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Physics and applications of conjugated polymers semiconductors

Physics and applications of conjugated polymers semiconductors. 孟心飛 交通大學物理所. 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. Conjugated polymer: organic semiconductor with direct bandgap of 2-3 eV. Outline. Overview Triplet exciton formation Field-effect transistor

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Physics and applications of conjugated polymers semiconductors

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  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

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