Improving the efficiency of OLED + other interesting results from UST

1. Improving the efficiency of OLED + other interesting results from UST Hoi-Sing Kwok Man Wong Ben-Zhong Tang (Chem) Cheng-Feng Qiu Hai-Ying Chen Zhi-Guo Meng Jacob Ho Eric Chan H J Peng Hong Kong University of Science & Technology Funding: RGC

2. Attributes of a good display • Energy efficiency • Viewing angle • Contrast ratio • Speed • Color saturation • Manufacturability, cost • Lifetime

3. Efficiencies - definitions • Internal quantum efficiency h • External quantum efficiency hext • Power efficiency hP (Lm/W) • Current efficiency hI(Cd/A) Luminous efficiency means something else hP = hIW / V hI = hext <hn f(l)> / W Green OLED has higher Lm/W and Cd/A than red and blue devices

4. Photopic response f(l)

5. Competitions • OLED competes with TFT LCD in monitor, camcorder, DSC applications, with STN for cell phone applications and with LED for lighting applications • LCD efficiency has improved to about 12 Lm/W • Reflective STN =  Lm/W • LED can achieve 100 Lm/W (red)

6. TFT LCD efficiency • CCFL is cw and has an efficiency of 80 Lm/W - averaged over RGB • Much progress in color filter, aperture ratio, polarizer recycling

7. OLED status • Standard Alq3/TPD - 1 Lm/W (1.5%) • Doping with efficient emitters - 10 Lm/W (7%) • Doping with triplet emitter - 70 Lm/W, 64 Cd/A (19%) • OLED efficiencies are quite close to best values of other technologies • Higher efficiencies are always needed

8. Factors in efficiency • To increase Lm/W, we need to reduce V and increase hext • hext = internal quantum efficiency x singlet branching ratio x coupling efficiency = h x B x C • B = ¼ (spin degeneracy = 2S+1) • C = x/n2 • Therefore, hext < 11% even if h =100% (1 singlet exciton  1 photon)

9. HKUST effort • New materials with higher h • Improve hole injection (to increase h) • Play with device structure (to improve C) • Device optimization (to reduce V)

10. New materials • MPS (1-methyl-1,2,3,4,5-pentaphenylsilole) • HPS (hexaphenyl silole) • Polymerized siloles • Liquid crystal OLED • External quantum efficiency reaches 8%

11. Siloles • An interesting family of compounds • Low PL in solution form • Large PL in solid form (aggregates emission) (MPS)1-methyl-1,2,3,4,5-pentaphenylsilole HPS (hexaphenyl silole)

12. Device structure

13. PL spectrum

14. EL spectrum

15. Comparison of emission spectra

16. L-V and I-V curves

17. Alq ( Å ) EL efficiency (Cd/A) Power Efficiency (Lm/W) Turn-on Voltage (V ) Spectral peak (nm) Max. Brightness (Cd/m2) 300 4.39 2.2 8 520 4105 170 8.07 3.78 10 520 2345 100 12 12.6 3.4 520 9234 Device performance • Devices with different Alq layer thicknesses

18. Very sensitive to Alq3 thickness

19. High efficiency OLED Measured at an output of 300 Cd/m2 Peak external quantum efficiency = 8%

20. Explanation • Alq3/LiF/Al forms a good electron injector – minimum thickness of Alq is needed • Electron mobility in silole is 100x that of Alq3(Chem Phys lett, 339(2001),Page 161-166) - Alq3 produces resistive loss • Hence there exists an optimal Alq3 thickness • Detailed rate equation modeling needed

21. Circuit model R I vo/R vo V vo L L ~ I ~ 1/R ~1/d d

22. TPD dependence

23. Luminance curve of the 7nm device

24. MPS is a singlet emitter

25. Angular distribution is Lambertian

26. Spectroscopic ellipsometry *Need n to calculate coupling efficiency

27. Interesting side-product: MPS is an amorphous semiconductor

28. HPS - similar results

29. PL and EL spectra

30. Comparing MPS and HPS

31. Summary for siloles • Max ext quantum efficiency = 8% • Max current efficiency = 22 Cd/A • Max power efficiency = 16 lm/W • Max at 100 Cd/m2 (no cheating) • Recently, NRL group also found efficiency OLED in another silole compound (2,5-bis-(2’,2”-bipyridin-6-yl)-1,1-dimethy-3,4-diphenyl-silole)

32. HKUST effort • New materials with higher h • Improve hole injection (to increase h) • Play with device structure (to improve C) • Device optimization (to reduce V)

33. New electrode structure • Improves hole injection efficiency by 10x • Improves quantum efficiency by 2.5x • Will disclose after patent is filed

34. HKUST effort • New materials with higher h • Improve hole injection (to increase h) • Play with device structure (to improve C) • Device optimization (to reduce V)

35. Coupling factor Integrating over the cone gives C = 1/2n2 More sophisticated analysis (Friend et al, JAP 88, 1073 (2000)) gives C = x/n2 where x (0.75, 1.2)

36. Tsutsui et al, Adv Mater 13, 1149 (01) • Used aerogel as substrate - decreased n to 1.03 • Enhanced output by 1.8x

37. Forrest et al, APL 91, 3324 (02) • Lens array on top of glass substrate • Idea - reduce TIR • Any structured surface will do - similar to solar cells - photonic structures

38. UST lens array / photonic crystal structure

39. Other ideas - structured emitter • Possible to control orientation of molecules? (liquid crystal OLED) • If the emission is directional to begin with, there is no need to reduce TIR • Falling leaves?

40. HKUST effort • New materials with higher h • Improve hole injection (to increase h) • Play with device structure (to improve C) • Device optimization (to reduce V)

41. Standard device optimization • Vary systematically all layer thicknesses

42. Optimization

43. Anode buffer layer

44. Mechanism for improvement

45. Summary for device structure • Many proposals to improve efficiency • Device optimization is needed and leads to better understanding of the function of the various layers

46. Announcement • First ever active matrix OLED display in Greater China (Mainland, Taiwan, HK) • Based on low temperature polycrystalline silicon technologies • Metal induced unilateral crystallization (MIUC) • Very uniform TFT without sophisticated compensation schemes

47. HKUST AMOLED

48. UV emitting OLED • Why UV? • Can make full color displays by PL of phosphors • PL can be 100% efficient • We made TPD OLED with ~1% external quantum efficiency

49. Device structure Anode/(TPD/GaN)n/TPD/cathode Vary n to find best configuration

50. I-V of MOLED