Improved ZnO Post-Treatment for High Performance Organic Solar Cell Materials

Abstract

Zinc oxide (ZnO) is a widely used electron transport layer for organic solar cells which has been optimized and established for the first generation of organic photovoltaic (OPV) materials. With the emergence of novel OPV materials which can reach up to 20% efficiency, several limitations of ZnO have become apparent. In particular, interactions of the active layer with ZnO under illumination can severely limit the device efficiency and stability. In this study, we investigate how various treatment options of ZnO like thermal annealing, ultraviolet exposure, as well as vacuum treatment can improve ZnO properties. Calcium tests show the release of reactive components form ZnO, and space charge limited current measurements allow to model energy level alignment using drift diffusion simulations. Crucially, permanent J_sc losses related to insufficient treatment of ZnO are observed for high performing material systems. An additional UV treatment step under vacuum is shown to significantly reduce those J_sc losses and allows using ZnO annealing temperatures of only 80°C.