Halide Engineering for Mitigating Ion Migration and Defect States in Hot-Cast Perovskite Solar Cells

Precise control of perovskite morphology to achieve large-sized grains with small-width grain boundaries is challenging and a pre-requisite for realizing high photovoltaic performance. Ion migration is another problem with perovskites, which hinders the device results. Herein, the incorporation of MABr into the mixed halide perovskite MAPbClxI3-x has been carried out to minimize ion migration by replacing the mobile I- ions in the perovskite with Br-. Moreover, the introduction of MABr results in high-quality and smooth films with large grains of average size ∼45 μm. An increasing ratio of MABr was added in the perovskite precursor solution, and devices with the architecture FTO/PEDOT:PSS/Perovskite/PCBM/Rhodamine-101/Ag were fabricated using the hot-casting technique. The device with an MABr ratio of 0.1 (0.1MABr) showed a superior power conversion efficiency of >18% compared to ∼14% of the control without any MABr. For higher ratios, the performance deteriorated due to the increase in band gap of the perovskite, which resulted in the reduction of short-circuit current density. For 0.1MABr, minimum defect states and trap density were obtained using various experimental techniques, supporting its optimum device performance. Furthermore, minimum recombination loss took place in the 0.1MABr device. Similarly, the applicability of this technique in large-area devices was studied, and an efficiency of ∼17% was obtained, which is the highest in the literature for devices with 2.5 cm2 dimensions. These innovative concepts and approaches are vital tools to achieve large-area grains for increasing the solar cell performance for commercial applications.