The energy level mismatch and unfavorable interfacial reaction between nickel oxide (NiOx) and organic cations of the perovskite are the main factors affecting the power conversion efficiency (PCE) of inverted perovskite solar cells. Herein, interlayer engineering via dual-functional alkaline hypophosphite is introduced between the nickel oxide hole transport layer (HTL) and the perovskite layer. The introduction of alkaline hypophosphite deepens the valence band maximum of NiOx, which better aligns the energy band of perovskite solar cells with the best energy level matching using sodium hypophosphite. In addition, the deposition of alkaline sodium hypophosphite on NiOx reduces the proportion of Ni3+ and slows down the accelerated degradation of the perovskite layer caused by the reaction of excess Ni3+ with organic cations in the perovskite. The open-circuit voltage (VOC) of devices treated with sodium hypophosphite increased from 1.01 to 1.08 eV with a power conversion efficiency of 19.92%. Moreover, the unencapsulated devices retained 80% efficiency for 1000 hours under ambient conditions (50-60% relative humidity), and demonstrate a feasible and effective strategy for the fabrication of efficient and air-stable inverted perovskite solar cells.
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