Impact of Rubidium and Cesium Cations on the Moisture Stability of Multiple-Cation Mixed-Halide Perovskites

Rubidium and cesium cations have been recently identified as enhancers for perovskite solar cell performance. However, the impact of these inorganic cations on the stability of the (FA<sub>0.83</sub>MA<sub>0.17</sub>)­Pb­(I<sub>0.83</sub>Br<sub>0.17</sub>)<sub>3</sub> perovskite crystal lattice has not been fully understood yet. Here, we show via in situ X-ray diffraction and energy-dispersive X-ray spectrometry measurements that the unsuitably small ionic radius of Rb<sup>+</sup> can lead to several nonphotoactive side-products. During the perovskite film synthesis, RbPb­(I<sub>1–<i>x</i></sub>Br<sub><i>x</i></sub>)<sub>3</sub> is formed, while exposure to humid air leads to the rapid formation of another hitherto unreported side phase (RbPb<sub>2</sub>I<sub>4</sub>Br). The formation of the Rb-rich side phases not only results in a loss of light absorption but also extracts bromide ions from the photoactive perovskite phase, thereby reducing its band gap. In comparison, the moisture-assisted formation of a CsPb<sub>2</sub>I<sub>4</sub>Br phase upon Cs-addition occurs on a significantly longer time scale than its Rb analog. While the incorporation of Cs<sup>+</sup> remains attractive for high-performance solar cells, the severe moisture-sensitivity of Rb-containing mixed-halide perovskites may create additional engineering challenges.