Quantifying and Reducing Ion Migration in Metal Halide Perovskites through Control of Mobile Ions

The presence of intrinsic ion migration in metal halide perovskites (MHPs) is one of the main reasons that perovskite solar cells (PSCs) are not stable under operation. In this work, we quantify the ion migration of PSCs and MHP thin films in terms of mobile ion concentration (N(o)) and ionic mobility (µ) and demonstrate that N(o) has a larger impact on device stability. We study the effect of small alkali metal A-site cation additives (e.g., Na(+), K(+), and Rb(+)) on ion migration. We show that the influence of moisture and cation additive on N(o) is less significant than the choice of top electrode in PSCs. We also show that N(o) in PSCs remains constant with an increase in temperature but μ increases with temperature because the activation energy is lower than that of ion formation. This work gives design principles regarding the importance of passivation and the effects of operational conditions on ion migration.