Stereoelectronic Effect from B-Site Dopants Stabilizes Black Phase of CsPbI₃

Phase stability remains a critical bottleneck hindering the use of CsPbI3 perovskites, which prefer to take on a non-perovskite, “yellow” phase at room temperature. The desired “black” phase perovskite, suitable for photovoltaic applications, can be obtained at elevated temperatures but readily reverts to the yellow phase under humid conditions and room temperature. B-site doping has shown some promise in improving the phase stability of perovskite nanocrystals; however, incorporating a sufficient amount of a single dopant into the perovskite lattice remains challenging. Here we report a dual-dopant strategy, where modest quantities of two divalent cations (Ca2+ and Mn2+) were simultaneously introduced in the fabrication of CsPbI3 films to circumvent the need to introduce a single dopant in high quantities. DFT is used to uncover the importance of dopant-based stereoelectronic effects in phase stabilization. The CaMn dual-doped films can retain the black phase for up to 16 days at room temperature and 40–60 %RH; the mono-substituted and non-substituted analogues reverted to the yellow phase in seconds under the same conditions, demonstrating the ability of dual doping to improve optoelectronic materials’ durability where a single dopant fails.