Download
physics and applications of conjugated polymers semiconductors n.
Skip this Video
Loading SlideShow in 5 Seconds..
Physics and applications of conjugated polymers semiconductors PowerPoint Presentation
Download Presentation
Physics and applications of conjugated polymers semiconductors

Physics and applications of conjugated polymers semiconductors

325 Views Download Presentation
Download Presentation

Physics and applications of conjugated polymers semiconductors

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Physics and applications of conjugated polymers semiconductors 孟心飛 交通大學物理所

  2. 感謝 • 洪勝富 清大電機系 • 施宙聰 清大物理系 • 許千樹 交大應化系 • 陳壽安 清大化工系 • 翰立光電研發部

  3. Conjugated polymer:organic semiconductor with direct bandgap of 2-3 eV

  4. Outline • Overview • Triplet exciton formation • Field-effect transistor • Multi-color LED

  5. Technologies of conjugated polymers • 1970-80, metallic conductivity reached by molecular doping • 1990, first polymer LED was made • 1998-99, polymer flat-panel-display was demonstrated, other opto-electronic devices are underway • Solution processing, large area, light-weight, high-brightness, flexible

  6. Science of conjugated polymers • 1D semiconductor • Electron-electron and electron-phonon enhanced in 1D • Quasi-particle: solitons, polarons .. • Complicated recombination • Spin-triplet exciton formation • Transport in disordered materials

  7. y x PPV semiconductor band structure E(k) C : 1s2 2s2 2p2  2s,2px,2py sp2 hybridization -bond  2pz-bond One -electron for each carbon atom

  8. LED Device Operation Conduction Valence

  9. Triplet exciton formation in polymer LED

  10. _ + _ Electron-hole pair + Coulomb capture Exciton (large binding energy) Radiative decay photon

  11. Total spin of exciton (electron-hole bound state) Electron spin = 1/2 , Hole spin = 1/2 Exciton spin = 0 (Singlet) 1 (Triplet)

  12. Spin-independent recombination γ= 3 Free electron-hole pair G 3 G Triplet Singlet Radiative: light Nonradiative: heat Ground State

  13. Not so simpleT.-M. Hong and H.-F. Meng, Phy. Rev. B, 63, 075206 (2001) Bottleneck RadiativeDecay Non-radiativeDecay

  14. G γG S T Ground State Detection of singlet and triplet excitons No quantitative relation available! Free electron-hole pair Induced absorption at near IR (1.3-1.6 eV) Visible light emission

  15. How do we measure γ ?Compare EL and PL rate equations EL : electric excitation PL : optical excitation

  16. 1. EL Rate equation G: generation rate for singlet exciton τs: singlet exciton lifetime τt: triplet exciton lifetime EL Free electron-hole pair G γG S T Ground State

  17. 2. PL Rate equation :intersystem crossing lifetime. PL Free electron-hole pair S T pump Ground State

  18. Steady-state • NsEL = NtPL

  19. Al 100nm Ca 10nm ITO Al MEH-PPV (100nm or 50nm) PEDOT 40nm ITO 80nm Glass MEH-PPV LED

  20. Experiment setup

  21. Optical table

  22. Infrared semiconductor probe lasers

  23. Cooling system (under construction)

  24. EL-induced absorption (EA) spectrum due to the triplet exciton

  25. Triplet and singlet exciton density linear

  26. Time-resolved PL, s=0.64 ns

  27. Phys. Rev. Lett., 90, 036601 (2003) • d : thickness of MEH-PPV. • Vbi : built-in voltage

  28. Two possible explanations 1. Field dissociation Free carrier continuum 0.1ev 1/4 3/4 0.3ev Phonon bottleneck 1ev 2. Quenching by polarons Ground state

  29. Conclusion • γ is not a constant but a strong universal function of the electric field • γ is much larger than 3 for intermediate bias and smaller than 3 for high bias • Triplet exciton formation is no longer the main limit for the efficiency of a LED operated under high bias

  30. Parallel transport and field effect transistors

  31. Light emitting polymers have very low carrier mobility

  32. Motivation for horizontal structure • Carriers transport by hopping in the sandwich structure – low mobility • Carriers transport along the backbone mostly in a horizontal device structure –high mobility Perpendicular transport (high mobility) Parallel transport (low mobility) j j Glass substrate Glass substrate

  33. Theoretical basis: High intrachain mobility can be achieved even with many conjugation defects Yi-Shiou Chen and Hsin-Fei Meng, Phys. Rev. B, 66, 035191 (2002)

  34. Parallel hole transport d d = 2 micron h = 100 nm Au h polymer Au glass

  35. Thermal coater

  36. Mask aligner for photo-lithography

  37. Spinner

  38. 1μm gold source/drain channel on glass or SiO2/ITO

  39. Interdigited1 μm channel

  40. ITO/PPV/Au sandwich device • Hole-only device • T=307K • SCLC model J= p=510-11m2/Vs =510-7cm2/Vs R1=CH3, R2=C10H21 PRB55,R656(1997)

  41. Space charge limited current • Steady state: J=nqE • Poisson’s eq.:  • ……Mott-Gurney law

  42. Ohmic: J=n0q pE There is little dependence between p and d. fixed T, variable SD distance d

  43. J-E plot • The slope of J-E curve = n0q p • n0 :extrinsic carrier density q:electron charge p: hole mobility  由n0 倒推回p p=3.810-3 cm2/Vs

  44. sandwich device:ITO/MEH-PPV/Ca/Al • bias>3V: SCLC J= =3 L =1200Å p =1.44×10-5cm2V-1s-1 • bias<3V: Ohmic J=n0q pE n0 =7.84×1021m-3

  45. Compare with other sandwich devices • Our horizontal device: p=3.810-3 cm2/Vs • Chen: p =1.44×10-5 cm2/Vs • Friend: p =2×10-7 cm2/Vs • Hegger: p =2.24×10-7 cm2/Vs

  46. T=297K There is little dependence between p and d. No domain down to 1 micron fixed T, variable d