Abstract Here in this paper, we demonstrate a facile technique for creating the mixed formamidinium (HN=CHNH3+, FA+) and methylammonium (CH3NH3+, MA+) cations in the lead iodide perovskite. This technique entails a facile drop-casting of formamidinium iodide (FAI) solutions on as-prepared MAPbI3 perovskite thin films under the controlled conditions, which leads to controllable displacement of the MA+ cations by FA+ cations in the perovskite structure at room temperature. Uniform and controllable mixed organic cation perovskite thin films without a “bi-layered” or graded structure are achieved. By applying this approach to photovoltaic devices, we are able to improve the performances of devices through extending their optical-absorption onset further into the infrared region to enhance solar-light harvesting. Additionally, this work provides a simple and efficient technique to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials for high-performance perovskite solar cells.
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