The Role of Mobile Ions in The Early Performance Degradation of Perovskite Solar Cells

In the last decade, perovskite semiconductors have triggered a revolution in solar cell research. However, there remain some critical issues concerning the stability of metal-halide perovskites, which need to be overcome to enable a large scale commercialisation of perovskite photovoltaics. While the rather poor environmental stability of these perovskites is usually attributed to their ionic nature rendering them sensitive to moisture and oxygen, the actual contribution of mobile ions to the total degradation loss under different environmental conditions is poorly understood. Here, we reveal that the initial degradation of perovskite semiconductors is largely the result of mobile ion-induced internal field screening - a phenomenon that has not been previously discussed in relation to the degradation of perovskite solar cells. The increased field screening leads to a decrease in the steady-state power conversion efficiency mainly due to a large reduction in current density, while the efficiency at high scan speeds (>1000 V/s) where the ions are immobilized is much less affected. We also show that interfacial recombination does not increase upon ageing, yet the open-circuit voltage decreases as the result of an increase in the mobile ion density upon ageing. Furthermore, similar ionic losses appear under different external stressors, in particular when there are free charges present in the absorber layer. Finally, we generalize these findings beyond triple cation solar cells by showing the same degradation mechanisms take place in perovskites with a range of different compositions, including methylammonium leadiodide (‘MAPI’), formamidinium cesium leadiodide (‘FACs’), and several mixed-halide perovskites with different bandgaps. This work reveals a key degradation mechanism, providing new insights into initial device degradation before chemical or extrinsic mechanical device degradation effects manifest, and it highlights the critical role mobile ions play therein. It highlights the importance of efficiently managing mobile ions and their effects to enable long term stability in perovskite-based photovoltaics and move towards commercialization.