Holmium rare earth metal ion incorporated and ambient-air processed all-inorganic γ-CsPbI2.5Br0.5 perovskite solar cells yielding high efficiency and stable performance

Cesium lead halide-based all inorganic perovskite solar cells (IPVSCs) are considered promising absorbers for photovoltaic applications owing to their exceptional thermal stability and suitability towards single, monolithic tandem and triple junction perovskite/perovskite tandem solar cells. However, their device performance and phase stability lag behind those of conventional organic–inorganic hybrid perovskite solar cells (HPVSCs). Here, we successfully incorporated different quantities of holmium (Ho3+) rare earth (RE) metal ions into the γ-CsPbI2.5Br0.5 perovskite. A noticeable improvement in power conversion efficiency (PCE) from 17.19% (γ-CsPbI2.5Br0.5) to 19.59% was attained for the 3% HoCl3-based IPVSC device. Furthermore, our ambient-air processed 3% HoCl3-based device maintains >95% efficiency over 140 h at 30% relative humidity. This investigation shows that the RE metal ion incorporation is an effective approach for stabilizing and improving the device efficiency simultaneously.