Preparation of planar CH₃NH₃PbI₃ thin films with controlled size using 1-ethyl-2-pyrrolidone as solvent

Recently, planar perovskite solar cells based on CH₃NH₃PbI₃ have attracted many researcher's interest due to their unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction comparing to the initial mesoporous structure. However, the preparation of planar perovskite films with high quality is still in challenge. In this paper, we developed a vapor-assisted solution process using a novel and green solvent of 1-Ethyl-2-pyrrolidone (NEP) instead of the traditional N, N-dimethylformamide (DMF) to construct a high-quality perovskite CH₃NH₃PbI₃ thin film with pure phase, high compactness, small surface roughness and controlled size. The phase evolution and growth mechanism of the perovskite films are also discussed. Utilizing the NEP of low volatility and moderate boiling point as solvent, we dried the PbI₂-NEP precursor films at different temperature under vacuum and then obtained PbI₂ thin films with different crystalline degree from amorphous to highly crystalline. The perovskite films with crystal size ranged from hundreds of nanometers to several micrometers can be prepared by reacting the PbI₂ films of different crystalline degree with CH₃NH₃I vapor. Moreover, planar-structured solar cells combining the perovskite film with TiO₂ and spiro-OMeTAD as the electron and holes transporting layer achieves a power conversion efficiency of 10.2%. - Highlights: • A novel and green solvent of 1-Ethyl-2-pyrrolidone (NEP) was used to construct high-quality perovskite CH₃NH₃PbI₃ thin film. • The CH₃NH₃PbI₃ grain with different sizes ranged from hundreds of nanometers to several micrometers can be obtained. • Planar-structured perovskite CH₃NH₃PbI₃ solar cells using NEP as solvent achieves a power conversion efficiency of 10.2%.