Control of the morphology of PbI₂ films for efficient perovskite solar cells by strong Lewis base additives

A two-step method is widely used to fabricate highly efficient perovskite solar cells. In a typical two-step method, CH₃NH₃PbI₃ perovskites are fabricated from PbI₂ films through an intercalation reaction between PbI₂ and CH₃NH₃I, which usually have a rough surface and residual PbI₂ due to a compact PbI₂ structure. Here, we introduce some strong Lewis bases into PbI₂/DMF solutions in the first step, which is helpful in controlling the morphology of PbI₂ films with a mesoporous structure. The mesopores in the PbI₂ films provide not only space for accommodating the volume expansion during the reaction between PbI₂ and CH₃NH₃I, but also channels for CH₃NH₃I solution to diffuse into the PbI₂ films, which helps eliminate the residual PbI₂ and the dissolution and recrystallization process. As a result, smooth perovskite films without residual PbI₂ are easily obtained. The power conversion efficiency, stability and reproducibility of the perovskite solar cells fabricated from the optimized mesoporous PbI₂ films improved simultaneously due to the improvement in perovskite films. This work reveals the key roles of solvents (boiling point, Lewis basicity) in controlling the microstructure of the PbI₂ films.