Surface defect engineering in sol-gel ZnO thin films: the role of UV-C irradiation and environmental factors in enhancing n-type conductivity

Abstract

Zinc Oxide (ZnO) thin films, prepared by the sol-gel method, are widely used in optoelectronic devices due to their high electron mobility and excellent optical properties. However, ZnO films often suffer from surface defects, which lead to a degradation in the performance of optoelectronic devices. Here, we systematically investigated the enhancement of n-type characteristics in ZnO thin films under various conditions through UV-C irradiation. Specifically, we analyzed the surface chemistry and n-type characteristics of ZnO films exposed to various environmental conditions, focusing on changes in oxygen species including oxygen ions, oxygen vacancies, and hydroxyl groups. UV-C light irradiation modified the surface chemistry by generating oxygen vacancies and desorbing adsorbed oxygen through photogenerated holes, thereby reducing surface defects and enhancing n-type conductivity. Thus, the ZnO films irradiated with UV-C, without exposure to an oxygen-rich environment, exhibited the highest n-type characteristics due to the removal of adsorbed oxygen. In contrast, the non-irradiated ZnO films exposed to an oxygen-rich environment had the lowest n-type characteristics because the additional adsorbed oxygen created new surface defects, resulting in the lowest electron mobility. Consequently, organic photovoltaics employing UV-C-irradiated ZnO films without exposure to an oxygen-rich environment achieved the best performance.