採用高折射率基板來提升發光電化學元件的光萃取量

Abstract

固態發光電化學元件(light-emitting electrochemical cells, LECs)和傳統有機發光元件(organic light-emitting devices, OLEDs)相比有許多優點，例如元件結構簡單可以使用溶液製程來製作元件，低操作電壓和可以使用惰性金屬作為陰極等優勢。然而發光電化學元件的效率仍有改善空間，例如提升光萃取量以滿足實際應用的要求。在所有被侷限在發光電化學元件的光學模態中，侷限在基板模態的光是最容易被萃取出來的，例如藉由簡單的將基板的出光表面磨霧，便可以萃取出基板模態中的光。因此增加基板模態的比例對於提升光萃取量是非常有效的方式。在此實驗中分析藉由應用不同折射率的材料作為基板時，發光電化學元件中光學模態比例的變化。當使用高折射率材料作為基板時，會有更多的光被侷限在基板模態中。此外，當基板與銦錫氧化物(indium tin oxide, ITO)層之間的折射率差異越小，可以讓被銦錫氧化物薄膜層侷限的光(波導模態)，其達到截止條件的波長範圍擴大。而減少銦錫氧化物薄膜層中波導模態的比例也有助於將光自發光電化學元件中萃取出來，因為將光從波導模態中耦合出來需要更複雜的技術才能達成目的。因此本實驗使用可以從市面上購買到的未拋光藍寶石當作基板製作元件，其元件的出光量和以玻璃為基板製作的元件相比提升了56%，而在銦錫氧化物薄膜層和藍寶石基板之間加入一層散射層，可以從基板模態中萃取更多的光，並使出光量提升71%，而藉由使用釕錯合物作為發光電化學元件的發光層材料可以使外部量子效率高達5.5%，此元件效率在現有的紅光電化學元件研究論文中是很高的值，證實了利用高折射率材料作為基板來提升發光電化學元件的出光量是一個既簡單又有效的方法。

Solid-state light-emitting electrochemical cells (LECs) show several advantages over conventional organic light-emitting devices (OLEDs) such as simple device structure compatible with solution processes, low operation voltage and capability of utilizing inert cathode metals. However, device performance of LECs must be improved, e.g. enhancing light extraction, to meet the requirements for practical applications. Among the optical modes trapped in LECs, light trapped in substrate mode is easier to be extracted, e.g., by simply roughing the output surface. Therefore, increasing the percentage of substrate mode is beneficial in improving light extraction. In this work, the contributions of optical modes in LECs employing substrates with various refractive indices are analyzed. Higher-refractive-index substrates are shown to trap more light in the substrates. Furthermore, smaller refractive index difference between substrate and indium tin oxide (ITO) layer increases the cutoff spectral range of light waveguided in ITO layer. Reducing the percentage of waveguide mode in ITO layer also facilitates light extraction since it requires more complicated methods for outcoupling light in waveguide mode. With commercially available unpolished sapphire substrates, light output of LECs is enhanced by 56%. When a scattering layer was inserted between ITO and sapphire substrate, more light in substrate mode can be extracted and 71% enhancement in light output is realized. High external quantum efficiency up to 5.5% is consequently obtained in LECs based on a ruthenium complex. Such device efficiency is among the highest reported values for red-emitting LECs and thus confirms that utilizing higher-refractive-index substrates would offer a simple and feasible approach to improve light output of LECs.