Optical: In situ monitoring during the synthesis of halide perovskite solar cells reveals formation kinetics and evolution of optoelectronic properties

The formation mechanism and the evolution of optoelectronic properties during annealing of chlorine-derived methylammonium lead iodide (MAPbI_3-xCl_x) are investigated in detail combining in situ and ex situ optical and structural characterization. Using in situ optical reflectometry we are able to monitor the evolution of the MAPbI_3-xCl_x phase as a function of time and processing temperature. The formation kinetics is fitted using an improved Johnson-Mehl-Avrami-Kolmogorov model and a delayed formation of MAPbI_3-xCl_x is found when chlorine is present in the precursor. This is verified by X-ray diffraction and X-ray fluorescence measurements. From absolute photoluminescence measurements we determine the implied V_oc during film formation, which exhibits a maximum at a specific time during the annealing process. In conjunction with ex situ time-resolved photoluminescence we deduce a decrease in the net doping density for increased annealing times, while the minority carrier lifetime stays constant. We thus demonstrate the potential of in situ optical spectroscopy to monitor and tailor the electronic properties of hybrid perovskites directly during film growth, which can be easily applied to different growth recipes and synthesis environments.