The inferior crystallinity and phase stability of CsPbI2Br films have severely hindered the development of carbon-based, all-inorganic perovskite solar cells (PSCs). Herein, we demonstrate the preparation of CsPbI2Br films by the top-seeded solution growth (TSSG) technique. It is performed through spin-coating of CH3NH3Br (MABr) atop CsPbI2Br precursor film prior to annealing, during which perovskite seeds are generated atop it. These perovskite seeds not only serve as nuclei to regulate the growth of CsPbI2Br grains but also provide additional Br– anions to generate a thin Br-rich layer atop the final CsPbI2Br film. The former contributes to the formation of CsPbI2Br film with full coverage, larger grains, higher crystallinity, and fewer electronic defects, while the latter gives rise to residual compressive strain along the film and thus markedly boosts its phase stability. Consequently, the optimized carbon-based, all-inorganic PSC exhibits a much better efficiency of 14.84% coupled with favored storage and operational stability.

All-inorganic perovskite CsPbIBr2 materials are promising for optoelectronics, owing to their upgraded ambient stability and suitable bandgap. Unfortunately, they generally undergo severe halide phase segregation under illumination, which creates many iodide-rich and bromide-rich domains coupled with significant deterioration of their optical/electrical properties. Herein, we propose a facile and effective strategy to overcome the halide phase segregation in the CsPbIBr2 film by modifying its crystalline grains with poly­(methyl methacrylate) (PMMA) for the first time. Such a strategy is proceeded by covering a PMMA layer on the substrate before deposition of the CsPbIBr2 film. Further investigations manifest that the CsPbIBr2 film with PMMA possesses larger grains, better crystallinity, and fewer traps than the one without any modification. Moreover, it holds the nearly eliminated halide phase segregation. Therefore, the carbon-based, all-inorganic CsPbIBr2 perovskite solar cell exhibits the much suppressed photocurrent hysteresis, coupled with an outstanding efficiency of 9.21% and a high photovoltage of 1.307 V.

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Halide exchange offers a versatile way to modify the properties of halide perovskites, but it is particularly challenging to slow the reaction rate to restrain defect growth in the products. Herein, we propose a slow halide exchange strategy to simultaneously fine-tune the optical and microstructural characteristics of CsPbIBr 2 films by physically pairing CsPbIBr 2 and CH 3 NH 3 PbI 3 films. Once a proper heating treatment is applied, halide exchange of Br − and I − ions between the films is activated, and the reaction rate can be well-controlled by the heating recipe, in which a high temperature can accelerate the exchange reaction, while a low temperature slows or stops it. By using an optimal halide exchange temperature (110°C) and time (2 h), the parent CsPbIBr 2 film was transformed into high-quality CsPbI 1+x Br 2−x film, featuring an extended absorption onset from 590 to 625 nm, coarsened grains, improved crystallinity, reduced surface roughness, suppressed halide phase segregation, and identical stability to the pristine film. Accordingly, the efficiency of a carbon-based, all-inorganic perovskite solar cell (PSC) was boosted to 10.94%, which was much higher than that of the pristine CsPbIBr 2 film (8.21%). The CsPbI 1+x Br 2−x PSC also possessed excellent tolerance against heat and moisture stresses.