摻合/側鏈基團/1,3,4-噁二重氮 (1,3,4- Oxadiazole)雜環基團影響高分子發光二極體的電激發光特性

Abstract

本論文主要研究方向分為三個部分：(1)聚甲基丙烯酸甲酯/聚【2-甲氧基-5-（2-乙基已氧基）-1,4-苯基已烯】摻合物物性及電激發光特性之研究；(2)側鏈基對卡唑（Carbazole）系列發光高分子之電激發光特性影響研究；(3) 新型態含1,3,4-噁二重氮（1,3,4- Oxadiazole）雜環基團聚亞醯胺高分子的電激發光及電子傳導特性影響研究。本論文第一部分為摻合PMMA來改進MEH-PPV為發光材之有機發光二極體發光效率，及有效降低其起動電壓。而隨著PMMA的含量改變，會使起動電壓逐漸的降低。元件系統利用Fowler-Nordheim tunneling theory的穿隧理論來模擬高分子發光二極體的光電行為，可得知隨著PMMA含量改變，其與鈣電極與MEH-PPV摻合PMMA之高分子LUMO能階的能障分別為0.23 eV (MEH-PPV)，0.12 eV (MEH-PPV:PMMA=9:1)和0.06 eV (MEH-PPV:PMMA=6:4)。而光激發光光譜與電激發光光譜相同，說明MEH-PPV摻合PMMA並未改變其能帶(Energy Gap)， 所以在摻合中並未有複合物(Complex)或複合激子(Exciplex)的產生。利用AFM及NSOM等儀器分析MEH-PPV及其摻合不同比例PMMA的表面型態。在AFM上視圖中可觀察到起伏明顯的圖形突出區域(Domain)。在NSOM實驗中之螢光光譜圖MEH-PPV則呈現一個亮區，而NSOM吸收光譜圖則提供我們驗證突出區域是否為PMMA的證據。PMMA在590nm區域是不會產生吸收的狀況，在AFM上視圖中可觀察到起伏明顯的圓形突出區域，故推論為PMMA。AFM及NSOM來綜合判斷，及比較整個比例大小，可得出圖3-14 PMMA散佈於MEH-PPV的示意圖。其表面型態類似在餅乾上散滿了芝麻。由亮度對電流圖，顯示稀釋效應並無存在此系統，本實驗中觀測到起動電壓隨著PMMA的含量增加而減低，摻合薄膜中PMMA扮演一個能有效降低能障的角色，X-ray繞射實驗存有一分子順向排列(orientation)所成繞射峰，且隨著PMMA的含量增加，其與鈣電極的能障逐漸降低，從0.23 eV降至0.06 eV。故推論電子易於從Ca電極中躍遷至PMMA層內，是由於PMMA產生順向排列之故。 本論文第二部分為用Wittig反應合成一系列含口卡口坐(carbazole)高分子來當作發光高分子，這些高分子在常用有機溶劑具有良好溶解性，故可輕易旋轉塗佈在各式各樣的載體之上，而成一均勻薄膜。在Wittig反應不同長度的側鏈基，造成不程度立體阻礙。而隨著側鏈基增長，其反式的比例即不斷降低。由於較低立體阻礙，在其整個Wittig反應過程，順式(trans- conformation)具有較低的能障。由於具有較低的能障，故隨著側鏈長度增長順式-CH=CH-生長較具優勢。螢光光譜圖及電激發光光譜中顯現隨鏈長長度增加產生紅移(red-shift)的現象。隨著鏈長長度增加，其起動電壓隨之降低，這是因LUMO能階隨著鏈長長度增加而減低，是故使電子注入的能障降低。元件的效能會因順式-CH=CH-比例增加而增加，是故側鏈基會藉由影響順反式的比例，而影響到這一系列高分子的電激發光與光激發光特性。 本論文第三部分為利用BAO 系列聚亞醯胺建立單層元件(ITO/PI/Al)及雙層元件(ITO/PPV-PVA/PI/Al)的高分子型發光二極體。所有聚亞醯胺均是可繞(flexible)且玻璃轉換溫度皆大於250℃，且具有良好的熱安定性及機械性質。BAO-6FDA可溶於極性(polor)溶劑，其餘聚亞醯胺不溶於任何所測試溶劑。全部聚亞醯胺均有螢光特性，且因dianhydride結構不同而有所變化。電激發光光譜及螢光光譜均顯示寬的發光波峰及相對單體大幅紅移現象。僅有兩種聚亞醯胺BAO-6FDA及BAO-ODPA有觀察到EL特性，另兩種BAO-PMDA及 BAO-BPADA可能因膜均勻性太差導致元件短路。在單層元件中BAO-ODPA比起BAO-6FDA有較高效率，雖然其有較高的起動電壓，量測其CIE色度座標值，分別為(0.45,0.33) (BAO-ODPA)和(0.32,0.22)(BAO-6FDA)。在BAO-ODPA聚亞醯胺雙層元件(ITO/PPV-PVA/PI/Al)中，BAO-ODPA同時扮演兩種效能：一是加強電子注入的特性、二是電洞束縛的功能(hole blocking effect) ，可以提升PPV-PVA發光層效率高達兩個級數，故含BAO 系列聚亞醯胺可同時當作電子傳導層或發光層。

In this article, we focus on three part of this dissertation. In the first part of this dissertation, a significant improvement on the electroluminescence threshold voltage from 5.5 volt down to 2.9 volt of poly〔2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene〕(MEH-PPV) in the Ca/polymer/ITO diode is presented by incorporating a non-conjugated PMMA into MEH-PPV. The threshold voltage of the material decreases with the poly(methyl methacrylate) (PMMA) content. The near-filed scanning absorption and photoluminescence micrographs show that the PMMA domains dispersed in the continuous MEH-PPV phase ressmble sesames embedded in the MEH-PPV pancakes. The resistivity of PMMA is two orders of magnitude lower than that of MEH-PPV. The excellent electrical conduction of PMMA in Ca/PMMA/ITO glass devices is not due to pin-hole defect mechanism, but may result from the electron hopping from Ca through the carbonyl groups in the PMMA. The distinguished improvement on the threshold voltage may be due to the combination of the electron hopping in the thin PMMA sesames and the thinner thickness effective of MEH-PPV in the light emitting layer. The second part of this dissertation, three carbazole-based copolymers with different long side chains were synthesized using the Wittig reaction. These three copolymers and model compound were characterized by nuclear magnetic resonance (NMR) spectroscopy to determine the cis/trans ratio. The cis/trans ratios decrease with the side chain length due to the variation in the degree of steric hindrance in the Wittig reactions. They influence the electroluminescence (EL) characteristics of the carbazole-based light emitting polymers. As the length of the side chain increased the wavelength of the emitted light increased proportionally whereas a decreased in the threshold voltages, 5.2 V, 4.8 V, 2.5 V was observed. However, EL intensity at same applied voltage increased with the length of the side chain. The energy levels of these three copolymers were also investigated to account for the phenomenon. The third part of this dissertation, four polyimides (PI) from 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (BAO) and four dianhydrides in light-emitting diodes (LED) are described. The PI films have high glass transition temperatures (Tg) > 250℃ and excellent thermal stability. The PL and EL characteristics of the BAO-containing PI (BAO-PI) are dianhydride structure dependent. Their PL and EL spectra are different from those of the monomers. The PL and EL spectra of the PI films exhibit peak broadening and red shifts. It may be due to the charge transfer in the PI. The CIE coordinates of BAO-6FDA and BAO-ODPA are located at (0.32, 0.22) and (0.45, 0.33) respectively. The latter has higher electroluminescence efficiency than the former, although the former has a lower threshold voltage. The other two PIs did not show EL due to the large surface roughness that leads to shorting in the devices. The BAO-ODPA can act not only as the light-emitting layer in the single layer LED ITO/PI/Al, but also as the electron transport and electron/hole blocking layer in the bilayer LED ITO/PPV-PVA/PI/Al. The incorporation of BAO-ODPA layer onto the PPV-PVA LED provides a significant improvement in the electroluminescence efficiency by two orders of magnitude.