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.