In-depth study of the effect of annealing temperature on the structural, chemical, and optical properties of MAPI thin films prepared by a one-step deposition method

The goal of this work is to study the effect of annealing temperature T_a on the properties of CH₃NH₃PbI₃ (MAPI) thin films deposited on glass substrate through structural, compositional, and optical characterization. The films were obtained by one-step deposition method in which an anti-solvent drip was implemented with some delay during spin coating of the precursor mixture, followed by thermal annealing to promote solvent evaporation and rapid crystallization of the film. Properties of the perovskite film after thermal annealing were characterized by different analytical methods. The morphology and roughness of the films were studied by scanning electron microscopy and atomic force microscopy. The crystalline phase was characterized by X-ray diffraction. The optical properties were also determined by UV–Vis spectroscopy. Finally, the elemental composition was analyzed by secondary ion mass spectroscopy. In the temperature range of 100–160 °C, variable average grain diameters between ~ 200 and ~ 500 nm were obtained. However, higher post-deposition thermal annealing temperatures produce clusters of PbI₂ between CH₃NH₃PbI₃ grain boundaries. XRD measurements showed a decrease in MAPI crystallite size and an increase in PbI₂ crystallite size and amount with increasing annealing temperature. In addition, higher T_a results in a modification of the absorption/reflectance spectra and a red shift of an optical band gap. The results of this study can be useful to produce thin films of MAPI with tunable optical and electronic properties for optimizing the performance of photovoltaic devices.