Chemically suppressing redox reaction at the NiOx/perovskite interface in narrow bandgap perovskite solar cells to exceed a power conversion efficiency of 20%

NiOx as a type of inorganic hole-transporting layer (HTL) material in narrow bandgap perovskite solar cells (NBG PSCs) showed exceptional stability but suffered a considerably poorer performance compared with NBG PSCs with commonly used PEDOT:PSS as the HTL. Herein, we found that redox reactions would occur at the interface between Ni3+ on the NiOx surface and the easily oxidized Sn2+ in the perovskite, causing considerable non-radiative recombination centers. On this basis, we proposed a bifacial reduction strategy at the interface to boost the performance of NBG PSCs. By using a reductive reagent ascorbic acid to reduce the Ni3+/Ni2+ ratio on the surface of NiOx beforehand, the possibility of contact between Ni3+ on the surface of NiOx and perovskite is chemically reduced substantially, suppressing the redox reaction between them as well as the non-radiative recombination at the interface. By applying this strategy, the device's power conversion efficiency is elevated from 17.81% to 20.48%, with 91% remaining after 1128 hours of storage in a nitrogen-filled glovebox.