採用波導耦合效應提升固態白光有機電化學元件效率

Abstract

固態白光有機電化學元件已經受到高度的關注，因為它可能應用於顯示及照明。雖然近幾年來白光有機電化學元件的效率已有顯著的改善，但為了能有更實際的應用，再進一步的提升仍是必須的。 在此研究中，我們藉由採用波導耦合效應的方式來提升白光有機電化學元件的效率。將兩個摻有二氧化鈦奈米粒子的透明光阻層嵌入於透明氧化銦錫電極和玻璃基板之間，而位於上層的透明光阻層，藉由調整半徑25 nm二氧化鈦粒子的濃度來調變其折射率，使ITO和位於下層的透明光阻層能達到有效的波導耦合作用，由於下層的透明光阻層含有半徑250 nm的二氧化鈦粒子，因此從ITO透過耦合作用而導出的光能被散射並重新導向正向出光。此外，被侷限在玻璃基板內的光也能穿透至位於下層的透明光阻層中，並經過散射而貢獻至正向出光。當被侷限在ITO和玻璃基板中的光能夠被有效的回收至正向時，採用波導耦合效應的白光有機電化學元件之外部量子效率和功率效率的峰值分別高達19.4 %和34.1 lm W–1，而這些效率為目前白光有機電化學元件中最高者，也證實了對於實現高效率白光有機電化學元件，採用波導耦合是個有效的方法。而具有散射作用的波導層結構之白光有機電化學元件除了能提升元件效率外，也因為在正向時能夠平均不同視角下的電激發光頻譜而改善了在照明應用上需要避免的顏色遷移現象。

Solid-state white light-emitting electrochemical cells (LECs) have received intense scientific attention owing to their potential applications in display and lighting. Although device efficiencies of white LECs have been improved significantly in recent years, further improvements are still required for practical applications. In this work, we demonstrate enhancing device efficiencies of white LECs by employing waveguide coupling. Two transparent photoresist (TPR) layers doped with TiO2 nanoparticles (NPs) are inserted between indium tin oxide (ITO) layer and glass substrate. By tuning the doping concentration of 25-nm TiO2 NPs in the upper TPR layer to adjust the refractive index, effective waveguide coupling between ITO layer and the lower TPR layer can be achieved. Since the lower TPR layer contains 250-nm TiO2 NPs, electroluminescence (EL) outcoupled from ITO layer can be scattered and redirected into the forward direction. Furthermore, the EL trapped in glass substrate can also transmit into the lower TPR layer and then is scattered to the forward direction. When the EL trapped in ITO layer and glass substrate can be effectively recycled into the forward direction, the peak external quantum efficiency and power efficiency obtained in white LECs employing waveguide coupling are up to 19.4 % and 34.1 lm W–1, respectively. These efficiencies are among the highest reported for white LECs and thus confirm that waveguide coupling would be useful for realizing highly efficient white LECs. In addition to enhanced device efficiencies, improved color migration of EL spectra, which is desired in lighting applications, can be obtained in white LECs with scattering waveguide layers since EL of different angles can be mixed in the forward direction.