Improved Carrier Management via a Multifunctional Modifier for High‐Quality Low‐Bandgap Sn–Pb Perovskites and Efficient All‐Perovskite Tandem Solar Cells

Abstract All‐perovskite tandem solar cells (TSCs) hold great promise in terms of ultrahigh efficiency, low manufacturing cost, and flexibility, stepping forward to the next‐generation photovoltaics. However, their further development is hampered by the relatively low performance of low‐bandgap (LBG) tin (Sn)–lead (Pb) perovskite solar cells (PSCs). Improving the carrier management, including suppressing trap‐assisted non‐radiative recombination and promoting carrier transfer, is of great significance to enhance the performance of Sn–Pb PSCs. Herein, a carrier management strategy is reported for using cysteine hydrochloride (CysHCl) simultaneously as a bulky passivator and a surface anchoring agent for Sn–Pb perovskite. CysHCl processing effectively reduces trap density and suppresses non‐radiative recombination, enabling the growth of high‐quality Sn–Pb perovskite with greatly improved carrier diffusion length of >8 µm. Furthermore, the electron transfer at the perovskite/C 60 interface is accelerated due to the formation of surface dipoles and favorable energy band bending. As a result, these advances enable the demonstration of champion efficiency of 22.15% for CysHCl‐processed LBG Sn–Pb PSCs with remarkable enhancement in both open‐circuit voltage and fill factor. When paired with a wide‐bandgap (WBG) perovskite subcell, a certified 25.7%‐efficient all‐perovskite monolithic tandem device is further demonstrated.