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  P =  I  / V   =  ext  h  f(  Green OLED has higher Lm/W and Cd/A than red and blue devices Internal quantum efficiency  External quantum efficiency  ext Power efficiency  P (Lm/W) Current efficiency  I (Cd/A) Luminous efficiency means something else

4. Photopic response f 

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 Alq 3 /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  ext  ext = internal quantum efficiency x singlet branching ratio x coupling efficiency =  x B x C B = ¼ (spin degeneracy = 2S+1) C =  /n 2 Therefore,  ext < 11% even if  =100% (1 singlet exciton  1 photon)

9. HKUST effort New materials with higher  Improve hole injection (to increase  ) 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. Devices with different Alq layer thicknesses Device performance 92345203.412.612100 2345520103.788.07170 410552082.24.39300 Max. Brightness (Cd/m 2 ) Spectral peak (nm) Turn-on Voltage (V ) Power Efficiency (Lm/W) EL efficiency (Cd/A) Alq ( Å )

18. Very sensitive to Alq 3 thickness

19. High efficiency OLED Measured at an output of 300 Cd/m 2 Peak external quantum efficiency = 8%

20. Explanation Alq 3 /LiF/Al forms a good electron injector – minimum thickness of Alq is needed Electron mobility in silole is 100x that of Alq 3 (Chem Phys lett, 339(2001),Page 161-166) - Alq 3 produces resistive loss Hence there exists an optimal Alq 3 thickness Detailed rate equation modeling needed

21. Circuit model I V vovo vovo v o /R d L L ~ I ~ 1/R ~1/d R

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/m 2 (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  Improve hole injection (to increase  ) 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  Improve hole injection (to increase  ) Play with device structure (to improve C) Device optimization (to reduce V)

35. Coupling factor Integrating over the cone gives C = 1/2n 2 More sophisticated analysis (Friend et al, JAP 88, 1073 (2000)) gives C =  n 2 where  (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  Improve hole injection (to increase  ) 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

51. Emission spectrum Same as PL spectrum

52. Luminance

53. More recent results

54. Summary OLED efficiency can be improved by many means: new materials, new electrodes, new structures It is now as efficient as TFT LCD for display application It is as efficient as LED for lighting applications