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.