Photocarrier dynamics in perovskite-based solar cells revealed by intensity-modulated photovoltage spectroscopy.

We studied perovskite photovoltaic devices with intensity-modulated photovoltage spectroscopy. Two coexisting relaxation times are found in accordance with the results of previous impedance spectroscopy (IS) measurements. The slower time constant is independent of the light power while the faster one is inversely proportional to the light power. We employed the surface polarization picture used in the IS analysis augmented by a plausible assumption that the surface polarization is proportional to the light intensity to explain the inverse power dependence of the fast time constant. Because the surface polarization results from the surface accumulated charges, its lateral (parallel to the electrode) distribution and dynamics should be known. We present evidence that the surface accumulated charges indeed form a two-dimensional layer, and have a finite binding energy and a diffusion length.