In the quest to further improve the efficiency of perovskite solar cells, the absorption layer in these devices is being extensively study. However, the optimization of the interface between the photoelectrode and perovskite layer is also essential for increasing overall efficiency. ZnO is a promising metal-oxide photoelectrode due to its versatility to make a variety of nanostructures (i.e. nanorods) with enhanced electron extraction rate derived from larger surface areas.
In order to increase the power conversion efficiency in the ZnO nanorods-based perovskite solar cells, improving the coverage of the MAPbI3 layer on ZnO is an essential step. During device fabrication, exposed ZnO nanorods in direct contact with the hole transfer layer (CuSCN) create a direct charge recombination pathway that negatively affects the charge transfer rate and collection efficiency.
To reduce carrier recombination caused by the poor coverage of the perovskite layer (CH3NH3PbI3, MAPbI3) on the surface of the ZnO photoelectrode, surface modification is performed on the exposed ZnO nanorods. After deposition and annealing of the MAPbI3 layer, a solid state CuSCN layer is spin-coated on top. The thickness of the MAPbI3 layer is modulated by controlling the length of ZnO nanorods, leading to different absorbances, charge extraction rate, and photocurrent density in the fabricated devices.
In this work, the effects of surface treatment pre-deposition of the MAPbI3 layer are observed and compared in the performance of perovskite solar cells. Several treatment methods are studied to maximize surface coverage of the perovskite layer on the ZnO nanorods. The distinct changes in surface morphology are observed by scanning electron microscope image characterization pre- and post-surface treatment. The modified ZnO surface shows improved power conversion efficiency in the treated perovskite solar cells.