Material Design of Transparent Oxide Semiconductors for Organic Electronics: Why Do Zinc Silicate Thin Films Have Exceptional Properties?

Abstract

Device application of organic electronics demands materials with seemingly incompatible properties. A material with a low work function, high mobility, high visible transparency, and chemical stability for electron injection/transport of organic light-emitting diodes is required. Typical n-type organic semiconductor materials with a low work function do not satisfy the requirements for high mobility, transparency, and chemical stability. In contrast, conventional transparent oxide semiconductors do not meet the criterion of low work function. A zinc silicate thin film has a low work function (approximate to 3.5 eV); it also satisfies the other requirements. The present paper elucidates the reasons for such exceptional properties. It is revealed that the zinc silicate thin films comprise ZnO nanocrystals, each of which is optically separated by an extremely thin amorphous ZnO-SiO2 layer. Such a unique nanostructure gives rise to the quantum-size effect of ZnO nanocrystals, leading to low work functions and hopping conduction; this enables it to meet the criterion for a low work function and also enables relatively higher mobility (0.3-1.0 cm(2) V-1 s(-1)) than those of the n-type organic semiconductors. Based on these findings, a material design idea is proposed to realize incompatible properties even using compositions of naturally abundant constituents.