The supporting information for the article "Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells" provides detailed experimental protocols and additional data to support the main findings. The materials section outlines the preparation of solutions for CsPb(BrₓI₁₋ₓ)₃ perovskites, including the use of high-purity precursors and the deposition processes. The XRD data section describes the measurement setup and conditions used for X-ray diffraction analysis. The photoluminescence (PL) measurements section details the changes in PL spectra over time at different illumination intensities and compositions of CsPb(BrₓI₁₋ₓ)₃. The device architecture section includes SEM images of the device structure and cross-sectional views. The steady-state Jsc section provides data on the short-circuit current density (Jsc) of representative devices used for external quantum efficiency (EQE) measurements. The references section lists key studies on the phase transitions, structural properties, and optoelectronic characteristics of cesium lead halide perovskites.The supporting information for the article "Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells" provides detailed experimental protocols and additional data to support the main findings. The materials section outlines the preparation of solutions for CsPb(BrₓI₁₋ₓ)₃ perovskites, including the use of high-purity precursors and the deposition processes. The XRD data section describes the measurement setup and conditions used for X-ray diffraction analysis. The photoluminescence (PL) measurements section details the changes in PL spectra over time at different illumination intensities and compositions of CsPb(BrₓI₁₋ₓ)₃. The device architecture section includes SEM images of the device structure and cross-sectional views. The steady-state Jsc section provides data on the short-circuit current density (Jsc) of representative devices used for external quantum efficiency (EQE) measurements. The references section lists key studies on the phase transitions, structural properties, and optoelectronic characteristics of cesium lead halide perovskites.