The supplementary materials for the paper "Efficient luminescent solar cells based on tailored mixed-cation perovskites" include various figures and tables that support the main findings. Figure S1 shows independent certification from Newport Corporation confirming power conversion efficiencies (PCEs) of 19.90% (backward scan) and 19.73% (forward scan), along with normalized electroluminescence quantum efficiency. Figure S2 presents photographs of two real devices, highlighting the active area, high reflectivity of the gold electrode, and the opaque appearance of the perovskite film. Figure S3 shows a histogram of solar cell efficiencies for 40 cells with the optimized PbI₂/FAI ratio of 1.05. Figure S4 presents an initial stability test of PSCs sealed with epoxy and stored in a desiccator in the dark. Figure S5 shows absorption spectra of perovskite films on a TiO₂/FTO substrate with varying PbI₂/FAI ratios. Figure S6 and S7 show top-view and XRD patterns of perovskite films with varying PbI₂/FAI ratios. Figure S8 shows normalized (001) peaks of PbI₂ phase with varying PbI₂/FAPbI₃ fractions. Figure S9 shows cross-sectional SEM images of perovskite films with varying PbI₂/FAI ratios. Figure S10 shows external electroluminescence quantum efficiency as a function of injection current for the device with PbI₂/FAI = 1.16. Figure S11 shows normalized PL spectra of perovskite films with varying PbI₂/FAI ratios. Figure S12 shows PL decay of perovskite films with varying PbI₂/FAI ratios. Table S1 provides photovoltaic parameters for PSCs measured using forward and backward scans. Table S2 provides photovoltaic parameters for PSCs stability under AM 1.5 G illumination. Table S3 provides composition of perovskite composite film determined by Rietveld refinement. The data was calibrated by Newport Corporation according to ISO 17025 standards, with uncertainties expanded using a coverage factor of δ = 2. The performance parameters reported apply only at the time of the test. Hysteresis was observed, and the final scan rate was 0.11 V/s.The supplementary materials for the paper "Efficient luminescent solar cells based on tailored mixed-cation perovskites" include various figures and tables that support the main findings. Figure S1 shows independent certification from Newport Corporation confirming power conversion efficiencies (PCEs) of 19.90% (backward scan) and 19.73% (forward scan), along with normalized electroluminescence quantum efficiency. Figure S2 presents photographs of two real devices, highlighting the active area, high reflectivity of the gold electrode, and the opaque appearance of the perovskite film. Figure S3 shows a histogram of solar cell efficiencies for 40 cells with the optimized PbI₂/FAI ratio of 1.05. Figure S4 presents an initial stability test of PSCs sealed with epoxy and stored in a desiccator in the dark. Figure S5 shows absorption spectra of perovskite films on a TiO₂/FTO substrate with varying PbI₂/FAI ratios. Figure S6 and S7 show top-view and XRD patterns of perovskite films with varying PbI₂/FAI ratios. Figure S8 shows normalized (001) peaks of PbI₂ phase with varying PbI₂/FAPbI₃ fractions. Figure S9 shows cross-sectional SEM images of perovskite films with varying PbI₂/FAI ratios. Figure S10 shows external electroluminescence quantum efficiency as a function of injection current for the device with PbI₂/FAI = 1.16. Figure S11 shows normalized PL spectra of perovskite films with varying PbI₂/FAI ratios. Figure S12 shows PL decay of perovskite films with varying PbI₂/FAI ratios. Table S1 provides photovoltaic parameters for PSCs measured using forward and backward scans. Table S2 provides photovoltaic parameters for PSCs stability under AM 1.5 G illumination. Table S3 provides composition of perovskite composite film determined by Rietveld refinement. The data was calibrated by Newport Corporation according to ISO 17025 standards, with uncertainties expanded using a coverage factor of δ = 2. The performance parameters reported apply only at the time of the test. Hysteresis was observed, and the final scan rate was 0.11 V/s.