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7/27/2019 1-Light Emitting Polymers Presentation
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Light Emitting PolymersBy: Dhruv Seshadri, Craig Lewis,Sai Kolluru, and Sen Jiao
EMAC 276Dr. John Blackwell
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History of Light Emitting Polymers
Friend, R.H., et. al. , Nature 397 (1999) 121.
• 1950’s Bernanose applied high voltage alternating current to thin
films
• 1960: Researchers at Dow prepared electroluminescent cells using
doped anthracene (pi-conjugated)
• Much work being continued today (UCSB one place where lots of
research happening)
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Light Emitting Devices
• Two Types
1. PLED (Polymer Light Emitting Diodes)
2. PLEC (Polymer Light Emitting Electrochemical cells)
•PLED vs OLED difference?
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PLED Structure
Anode such as
Indium Tin
Oxide
Conductive Hole
Transport Layer
Inorganic materials: Li,Ca, Mg
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Pi-Conjugated Polymers
Polyflourene
Poly phenylenevinylenes (PPV)
Poly(N-vinylcarbazole)
Polythiophene
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Properties of Conjugated Polymers
Bao, et. al Thin Solid Films 323 (Dec. 1999) 239-242. PDF file.
• Exist as Semiconductors or
insulators in undoped state
• Band gap greater than 2eV.
• Oxidative doping enhances
conductivity.
• Charged organic backbone is
unstable in moisture
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Band Gap vs. Color
Source: http://cms.tnw.utwente.nl/polymers/conj_pol.htm
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Organic Light Emitting Diode (OLED)
Sources: engadget.com; gizmodo.com; wired.com
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OLED Advantages
• Can be printed on flexible substrate such as PET
• Excellent for large area lighting applications
•Possibility for roll to roll processing
Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Flexible light-emitting diodes made from soluble conducting polymers. Nature. 1992, 357, 477-479
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OLED Structure
Source: How Stuff Works. How OLEDs Work. http://electronics.howstuffworks.com/oled4.htm (accessed April 8, 2012).
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Light Creation Process
Source: How Stuff Works. How OLEDs Work. http://electronics.howstuffworks.com/oled4.htm (accessed April 8, 2012).
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Pi-Conjugated Polymers
Polyflourene
Poly phenylenevinylenes (PPV)
Poly(N-vinylcarbazole)
Polythiophene
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Polyfluorene
• Conjugation leads to excellent conductivity
• Color and solubility can be controlled with electron donating and
withdrawing groups
• Substituents allow for emission of light across entire visible spectrum
• Soluble in most organic solvents
Leclerc, M. Polyfluorenes: Twenty Years of Progress. J. Polym. Sci. A1. 2001, 39, 2867-2873.
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Polyfluorene Derivatives
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Challenges with Polyfluorenes
• Chemical Degradation
• Formation of carbonyl groups causes surface roughness
• Physical Degradation
• Aggregation leads to excimer formation and quenched fluorescence
Bliznyuk, V. N.; Carter, S. A. Electrical and Photoinduced Degradation of Polyfluorene Based Films and Light-Emitting Devices. Macromolecules. 1998, 32, 361–369.
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• Molecular Formula: (C8H6)
• Appears as a Yellow Solid
• Has P21 symmetry with a monoclinic unit cell
conformation• Used as electron donating material in organic
cells
Skotheim, T. A. et al. Handbook of Conducting Polymers, 2nd ed.; CRC Press: New York, 1997; pp 343-351.
Poly phenylenevinylene (PPV)
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PPV History
Shao, et. al. Advanced Materials 19 (2007) 365–370. PDF file.
• 1968: first synthesized by Wessling at
Dow
• 1989: used as emissive layer for polymer
LED
• 1990: Friend Research group at
Cambridge achieved green-yellow EL
using PPV
• Hoechst group expanded Friend’s
research to look into color LED’s.
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PPV History (cont.)
Bao, et. al Thin Solid Films 323 (Dec. 1999) 239-242. PDF file.
• 1991, Heeger and co-workers at UCSB announced EL
application of a soluble derivative of PPV, MEH-PPV, band
gap energy of about 2.2 eV.
• 1992: Cambridge Display Technology (CDT) to commercialize
this technology.
• Much has happened and will happen
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Why PPV is Studied?
• Small optical band gap and bright yellow fluorescence.
•Doped to form electrically conductive materials.
• Physical and electronic properties can be changed due
to inclusion of functional side groups
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PPV Properties
Wang, Haiqiao, et al. Journal of Applied Polymer Science 83 (2002) 2195-2200. PDF file
• Example of a PLED
• Insoluble in water
• Only polymer processed into a highly ordered crystalline
thin film
• Short conjugated length gives a pure blue spectrum
• Non-conjugated block provides good solubility
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Poly(N-vinylcarbazole)
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Introduction
• Poly(N-vinylcarbazole), abbreviated as
PVK
• Typical light emitting polymer
• Well known as an organic electroactive
material
• Commonly applied for photorefractive
and electroluminescent devices, such as
organic light emitting diodes
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Synthesis
Figure 1: Structure of Poly(N-
vinylcarbazole) (PVK)
(Constructed in Chemdraw)
• Monomer: vinylcarbazole
• Performed in bulk, in solution, in
suspension or in precipitation
•Polymerized by radical and cationicinitiation both in vinyl group and
benzene ring
• Stabilized electron-deficient
centers by resonance involving the
non-bonding electron pair on the
nitrogen atom
• Product: Conducting, colorless
PVK; dark green color also
possible
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Properties
• Photoconductivity
• Charge-transfer complexes
• Photoluminescence
• Electroluminescence
• Chemical stability
• Thermal stability
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Application - OLED
Figure 2: Schematic representation
of organic light emitting diode (T.M.
El-Agez et als)
• OLED: Organic Light Emitting Diode
• Sandwiched organic thin films (single or
multiple) layers between the electrodes
•Transparent indium tin oxide (ITO) anodeand metallic cathode
• When voltage is applied, charge carriers
are injected from the electrodes
• Doped PVK is excited by the injectedcharge carriers to form excitons, which in
turn give photons due to the hole-electron
combination.
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Emitting Mechanism
Figure 3: Steps for energy transfer
and charge transport in PVK:Alq3
blend films (H. Jin et al.)
Doped PVK: PVK:Alq3 blend films
a) Photogeneration of excitons in PVK
upon absorption
b) Electron transfer from PVK to Alq3,
leaving an electron on Alq3 and a
hole on PVK
c) Energy transfer from PVK to Alq3,
recombination occurs, thus emittingphotons as light.
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Advantages:
• Excellent emissive ability
• Color tuning, various colors possible
•Durability
• Ease of deposition
Disadvantages:
• Poor mechanical property, stiff and brittle, can be improved by
copolymerization with suitable monomers
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Polythiophene
…the polymer that will electrify you.
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History of the polymer
…yeah it’s fascinating.
• Relatively new to the field of conductive
polymers.
• Developed immensely over the past few
decades, credited to its improvement are
Nobel Prize in Chemistry to Alan Heeger, Alan
MacDiarmid, and Hideki Shirakawa.
• Purpose of the polymer is two-fold.
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Properties of the Polymer
…colorfully electrifying. 1) Electrical conductivity
• Result of delocalization of electrons along the
polymer ’s backbone. Known to be a
“synthetic metal.”
2) Optical properties
• Respond to environmental stimuli resulting in
changes in color in response to solvent,
temperature, applied potential.
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Mechanism
…it knows how to twist.
• Color changing optical properties and electric conductivity have thesame mechanism.
• Twisting of the polymer’s backbone, disrupting conjugation.
• Conducting polymers have electrons that are delocalized alongconjugated backbones
• Results in conjugated polymers (process similar to other LEPs).
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Removal of two electrons (p-doping) from aPT chain produces Bipolaron.
Bipolaron = a bound pair of two polarons, amacromolecular chain containing twopositive charges in a conjugated system.
Conjugated Polythiophene structure.
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Synthesizing Polythiophenes
• Electrochemical Synthesis
• Most common way of synthesizing PTs.
• Applying a potential across a solution of the monomer to be
polymerized (electrochemical polymerization).
• Convenient: does not need to be isolated or purified.
• Problem: produces polymers with undesirable linkages.
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Synthesizing continued…
• Chemical Synthesis
• Accomplished through using oxidants or cross-coupling catalysts.
• Advantage over electrochemical synthesis: a greater selection of
monomers.
• Oxidative polymerization has been very successful using ferric
chloride (in less demanding environment)
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Applications: PEDOT-PSS
• Antistatic coating through oxidative polymerization
prepared on commercial scale using ferric chloride.
• PEDOT-PSS: antistatic coating, transparent and
colorless, prevents electrostatic discharges using film
rewinding, and reduces dust buildup on negatives after
processing PEDOT (currently most commercialized
outcome of PT research).
• Electrochromic properties used in windows and mirrors
saves billions.
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Research Based Applications
• Field-effect transistors
• Electroluminescent devices
• Solar cells
• Photochemical resists
• Non-linear optic devices
• Batteries
• Chemical sensors
• AND OBVIOUSLY DIODES!
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Conclusion
• All of the polymers presented here have similar
mechanisms to produce exciting yields in this field.
• The field of Light Emitting Polymers is still relatively
new and has a great potential in terms of research.
• Not all applications are commercialized and many
current applications have limited potential (youpotential polymer PhDs, this is a good field to go
into).
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We thank you for your kind,
generous, undivided, and very
enthusiastic attention.
QUESTIONS?
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