2, National Renewable Energy Laboratory, Golden, Colorado, United States
Perovskite solar cells based on organic-inorganic materials, which have the perovskite crystal structure, present superb power conversion efficiency (PCE) exceeding 22%. Demonstration of the high efficiency devices in a short time is in general attributed to developing of various film formation methods. One of the well-known processes for the film formation is that pouring an anti-solvent during spin-coating of a perovskite material to make an intermediate film which turns to the final perovskite film. This film formation method makes it possible to make high quality methylammonium lead iodide (MAPbI3) thin films with well-crystallized large grains and uniform film thicknesses. It is known that intermediate phase formers should have lone pair electrons to easily participate in forming the Lewis adduct. In this work, we demonstrate that N-methyl-2-pyrrolidone or trimethyl phosphate with methylammonium and lead iodide make intermediate films, and finally form high quality MAPbI3 films. Dimethyl sulfuroxide, N-methyl-2-pyrrolidone, and trimethyl phosphate form different intermediate phases, and finally transform to smooth perovskite films with well-crystallized grains. Perovskite solar cells based on the three different films were fabricated, and the performances of the cells as well as the film formation principles were investigated.