Raman spectroscopy as a probe of local strain in perovskite solar cells

Hybrid organometallic perovskites are a promising material for thin-film solar cells. Strain can develop due to substrate thermal expansion mismatch during processing, presence of grain boundaries, and even absorption of light [J. Zhao et al., Sci. Adv. 3, eaao5616 (2017)]. These strains may be inhomogeneous and can affect carrier mobility, non-radiative recombination etc. and may be related to degradation. 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 and other materials [D. A. Strubbe et al., Phys. Rev. B 92, 241202(R) (2015)]. We study vibrational properties of different phases of methylammonium lead iodide (CH₃NH₃PbI₃) structures and calculate the slope of the shift of each mode for applied strains along different crystal directions to provide a calibration curve for measuring local strain. Our study also gives insight into the interaction between strain, structural changes and vibrational modes which may help to understand the degradation that plagues these materials.