Photoinduced ion-redistribution in CH3NH3PbI3 perovskite solar cells

We use photoinduced absorption spectroscopy (PAS) to study the ionic motion in CH3NH3PbI3 perovskite solar cells, consisting of indium tin oxide (ITO)/NiOx/perovskite/phenyl-C-61-butyric-acid-methyl ester (PCBM)/aluminum-doped zinc oxide (AZO)/ITO. We observed a slow (similar to 50 mHz) spectral blue shift (similar to 10(-4) eV) under modulated 520 nm illumination, which we interpreted in terms of the modulation in the bulk ion density. Numerical simulation shows that the mobile ion moves in and out from the double layers at the perovskite/charge transport layer interfaces in order to recover the bulk charge neutrality tipped off-balance by the photocarriers. The diffusion coefficient of the ion is 10(-10) to 10(-11) cm(2) s(-1), when we assume that the characteristic time constant of the ion motion is governed by the diffusion.