Photoinduced ion-redistribution in CHNHPbI perovskite solar cells.

We use photoinduced absorption spectroscopy (PAS) to study the ionic motion in CHNHPbI perovskite solar cells, consisting of indium tin oxide (ITO)/NiOx/perovskite/phenyl-C-butyric-acid-methyl ester (PCBM)/aluminum-doped zinc oxide (AZO)/ITO. We observed a slow (∼50 mHz) spectral blue shift (∼10 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 to 10 cm s, when we assume that the characteristic time constant of the ion motion is governed by the diffusion.