Surface n-type band bending for stable inverted CsPbI₃ perovskite solar cells with over 20% efficiency

Inverted CsPbI₃ perovskite solar cells offer merits for tandem and indoor photovoltaics. Compared to the regular structure (n-i-p), realizing high-efficiency inverted devices is still challenging due to severe energy losses at the contact of the p-type perovskite and n-type electron transporting layer (ETL). Herein, we develop a surface n-type band bending strategy to mitigate such energy loss. It is discovered that the post-deposition of propylamine hydrochloride induces a p to n-type transition at the CsPbI₃ surface to accelerate the separation and extraction of the electrons while passivating the surface defects. Consequently, the power conversion efficiency (PCE) of inverted CsPbI₃ devices reaches record-high values of 20.17% (19.67% on average) under 1-sun equivalent illumination and 38.93% under indoor LED light (1000 lux, 307 μW cm⁻²). Moreover, such inverted devices maintain ∼97% PCE after light soaking for 792 h at the open circuit condition while the control samples degrade to ∼11% in 552 h.