Partial Substitution of Pb in CsPbI as an Efficient Strategy To Design Fairly Stable All-Inorganic Perovskite Formulations.

All-inorganic lead halide perovskites, for example, CsPbI, are becoming more attractive for applications as light absorbers in perovskite solar cells because of higher thermal and photochemical stability as compared to their hybrid analogues. However, a specific drawback of the CsPbI absorber consists of the rapid phase transition from black to yellow nonphotoactive phase at low temperatures (e.g., <100 °C), which is accelerated under exposure to light. Herein, an experimental screening of an unprecedently large series (>30) of metal cations in a wide range of concentration has allowed us to establish a set of Pb substitutes, facilitating the crystallization of the photoactive black CsPbI phase at low temperatures. Importantly, the appropriate Pb substitution with Ca, Sr, Ce, Nd, Gd, Tb, Dy, Er, Yb, Lu, and Pt cations has led to a spectacular enhancement of the film stability under realistic solar cell operation conditions (∼1 sun equivalent light exposure, 50 °C). Optoelectronic, structural, and morphological effects of partial Pb substitution were investigated, providing a deeper insight into the processes underlying the stabilization of the CsPbI films. Several CsPbMI systems were evaluated as absorber materials in perovskite solar cells, demonstrating encouraging light power conversion efficiency of 11.4% in preliminary experiments. The obtained results feature the potential of designing efficient and stable all-inorganic perovskite solar cells using novel absorber materials rationally designed via compositional engineering.