Quantitative analysis of FAI-diffusion in sequentially evaporated FAPbI₃ perovskite thin films

Physical vapor deposition of perovskite solar cells is gaining increasing importance due to its up-scaling potential applicability for the industrial manufacturing. Especially the layer-by-layer sequential approach allows for a more precise process and stoichiometry control and offers cross-contamination free deposition. However, a detailed analysis of perovskite film formation and growth is usually only done qualitatively, with a limited number of studies addressing this aspect. We want to start filling the gap of lacking quantitative evaluations by investigating the phase evolution inside the PbI₂-FAI diffusion couple during deposition and annealing with an in situ X-ray diffraction (XRD) system. The observed diffraction intensity transients allow us to calculate the diffusion coefficient of the diffusing species for different isothermal annealing temperatures. With a derived Arrhenius plot, the activation energy and preexponential factor for the diffusion constant is determined. This report describes the mathematical model underlying the evaluation as well as application to a PbI₂-FAI (FA⁺: formamidinium CH(NH₂)₂⁺) diffusion couple, i.e. two initially separated layers brought into contact, enabling interdiffusion and reactive perovskite formation upon annealing. We find a linear trend in the Arrhenius plot, resulting in an activation energy of 0.83 eV. A variation of the initial parameters shows only minor activation energy changes, indicating a robust underlying mathematical model.