Stress Effects on Vibrational Spectra of a Cubic Hybrid Perovskite: A Probe of Local Strain

Inhomogeneous
strain may develop in hybrid organic metal-halide
perovskite thin films due to thermal expansion mismatch with a fabrication
substrate, polycrystallinity, or even light soaking. Measuring these
spatially varying strains is difficult but of prime importance for
understanding the effects on carrier mobility, non-radiative recombination,
degradation, and other optoelectronic properties. Local strain can
be mapped using the shifts in vibrational frequencies using Raman
or infrared microscopy. We use density functional theory to investigate
the effect of uniaxial strain on the vibrations of pseudo-cubic methylammonium
lead iodide (CH3NH3PbI3)­and identify
the vibrational modes most
favorable for local strain mapping (86, 97, 1457, and 1537 cm–1) and provide calibration curves. We explain the origin
of the frequency changes with strain using dynamical matrix and mode
eigenvector analysis and study strain-induced structural changes.
We also calculate mode Grüneisen parameters, giving information
about anharmonicity and anisotropic negative thermal expansion as
recently reported for other phases. Our results provide a basis for
strain mapping in hybrid perovskites to further the understanding
and control of strain and improve stability and photovoltaic performance.