Surface states of ZnO nanoparticles effect on the performance of inverted-organic solar cells

ZnO is a promising material used as the electron transport layer in the inverted organic solar cells (IOSCs). However, the electrical or photoelectric properties of ZnO nanoparticles are governed by the surface states of the nanoparticles. Here, we demonstrate that the large number of hydroxyl (-OH) existed on the ZnO nanoparticles films have a vast impact on the performance of IOSCs with the structure of ITO/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C-61 butyric acid methyl ester (PCBM)/MoO3/Ag. The surface hydroxyl groups depredate active layer via elevating photocatalytic activity of the ZnO, hence deteriorate the device performance. Experimental results show that hydroxyl groups can be effectively detached from ZnO film by annealing. Hydroxyl groups detach more with increasing annealing temperature, resulting in less degradation of the active layer. Therefore, the efficiency is significantly improved due to increased photo-current density and decreased series resistance of IOSCs. The best device exhibits a power conversion efficiency of 3.05% after annealing at 150 degrees C.