Lead halide perovskites have emerged as new optoelectronic materials owing to their outstanding optical properties. There has been increased interest in their temperature-sensitive optical properties and new optical applications have been proposed thereby. Here, we report the origin of the unusual negative thermo-optic coefficient of the halide perovskite $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbC}{\mathrm{l}}_3$, i.e., a decrease in the refractive index by an increase in temperature. From the temperature dependences of the absorption spectrum and the lattice constant and using the Lorentz oscillator model, we conclude that the negative thermo-optic coefficient below the absorption edge is predominantly determined by the large thermal expansion coefficient inherent to this soft material system. This work demonstrates that the negative thermo-optic coefficient is a distinctive phenomenon reflecting the unique electronic and lattice properties of halide perovskites.

• Received 3 April 2020
• Revised 9 June 2020
• Accepted 24 June 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.074604

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Published by the American Physical Society