high frequency region. As can be seen, the values of exponent ―s‖ are varying
from 0.029 to 1.81 (±0.004).
Generally, all samples shows conduction mechanism at high or low frequencies. At low and high
frequencies all the samples demonstrated QMT and SLPL conduction mechanism respectively,
except Poly(Chol-7-MMA-co-MMA) sample which showed CBH mechanism at low
frequencies. Furthermore, angular frequency dependent measurement of Poly(Chol-10-MMA)
110
Figure 6-15: Cole–Cole plots of the dielectric constant (ε"– ε')
sample has shown three different conductivity mechanisms. The significance of this sample is, it
reaches saturation at high frequencies with no slope and parameter s value near to zero, which
indicates DC conductivity behavior of the sample at higher frequencies. This behavior of the
sample suggests its suitability for the development efficient organic photovoltaic devices
operative at higher frequencies.
Performance of an Organic Field Effect Transistor is function of the properties of its gate
dielectric films. Peng et al. [326] have realized all organic field effect transistors using a variety
of polymer dielectrics and change in the field effect mobility values of the devices was correlated
with varying dielectric constants values of dielectric films. Dielectric constants and other
promising dielectric properties of the investigated SCLC polymers suggests their suitability for
the development of high frequency operated flexible Organic Field Effect Transistors (OFETs)
for large area sensing and other advanced applications.
111
Figure 6-16: Specific conductivity of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA) (n= 3,
7, and 10); inset: High resolution plots of specific conductivity of samples in high frequency regime
Electrical characterization of a series of SCLC type homopolymers (poly(Chol-n-MMA)) and
copolymers (poly(Chol-n-MMA-co-MMA)) with flexible methylene spacer (n = 3, 7, and 10)
between the poly(methyl methacrylate) backbone and cholesteryl mesogen is undertaken in this
work. Real and imaginary dielectric constants, absorption coefficient, relaxation time, dielectric
strength, and critical frequency values of the polymers were determined using parallel plate
impedance spectroscopic technique. The dielectric constant values of the polymers were found to
increase with decreasing aliphatic methylene spacer length. εʺ– εʹ curves (Cole-Cole plots) of the
polymers show the depressed semicircles indicating non-Debye type relaxation behavior of the
polymers except for the copolymer and homopolymer with n=3, which showed nearly Debye
type relaxation properties. Detailed conductivity analysis revealed that the polymeric films
followed QMT and CBH conductivity mechanisms at low frequency regime and the SLPL and
DC conductivity mechanisms at high frequency regions. Furthermore, homopolymer with n= 10
showed the saturation or DC conductivity behavior at higher frequencies which represents its
suitability for the development of high frequency organic photovoltaic devices.
112
Figure 6-17: Specific conductivity versus frequency plot of Poly(Chol-n-MMA) and poly(Chol-n-
MMA-co-MMA) (n= 3, 7, and 10).
113
Chapter-7: Conclusions and Future Work 7 8
7.1 Conclusions
The research work was mainly focused on the fabrication and characterization of solution
processed semiconducting films based electronic devices. The solution processed films based
devices are preferred over conventional microelectronic devices due to their low cost, ease of
processing, flexibility and large area sensing applications. The important results obtained from
the research work presented in this dissertation are summarized as:
1. Polymer-Fullerene BHJ based organic solar cells were fabricated using three different
batches of MDMO-PPV polymer with a motivation to investigate the effect of polymer
intrinsic properties i.e. Molecular Weights, Polydispersity values, Charge Carrier
Densities and HOMO/LUMO Levels, on the performance of these devices. Polymer
batches with significant varying intrinsic properties were used for this investigation.
Structural and optical properties of MDMO-PPV:PCBM blends based nanolayers
developed using three different batches of polymer, were studies using Atomic Force
Microscopic and UV-Vis absorption spectroscopic. Current Density-Voltage (J-V)
characteristics of the devices were investigated under an UV-Vis illumination intensity of
100mW/cm2 and their results were compared. Important findings of the work are as
follows:
a. Differences in the nano morphology of organic films, casted using three different
batches of polymers were observed which have significantly influenced on Jsc and
FF parameters of the devices.
b. Efficiency of the solar cell, realized using polymer with moderate Molecular
Weights (Mn= 85 kg/mol and Mw= 1400 kg/mol), and highest Polydispersity
(16.8) and Charge Carrier Density (3.2×10-17
cm-3
) values, was found maximum,
which was attributed due to higher photon absorption capability and favorable
morphology of its photoactive layer.
2. Organic BHJ based Transistor‘s investigation in metal semiconductor metal architecture
known as MESFET architecture was undertaken for the first time for photo, strain/ bend
and displacement sensing applications. Transistors were fabricated on Glass and flexible
114
substrates. Drain/ Source and Gate electrodes of the devices were made using Silver and
Aluminum, respectively. Important findings of this work are summarized as follows:
a. Gate electrode and Drain/Source electrodes showed Schottky and Ohmic type
contacts with the semiconducting/active layers of the devices, respectively.
b. Semiconducting layer of the devices which was the blend of poly(3-
hexylthiophene)(P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM)
demonstrated Schottky and Ohmic type contacts with Gate and Drain/Source
electrodes, respectively, and showed ambipolar properties which was discussed
with the help of energy band diagram.
c. Semiconducting layer of the devices showed ambipolar properties
d. MESFET devices low voltage operation
e. Drain current was found to increase with the illumination intensity
f. Photo responsivity and sensitivity values of BHJ based MESFETs were found
higher as compared to single organic semiconductor based photo-MESFETs
g. Photo responsivity and sensitivity values of the devices were found to decrease
with increase in negative gate voltage
h. Photo sensitivity was found to increase when P3HT:PCBM blend based
semiconducting layer mixed with Red Methyl Dye. However gate leakage current
and photo sensitivity values of the devices were reduced with Red Methyl
blending.
3. Influence of temperature and frequency variation on the dielectric and transport
properties of Graphene Oxide (synthesized using modified Hummers Method) was
investigated using impedance spectroscopic technique. Graphene Oxide films were
prepared using drop casting method on Indium Tin Oxide (ITO) coated glass substrate.
Atomic force microscopy was used to characterize the films microstructure and surface
topography. Electrical characterization was carried out using LCR meter in frequency
regime (100 Hz to 10 MHz) at different temperatures. Important findings of the work are
as follows:
a. AC conductivity ζac of the films‘ varied with angular frequency, ω as ωS, with S
< 1. Temperature and frequency variation has significantly influenced on the
electrical properties of GO film, in particular at low frequency regions.
115
b. GO film contains DC and CBH conductivity mechanisms at low and high
frequency ranges, respectively.
4. Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics, developed using sol-gel route were
electrically characterized using parallel plate impedance spectroscopic technique.
a. The microstructure of ceramics was found to be dependent on Mg doping content.
The x = 0.00 composition showed a relaxor behavior in the frequency regime
from 100 Hz to 1 MHz.
b. The magnitude of tan and ɛr of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples were
decreased with an increase in Mg doping content.
c. The x = 0.25 composition was found to exhibit a high ɛr (4261) and tan (0.05)
around room temperature at 100 kHz frequency.
d. Complex impedance spectroscopic analysis revealed that PbTi0.5Zr0.3(Co1-
xMgx)0.2O3 (x = 0.000.85) exhibited both grain boundary and bulk contributions.
e. The total resistance was decreased with an increase in temperature showing a
typical ceramic behavior.
f. The AC conductivity was increased with an increase in temperature due to the
production of oxygen vacancies associated with doping of aliovalent dopants
(Co3+
, Mg2+
) on Ti4+
/Zr4+
.
5. Electrical characterization of a series of SCLC type homopolymers (poly(Chol-n-MMA))
and copolymers (poly(Chol-n-MMA-co-MMA)) with flexible methylene spacer (n = 3, 7,
and 10) between the poly(methyl methacrylate) backbone and cholesteryl mesogen is
undertaken in this work. Important findings are as follows are as follows:
a. The dielectric constant values of the polymers were found to increase with
decreasing aliphatic methylene spacer length.
b. εʺ– εʹ curves (Cole-Cole plots) of the polymers show the depressed semicircles
indicating non-Debye type relaxation behavior of the polymers except for the
copolymer and homopolymer with n=3, which showed nearly Debye type
relaxation properties.
c. Detailed conductivity analysis revealed that the polymeric films followed QMT
and CBH conductivity mechanisms at low frequency regime and the SLPL and
DC conductivity mechanisms at high frequency regions.
116
d. Homo-polymer with n= 10 showed the saturation or DC conductivity behavior at
higher frequencies.
It is expected that detailed characterization of solution processed semiconducting films based
devices will provide potential for the development of low cost large area flexible sensor arrays
and other low power electronic devices in future.
7.2 Future Work
In order to fully understand and explore the potential applications of solution processed films
based microelectronic devices for industry; the current work can be further extended into the
following directions:
1. Polymer-Fullerene BHJ based Solar Cells were fabricated to analyze the effect of
polymer intrinsic properties on the photo conversion efficiency of the devices. Since,
performance of the solar cells is function of the structural properties of photo sensitive
semiconducting films. Therefore this work could be further extended to investigate the
effect of polymer intrinsic properties on the surface profiles of the semiconducting films.
2. Organic BHJ based MESFETs, having simpler transistor architectures, were fabricated
for the first time and investigated for photo, strain/bend and displacement sensing
applications. This work can be extended further into the following directions, in order to
further understand physics underlying these structures and more sensing applications for
low cost developments in future:
a. Investigation of the effect of transistor‘s physical parameters such as channel
length, channel width, semiconducting and contact layer thicknesses on the
performance of the devices
b. Investigation of the effect of humidity, temperature, various gases and tensile
forces on the electrical properties of the devices
c. Fabrication of small molecules semiconducting layers based BHJ MESFET and
their comparison with the investigated devices
d. Fabrication and characterization of solution processed BHJ films based dual gate
MESFETs
e. Simulation of the devices using device physics simulator, SILVACO TCAD
117
3. Simulate the fabricated Organic Solar Cells and Transistors using SILVACO TCAD so as
to further understand the physics of the devices.
4. Investigate the effect of elevated temperatures (higher than 100 ºC) and humidity
variations on the electrical conduction and dielectric properties of Graphene Oxide films
5. Study the effect of temperature on the electrical properties of SCLCP films.
118
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