Raman Spectra as a probe of localized strain in perovskite solar cells

Hybrid organometallic perovskites are a promising new material for cheap, flexible, and high-efficiency photovoltaics, although a major obstacle preventing commercialization is rapid degradation under illumination. During processing, strain may develop in the material due to thermal expansion mismatch with the substrate. Polycrystallinity in thin films may result in inhomogeneous strains around grain boundaries, which in turn can affect carrier mobility, non-radiative recombination etc. Absorption of light can also generate strain within the perovskite material, which may be related to degradation processes. Vibrational frequencies are shifted due to strain in a material and we want to enable use of this phenomenon to map local stress-strain behavior within a perovskite material via Raman spectroscopy, as is done in crystalline silicon. We calculate the slope of the shift for applied strains along different crystal directions to provide a calibration curve for measuring local strains. Our study also gives insight about vibrational mode characters which may help to understand degradation in perovskites.