Simultaneous dual-interface and bulk defect passivation for high-efficiency and stable CsPbI2Br perovskite solar cells

All-inorganic CsPbI2Br is a promising candidate to produce the optimized balance between efficiency and stability. Unfortunately, the CsPbI2Br perovskite films prepared by solution methods often show numerous defects on the grain surfaces and a tendency for phase transition. In this work, we discovered that the proper amount of NiI2 additive enables Ni2+ doping in the perovskite interstitial lattice, while additional Ni2+ ions serve as passivation agents on the grain surfaces; in particular, a profiled distribution appears from the film bulk to both the upper and lower surfaces. The special Ni2+ distribution results in an optimized TiO2 surface for perovskite growth, better perovskite film quality, superior charge extraction capability, and effective suppression of interfacial recombination. As a result, the CsPbI2Br perovskite solar cell (PSC) efficiency is increased to 15.88%, among the highest for its type. Also, the special Ni2+ distribution endows the PSC with improved moisture tolerance. This work provides a promising strategy to overcome the surface/bulk instability issues common in PSCs.