Stable wide-bandgap perovskite solar cells for tandem applications

Tandem solar cells (TSCs) are an effective device architecture for surpassing the Shockley-Queisser (SQ) limit of single-junction solar cells. Owing to their excellent optoelectronic properties and solution processability, wide-bandgap (Eg ≥ 1.60 eV) perovskites are a unique class of top-cell materials for tandem applications. They can be paired with a range of bottom-cell materials such as Si, narrow-gap perovskites, organic semiconductors, and CIGS to create high-efficiency, low-cost TSCs. Notably, perovskite/Si and perovskite/CIGS TSCs have already outperformed their single-junction counterparts. However, the operational lifetimes of perovskite-based monolithic TSCs are generally limited by the instability of the wide-bandgap perovskites, hampering the progress toward commercial applications. In this review, we critically examine the recent advances in tackling the stability challenge of wide-bandgap perovskite cells for tandem applications. Factors affecting the stability of wide-bandgap perovskite materials, exemplified using organic-inorganic mixed-halide and all-inorganic perovskites, are analyzed with an emphasis on phase segregation and transition. These are accompanied by a discussion of possible strategies for improving the operational stability of perovskite-based monolithic TSCs. Finally, we outline the challenges ahead and opportunities in this direction, with an aim to broaden the horizons of designing stable wide-bandgap perovskite cells for tandem applications.