Surface Passivation of FAPbI3-Rich Perovskite with Cesium Iodide Outperforms Bulk Incorporation

Metal halide perovskites (MHPs) have shown an incredible increase in efficiency, reaching as high as 25.7%, which now competes with traditional photovoltaic technologies. Herein, we excluded CsX and RbX (X = I-, Br-, Cl-), the most commonly used cations to stabilize alpha-FAPbI3, from the bulk of perovskite thin films and applied them on the surface as passivation agents. Extensive device optimization led to a power conversion efficiency (PCE) of 24.1% with a high fill factor (FF) of 82.2% upon passivation with CsI. We investigated in depth the effect of CsI passivation on structural and optoelectronic properties using X-ray diffraction (XRD), angle-resolved X-ray photoelectron spectroscopy (ARXPS), Kelvin probe force microscopy (KPFM), time-resolved photoluminescence (TRPL), photoluminescence quantum yield (PLQY), and electroabsorption spectroscopy (TREAS). Furthermore, passivated devices exhibit enhanced operational stability, with optimized passivation with CsI leading to a retention of similar to 90% of the initial PCE under 1 sun illumination with maximum power point tracking for 600 h.

Automated summary

CsX and RbX were excluded from the bulk of the perovskite thin films and used as passivation agents on the surface, resulting in a high power conversion efficiency of 24.1% and a fill factor of 82.2% when CsI was used as the passivation agent. The effect of CsI passivation on structural and optoelectronic properties was extensively investigated using various characterization techniques.