側鏈含有奈米結構之高分子電激發光二極體

Abstract

高分子發光二極體(PLED)是未來發展成大平面的顯示器的重要技術，而大部份的電激發光高分子，由於具有豐富的π電子，因此電洞注入特性和傳輸電洞的能力遠比電子注入特性和傳輸電子的能力來的有效率。 近期不少研究著重於開發高效率且穩定的發光材料，其中又以藍光材料最受重視。 聚茀（Polyfluorene）及其衍生物由於包含一剛性且共平面的雙苯環結構，所以表現出特殊的物理和化學性質; 然而，無論是PPV 或者是PF系列元件在製成薄膜時，由於材料累積濃度過高造成分子堆疊或產生excimer，嚴重影響光色以及降低發光效率; 此外，PF系列由於C9位置容易產生氧化現象(keto defect)，也會改變元件原有穩定的發光光色。 為了改善這些缺點，選擇導入多立面體聚矽氧烷(POSS) 於高分子材料之側鏈期望以防止氧化及減少分子堆疊，使得高分子的光色及熱穩定性進一步改良，可廣泛地應用在顯示器的發光材料上。此方法所合成之高分子奈米複合材料可提升發光高分子之發光效率、元件效率並提升其耐熱度及穩定性。 在本文的第二及第三章節，我們將分別討論POSS在Polyfluorene(PF) 與 Polyphenyl vinylene (PPV)中所扮演的角色。 另外一部分，有鑒於高分子內部螢光發光效率最高僅達25%的物理限制，高分子與無機材料的結合便受到了矚目﹔雖近期有磷光高分子發光二極體之開發，但合成時必須使用重金屬，來源恐不穩定。因此，本研究嘗試以膠體化學法合成半導體材料量子點 (其直徑小於10 奈米)製備一系列S-CdS/PF-GX (X=1, 2)之奈米複合發光材料，並藉由實驗證實，導入少量改質的量子點(S-CdS)，不但可有效地提昇螢光及電激發光效率至原來之兩倍至三倍，同時也增強原本材料在製程元件後的穩定度及電性。在第五章中我們選擇用金奈米粒子並對其特性作進一步的探討。含有1wt %的金奈米粒子之聚茀高分子共聚物提高了原本的量子效率與光學穩定性；同時在元件部份，與純聚茀高分子共聚物比較，高分子藉由鍵結金奈米粒子之元件也具有較優異的表現。

The main objective of this dissertation is to study the performance of polymer light emitting diodes involving luminescent polymers incorporating different kinds of inorganic segment in their side chains. In the introduction of this dissertation, we gave an explanation on the historical evolution of polymer nanocomposites light emitting diodes and summarized the literatures in the recent years. In the chapter 2, we have synthesized polyhedral silsesquioxane-tethered polyfluorene copolymers, poly(9,9´-dioctylfluorene-co-9,9´-bis[4-(N,N-dipolysilsesquioxane) aminophenyl]fluorene) (PFO-POSS), that have well-defined architectures using Suzuki polycondensation. This particular PFO-POSS molecular architecture increases the quantum yield of polyfluorene significantly by reducing the degree of interchain aggregation; in addition, these copolymers exhibit a purer and stronger blue light by preventing the formation of keto defects. The PPV-POSS molecular architecture also increases the quantum yield significantly by reducing the degree of interchain aggregation were discussed in Chapter 3. This particular molecular architecture of POSS-PPV-co-MEHPPV copolymers possesses not only a larger quantum yield (0.85 vs. 0.19) but also higher degradation and glass transition temperatures relative to those of pure MEHPPV. The maximum brightness of a double-layered-configured light emitting diode (ITO/PEDOT/emissive polymer/Ca/Al) incorporating a copolymer of MEHPPV and 10 mol% PPV-POSS was five times as large as that of a similar light emitting diode incorporating pure MEHPPV (2196 vs. 473 cd/m2).

The presence of a low percentage of thiophenol-modified cadmium sulfide (S-CdS) nanoparticles in the benzoxyl-dendritic structure of a copolyfluorene (PF-GX) substantially improves the efficiency of its light emission were discussed in Chapter 3. The enhancements in photoluminescence and electroluminescence arise mainly from a reduction in the degree of energy transfer from the excited polymer chains to their neighboring polymer chains in the ground state; i.e., there is an increase in the inter-polymer chain distance when CdS nanoparticles are present. We have prepared highly luminescent dendron-substituted copolyfluorenes that incorporate surface-modified cadmium sulfide nanoparticles. A small percentage of these nanoparticles can be incorporated into the dendritic structures upon tailoring the interfaces between the ligands on the nanoparticles and the dendritic structures in the copolyfluorene. Both the photoluminescence and electroluminescence efficiencies of the polymer nanocomposites are dramatically enhanced. Moreover, in order to know the effect to some other nanoparticles, we have tethered gold nanoparticles (Au NPs) to the side chains of poly{2,7-(9,9´-dioctylfluorene)-co- 4-diphenylamino-4´- bipenylmethylsulfide} (PF-DBMS) through its ArSCH3 anchor groups. The presence of 1 wt% of the tethered gold NPs led to a reduction in the degree of aggregation of the polymer chains, resulting in a 50% increase in its quantum yield. The electroluminescence of a 1wt% Au/ PF-DBMS device was three times higher in terms of its maximum brightness and its full-width-at-half-maximum emission peak was much narrower than that of a pure PF-DBMS device. These phenomena arise from the photooxidation suppression, hole blocking, and electron transport enhancing effects of the Au NPs were also demonstrated in Chapter 5.