以無機量子點及溶膠-凝膠製程製備高效能奈米有機發光元件

Abstract

本研究主要分成兩部分，以探討引進無機量子點(Quantum Dots)及溶膠凝膠製程(Sol-Gel process)，進而有效提升有機發光二極體之效能。於第一部份，採用鎳催化-Yamamoto聚合及鈀-催化Suzuki(鈴木)偶合法，配合適當的苯基溴(aryl bromide)以及硼酯類(bronic ester)取代基單體，可成功的聚合出一系列含有硫的有機聚芴同元及異元高分子(PFS, PF1, PF3及PF4)。另外合成三種不含硫的聚芴高分子(PF2, PF5及PFC6)與上述含硫高分子以茲區別。這些聚芴高分子則透過FT-IR、元素分析、DSC、TGA、螢光光譜(PL)以及電激發光實驗來做為其定性上的分析。其中PFS, PFC6, PF1 – PF3可發藍光(440-468 nm)，而 PF4 – PF5則發黃綠光(540 nm)。迴火實驗證實，相較於同結構的聚芴高分子(PFC6)，含硫聚芴高分子的表現出較佳的抗氧化性(PFS)，連帶抑制了傳統聚芴高分子常見的光譜偏移問題（keto defect）。電激發光元件可驗證此系列含硫聚芴高分子表現出相當良好的光譜穩定性，在高電壓的操作之下並不會有光譜偏移的現象發生。以高分子PF1所製備的多層電激發光元件 ITO/PEDOT/ PF1/CsF/Al為例，於電壓8V時可達最大亮度2991 cd/m2，並於電流密度75 mA/cm2時達到最大效率1.36 cd/A。另一方面，經由配位基交換製程，此硫原子中的孤對電子可與量子點的空價軌域行配位共價鍵鍵結。以此有機/無機(聚芴高分子/量子點)混成奈米複合材料所製成的電激發光元件，相較於原本未含硫的聚芴高分子來說，表現出更佳的亮度及效率。

以有機半導體所製成的電激發光二極體擁有極佳的亮度及效率，同時具有可商業化的潛力。然而其潛在問題為薄膜態時，發光分子之間經常會彼此堆疊並降低放光效率。在本論文的第二部份，導入了兩個以芴環為主體，以三苯基胺或蒽作為側取代基的藍色發光體，透過溶膠-凝膠製程(sol-gel process)在酸性的環境下與矽源反應（TOES），並研究其自組裝後所形成中孔洞薄膜間的分子吸引力關係。此以芴環為主的兩性藍光界面活性劑，配合矽氧烷所形成的共自組裝薄膜，可有效的將發光分子固定在矽烷所形成的奈米孔洞之中，進而避免分子之間堆疊，以提高放光效率。以此奈米薄膜所製成的電激發光元件，可簡單的利用旋轉塗佈溶液製程而得，而其表現相較於原本未加矽烷的薄膜來說，亦有大幅的增加。繼螢光系統之後，本實驗引入無機磷光銥錯合物，透過簡單的有機合成方法，在三步驟反應之後得到帶有三個長碳鏈羥基的天藍色最終單體。利用帶有唑(carbazole)長碳鏈羥基小分子與此銥錯合物單體，依照不同的重量比例，在與矽烷作用後所得到的奈米薄膜，可有效的製備出高效能的客體/主體有機電激發光元件。此手法將可望建構出一種嶄新的製備奈米有機電激發光元件的製程，以簡化複雜的共蒸鍍及摻混製程，同時增進元件本身的效能。

The goal of this study is aimed to improve the performance of the light emitting diodes by introducing inorganic quantum dots and sol-gel process. In the first part, a new series of sulfide-containing polyfluorene homopolymers and copolymers (PFS, PF1, PF3 and PF4) comprised of aryl bromide and bronic ester moieties were synthesized by Ni(0)-mediated Yamamoto coupling and palladium-catalyzed Suzuki polymerizations. Three other polyfluorenes (PF2, PF5 and PFC6) without sulfur atom in the alkyl side chains were also synthesized by a similar method for comparison purpose. These fluorene-based polymers were characterized using FT-IR spectroscopy, elemental analysis, DSC, TGA, photoluminescence (PL) spectroscopy. The synthesized polymers PFS, PF1-PF3 emit blue light at around 440-468 nm, while copolymers PF4 and PF5 emit green light at 540 nm. In the annealing experiments, these polymer films show better stability against thermal-oxidation than polymer PFC6. Sulfide-containing polymers show not only good electroluminescent color stability, but their EL spectra also remain unchanged at high driving voltage. A double-layer electroluminescent device with the configuration of ITO/PEDOT/PF1/CsF/Al exhibited a stable sky-blue emission with CIE (0.21, 0.23) at 10 V, which showed a maximum brightness of 2991 cd/m2 at 8 V (75 mA/cm2) and a maximum efficiency of 1.36 cd/A. Finally, by ligand exchange process, the sulfur element could form coordination bonding with quantum dots, and PLED devices using these new QDs-containing organic/inorganic hybrid materials as light emitting layers exhibited superior or comparable EL performance compared to those without quantum dots.

Organic semi-conductors show efficient electroluminescence which has led to their commercialization in LEDs. However, they have been marred by the thorniest problem of solid-state quenching. In the second part, we report the synthesis and characterization of two fluorene-based blue amphiphilic emitters containing triphenylamine or anthracene side groups. The formation of the hybrid meso-structured nanocomposites by sol-gel co-assembly with tetraethyl ortho-silicate was demonstrated, and the molecular interactions within the mesophases were studied. The blue light luminescent films made of fluorene-based amphiphile/silica co-assembled nanocomposite have been successfully prepared with enhanced emission. Different kinds of light emitting devices based on these nanocomposites showed improved efficiencies several times higher than the corresponding pristine chromophores. Furthermore, we report the synthesis and characterization of cyclometalated iridium complex which emits sky-blue light. The hybrid meso-structured nanocomposites by sol-gel co-assembly with tetraethyl ortho-silicate and the molecular interactions within the mesophases were also demonstrated. Electroluminescent devices were fabricated using carbazole-based precursor and iridium complex act as host/guest system through co-assembled sol-gel process. Light emitting devices based on these nanocomposites showed improved efficiencies several times higher than similar chromophore elaborated in the literature. The demonstration of nano-sized chromophoric amphiphiles/silica architecture may offer an easier strategy for fabricating high-efficiency phosphorescent OLEDs.