The Possible Side Reaction in the Annealing Process of Perovskite Layers.

The high purity of light harvesting layers is one of the core issues for highly efficient perovskite solar cells. The perovskite precursor solution and the crystallization growth of thin films have been extensively studied in the past few years. Herein, we have unveiled some side reactions that occur during the evaporation of the residual solvent in the spin-coated films at elevated temperature, forming N-methyl formamidium iodide and N,N'-dimethyl formamidium iodide. Such side reactions will consume the precursor materials and then produce a secondary phase in the perovskite films, which is detrimental for the performance improvement. We have also found that a combination of room temperature aging and vacuum treatment of the spin-coated wet film is conducive to eliminate the side reactions and improve the perovskite phase purity, reaching an efficiency of 20.98%.