降低使用非活性金屬電極有機電激發光元件之操作電壓

Abstract

高分子有機發光二極體（polymer light emitting diodes, PLED）是近年來最有工業發展潛力的應用項目之一，其良好的光電特性，廣泛應用於發光二極體的製作。但仍有許多問題尚未解決，高分子發光二極體必須使用鋇、鈣等低功函數金屬之陰極電極材料，以降低有機半導體發光材料與金屬電極之間的能帶位障，使陰極電子有效注入有機半導體材料中，使操作電壓降低。但因低功函數金屬材料極易與大氣中水氣及氧氣產生氧化反應而會影響元件的穩定性，故必須要有相當嚴格的封裝要求，此必定會使元件可撓性降低。 吾人於本實驗中在高分子發光材料聚芴（polyfluorene, PF）中加入高分子材料聚氧化乙烯（poly ethylene oxide, PEO）及離子性鹽類LiSO3CF3（LiTf），並以鋁（aluminum）為陰極金屬，利用PEO作為離子傳輸介質（ionic transport medium）並由LiTf提供離子，可增加載子注入效率，使驅動元件發光達亮度為1燭光/平方公尺（cd/m2）的電壓從6 V降為4.7 V。 而後吾人又將上述實驗元件導入主客體（host-guest）發光系統，於螢光主體發光材料中加入磷光客體材料[Ir(ppy)2(dasb)]PF6，增加元件發光效率，使元件亮度為1nit的電壓從6 V下降為2.9 V，且元件外部量子效率（external quantum efficiency）的最大值達到1.6 %。 對於節能以及可撓光電應用，於有機高分子發光材料中加入高分子材料及離子性鹽類，並導入主客體發光系統，可有效增加載子注入效率及增加元件發光效率，改善高分子材料高操作電壓及載子傳輸不平衡，且提供低電壓操作特性，使用較不易氧化之鋁電極，可減少封裝成本。

Because of superior optical properties, polymer light emitting diode(PLED) are promising candidates in commercial applications. But there are still many unresolved issues for PLEDs. PLED must use low-work- function metal electrode materials like Ba and Ca to reduce the energy barrier between organic semiconductor materials and metal electrode, and thus electrons can be effectively injected into organic semiconductor materials, lowering operating voltage decreases. Unfortunately low work function metal materials are sensitive to water and oxygen in the atmosphere and thus the stability of device deteriorates. To lower the operation voltage with stable electrodes, we blend poly ethylene oxide (PEO) and ionic salts LiSO3CF3 (LiTf) in polymer light-emitting material polyfluorene (PF) with aluminum as cathode materials. PEO are used for ion transport medium and LiTf provide ions for forming conductive doping regions, and the turn-on voltage of the device is thus reduced from 6 V to 4.7 V. But the device luminous efficiency and quantum efficiency are reduced at the same time. Blending phosphorescent guest material [Ir(ppy)2(dasb)]PF6 in PF host increases the device efficiency of the device. The turn-on voltage was reduced from 6 V to 2.9 V but the maximum quantum efficiency remained at 1.6 %. For energy-saving and flexible optoelectronic applications, host-guest system can not only effectively increase the device efficiency but also lower operating voltage. Furthermore, using aluminum cathode material, which is relatively inert, can lower packaging cost for commercial applications.