Optically tunable intramolecular charge transfer dyes for vacuum deposited bulk

heterojunction solar cells

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Nanotechnology 19 (2008) 424008 (8pp) doi:10.1088/0957-4484/19/42/424008

Optically tunable intramolecular chargetransfer dyes for vacuum deposited bulkheterojunction solar cellsChris S K Mak1, Qing Yun Leung1, Wai Kin Chan1 andAleksandra B Djurisic2

1 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong,People’s Republic of China2 Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong,People’s Republic of China

E-mail: waichan@hku.hk

Received 3 April 2008, in final form 6 July 2008Published 25 September 2008Online at stacks.iop.org/Nano/19/424008

AbstractWe present the tunability of the photophysical and electrochemical properties of a series ofintramolecular charge transfer compounds by facile molecular design and synthesis. Thephotovoltaic performances based on these sublimable materials and C60 bulk heterojunctioncells are compared and reported. The structural modification of the charge transfer dyes alterednot only the electronic properties, but also the morphology of the bulk heterojunction thin films,as revealed by AFM and SEM studies. Addition of PEDOT:PSS between the ITO and thephotoactive layer improved the hole injection from the photosensitizer into the anode, and theoverall power conversion efficiency is also enhanced.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Organic photovoltaics open up a potential alternative tothe currently used silicon based solar cells for generatingelectricity from sunlight [1–5]. Thin organic film offerstunable optical and electrical properties, simple and lowcost material processing and excellent mechanical flexibility.Rapid development in solution processable polymer/fullereneblend cells had led to efficiencies up to 5% [6–8]. Organicsolar cells based on vacuum deposition of small moleculedyes have received unabated attention since a 0.95% bilayerheterojunction device was reported by Tang [9]. Considerableadvances have been achieved after a decade, and powerconversion efficiency (PCE) was boosted up to 3–5% [10–14].The PCE mainly depends upon the light harvesting abilityof the active materials, exciton dissociation at the donor–acceptor interface and change transport and collection at theopposing electrodes. However, the short exciton diffusionlength and relatively low absorptivity in organic materialslimits the thickness of the active material in photovoltaiccells. Materials that possess large absorptivity and broad

absorption coverage (or having a complementary absorption tothe currently used solar cell sensitizers) in the solar spectrumare therefore essential.

Significant progress has been made in the fieldof optoelectronics based on charge transfer materials.Numerous studies on low bandgap polymers solar cellpossessing charge transfer moiety demonstrated encouragingefficiency [15–17]. We have demonstrated the use ofsublimable rhenium complexes that exhibit low absorptionenergies and complementary optical absorption to othermaterials in the device [18, 19]. We have also attempted tobroaden the absorption of conjugated polymers by introducingmultiple light absorption units to the molecule [20]. Recently,Roncali and co-workers reported a promising heterojunctionsolar cell with extended absorption spectrum in the red regionbased on intramolecular charge transfer (ICT) triphenylaminederivatives [21, 22]. The absorption energy can be loweredby the interaction between the donor and acceptor withinthe molecule, thus reducing the bandgap to match the solarspectrum. In this paper, we report the optical and electronicproperties of a series of ICT dyes based on pyrazine and

0957-4484/08/424008+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 424008 C S K Mak et al

terthiophene motifs. The important aspect of this workis to demonstrate the tunability of absorption colour andHOMO–LUMO energy of the materials via facile structuralmodification. We also anticipate the opportunity of the useof these ICT dyes as sublimable materials for photovoltaicdevices.

2. Experimental details

2.1. General

All reagents were purchased from Aldrich, Strem andLancaster and used as received. All reactions were preformedunder a nitrogen atmosphere unless specified otherwise. HPLCgrade acetonitrile (Fluka) was used and dichloromethaneand hexane were dried from alumina columns. Columnchromatography was performed using Merck 9385 silica gel60 (0.040–0.063 mm). 1H and 13C NMR spectra wererecorded on Bruker DPX-300 (300 and 75 MHz respectively)and Bruker DRX-400 (400 and 100 MHz respectively) NMRspectrometers. Positive-ion electron impact (EI) mass spectrawere recorded on a Finnigan MAT 95 mass spectrometer.Elemental analyses were performed at the ButterworthLaboratories Limited, United Kingdom. Cyclic voltammetrywas carried out using a Princeton Applied Research 273Apotentiostat. A Teflon shrouded 3 mm glassy carbon electrodewas used as the working electrode and platinum foils wereemployed as counter and pseudoreference electrodes. Thereference potential was calibrated internally with ferrocene(Fc/Fc+). The solvent was anhydrous and deoxygenated withAr prior to the measurement. 0.1 M of tetrabutylammoniumtetrafluoroborate (TBABF4) was used as the supportingelectrolyte and the scan rate of all the measurements was100 mV s−1. Solution and thin film UV/vis spectra wereobtained using a Varian Cary 50 Bio spectrophotometer. Forsolution photoluminescence measurements, the samples wereexcited with the 532 nm line of a diode laser. The emittedlight was collected with an optical fibre and recorded by a512-USB CCD array (Control Development Inc.). Atomicforce microscopy (AFM) images were obtained under ambientcondition using a Digital Instrument Nanoscope IIIa controllerand an E-scanner. The amplitude and phase of the oscillatingcantilever operating in tapping mode were simultaneouslymonitored. The scanning rate and the drive frequency were0.6–0.7 Hz and 263 kHz, respectively.

2.2. Synthesis

The 2,3-diaryl-5,7-di(thien-2-yl)thieno[3,4-b]pyrazines (1–3)were prepared by the general method described by Kitamuraet al [23]. 3′,4′-diamino-2,2′:5′,2′′-terthiophene (0.90–3.0 mmol) and 1,2-diketones (1.2 equiv.) were dissolved inethanol (100 ml). The mixture was stirred at room temperatureunder N2 for three days. The solid was filtered and washedwith MeOH. The crude product was then purified by silicacolumn chromatography with DCM/hexane (1:1) as eluent andfollowed by recrystallization from DCM/MeOH.

2.2.1. Synthesis of 2,3-diphenyl-5,7-di(thien-2-yl)thieno[3,4-b]pyrazine (1). Dark purple solid (71.1%). 1H NMR(400 MHz, CDCl3): δ7.12 (dd, 2H, ArH, J = 3.7, 5.1 Hz),7.32–7.41 (m, 8H, ArH), 7.59–7.61 (m, 4H, ArH), 7.69 (dd,2H, ArH, J = 0.9, 5.9 Hz); 13C-NMR (75 MHz, CDCl3):δ125.1, 125.3, 127.1, 127.7, 128.5, 129.5, 130.4, 135.0, 137.9,139.4, 153.3; IR (KBr disc): νmax = 3095 (w), 1497 (m),1431 (m), 1356 (m), 1269 (s), 1156 (w), 1082 (s), 988 (m),920 (m), 843 (s), 768 (s), 696 (vs), 617 (w), 531 (m),477 (s) cm−1; EIMS: m/z(%): 452.0 (100%) [M+]; elementalanalysis calcd (%) for C26H16N2S3 (452.62): C 68.99, H 3.56,N 6.19; found C 68.82, H 3.56, N 6.16.

2.2.2. Synthesis of 2,3-di(thien-2-yl)-5,7-di(thien-2-yl)thieno[3,4-b]pyrazine (2). Black needle-shaped solid (65.1%). 1HNMR (300 MHz, CDCl3): δ 7.04 (dd, 2H, ArH, J = 3.8,5.0 Hz), 7.13 (dd, 2H, ArH, J = 3.7, 5.1), 7.36 (dd, 2H,ArH, J = 1.0, 3.7 Hz), 7.42 (dd, 2H, ArH, J = 1.0,5.1 Hz), 7.53 (dd, 2H, ArH, J = 1.0, 5.0 Hz), 7.67 (dd, 2H,ArH, J = 1.0, 3.7 Hz); IR (KBr disc): νmax = 3100 (w),1514 (m), 1422 (s), 1268 (m), 1277 (s), 1238 (m), 1057 (m),901 (w), 841 (s), 696 (vs), 610 (w), 475 (s) cm−1; EIMS:m/z(%): 464.0 (100%) [M+]; elemental analysis calcd (%) forC22H12N2S5 (464.67): C 56.86, H 2.6, N 6.03; found C 56.20,H 2.87, N 5.98.

2.2.3. Synthesis of 10,12-di-thiophen-2-yl-11-thia-9,13-diaza-cyclopenta[b]triphenylene (3). Dark green solid (76.2%).IR (KBr disc): νmax = 3064 (w), 1499 (s), 1447 (m),1349 (vs), 1300 (vs), 1229 (m), 1111 (m), 1048 (s), 980 (w),843 (s), 760 (vs), 689 (vs), 538 (m), 473 (s) cm−1; EIMS:m/z(%): 450.0 (100%) [M+]; elemental analysis calcd (%)for C26H14N2S3 (450.60): C 69.3, H 3.13, N 6.22; found C69.33, H 3.11, N, 6.13.

2.3. Device fabrication

ITO glass substrates (15 � �−1) were cleaned by sonicationin detergent, distilled water, acetone, isopropanol, and dried inan oven. The substrates were then cleaned with UV ozone for300 s. Organic thin film devices were thermally evaporatedunder high vacuum (10−6 Torr). The device structures ofthe present study are ITO/ICT dye (50 A)/ICT dye:C60 (1:1,100 A)/C60 (300 A)/BCP (25 A)/Al and ITO/PEDOT:PSS(400 A)/ICT dye (50 A)/ICT dye:C60 (1:1, 100 A)/C60

(300 A)/BCP (25 A)/Al. PEDOT:PSS was spin-coated onITO and dried under vacuum at 80 ◦C for an hour prior tothermal evaporation of organic dyes. A thin layer of ICT dye(50 A) was evaporated at a rate of 0.2 A s−2 and the bulkheterojunction of the ICT dye/C60 layer with ratio 1:1 wasobtained by co-evaporation of both materials at the same rate of0.2 A s−2; the overall thickness is 100 A. Aluminium cathode(800 A) was evaporated through a shadow mask to define theactive area of the devices (2 mm diameter circles). The overallactive layer thickness was measured with a Veeco Dektak3ST surface profilometer. The current–voltage characteristicswere measured using a Keithley 2400 sourcemeter. For whitelight efficiency measurements (at 100 mW cm−2), Oriel 66002

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Nanotechnology 19 (2008) 424008 C S K Mak et al

Scheme 1. Molecular structures of compounds 1–3.

solar light simulator with an AM 1.5 filter was used. Thelight intensity was measured with a Molectron Power Max500D laser power meter. For the measurement of the incidentphoton-to-current efficiency (IPCE), a different wavelength oflight was selected with a Thermo Oriel 257 monochromator.The photocurrent generated was measured with a Keithley2400 sourcemeter under ambient condition. The light intensitywas measured with a Newport 1830-C optical power meterequipped with a 818-UV detector probe.

3. Results and discussion

3.1. Photophysical and electrochemical properties

The molecular structures of compounds 1–3 are shown inscheme 1. The electronic absorption spectra of the threecompounds in CHCl3 showed a strong absorption peak ataround 350 nm, originating from the π–π∗ transition. Thebroad absorption band in the visible region is ascribedto the ICT transition from the terthiophene donor to thethienopyrazine acceptor (figure 1). The transition bandexhibits solvatochromic shifts in both absorption and emissionspectra, indicating its charge transfer nature (figure 2). TheICT absorption peak of 1 spanned from 545 nm in acetoneto 560 nm in toluene. This small shift shows that thedipole moment difference between the ground state (μg) andthe excited state (μe) is small. The hypsochromic shiftreveals that μg > μe [24, 25]. A similar solvatochromiceffect was observed for 2 and 3. The absorption colourcan be tuned towards the infrared region by changing theperipheral substituents on the thienopyrazine. The ICT bandof compounds 2 and 3 red shifted by 30 nm and 85 nm,respectively, with respect to that of 1 (table 1). The red shift ofthe absorption maximum in 2 is attributed to the stabilizationof the LUMO by the electron donating thienyl groups on thethienopyrazine motif. This observation is in good agreementwith the cyclic voltammetry results. Further reduction of

Figure 1. UV–vis absorption spectra of compounds 1–3 in CHCl3

and the solution colours (1 × 10−4 M) of the three compounds aredisplayed in the inset photograph. The solution colours ofcompounds 1, 2 and 3 are purple, grey and sky-blue, respectively.Cyclic voltammograms of 1–3 with scan rate of 100 mV s−1 (versusFc/Fc+) in acetonitrile are shown in the inset.

optical bandgap is observed in compound 3 by extending theπ conjugation with phenanthrene moiety on the thienopyrazineunit. In particular, the LUMO is stabilized to a greater extent inthe more π -delocalized system and the absorption maximumpeaked at 635 nm with an onset close to 750 nm. The sameabsorption trend has been observed for the three compoundsdeposited as thin films by thermal evaporation. A red shift(around 20–30 nm) of the film absorption with respect tothe solution spectra (see figure 6) is expected since the π–π

stacking of the conjugated molecules in the solid state shiftsthe absorption to lower energy.

The redox properties of 1–3 were studied and theresults are summarized in table 1. All the compoundsfeature a quasi-reversible oxidation at around 0.3–0.5 Vversus a ferrocene/ferrocenium (Fc/Fc+) redox couple, whichcorresponds to the oxidation of the thienyl fragment. Thethienopyrazine acceptor shows a reversible reduction in therange −1.39 to −1.72 V versus Fc/Fc+. The oxidationpotentials (Eox) among the three compounds are comparable.In contrast, the reduction potential (Ered) was significantlyaltered by changing the substituents on the electron acceptingthienopyrazine motif. The electron rich substituents onthe thienopyrazine unit lowered the reduction potential incompounds 2 and 3. Compound 3 possesses a phenanthrenegroup, and the Ered is higher than that of the non-conjugatedcongener 1 by ∼0.3 eV. The result suggests that the electronaccepting ability increases by replacing the diphenyl groupswith more electron rich substituents.

3.2. Surface morphology

The surface morphology of an ICT dye/C60 bulk heterojunc-tion active layer was studied by atomic force microscopy(AFM) and scanning electron microscopy (SEM). The bulkheterojunction thin films (ICT (5 nm)/ICT dye:C60 (1:1,10 nm)) were prepared by thermal evaporation under the samecondition as the photovoltaic devices. Figure 3 shows the AFM

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Table 1. Photophysical and electrochemical properties and power conversion efficiency (PCE) of compounds 1–3.

λabs (nm)(log ε) λonseta (nm) λem

b (nm) Eoxc (V) Ered

c (V) HOMOd eV LUMOd eV Ege (eV) PCEf (%)

1 335 (4.63), 550 (3.85) 657 706 0.43 −1.72 −5.23 −3.08 2.15 0.822 370 (4.56), 580 (3.76) 687 747 0.48 −1.55 −5.28 −3.25 2.03 0.793 360 (4.66), 635 (3.75) 748 803 0.36 −1.39 −5.16 −3.41 1.75 0.28

a The onset value of the absorption is determined by extrapolating the leading edge of the absorption band to the baseline.b Measured in CHCl3 solution at a concentration of 1 × 10−4 M with 532 nm excitation.c Cyclic voltammograms were recorded with scan rate of 100 mV s−1 (versus Fc/Fc+) and the sample concentration was 1 × 10−4 M inacetonitrile with 0.1 M TBABF4 as supporting electrolyte.d Calculated from the CV data.e Eg = HOMO–LUMO.f Device configuration: ITO/PEDOT:PSS (400 A)/ICT dye (50 A)/ICT dye:C60(1:1, 100 A)/C60 (300 A)/BCP (25 A)/Al.

Figure 2. Absorption and photoluminescence spectra of compound 1in various solvents.

images of compounds 1–3 with C60 bulk heterojunction film.A thin film of compound 1/C60 exhibits a well-packed andsmooth film with a root-mean-square roughness of 0.8 nm, andthe average grain size is about 20 nm. In contrast, compounds2 and 3 produced a very rough film; the root-mean-squareroughnesses of compound 2/C60 and compound 3/C60 films

were 3.3 and 6.8 nm, respectively. The average grain size wasabout 100 nm. The results revealed that aggregation occurredin the bulk heterojunction of films 2 and 3. The surfacemorphology of films 2 and 3 was further investigated by SEM.Spherical nanostructures with diameter about 100 nm wereobserved (figure 4). Significant phase separation was observedfrom the SEM images of film 3 which may lead to the poorcharge transport and as a consequence to poor PCE.

3.3. Photovoltaic performances

The maximum open-circuit voltage (Voc) can be estimatedby the energy difference between the HOMO of the donorand the LUMO of the acceptor, but that value of Voc thatcan be achieved in practice is typically lower due to severalfactors [26, 27]. The design principle behind the seriesof ICT dyes in the present work is to reduce the bandgapof the electroactive materials in order to match the solarspectrum without affecting the Voc of the photovoltaic device.Therefore, the LUMO energy of the ICT dyes is tuned byvarying the auxiliary groups on the thienopyrazine acceptor,while the HOMO level remains nearly the same (table 1).Bulk heterojunction solar cells based on the ICT dyes asphotosensitizer and hole transport material were fabricatedby thermal evaporation and the photovoltaic performanceswere examined under simulated AM 1.5 solar illumination(100 mW cm−2) and in the dark. The current density–voltage(J–V ) characteristics of compounds 1–3 were measuredat ambient condition in a device configuration of ITO/ICTdye (50 A)/ICT dye:C60 (1:1, 100 A)/C60 (300 A)/BCP(25 A)/Al. A layer of ICT dye was deposited on the ITO inorder to prevent the direct contact of the bulk heterojunctionwith the electrode. C60 was used as electron acceptor. Athin exciton blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) was inserted between the active layerand the cathode to confine the excitons and improve electroncollection [28, 29]. The fill factor (FF) and Voc increased by14% and 100 mV, respectively, after introducing the BCP layer.The thickness of BCP was constrained within 50 A by thecurrent density. Under one sun illumination, cell 1 (figure 5(a))exhibits short-circuit current density (Jsc) = 2.5 mA cm−2,Voc = 480 mV and FF = 0.36, resulting in a PCE of 0.43%.The AFM study showed that the bulk heterojunction of cell 1has excellent surface morphology. The average grain size isabout 20 nm, which is within the exciton diffusion length

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Figure 3. AFM height and phase images of ICT dye/C60 bulk heterojunction thin film (15 nm) of 1 ((a) and (b)), 2 ((c) and (d)) and3 ((e) and (f)).

of most organic molecules. Therefore, the photo-generatedexciton from 1 can efficiently diffuse to the donor–acceptorinterface for charge separation. Aggregation was observedin the compound 2/C60 bulk heterojunction. However, cell2 still exhibits Jsc = 3.1 mA cm−2, Voc = 460 mV andFF = 0.42, demonstrating a PCE of 0.60%. The lowerabsorption energy exhibited by 2 compared with 1 results inmore efficient harvesting of photons in the red region. Theenhancement of the photocurrent and FF may be due to theextra thienyl groups on the thienopyrazine unit that facilitatethe hole transport. The highest device performance would beexpected for compound 3 as it possesses the smallest bandgapand the planar structure would facilitate electron delocalizationand thus charge transport. Surprisingly, cell 3 presents thelowest device efficiency, Jsc = 1.7 mA cm−2, Voc = 200 mVand FF = 0.39, resulting in a PCE of 0.13%. The FFs for

both cell 1 and 3 are comparable, implying that the resistancesare similar in the two devices. The discrepancy between theprediction from the photophysical and electrochemical resultsand the device performance of cell 3 can be explained by thethin film morphology. Although the absorption and HOMO–LUMO energies of compound 3 have been tuned to fit thecriteria of our PV cells by simple chemical modification, thechange of molecular structure can also affect the molecularpacking and the intermolecular interaction in thin film. Thesmaller Jsc of cell 3 is mainly due to the aggregation ofthe organic molecules as revealed by the SEM and AFMimages. The highly planar structure of 3 would favour theπ–π stacking of the molecules and thus promote aggregation.This observation reveals that the electronic properties of aphotosensitizer are not the only factor of concern for materialdesign, but also the molecular interaction, which would affect

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Figure 4. SEM images of ICT dye/C60 bulk heterojunction thin film(15 nm) of (a) 1, (b) 2 and (c) 3.

the film morphology and thus the device performance. On theother hand, the device efficiency of cell 3 is limited by theVoc, and the decrease in Voc may be due to the offset energyof compound 3 being small [26].

The photo-response in the external quantum efficiency(EQE) spectrum was extended from the UV to the visibleregion. UV–vis absorption spectra of pure ICT dye film,ICT dye/C60 (1:1) bulk heterojunction film, photovoltaicdevice and IPCE spectra of compounds 1–3 are depictedin figure 6. The device configuration of the PV cells isITO/PEDOT:PSS/ICT dye/ICT dye:C60/C60/BCP/Al. TheEQE maxima were located at ∼440 and 600 nm, contributedfrom the π–π∗ transition of 1 and C60 and the charge transfertransition, respectively. Although the absorption originating

Figure 5. J –V characteristics of the (a) ITO/ICT dye/ICTdye:C60/C60/BCP/Al and (b) ITO/PEDOT:PSS/ICT dye/ICTdye:C60/C60/BCP/Al with 1 (cell 1, solid line), 2 (cell 2, dashedline) and 3 (cell 3, dotted line) as electroactive material under onesun AM 1.5 G simulated illumination.

from compound 1 was overwhelmed by that of C60 in thefilm absorption spectrum of cell 1 in the blue region, theEQE contribution from the charge transfer state of 1 was stillobvious. The photo-response generated from 1 in the visibleregion is over 6% at around 550 nm with leading edge downto ∼750 nm, which is in good agreement with the absorptionspectrum of 1. Similar results have been obtained in the caseof compounds 2 and 3, although the absorption intensity ofthe pure and bulk heterojunction films is lower. The peakat ∼600 nm in the IPCE spectra originates from the chargetransfer transition of the dye molecule. The broadening of thispeak up to 700 nm is due to the absorption of the C60 layer(30 nm) [21]. The increase of the absorption ∼730–800 nm inthe PV device absorption trace is due to an artefact from theUV–vis spectrometer.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) exhibits high conductivity (∼1–10 S cm−1)

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Figure 6. UV–vis absorption spectra of pure ICT dye film (dottedline), ICT dye/C60 (1:1) bulk heterojunction film (dashed line),photovoltaic device (solid line) and IPCE spectra (empty triangles)with compound (a) 1, (b) 2 and (c) 3. The device configuration of thePV cells is ITO/PEDOT:PSS/ICT dye/ICT dye:C60/C60/BCP/Al.

and intrinsically high work function. It has been widelyemployed as the hole transporting and hole injecting/collectingmaterial in OLEDs and OPVs. A thin layer of PEDOT:PSS(40 nm) was spin-coated on top of ITO prior to depositionof the photoactive layer by thermal evaporation in order toimprove the device efficiency. The photovoltaic performancesof the PV cells with the insertion of PEDOT:PSS layer are

summarized in table 1. The PCE and Voc of the threecompounds were increased after using PEDOT:PSS as the holetransport layer to facilitate the charge transport of the activelayer (figure 5(b)). The addition of PEDOT:PSS layer reducesthe hole collection barrier (referring to the HOMO energiesof the three compounds in table 1) from the photoactive layerto the anode (that work function of ITO is 4.8 ± 0.1 eV, andthat of PEDOT:PSS is 5.2 ± 0.1 eV [30]) in the photovoltaicdevices based on the three compounds in this study. As aconsequence, less space charge is accumulated near the anodeand the charge collection efficiency is improved [31]. Wehave observed that the Voc of our ICT/C60 bulk heterojunctiondevices has been increased after inserting a PEDOT:PSS layerbetween ITO and the light harvesting layer. Snaith et alhave reported that the increase of energy barrier of the chargeinjection from the electrodes into the photoactive layer wouldlead to an increase of the open-circuit voltage of a polymerblend photovoltaic device. This is because of the increase inthe shunt resistance of the theoretical photodiode circuit [32].This result revealed that the hole injection barrier from thePEDOT:PSS into the photoactive layer has been reduced andthe overall PCE efficiency of the devices with PEDOT:PSS hasbeen enhanced.

4. Conclusions

In summary, we have demonstrated the tunability ofphotophysical and electrochemical properties of a series of ICTcompounds by varying peripheral substituents on the acceptorunit. Thermally evaporated photovoltaic devices based on thebulk heterojunction of C60 and ICT materials exhibit promisingdevice efficiencies even with a thin layer of photosensitizer andsimple device configuration. The structural modification of thecharge transfer dyes not only altered the electronic properties,but also the morphology of the bulk heterojunction thin films.Study of the photovoltaic performances with different deviceconfigurations is in progress and detailed investigation on theroll-off of Voc in the devices is needed.

Acknowledgments

The project was supported by the Research Grants Council ofthe Hong Kong Special Administrative Region, China (projectNos HKU 7010/05P, 7008/07P and CAV Grant HKU 2/05C).Partial financial support from the Strategic Research Theme,Seed Funding For Basic Research For New Staff and SmallProject Fund (administrated by The University of Hong Kong)are also acknowledged.

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