N-methyl-2-pyrrolidone as an Excellent Coordinative Additive with a Wide Operating Range for Fabricating High-Quality Perovskite Films

We demontrate in this work that N-methyl-2-pyrrolidone (NMP) is a superior alternative to dimethyl sulfoxide (DMSO) as an additive in precursor solution for fabrabricating high-quality pervoskite films for solar cells. While the DMSO-assisted method always requires the films to be prepared with a narrow and restricted PdI2:DMSO ratio, NMP provides a much wider window of PbI2/NMP ratio for preparing high-quality perovskite films. The systematic studies on the structures of intermediated obtained with different PbI2/NMP ratios reveal that only one kind of intermediate forms regardless of the Pb2+/NMP ratio between 1 to 4, which is totally different from the situation of PbI2/DMSO system. But the perovskite solar cell devices fabricated with different Pb2+/NMP ratios all display similar performances to the counterpart fabricated with 1:1 Pb2+/DMSO. Besides the MAPbI3 (MA = CH3NH3+) system, NMP is demonstrated as an excellent coordinative additive for the fabrication of high-quality films of triple-cation (Cs+/ formamidinium /MA) perovskite .