Triplet Exciton Dynamics in π–Conjugated Polymer Light-Emitting Diodes Studied by Modulated-Induced Absorption Spectroscopy

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研究生：	廖華賢 Hua-Hsien Liao
論文名稱：	Triplet Exciton Dynamics in π–Conjugated Polymer Light-Emitting Diodes Studied by Modulated-Induced Absorption Spectroscopy 以調制誘發吸收光譜研究共軛高分子發光二極體中三重態激子之動力學
指導教授：	洪勝富 Shang-Fu Horng 孟心飛 Hsin-Fei Meng
口試委員:
學位類別：	博士 Doctor
系所名稱：	電機資訊學院 - 電子工程研究所 Institute of Electronics Engineering
論文出版年：	2008
畢業學年度：	96
語文別：	英文
論文頁數：	120
中文關鍵詞：	誘發吸收、內部系統轉換、三重態、聚芴、銥錯合物、共軛高分子發光二極體
外文關鍵詞：	induced absorption, PA, ISC, polyfluorene, Ir-complex, triplet exciton
相關次數：	點閱：2 下載：0
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We use modulated induced absorption spectroscopy technique to detect non-radiative triplet excitons in polymer light-emitting diodes and study three important mechanisms in organic semiconductor photo-Physics, including: the triplet to singlet exciton ratio, Dexter energy transfer between triplet excitons of a conjugated polymer and phosphorescent dopants, how does the intersystem crossing efficiency of a conjugated polymer be affected by phosphorescent dopants. Chapter 3 studies the formation and decay of triplet excitons in a polyfluorene light-emitting diode. A strong suppression of the triplet exciton density relative to the singlet by voltage is observed. Through an unique independent measurement on the triplet exciton lifetime if is shown that the suppression solely comes from triplet exciton quenching by current injection. The triplet-to-singlet exciton formation ration is independent of voltage as well as temperature, implying a spin-independent exciton formation. Chapter 4 uses photo-induced absorption and photoluminescence to study the Dexter energy transfer in Ir-complex/polyfluorene blend system. It is found that some Ir-complex doped system has clear Dexter energy transfer evidence. The dominant mechanisms are aggression and dopant lifetime, but the triplet confinement is not the necessary criterion. In Chapter 5, photo-induced absorption is used to study the intersystem crossing (ISC) from singlet to triplet states of two polyfluorenes (PFs) after doped with Ir-complex. It is found that the triplet exciton lifetime of PFs is reduced by the dopants. But instead of decreasing, the population density of PF triplet exciton increases by almost one order of magnitude. The finding shows that the ISC rate can increase over 100 times due to the spin-orbital interaction with the Ir ions.

我們利用調制誘發吸收光譜技術來偵測共軛高分子發光二極體中不發光的三重態激子的行為，藉此來研究有機半導體光物理中三個重要的機制: 三重態激子與單重態激子生成之比例、共軛高分子三重態與磷光雜質三重態間的Dexter能量傳遞機制、共軛高分子單重態與三重態間的系統轉換效率如何受磷光雜質摻雜的影響。第三章研究聚芴高分子發光二極體中三重態激子的形成與衰變，研究發現三重態激子在高電壓下會有很強的抑制產生與單重態激子十分不同，其因為在高電壓下，有大量的電流注入，三重態激子被電流載子給消滅；我們同時也發現三重態激子與單重態激子的生成比例是不隨電壓或溫度而變的，此意味著在共軛高分子發光二極體中激子的形成是與自旋無關的。第四章研究在銥錯合物摻雜聚芴系統中聚芴的三重態到銥錯合物的三重態的Dexter能量傳遞的行為；研究發現僅有某些銥錯合物在聚芴系統中才會Dexter能量傳遞的行為，而其主宰的機制並非先前大眾所認知的三重態能量限制條件，而是由高分子聚集效應與磷光雜質的生命期所主導。第五章以光誘發吸收光譜技術研究兩種聚芴高分子摻雜綠光銥錯合物後，聚芴的單重態到三重態的內部系統轉換效率的改變；研究發現在摻雜後，聚芴的三重態生命期減短，但出乎意料的是其三重態的態密度並沒有隨之降低反而有一個數量級的增加，其原因為單重態到三重態的內部系統轉換受銥錯合物的影響效率增加了100倍。

ACKNOWLEDGMENTS
ABSTRACT OF THE DISSERTATION
Chapter 1 Introduction……………………………………………………1
1-1    Background…………………………………………….1
1-2    Motivation……………………………………………...4
1-2-1    Triplet to singlet exciton ratio γ………………… 5
1-2-2    Dexter energy transfer between triplet states of polymer and phosphorescent dopant………….6
1-2-3    Dexter energy transfer between triplet states of polymer and phosphorescent dopant………….8
1-2-4    Modulated induced absorption spectroscopy………9
1-3    The structure of this dissertation……………………..11
Chapter 2 Theoretical Background and Experimental method………………………………………………………12
2-1  Polarons and Excitons………………………………...12
2-1-1 Polarons……………………………………………12
2-1-2 Excitons……………………………………………12
2-1-3 Frenkel and Mott-Wannier excitons……………….13

2-2    Two-Fermion System: Spin Singlet and Triplets……..14
2-3    Structure and operation of polymer LED…………….18
2-4    Quantum efficiency…………………………………..20
2-5    What is induced absorption spectroscopy?..................23
2-6    CW Modulated Induced Absorption Spectroscopy ….25
2-7    Experiment setup……………………………………..30
2-7-1    Instruments………………………………………..30
2-7-2    Device structures………………………………….31
2-7-3    Lock-in amplifier working principle……………...32
2-7-4    Photo-induced absorption (PA)…………………...36
2-7-5    Electroluminescence-induced absorption (EA) …..40

Chapter 3 Triplet Exciton Formation and Decay in Polyfluorene Light-Emitting Diodes………………………………………42

3-1 Introduction……………………………………………..42
3-2 Experiment……………………………………………...42
3-3 Results…………………………………………………..43
3-3-1 PA and EA spectra in previous literatures…………43
3-3-2 Triplet exciton PA spectrum in Pristine PFO film...46
3-3-3 Triplet exciton EA spectrum in a PFO PLED device…………………………………………………….47
3-3-4 Triplet exciton suppression in EA spectra………...51
3-3-5 Voltage dependent of triplet exciton lifetime  …53
3-3-6    is independent of temperature T……………..57
3-4 Conclusions …………………………………………….60

Chapter 4 Triplet Exciton Energy Transfer in Polyfluorene Doped with Heavy Metal Complexes……………………………………61

4-1 Introduction………………………………………….61
4-2 Experiment…………………………………………..63
4-3 Modulation Spectra………………………………….64
4-4 Morphology and concentration effects………………74
4-5 Electroluminescence………………………………...78
4-6 Discussions…………………………………………..80
4-7 Conclusions………………………………………….81

Chapter 5 Large Enhancement of Intersystem Crossing in Polyfluorenes by Iridium-Complex Doping………………………………..82

5-1 Introduction…………………………………………82
5-2 Experiment………………………………………….83
5-3 PA Frequency-modulation ...……………………….84
5-3 Fitting method ……………………………………...87
5-5 Rate equations for ISC process……………………..90
5-6 Another proof for large ISC enhancement………….92
5-7 Discussion…………………………………………..98
Chapter 6 Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer……………………………100
          6-1 Introduction……………………………………….100
          6-2 Experiment………………………………………...102
            6-3 Enhancement of Solar cell efficiency by Cs2CO3…103
       6-4 UPS results………………………………………...107
            6-5 XPS results………………………………………...109
            6-6 Discussion………………………………………….111
            6-7 Conclusion…………………………………………112
APPENDIX
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