Additive Engineering for Mixed Lead–Tin Narrow-Band-Gap Perovskite Solar Cells: Recent Advances and Perspectives

Low-cost perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs) of >25% are considered as the most promising replacement for commercial silicon-based solar cells to realize a sustainable future. To break the theoretical PCE limits of single-junction PSCs, all-perovskite tandem solar cells consisting of a narrow-band-gap bottom subcell and a wide-band-gap top subcell have attracted particular attention recently. Mixed Pb–Sn perovskites with narrow band gaps have received great attention as an efficient light harvester in the bottom subcell of all-perovskite tandem solar cells as a result of the reduced toxicity, high light-absorbing capability, and matched current with the wide-band-gap top subcells. However, mixed Pb–Sn narrow-band-gap PSCs suffer from low PCEs, inferior stability, and high open-circuit voltage (Voc) loss, owing to the high defect amount and inferior perovskite film quality induced by the detrimental oxidation of Sn2+ cations and the rapid crystallization of perovskite crystals. Herein, the recent advances about the additive engineering for mixed Pb–Sn narrow-band-gap PSCs are reviewed by demonstrating the origins and unique features of Pb–Sn narrow-band-gap perovskites. Additionally, several strategies to improve PCEs and durability of Pb–Sn narrow-band-gap PSCs through additive engineering are proposed, including Sn2+ cation stabilization, heterojunction construction, crystallization control, surface/grain boundary passivation, film morphology control, carrier dynamics modulation, and gradient-distributed film formation. Furthermore, the existing challenges and future directions are also presented, aiming to provide important insights for designing and developing efficient and stable single-junction narrow-band-gap PSCs and all-perovskite tandem solar cells.