Abstract
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.
Highlights
Lead halide perovskites have attracted attention as a promising class of materials for optoelectronics [1,2]
We report that the large thermal expansion inherent to this soft material system leads to the negative thermo-optic coefficient of the halide perovskite MAPbCl3 (MA = CH3NH3), by comparing the temperature dependences of the optical absorption spectrum and the lattice constant and performing an analysis based on the Lorentz oscillator model
Our results demonstrate that halide perovskites are unusual semiconductors in which a large thermal expansion coefficient results in a large negative thermo-optic coefficient
Summary
Lead halide perovskites have attracted attention as a promising class of materials for optoelectronics [1,2]. It was revealed that the halide perovskite exhibits a distinct decrease in refractive index with increasing temperature, i.e., a large negative thermo-optic coefficient [22] This behavior is rather unique; it is opposite to the positive thermo-optic coefficients usually observed in conventional inorganic semiconductors (e.g., GaAs and Si). Previous studies have reported that halide perovskites show a blueshift in optical absorption-edge energy with temperature [4,23], which is contrary to the redshift observed in conventional inorganic semiconductors [24] This opposite shift in the absorption edge may have an impact on the sign of the thermo-optic coefficient. The present work demonstrates that the negative thermo-optic coefficient of halide perovskites strongly reflects the unique thermophysical properties of this class of materials
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