Improving crystallization and photoelectric performance of CsPbI₂Br perovskite under ambient air via dynamic hot-air assisted recrystallization strategy

Abstract

Organic cations such as methylamine or formamidine in organic-inorganic hybrid perovskites readily reactive with oxygen or water at high temperatures, leading to the final decomposition of the perovskite structure into organic gases and lead iodide. In order to overcome the instability problem of organic cations, researchers have invested in the study of heat-resistant all inorganic CsPbX<sub>3</sub> (X is Cl, Br, I or mixed halide) perovskites. Mixed halide perovskite CsPbI<sub>2</sub>Br, due to its excellent thermal stability and suitable bandgap (1.90 eV), has attracted much attention as a top cell in semi-transparent solar cells and tandem cells. Although high-performance CsPbI<sub>2</sub>Br solar cells could be prepared through glove boxes, this preparation method not only increased costs but also had complex manufacturing processes, unsuitable for low-cost commercial manufacturing. However, CsPbI<sub>2</sub>Br thin films prepared in ambient air were susceptible to humidity, resulting in low coverage, poor crystallization quality, numerous pinholes, and easy transformation into non perovskite phases. To overcome the trouble of pervoksite fabrication in ambient air, one feasible way is dynamic hot-air assisted strategy to reduce the moisture around the films as much as possible. However, the hot air accelerated the evaporation rate of solvent, resulting in a decrease of grain size. In order to improve the issues of 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 were recrystallized 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 has been prepared via DHR strategy. The dynamic hot-air process could 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 resulted in the significantly decrease of grain size (R<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 increased the grain-to-grain contact areas, facilitating complete recrystallization. Thus, the recrystallization process could significantly increase the grain size (R<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 to achieve 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.