Annealing-induced chemical and structural changes in tri-iodide and mixed-halide organometal perovskite layers

The annealing process is crucial for obtaining high-quality perovskite layers used in highly efficient planar perovskite solar cells. In this study, we have investigated the annealing-induced chemical and structural changes of tri-iodide (TI) and mixed-halide (MH) organometal perovskite layers using infrared absorption spectroscopy, scanning electron microscopy and X-ray diffraction measurements. For TI layers, the solvent molecules, dimethylformamide (DMF), remained in the form of the PbI₂/DMF compound after drying at room temperature. During annealing, the DMF evaporated to form PbI₂ crystals. When the MH perovskite film was annealed, both CH₃NH₃PbCl₃ and CH₃NH₃PbI₃ crystals were initially formed from an amorphous phase. With further annealing, the CH₃NH₃PbI₃ crystals gradually grew through the incorporation of source materials supplied from the CH₃NH₃PbCl₃ crystals and the amorphous phase and the slow evaporation of methylammonium (MA) and chloride ions. The resultant MH perovskite layer after annealing was mainly composed of large CH₃NH₃PbI₃ grains with a trace of chloride ions. We suggest that the difference in composition and structure leads to different charge transport properties of the TI and MH perovskite layers.