Reactive modification of zinc oxide with methylammonium iodide boosts the operational stability of perovskite solar cells

Low operational stability of perovskite solar cells represents a major obstacle for practical implementation of this technology. In that context, ZnO was considered as a promising electron transport material with suppressed oxidizing properties as compared to SnO2 and TiO2. However, the first studies revealed the chemical instability of the interface between lead halide perovskites and zinc oxide, whereas the underlying reasons are still under active debates. Still, the interfacial instability issues made ZnO a largely overlooked electron transport layer despite its excellent optoelectronic properties. This study presents a novel approach to the zinc oxide surface modification with methylammonium iodide that suppresses further reactions with the adjacent perovskite absorber layer. Consequently, the solar cells based on CH3NH3PbI3 exhibited largely improved thermal stability. Furthermore, the application of Cs0.12[HC(NH2)2]0.88PbI3 as absorber material in devices with the modified ZnO electron transport layer resulted in 82% retention of the initial efficiency after aging for 2100 h at 50 mW cm-2 and 65 ?C. On the contrary, the reference cells fabricated with the pristine non-modified ZnO degraded completely under the same conditions. We attribute the revealed stabilization effect of the methylammonium iodide treatment to passivation of the reactive ZnO surface and inhibiting the parasitic interfacial chemistry leading to the lead iodide formation. To our best knowledge, here we have demonstrated the highest operational stability for the perovskite cells when using ZnO-based ETL