臺灣博碩士論文加值系統

PPV與PF系高分子具有優良的螢光特性與適當的半導性，是最被廣為應用於電激發光的共軛高分子。文獻上對於發光高分子中載子傳遞、陷阱捕捉、釋放及再結合的研究中仍有許多待釐清之處，如載子陷阱類型的判斷(電子或電洞)、載子陷阱對遷移率的實際影響及陷阱釋放電流與再結合放光等相關報導十分缺乏。此外，由於共軛高分子除具有半導體特性之外，亦有傳統高分子分子鏈鬆弛的現象，故在進行研究時，情況更為複雜，也因如此，該方面之研究設備亦因規格化困難而十分缺乏。因此，本研究中以自行組裝之熱激發電流(thermally stimulated current, TSC)、熱激發放光(thermally stimulated luminescence, TSL)、飛行時間載子遷移率量測系統(Time-of-flight, TOF)以及時間解析電激光譜量測系統(time-resolved electroluminescence spectroscopy, TREL)以探討載子傳遞與陷阱之課題。研究中利用飛行時間原理為基礎的TSC量測法，可以清楚的判別MEH-PPV中電子與電洞陷阱。其中電洞陷阱活化能為0.1-0.4 eV，峰值約在210 K，由於此陷阱濃度不隨空氣引入而改變，亦與高分子形態無關，故可能是合成時即已存在的外來雜質；電子陷阱之活化能為 0.45-0.5 eV，峰值約出現在300 K，由於此陷阱會隨著氧氣的引入而增加，且具可逆性，故證實為氧分子所造成。值得注意的是，此峰值之溫度位置隨電場增強，有略往高溫移動的趨勢，此現象在TSC量測中從未被報導，該現象表示在此電場範圍內，電子遷移率會隨電場上升而下降，此現象暗示對電子傳遞而言，位置亂度主宰其傳遞行為。
由TSC觀察電子與電洞在MEH-PPV中遷移情形與由TOF中所觀察的結果相符，電子在MHE-PPV中傳遞不易，無法傳遞通過1.5 μm的薄膜；電洞傳遞性質則較佳，可順利的傳遞通過厚達8μm的薄膜。以TSC量測MEH-PPV之去極化電流，可以清楚的觀察側鏈及主鏈的鬆弛現象，顯示TSC量測系統對分子鏈鬆弛現象之高靈敏度，電子與電洞釋放溫度的起始值與範圍分別與主鏈及側鏈的鬆弛溫度吻合，其活化能與溫度的變化趨勢亦相同，此結果表示分子鏈運動會引發載子的釋放。
氧分子在MEH-PPV中，補捉電子的形式為應為O2-，由於其電子親和力(0.89eV)小於MEH-PPV主鏈之電子親和力(2.8 eV)，故電子仍會傾向存在共軛主鏈上而部分被氧分子吸引而形成MEH-PPV…e…O2之錯合物，由於電子釋放的起始溫度與主鏈鬆弛的起始溫度相同，表示只要主鏈開始運動，電子即可以脫離氧分子的補捉，但因再補捉(retrapping)現象嚴重，增加了在此處進行非放射性再結合的機率，此現象將造成元件效率下降。
以同時量測TSC與TSL的方式發現MEH-PPV中TSL放光為geminate pair所貢獻，此物種為激子產生後至解離成自由載子前的中間體。本研究首次採用分段式激發法進行TSL量測，證明geminate pair 在形成後，可以用光激發的方式將其釋放，所以不同波長激發時造成不同強度的TSL是一種動態平衡的結果，激發光一方面會產生激子進而解離成geminate pair，同時亦會將已形成的geminate pair釋放，此現象暗示了geminate pair可能具有一特定吸收，此發現將有助於了解螢光產生的過程及其解離成載子的機制。
以光電流遷移理論模型(photocurrent transient equation, PTE)模擬飛行時間法所量測之非分散性、分散性及高度分散性遷移光電流，可同時得到遷移率( mobility, μfit)與擴散係數 (diffusion coefficient, D)，在不同系統及不同形態的高分子中皆能得到很好的模擬結果。其中μfit與光電流中以t1/2所決定的遷移率μ1/2相近，可以代表平均的載子遷移率，若以偏離係數Dv=Dq/μkT表示偏離Einstein’s Law的程度，在所探討之光電流中，皆有嚴重偏離Einstein’s Law的情形，而此偏離程度與光電流拖尾係數W (tail-broadening parameter, W=(t1/2-t0)/t1/2)所決定的光電流分散程度完全符合，因此，Dv可以作為光電流分散程度的指標，此結果顯示偏離Einstein’s Law的程度越大，其傳遞光電流越為分散性。由於D可以表現載子於不同系統及形態之發光高分子中遷移的現象，故此擴散係數D包含了所有造成載子遷移時分散的原因。
在TOF量測中，僅引入1%光紅銥(Ir)金屬錯合物的PFOR1其電洞遷移率相較於PFO，下降了兩個數量級，而當含量增加為12%時，電洞遷移率更低。這表示側鏈的Ir錯合物具有電洞阱的功能。在TSC量測中偵測不到任何陷阱電流，原因是Ir 錯合物的HOMO與LUMO介於PFO的能階中，所以不僅是電洞而亦是電子的陷阱，故在此處進行再結合的機率極高，載子皆經由發射性衰退回到基態，故無法以TSC偵測其電流。
在PF側鏈引進Cz基團後(CzPF)，雖然該基團具電洞傳遞特性，但引入後卻造成電洞遷移率下降約一個數量級，藉由光激發及電場激發TSC之量測，發現引進Cz後會形成電洞陷阱，其TSC圖譜與poly (N-vinylcarbazole) (PVK)相似，兩者的陷阱溫度位置與活化能亦相近，故推測，此電洞陷阱為Cz-Cz 二聚體(dimer)所造成。
時間解析電激光譜中，經退火處理之MEH-PPV發光二極體，在脈衝電壓操作初始時，單體單元放光比例隨時間增加，在電壓移除後，單體單元之放光較聚集放光先衰退，此現象代表聚集有載子陷阱及再結合中心之功能，但因其能階與單體單元之能階相近，故陷阱活化能極小，因此在TSC中無法觀察到此陷阱電流。

Poly(phenylene vinylene) (PPV), polyfluorene (PF) and their derivatives are the most popular electroluminescent polymers due to their semi-conductive and good fluorescent properties. However, the charge transport, trapping, detrapping and recombination mechanisms have not been well known so far. Issues such as the assignment of trap polarity (hole or electron), the exact effect of the trap states on the charge mobility, and the relationships between detrapping carriers and radiative recombination… etc, are rarely discussed in documents. This is because that both the semi-conduction capabilities and chain relaxation complicate these questions. As a result, standardized instruments for these researches are not commercial available now. In this research, therefore, we used homemade apparatus assemblies for thermally stimulated current (TSC), thermally stimulated luminescence (TSL), time-of-flight (TOF), and time resolved electroluminescence spectroscopy (TREL) to investigate the charge transport and trapping mechanisms of conjugated polymers.
By using TOF-based TSC, the trap states of hole and electron in MEH-PPV can be clearly assigned. The hole trap is located at about 210 K with an activation energy of 0.1-0.4 eV. This trap state is not affected by the ambient air and the change of morphology, which is attributed to the extrinsic impurities. The observed electron trap is located at about 300 K with an activation energy of 0.45-0.5 eV. Since the trap concentration increases by the exposure to oxygen and is reversible, it is attributed to the molecular oxygen. It is worth noting that the trap peak shifts to higher temperature as the drain field increases. This behavior is firstly observed by TSC measurement and indicates that electron mobility decreases as field increases. It can be attributed to the field-induced localization or positional-disorder dominant transport.
The transport properties observed in TSC measurement are in agreement with those from TOF measurement. In MEH-PPV, electron transport is poor and electrons can not move across a 1.5-μm-thick film. Contrarily, holes can travel through a 8-μm-thick film, indicating a better transport. The relaxation currents for side chain and main chain can be observed unambiguously by TSC measurement, indicating that TSC is chain-relaxation-sensitive technique. The peak location and activation energies of detrapping current for electrons and holes are in agreement with those of side chain and main relaxations, implying that chain motion can induce carrier detrapping.
The most possible form for molecular oxygen to catch an electron is “O2- “. Since the electron affinity of O2- (0.89eV) is smaller than that of MEH-PPV (2.8 eV), an electron prefers to stay on the conjugated main chain but slightly attracted by the adjacent molecular oxygen. A complex of MEH-PPV…e…O2 is likely to form. The onset temperature for electron detrapping is close to that of main chain relaxation, indicating that electron can escape from the attraction of oxygen as long as the main-chain relaxations start. The electron could be released at room temperature. However, serious retrapping makes these sites readily for charge recombination and then non-radiative decay. This will result in low efficiencies for electroluminescence devices.
By the simultaneous measurement of TSC and TSL on MEH-PPV, the TSL emission contributed by geminate pairs (an intermediate between exciton and free carrier) can be investigated. In this research, the “two step excitation” method is firstly applied and we find that geminate pairs could be dissociated by the incident light. The wavelength dependent TSL is a result of dynamic balance. The incident light not only generates excitons to form geminate pairs but also dissociate them at the same time. This behavior implies that the geminate pairs may have characteristic absorption profile. This observation is very helpful to investigate carrier generation process.
By fitting of photocurrent transient equation (PTE) to the non-dispersive, dispersive, and highly dispersive photocurrents, the charge mobility μfit and diffusion coefficient D can be obtained at the same time. PTE can perfectly fit the experimental data of different polymers and their films with different morphology. The thus obtained mobility is close to those determined by t1/2 (from the TOF results), which is close to the average mobility. By defining a deviation parameter Dv=Dq/μkT, large deviation from Einstein’s law is observed in all of the investigated polymers. The degree of this deviation is in agreement with the tail-broadening parameter W, which is widely used parameter for dispersion in photocurrent transients. Therefore, Dv can be regarded as an indicator of dispersion. This result also reveals that the larger deviation from Einstein’s law, the more dispersion in the transport behaviors. Since the D value characterizes the photocurrent transients in different type of polymers and films with different morphology, it lumps all factors together that cause the dispersion in carrier transport.
From the TOF measurement, the copolymer PFOR1 (containing only 1% red Ir complex) has the hole mobility 2 orders of magnitude lower than PFO; and PFOR12 (with 12 % Ir complex) has an even lower hole mobility. These results indicate the occurrence of hole trapping on the side-chain Ir complex. In the TSC measurement, however, no trap currents were found. The reason is that Ir complex is an efficient recombination center because the HOMO and LUMO lay between those of the main chain (PFO), which permits both hole and electron to be trapped and recombine at these sites.
Cabarzole is a hole-transport material. However, hole traps are formed and the hole mobility decreased about one order for carbazole-grafting polyfluorene (CzPF), as compared with that of PFO. By using photoexcitation and field-induction TSC, two hole traps were found and the distribution of trap currents and activation energies are in agreement with that in PVK. This two trap states are attributed to two types of Cz-Cz dimer.
From the result of time-resolved electroluminescence (TREL) measurement, the monomeric emission from the annealed device increases with time in the initial operation period but decreases with time after the bias is off. This behavior indicates that aggregates are sites for charge trapping and recombination and function as dopant. Since the activation energy of aggregates is quite small so that the detrapping currents are difficult to be detected by TSC measurement.

摘要(中文) I
摘要(英文) V
目錄 IX
圖目錄 XIV
表目錄 XXI

第一章緒論
1-1前言 1
1-2 共軛導電高分子之電子狀態 3
1-3 螢光理論 8
1-3-1 螢光的成因 8
1-3-2 影響螢光的因素 9
1-3-3 螢光的能量轉移 12
1-4 金屬半導體理論 13
1-4-1 界面接合 13
1-4-2 電流傳遞過 14
1-5 本文目的 16

第二章 文獻回顧
2-1 應用於發光二極體之高分子 17
2-1-1 PPV系高分子- 17
2-1-2 poly(para-phenylene)s PPP系高分子 18
2-1-3 Poly(fluorene) PF系高分子- 18
2-1-4 白光發光二極體 18
2-1-5 電洞傳遞層 19
2-1-6 電子傳遞層 20
2-1-7 具傳遞基團的發光層 21
2-2 高分子發光二極體元件物理 25
2-2-1 發光二極体之發光原理 25
2-2-2 載子注入與傳遞機制- 27
2-2-2-1 注入限制電流 27
2-2-2-2 空間電荷限制電流 28
2-2-2-3 Poole-Frenkel relationship- 28
2-2-2-4 Bässler formalism 29
2-2-2-5 Einstein relationship- 30
2-2-2-6 Onsager theory 30
2-3 元件物理量測方法: 載子遷移率與陷阱之量測 31
2-3-1 載子傳遞 31
2-3-1-1飛行時間法(Time-of-Flight, TOF) 31
2-3-1-2 Modified TOF 34
2-3-1-3 Dark Injection (DI) 35
2-3-1-4 阻抗分析儀Impendence analyzer 37
2-3-1-5載子注入效率 37
2-3-2 載子陷阱 38
2-3-2-1激發電流(Thermally Stimulated Current, TSC) 38
2-3-2-2激發放光(Thermally Stimulated luminescence, TSL) 39
2-4 載子傳遞與陷阱之回顧 40
2-4-1載子傳遞 40
2-4-1-1 PPV 系高分子 40
2-4-1-2 PF系高分子 41
2-4-1-3 其他有機分子 43
2-4-2 載子陷阱 45
2-4-3 熱激發放光- 46
2-5 文獻分析 49

第三章 實驗
3-1 高分子之合成 52
3-1-1 MEH-PPV之合成 52
3-1-2 PFO、CzPF、PFOR之合成 52
3-1-3 Gm之合成 52
3-2 儀器設備 53
3-3 元件的製作 54
3-3-1 ITO玻璃基材之蝕刻與清潔 54
3-3-2 發光高分子薄膜之塗佈 54
3-3-3 金屬之蒸鍍 55

第四章 MEH-PPV中載子陷阱之研究
4-1 前言 56
4-2 實驗部分 58
4-2-1 TSC/TSL量測設備及實驗流程 58
4-2-2 實驗流程 58
4-2-3 分析方法 60
4-3 電場誘發TSC 61
4-4 光激發TSC 65
4-4-1 陷阱類型判別 66
4-4-2 電場強度對TSC電流峰值位置之影響 75
4-5 陷阱之成因 77
4-6 陷阱活化能分析 80
4-7 載子釋放機制 82
4-8 TSL量測 85
4-9 結論 86

第五章 MEH-PPV陷阱放光之研究
5-1 前言 87
5-2 實驗裝置 90
5-3 MEH-PPV 之TSC/TSL同步量測圖譜 90
5-4 激發條件與TSL圖譜之關係 92
5-4-1 不同激發光對TSL之影響 92
5-4-2 膜厚與TSL圖譜之關係 95
5-5 TSL 強度變化與波長之分佈 96
5-6 分段式激發TSL 98
5-7 TSL光譜之量測 100
5-8 �皕鰱E發後放光 (Isothermal afterglow) 102
5-9 活化能分析 103
5-10 高分子形態對TSL的影響 105
5-11 結論 107

第六章 發光高分子之載子遷移率與擴散係數之研究
6-1 前言 108
6-2 實驗部分 110
6-2-1 飛行時間載子遷移率量測系統 (time-of-flight, TOF) 110
6-2-2 光電流遷移理論模型(PTE) 111
6-2-3 理論模型之模擬與參數配適(parameter fitting)流程 112
6-3 電洞在不同形態之MEH-PPV遷移光電流 113
6-4 電洞在PFO中之遷移光電流 116
6-5 電子與電洞在m-DMOP-PPV (Gm)中之傳遞光電流 120
6-6 光電流理論模型之模擬及參數配適 (parameter fitting) 122
6-6-1 不同形態之MEH-PPV薄膜中電洞遷移光電流模擬與參數配適 122
6-6-2 電洞在PFO中遷移光電流之模擬與參數配適 129
6-6-3 電洞與電子在Gm中遷移光電流之理論模擬與參數配適 132
6-7 發光高分子中載子傳遞偏離Einstein 關係式之現象 136
6-8 結論 140

第七章 發光高分子中陷阱與載子傳遞關係之研究
7-1 前言 141
7-2 實驗部分 143
7-2-1 飛行時間量測遷移率 143
7-2-2 熱激發電流量測 143
7-2-3 時間解析電激光譜量測系統 143
7-3 PFOR中電荷傳遞與陷阱機制 145
7-3-1 PFOR之載子遷移率 145
7-3-2 PFOR之熱激發電流 (TSC) 149
7-4 CzPF中電荷傳遞與陷阱機制 152
7-4-1 CzPF 之載子遷移率 152
7-4-2 CzPF 與PVK之陷阱量測 152
7-5 MEH-PPV 之時間解析電激光譜 (TREL) 157
7-6 結論 164

第八章 總結與未來展望 165

本研究之原創性工作 167
參考文獻 168
自傳 180
著作目錄 181

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