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 0.83 MA 0.17 )Pb(I 0.83 Br 0.17 ) 3 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 + can lead to several nonphotoactive side-products. During the perovskite film synthesis, RbPb(I 1– x Br x ) 3 is formed, while exposure to humid air leads to the rapid formation of another hitherto unreported side phase (RbPb 2 I 4 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 2 I 4 Br phase upon Cs-addition occurs on a significantly longer time scale than its Rb analog. While the incorporation of Cs + remains attractive for high-performance solar cells, the severe moisture-sensitivity of Rb-containing mixed-halide perovskites may create additional engineering challenges.