The bandgap tunability of organic–inorganic hybrid perovskite materials makes them ideal light absorbers in tandem configuration, enabling breakthroughs beyond the theoretical efficiency limit of single-junction solar cells. Representatively, the monolithic perovskite/silicon tandem solar cell has achieved an astounding recorded efficiency of 34.6%. As the top cell absorbers in tandem solar cells, wide-bandgap (WBG) perovskites obtained by on-demand regulation of the iodide–bromine (I–Br) ratio, are responsible for capturing high-energy photons to reduce thermalization losses. However, typical I–Br WBG perovskites suffer severe intrinsic instability, primarily due to halide segregation when exposed to light and/or heat. The instability compromises both the efficiency and durability of tandem devices, posing a major barrier to technology commercialization. In this short review, we first provide a classified overview of recent efforts aimed at inhibiting halogen segregation in WBG perovskites, with a particular focus on the often-overlooked issue of cationic segregation. Furthermore, we highlight that stability could be enhanced by strengthening the interactions between anionic and cationic components and/or employing new constituents in the material formula beyond the I/Br system. Finally, we discuss the primary challenges and offer perspectives on advancing stable WBG perovskites for efficient and stable tandems.

Cu(In_1−xGa_x)Se₂ (CIGS) is a promising and ideal material for bottom cell in tandem solar cells, which can break the double junction solar cell’s Shockley–Queisser theoretical efficiency to above 40%. However, the high-efficiency CIGS solar cells deposited by the 3-stage process is normally double grading, leading to an incomplete absorption in the bottom cells. In this study, single bandgap grading and quasi-flat bandgap CIGS solar cells are proposed and fabricated for perovskite/CIGS 4-terminal tandem solar cells, which are more favorable for long-wavelength absorption and higher short-circuit current density J_sc. Various characterizations have been performed to investigate the crystallization, crystal defects, composition depth profile, and carrier dynamics of the CIGS thin films. Our study reveals that the performance of CIGS solar cells with high Ga content is worse than expected. Using bandgap engineering, we can obtain CIGS solar cell with an efficiency above 16.5% regardless of the GGI [Ga/(Ga + In)] varying from 0.27 to 0.40. However, CIGS solar cells with less Ga content and low bandgap exhibit superior long-wavelength spectral response, making them more suitable for bottom cell applications in tandem solar cells. In combination with an 18.9% semitransparent inorganic perovskite solar cell, a 25.6% perovskite/CIGS tandem device in 4-T configuration is demonstrated.