Acetate-Based Crystallization Kinetics Modulation of CsPbI₂Br for Improved Photovoltaic Performance

Inorganic CsPbI₂Br perovskite has emerged to be a promising candidate for photovoltaic materials, while developing additive engineering strategies to address the issues of defect induced crystalline phase transformation from a photoactive perovskite phase to a nonperovskite phase has been a challenge for achieving their outstanding optoelectronic properties and thermal stability. In this work, we demonstrate an effective acetate mediation strategy for significantly retarding the crystallization process and thus the phase transformation, which leads to a much improved optoelectronic performance for the solar cells. It was demonstrated that the addition of an acetate like cobalt(II) acetate (Co(Ac)₂) or zinc acetate (Zn(Ac)₂) could lead to a significantly retarded crystallization process for the perovskite films, as well as the formation of the flat, dense, and defect-free perovskite film. We propose that the perovskite crystal grows following the Ostwald ripening mechanism at an annealing temperature of 270 °C, during which the acetate in the perovskite film would be squeezed from the boundaries by grains growing because the acetate cannot grow through the crystalline material. On the basis of the proposed strategy of crystallization kinetic modulation with acetate, the photon conversion efficiency (PCE) of the champion device with the acetate modified perovskite film can reach as high as 15.04%, and with a superb fill factor of 80.46%. We thus believe that our work could provide an effective alternative for the design of high-performance inorganic CsPbI₂Br perovskite solar cells.