TiO₂ passivation for improved efficiency and stability of ZnO nanorods based perovskite solar cells

Zinc oxide (ZnO) has been demonstrated to be a superb electron selective contact material in photovoltaic devices for its high electron mobility and various accessible nanostructures. However, issues of severe charge recombination and thermal instability occurring at the perovskites/ZnO interface hinder its application on perovskite solar cells. Herein, we report a strategy of TiO₂ passivation onto the surface of ZnO nanorods (NRs) using a wet-chemical method, where a device structure FTO/ZnO NRs/TiO₂ passivation layer/CH₃NH₃PbI₃/spiro-OMeTAD/Ag is adopted. Based on the proposed strategy, an overall power conversion efficiency (PCE) of 13.49% is achieved mainly due to the improved open-circuit voltage (V_oc) of 1.02 V, shirt-circuit current density (J_sc) of 20.69 mA cm^-2, and fill factor (FF) of 0.64, which are much higher than those of bare ZnO NRs-based devices. Interestingly, TiO₂ passivated samples show much better long-term device stability than those without passivation, where TiO₂ acts as a buffer layer with improved thermal stability owning to reduced chemisorbed hydroxyl groups as indicated by X-ray photoelectron spectroscopy.