Template-Assisted Formation of High-Quality α-Phase HC(NH₂)₂PbI₃ Perovskite Solar Cells

Formamidinium (FA) lead halide (α-FAPbI₃) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure α-FAPbI₃ phase because of its easy transformation into a nondesirable δ-FAPbI₃ phase. In the present study, a "perovskite" template (MAPbI₃-FAI-PbI₂-DMSO) structure is used to avoid and suppress the formation of δ-FAPbI₃ phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI-PbI₂-DMSO film with FAI before annealing. In situ X-ray diffraction in vacuum shows no detectable δ-FAPbI₃ phase during the whole synthesis process when the sample is annealed from 100 to 180 °C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much-enhanced short-circuit current density (  J _sc) of 24.99 mA cm^-2 and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI₃, with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.