Cs⁺ incorporation into CH₃NH₃PbI₃ perovskite: substitution limit and stability enhancement

In this study we systematically explored the mixed cation perovskite Cs_x(CH₃NH₃)_1-xPbI₃. We exchanged the A-site cation by dipping MAPbI₃ films into a CsI solution, thereby incrementally replacing the MA⁺ in a time-resolved dipping process and analysed the resulting thin-films with UV-Vis, XRD, EDAX, SEM and optical depth-analysis in a high-throughput fashion. Additional in situ UV-Vis and time-resolved XRD measurements allowed us to look at the kinetics of the formation process. The results showed a discontinuity during the conversion. Firstly, small amounts of Cs⁺ are incorporated into the structure. After a few minutes, the Cs content approaches a limit and grains of δ-CsPbI₃ occur, indicating a substitution limit. We compared this cation exchange to a one-step crystallisation approach and found the same effect of phase segregation, which shows that the substitution limit is an intrinsic feature rather than a kinetic effect. Optical and structural properties changed continuously for small Cs incorporations. Larger amounts of Cs result in phase segregation. We estimate the substitution limit of Cs_xMA_1-xPbI₃ to start at a Cs ratio x = 0.13, based on combined measurements of EDAX, UV-Vis and XRD. The photovoltaic performance of the mixed cation perovskite shows a large increase in device stability from days to weeks. The initial efficiency of mixed Cs_xMA_1-xPbI₃ devices decreases slightly, which is compensated by stability after a few days.