Small Molecule Passivation Leading to Efficient Hole Transport Layer‐Free Sn–Pb Mixed Perovskite Solar Cells with High Open‐Circuit Voltage

There have been a number of remarkable signs of progress achieved in tin–lead mixed narrow‐bandgap perovskite solar cells (PSCs) due to the high theoretical power conversion efficiency (PCE) and their promising application in tandem devices. Indeed, Sn–Pb mixed PSCs without a hole transport layer (HTL) also have been more attractive owing to lower cost and simplification of the device structure. However, the defects in perovskite film introduced by Sn 2+ oxidation severely restrict device efficiency and stability.Herein, a small organic molecule, 4,4'‐sulfonyldiphenol, is employed to passivate perovskite ( E g = 1.26 eV) surface to decrease the interfacial defects and suppress the nonradiative carrier recombination. Furthermore, by regulating energy‐level alignment, charge carrier extraction is greatly facilitated. The device performance is significantly enhanced in that the champion PCE is enlarged to 21.43% with an open‐circuit voltage ( V oc ) of 0.876 V from only 18.02% with a V oc of 0.770 V. The stability of unencapsulated devices is improved substantially as well while retaining 80% PCE of its initial value after being stored in the glovebox for around 600 h. This facile but highly effective strategy successfully proposes the promising development of HTL‐free Sn–Pb mixed PSCs.