1-Light Emitting Polymers Presentation

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



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)



Light Emitting Devices

• Two Types

1. PLED (Polymer Light Emitting Diodes)

2. PLEC (Polymer Light Emitting Electrochemical cells)

•PLED vs OLED difference?



PLED Structure

Anode such as

Indium Tin

Oxide

Conductive Hole

Transport Layer

Inorganic materials: Li,Ca, Mg



Pi-Conjugated Polymers

Polyflourene

Poly phenylenevinylenes (PPV)

Poly(N-vinylcarbazole)

Polythiophene



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



Band Gap vs. Color

Source: http://cms.tnw.utwente.nl/polymers/conj_pol.htm



Organic Light Emitting Diode (OLED)

Sources: engadget.com; gizmodo.com; wired.com



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



OLED Structure

Source: How Stuff Works. How OLEDs Work. http://electronics.howstuffworks.com/oled4.htm (accessed April 8, 2012).



Light Creation Process

Source: How Stuff Works. How OLEDs Work. http://electronics.howstuffworks.com/oled4.htm (accessed April 8, 2012).



Pi-Conjugated Polymers

Polyflourene

Poly phenylenevinylenes (PPV)


Polythiophene



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.



Polyfluorene Derivatives



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.



• 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)



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.



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



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



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






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



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



Properties

• Photoconductivity

• Charge-transfer complexes

• Photoluminescence

• Electroluminescence

• Chemical stability

• Thermal stability



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.



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.



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



Polythiophene

…the polymer that will electrify you.



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.



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.



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).



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.



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.



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)



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.



Research Based Applications

• Field-effect transistors

• Electroluminescent devices

• Solar cells

• Photochemical resists

• Non-linear optic devices

• Batteries

• Chemical sensors

• AND OBVIOUSLY DIODES!



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).



We thank you for your kind,

generous, undivided, and very

enthusiastic attention.

QUESTIONS?

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1-Light Emitting Polymers Presentation

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