All‐Perovskite Multi‐Junction Solar Cells: The Rising Star of Ultrahigh‐Efficiency Photovoltaics

All‐perovskite multi‐junction solar cells, integrating multiple light absorbers with complementary bandgaps, provide an effective pathway to break through the power conversion efficiency (PCE) limits of single‐junction photovoltaics. However, they are still in the infancy stage. Several critical challenges include the facile oxidation of Sn 2+ and difficult‐to‐control crystallization dynamics in narrow‐bandgap perovskites, as well as significant open‐circuit voltage deficits in wide‐bandgap perovskites. These intrinsic limitations result in experimental PCEs far below their theoretical values, highlighting the urgent need for a systematic review in this field. Herein, research progress are comprehensively summarized in all‐perovskite multi‐junction solar cells, elucidating the underlying mechanisms of performance losses. Furthermore, various optimization strategies are critically discussed for perovskite absorbers, charge transport materials, and interconnecting layers. Finally, future perspectives for this emerging photovoltaic technology are presented.