Light Emitting Polymers

1. Light Emitting Polymers By: Dhruv Seshadri, Craig Lewis, Sai Kolluru, and Sen Jiao EMAC 276 Dr. John Blackwell

2. History of Light Emitting Polymers • 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) Friend, R.H., et. al. , Nature 397 (1999) 121.

3. Light Emitting Devices • Two Types • PLED (Polymer Light Emitting Diodes) • PLEC (Polymer Light Emitting Electrochemical cells) • PLED vs OLED difference?

4. PLED Structure Inorganic materials: Li, Ca, Mg Conductive Hole Transport Layer Anode such as Indium Tin Oxide

5. Pi-Conjugated Polymers Polyflourene Poly phenylenevinylenes (PPV) Poly(N-vinylcarbazole) Polythiophene

6. Properties of Conjugated Polymers • Exist as Semiconductors or insulators in undoped state • Band gap greater than 2eV. • Oxidative doping enhances conductivity. • Charged organic backbone is unstable in moisture Bao, et. al Thin Solid Films 323 (Dec. 1999) 239-242. PDF file.

7. Band Gap vs. Color Source: http://cms.tnw.utwente.nl/polymers/conj_pol.htm

8. Organic Light Emitting Diode (OLED) Sources: engadget.com; gizmodo.com; wired.com

9. 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

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

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

12. Pi-Conjugated Polymers Polyflourene Poly phenylenevinylenes (PPV) Poly(N-vinylcarbazole) Polythiophene

13. 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.

14. Polyfluorene Derivatives

15. 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.

16. Poly phenylenevinylene (PPV) • 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.

17. PPV History • 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 • Hoechstgroup expanded Friend’s research to look into color LED’s. Shao, et. al. Advanced Materials 19 (2007) 365–370. PDF file.

18. PPV History (cont.) • 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 Bao, et. al Thin Solid Films 323 (Dec. 1999) 239-242. PDF file.

19. 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

20. PPV Properties • 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 Wang, Haiqiao, et al. Journal of Applied Polymer Science 83 (2002) 2195-2200. PDF file

21. Poly(N-vinylcarbazole)

22. 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

23. Synthesis • Monomer: vinylcarbazole • Performed in bulk, in solution, in suspension or in precipitation • Polymerized by radical and cationic initiation 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 Figure 1: Structure of Poly(N-vinylcarbazole) (PVK) (Constructed in Chemdraw)

24. Properties • Photoconductivity • Charge-transfer complexes • Photoluminescence • Electroluminescence • Chemical stability • Thermal stability

25. Application - OLED • OLED: Organic Light Emitting Diode • Sandwiched organic thin films (single or multiple) layers between the electrodes • Transparent indium tin oxide (ITO) anode and metallic cathode • When voltage is applied, charge carriers are injected from the electrodes • Doped PVK is excited by the injected charge carriers to form excitons, which in turn give photons due to the hole-electron combination. Figure 2: Schematic representation of organic light emitting diode (T.M. El-Agez et als)

26. Emitting Mechanism • Doped PVK: PVK:Alq3 blend films • Photogeneration of excitons in PVK upon absorption • Electron transfer from PVK to Alq3, leaving an electron on Alq3 and a hole on PVK • Energy transfer from PVK to Alq3, recombination occurs, thus emitting photons as light. Figure 3: Steps for energy transfer and charge transport in PVK:Alq3 blend films (H. Jin et al.)

27. 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

28. Polythiophene…the polymer that will electrify you.

29. 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.

30. 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.

31. Mechanism…it knows how to twist. • Color changing optical properties and electric conductivity have the same mechanism. • Twisting of the polymer’s backbone, disrupting conjugation. • Conducting polymers have electrons that are delocalized along conjugated backbones • Results in conjugated polymers (process similar to other LEPs).

32. Removal of two electrons (p-doping) from a PT chain produces Bipolaron. Bipolaron = a bound pair of two polarons, a macromolecular chain containing two positive charges in a conjugated system. Conjugated Polythiophene structure.

33. 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.

34. 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)

35. 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.

36. Research Based Applications • Field-effect transistors • Electroluminescent devices • Solar cells • Photochemical resists • Non-linear optic devices • Batteries • Chemical sensors • AND OBVIOUSLY DIODES!

37. 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 (you potential polymer PhDs, this is a good field to go into).

38. We thank you for your kind, generous, undivided, and very enthusiastic attention.QUESTIONS?