Heterogeneity of Light-Induced Open-Circuit Voltage Loss in Perovskite/Si Tandem Solar Cells

Mixed-halide perovskite-silicon tandem solar cells have demonstrated great potential in achieving >40% efficiencies. However, light-induced phase segregation makes the commercialization of mixed-halide perovskite-silicon tandem solar cells difficult. Here, we unveil the impact of phase segregation in the 1.67 eV bandgap Cs(0.17)FA(0.83)Pb(I0.80Br0.20)(3) on the nanoscopic heterogeneity across the film through photoconductive atomic force microscopy. By measuring I-Vcurves at both grain boundaries (GBs) and grain interiors (GIs) with nanoscopic resolution, we identified that iodide-rich phases primarily segregate at defect-enriched GBs under continuous illumination, causing a more significant local open-circuit voltage (V-OC) decrease than GIs. It also results in short-circuit current density and fill factor losses in both single-junction and perovskite/silicon tandem devices after extended illumination. We show that introducing fluorophenylethylammonium iodide (FPEAI) during film fabrication improves device performance and light stability by suppressing phase segregation and passivating defects, thus preventing GB decreases in V-OC and macroscopic device performance losses.