Inherent Stability Challenges of Ultrawide‐Bandgap Halide Perovskite for Multijunction Solar Cells

Ultrawide‐bandgap (UWBG, E g ≥ 1.8 eV) perovskites serve as indispensable top‐cell absorbers in perovskite–perovskite–silicon (PPS) triple‐junction (TJ) configurations and play a decisive role in approaching the theoretical efficiency limit of approximately 49.6%. Halide mixing, such as incorporation of Br − or Cl − , is a conventional approach to widen the bandgap; however, it causes thermodynamic instabilities due to their high mixing enthalpies and pronounced lattice distortion. While such instabilities are also observed in conventional wide‐bandgap (WBG, E g ≥ 1.65 eV) perovskites, they are significantly exacerbated in UWBG compositions, leading to accelerated light‐induced phase segregation, lattice strain, acceleration of ion migration, and generation of nonradiative recombination sites. This review focuses on the thermodynamic, structural, and interfacial origins of instability in UWBG perovskites and systematically analyzes the stabilization strategies proposed to overcome these limitations. It further discusses how these strategies are implemented in perovskite–perovskite–silicon (PPS) triple‐junction solar cells (TJSCs) to enhance device performance and operational stability and outlines future research directions toward realizing high‐efficiency, long‐term‐stable UWBG top cells and PPS TJSCs.