Improving crystallization and photoelectric performance of CsPbI<sub>2</sub>Br perovskite under ambient air via dynamic hot-air assisted recrystallization strategy

Abstract

CsPbI<sub>2</sub>Br thin films prepared in ambient air are susceptible to humidity, resulting in low coverage, poor crystallization quality, numerous pinholes, and easy transformation into non perovskite phases. To overcome the troubles of pervoksite fabrication in ambient air, a feasible way is to reduce the moisture around the films as much as possible according to dynamic hot-air assisted strategy. However, the hot air accelerates the evaporation rate of solvent, resulting in the decrease of grain size. In order to improve the crystal growth and long-term stability in dynamic hot-air assisted strategy, in this work, we present a dynamic hot-air assisted recrystallization (DHR) strategy to prepare high-quality CsPbI<sub>2</sub>Br thin films in ambient air (i.e. the CsPbI<sub>2</sub>Br thin films prepared via dynamic hot-air strategy are recrystallized by using a green solvent (methylamine acetate) with high viscosity coefficient). Under ambient air with high humidity (RH＞60%), the CsPbI<sub>2</sub>Br thin film with high coverage, (100) preferred orientation, large average grain size, and stable structure is prepared via DHR strategy. The dynamic hot-air process can effectively reduce the moisture around the film and increase the nucleation sites in the precursor solution, thereby improving the coverage of the film. However, this process inevitably results in the significant decrease of grain size (<i>R</i><sub>ave</sub>= 0.32 μm) (i.e. more grain boundaries), exacerbating non-radiative recombination of carriers associated with trap states at these boundaries. The high coverage increases the grain-to-grain contact area, facilitating complete recrystallization. Thus, the recrystallization process can significantly increase the grain size (<i>R</i><sub>ave</sub> = 2.63 μm) and obtain a (100) preferred orientation (<i>I</i><sub>(110)</sub>/<i>I</i><sub>(200)</sub> = 0.006), resulting in high photoluminescence intensity and long fluorescence lifetime (118 ns). The unencapsulated CsPbI<sub>2</sub>Br perovskite solar cell (PSC) optimized via DHR strategy with low hysterescence factor (2.34%) and high repeatability exhibits a high power conversion efficiency (PCE = 17.55%), which is higher than those of most CsPbI<sub>2</sub>Br PSCs prepared in ambient air and gloveboxes previously reported. Moreover, the unencapsulated CsPbI<sub>2</sub>Br PSC possesses an excellent storage stability under ambient air with high humidity (RH > 60%), remaining 96% of the original PCE after aging 40 days. This provides a promising approach for achieving high-performance and long-term stable CsPbI<sub>2</sub>Br films under ambient air with high humidity, which is expected to promote the commercialization process of perovskite/silicon tandem cells and semi-transparent devices.