CONVERSION OF PbS THIN FILMS TO PbI₂ AND SUBSEQUENTLY TO MAPbI₃ PEROVSKITE

Methylammonium lead halide perovskites (MAPbI₃) are rapidly emerging as promising materials for high efficiency optoelectronic applications due to their large absorption coefficient over high spectral range, high charge carrier mobility, small exciton binding energies and long exciton diffusion length ^[1]. However, stability and long-term performance under operating conditions have been the main issues that need to be addressed before commercial perovskite-based solar cells could be introduced ^[2]. It has been previously reported that there is a significant effect of the preparation process on the morphology and photovoltaic properties of the resultant perovskite ^[3]. This research shows successful conversion of PbS thin films to PbI₂ and afterwards to MAPbI₃, and focuses on the effect of the conversion parameters on the perovskite films.

A two-stage conversion process was performed: First, PbS thin films with a (100) columnar morphology were treated with polyiodide (iodine and potassium iodide) solutions, resulting in the formation of PbI₂ films with different grain size. The columnar morphology was chosen due to the high density of grain boundaries, which facilitates the conversion process. Finally, the PbI₂ films were immersed in a solution of methylammonium iodide (MAI) dissolved in isopropanol. This procedure resulted in two perovskite films with different grain sizes.

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2. Jacobsson, T. J. et al. Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using x-ray diffraction. Inorganic chemistry 54.22 (2015): 10678-10685.
3. Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells.Nature499.7458 (2013): 316-319.