Uniform stepped interfacial energy level structure boosts efficiency and stability of CsPbI${}_2$Br solar cells

All-inorganic CsPbI${}_2$Br perovskite has attracted great attention as an absorber for perovskite solar cells (PSCs) due to its excellent thermal and light resistance. However, its device performance is restricted by the large energy level offset between CsPbI${}_2$Br and the most commonly used hole-transporting layer (HTL). Herein, multicarbazolyl-substituted benzonitrile (4t-5CzBn) is inserted into the interface between CsPbI${}_2$Br and HTL to form a uniform stepped (0.24 eV) interfacial energy level structure, which reduces the energy loss and boosts the hole extraction of CsPbI${}_2$Br PSCs. The incorporation of 4t-5CzBn induces the increase in open-circuit voltage and fill factor from 1.256 V and 74.5% to 1.335 V and 82.3%, respectively. The optimized device achieves a power conversion efficiency of 17.34%, which is among the highest reported values of CsPbI${}_2$Br PSCs. Besides the energy level tuning effect, the tert-butyl groups in 4t-5CzBn improve the moisture-resistance of CsPbI${}_2$Br PSCs. The unencapsulated device maintains over 75% of its initial efficiency after 700 h storage in air. These results demonstrate that the rational tuned energy level step benefits the performance improvement of CsPbI${}_2$Br PSCs. (© 2021 Wiley‐VCH GmbH)