Optimizing reaction kinetics of sequential deposition technique for ambient air and solution processed hybrid perovskite thin films

In this work, the reaction kinetics of sequential deposition technique were optimized to produce large area (~ 1 cm²), uniform and pinhole free hybrid CH₃NH₃PbI₃ perovskite (MAPbI) thin films in ambient air; at room temperature and a relatively high humidity level of ~ 50%. The MAPbI thin films were grown upon a thin passivation layer of CH₃NH₃I (MAI) using conventional two step deposition scheme. The films were characterized using Scanning Electron Microscope (SEM), X-ray diffraction (XRD), Electroimpedance Spectroscopy, Current–Voltage and DC-Hall effect measurements. The resultant MAPbI thin films demonstrated negligible amount of unreacted PbI₂ because of the sandwich structure where PbI₂ layer was embedded in-between two MAI layers. Thin films showed a decreased resistance (~ 14 MΩ), increased intrinsic charge carrier concentration (~ 9.3 × 10¹⁵ cm⁻³) and higher charge carrier mobilities (~ 195 cm² V⁻¹s⁻¹). The decrease in resistance and increase in charge carrier mobilities were attributed to the reduced trap sites and grain boundaries in MAPbI thin film front face due to the MAI passivation layer. The increase in charge carrier density was a consequence of MAI passivation layer on the energy levels tuning of MAPbI. The MAPbI thin films also showed promising stability of above 70%; after 8 weeks of ambient air storage and without any encapsulation.