Effects of Small Polar Molecules (MA⁺ and H₂O) on Degradation Processes of Perovskite Solar Cells

Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA⁺ and H₂O). CF study shows that short-circuit current density of the PSCs is increased by H₂O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA⁺ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film.