This study presents cesium lead halide perovskites with improved stability for tandem solar cells. The materials were synthesized using dry DMSO and precursors in an inert atmosphere. Films were coated with polystyrene in chlorobenzene, and hole transport layers were spun on ITO-coated glass substrates. Electron transport layers were deposited using PCBM in anhydrous dichlorobenzene. A control sample of (MA)PbI3 was prepared for thermal stability testing. Films were cleaned and treated with UV-ozone before being transferred to a dry-air box. Indium tin oxide (ITO) coated glass substrates were purchased and cleaned similarly. Bathocuproine and aluminum were evaporated for device fabrication. X-ray diffraction patterns were measured using a Panalytical X'Pert Pro Diffractometer. Photoluminescence spectra of CsPb(BrxI1-x)3 were measured at different illumination intensities. At 100 mW/cm², peak position was stable but intensity decreased for 0.2 ≤ x ≤ 0.4. For x = 0.6, the peak shifted to longer wavelengths with increased intensity. For x = 0.8, two initial peaks formed, with a third peak emerging at intermediate wavelengths. For x = 1, peak position was stable and intensity increased. At 1 W/cm², peak position and intensity were stable for x = 0, but decreased for x = 0.33. Scanning electron microscopy images showed pinholes on the device surface. The device architecture was schematically and cross-sectionally illustrated. Steady-state Jsc of a representative device was measured for EQE analysis. References to previous studies on phase transitions and properties of perovskites are included.This study presents cesium lead halide perovskites with improved stability for tandem solar cells. The materials were synthesized using dry DMSO and precursors in an inert atmosphere. Films were coated with polystyrene in chlorobenzene, and hole transport layers were spun on ITO-coated glass substrates. Electron transport layers were deposited using PCBM in anhydrous dichlorobenzene. A control sample of (MA)PbI3 was prepared for thermal stability testing. Films were cleaned and treated with UV-ozone before being transferred to a dry-air box. Indium tin oxide (ITO) coated glass substrates were purchased and cleaned similarly. Bathocuproine and aluminum were evaporated for device fabrication. X-ray diffraction patterns were measured using a Panalytical X'Pert Pro Diffractometer. Photoluminescence spectra of CsPb(BrxI1-x)3 were measured at different illumination intensities. At 100 mW/cm², peak position was stable but intensity decreased for 0.2 ≤ x ≤ 0.4. For x = 0.6, the peak shifted to longer wavelengths with increased intensity. For x = 0.8, two initial peaks formed, with a third peak emerging at intermediate wavelengths. For x = 1, peak position was stable and intensity increased. At 1 W/cm², peak position and intensity were stable for x = 0, but decreased for x = 0.33. Scanning electron microscopy images showed pinholes on the device surface. The device architecture was schematically and cross-sectionally illustrated. Steady-state Jsc of a representative device was measured for EQE analysis. References to previous studies on phase transitions and properties of perovskites are included.