A Facile Way to Form a Graded Halide Layer in Perovskite Solar Cells

A light harvest layer composed of gradual change from formamidinium lead triiodide (FAPbI₃) to methylammonium lead triiodide (MAPbI₃) was fabricated using a novel two-step process. That is, a graded halide layer structure without extra processing steps is demonstrated. Conventionally, in the fabrication of MAPbI₃ perovskite solar cells (PSCs) using two-step process, PbI₂ layer was the first deposited on a mesoporous TiO₂ coated substrate. The methylammonium iodide (MAI) solvent was then spin-coated on the surface of PbI₂ layer and heated to form the MAPbI₃ perovskite layer. Double perovskite layers such as FAPbI₃ plus MAPbI₃ requires twice of the second step which FAI and MAI should be spin-coated individually. This can be tedious and time consuming. We report here a facile way to form a graded perovskite layer, consisting FAPbI₃ to MAPbI₃, in a single step. FAI was first added into dimethylformamide (DMF) solution that was used to form PbI₂ layer, then MAI solution was dripped on top of the FAI/PbI₂ layer. The graded perovskite layer structure (FAPbI₃/MAPbI₃) in a gradient manner are readily formed, where the structure is confirmed by EDS to be FTO/compact TiO₂/mesoporous TiO₂/FAPbI₃(thin)/MAPbI₃/Spiro-OMeTAD/Ag. The J_sc, and V_oc of solar cells with this graded perovskite layer are enhanced and the efficiency increases from 11.62% to 14.06%.