Phase degradation of all-inorganic perovskite CsPbI 2 Br films induced by a p-type CuI granular capping layer

It is necessary to evaluate the interactions between the different functional layers in optoelectronic devices to optimize device performance. Recently, the I-rich all-inorganic perovskite CsPbI 2 Br has attracted tremendous attention for use in solar cell applications because of its suitable band gap and favorable photo and thermal stabilities. It has been reported that the undesirable phase degradation of the photoactive a phase CsPbI 2 Br to the non-perovskite δ phase could be triggered by high humidity. To obtain stable devices, it is thus important to protect CsPbI 2 Br from moisture. In this paper, CuI, a non-hygroscopic p-type hole-transporting material, is found to induce the phase degradation of α-CsPbI 2 Br to the δ-CsPbI 2 Br. The rate and extent of phase degradation of CsPbI 2 Br are closely associated with the heating temperature and coverage of a CuI granular capping layer. This discovery is different from the widely reported water-induced phase degradation of CsPbI 2 Br. Our work highlights the importance of careful selection of hole-transporting materials during the processing of I-rich all-inorganic CsPbX 3 (X=Br, I) perovskites to realize high-performance optoelectronic devices.