高分子發光二極體(Polymer Light-Emitting Diode, PLED)的發光方式是藉由從陰、陽極分別注入電子與電洞，並在發光層中再結合，進而發出不同的光色，因此電荷的注入能力與傳輸速率的平衡是影響元件發光效率最重要的因素。然而，在大部分的有機共軛材料中電洞通常較電子有更好的注入及傳輸能力，因此提升電子的注入、傳輸能力將是增進PLED元件效率最有效的方法之一。許多研究利用低功函數金屬作為陰極以提升電子注入能力，但其在大氣中穩定性差，又須以真空蒸鍍的方式製備，成本較高且蒸鍍條件不易控制，故可利用濕式製程成膜的有機電子注入材料近來備受重用。
本研究使用商業化的聚乙烯醇(PVA: 水解程度為78 %)作為電子注入材料，並導入鹼金屬弱酸鹽類來提升PLED元件的效率。 PVA高極性的羫基側鏈能與鹼金屬陽離子作用，幫助鹼金屬鹽類利用濕式製程成膜，增進電子自陰極注入，使發光層內的載子數量更加平衡，大幅增進元件效能。而其特殊的溶解特性更可利用濕式製程(Spin-Coating)在發光層上塗佈成膜，降低製作成本。由於電子注入層具有顯著的效果，因此元件只需以水、氧穩定性高的高功函數金屬鋁(Al)作為電極，即可達到高效率高分子發光二極體之目的。本研究選用之共軛發光材料為Super Yellow (SY)，從能階上可看出PVA與之搭配後，將具有電子注入與電洞阻隔之效果。單純以PVA作電子注入層，最大亮度由395 cd/m2提升至5518 cd/m2，起始電壓由5.3 V下降至3.2 V；最大電流效率由0.06 cd/A增加至2.64 cd/A，提升44倍；而最大功率效率則由0.02 lm/W提升至1.41 lm/W，提升至少70倍。加入鹼金屬鹽類後，以30 wt%碳酸鈉(Na2CO3)的元件效果最佳，最大亮度由未添加電子注入層的395 cd/m2提升至20563 cd/m2，起始電壓由5.3 V下降至2.6 V；最大電流效率由0.06 cd/A增加至6.83 cd/A，提升114倍；而最大功率效率則由0.02 lm/W提升至3.66 lm/W，整整提升183倍。整體元件效率、最大亮度均有明顯增加，而起始操作電壓也明顯下降，達到高效率高分子發光二極體之目的。元件效率提升之原因推測是因電子注入層與陰極具有高接觸表面積，因此提升電子注入及電洞阻隔能力，本研究以原子力顯微鏡、Hole-only元件、Electron-only元件及光電測量來證明各種使元件效率提升之假設。

A balanced charges injection of devices is very important to increase the efficiency of the polymer light emitting diodes (PLEDs). Some non-conjugated polymers such as poly(ethylene oxide) (PEO) and ploy(ethylene glycol) dimethyl ether (PEGDE) have been used as potential electron injection layer (EIL) to overcome the problem of lower electron mobility in the organic materials. However, a complicated and costly facility was required to fabricate these kind of electron injection materials by thermal evaporation. In this study, we have successfully used water/alcohol soluble poly(vinyl alcohol) (PVA) as an efficient EIL and it is applicable in fabricating multilayer PLEDs by low-cost wet processes such as spin-coating. With PVA (0.9 mg/ml) as EIL, device achieved higher maximum brightness (5518 cd/m2) and current efficiency (2.64 cd/A) than the device without EIL (395 cd/m2, 0.06 cd/A) due to the enhanced electron injection ability and hole blocking ability of PVA. Additionally, PVA acted as a binder with alkali metal salts to fabricate a thin film on the top of emitting layer by spin-coating and the performance of devices with PVA+alkali metal salts (30 wt%) as EIL were all improved compared with the device with pure PVA as EIL because of the promoted electron injection ability. Especially, the device with PVA+Na2CO3 as EIL achieved highest current efficiency (6.83 cd/A) and luminous power efficiency (3.66 lm/W), which has 114 and 183 times increased than the device without EIL (0.06 cd/A, 0.02 lm/W). These results shows that using PVA as EIL is a promosing strategy for applications of solution-processed mutilayer PLEDs.

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