Fabrication of mixed perovskite organic cation thin films via controllable cation exchange

Here in this paper, we demonstrate a facile technique for creating the mixed formamidinium(HN = CHNH_3~+, FA~+)and methylammonium(CH_3NH_3~+, MA~+) cations in the lead iodide perovskite. This technique entails a facile drop-casting of formamidinium iodide(FAI) solutions on as-prepared MAPbI_3 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.