Ethanol-based green-solution processing of α-formamidinium lead triiodide perovskite layers

The use of non-toxic or less-toxic solvents in the mass production of solution-processed perovskite solar cells is essential. However, halide perovskites are generally not completely soluble in most non-toxic solvents. Here we report the deposition of dense and uniform α-formamidinium lead triiodide (α-FAPbI3) films using perovskite precursor solutions dissolved in ethanol-based solvent. The process does not require an antisolvent dripping step. The combination of a Lewis base, such as dimethylacetamide (or dimethylsulfoxide), and an alkylammonium chloride (RNH3Cl) in ethanol results in the stable solvation of FAPbI3. The RNH3Cl added to the FAPbI3 precursor solution is removed during spin-coating and high-temperature annealing via iodoplumbate complexes, such as PbI2·RNH2 and PbI2·HCl, coordinated with dimethylacetamide (or dimethylsulfoxide). It is possible to form very dense and uniform α-FAPbI3 perovskite films with high crystallinity by combining several types of RNH3Cl. We obtain power conversion efficiencies of 24.3% using a TiO2 electrode, and of 25.1% with a SnO2 electrode.