Carrier–Phonon Interaction Induced Large Negative Thermal-Optic Coefficient at Near Band Edge of Quasi-2D (PEA)₂PbBr₄ Perovskite

The soft and polar nature of quasi-2D (PEA)₂PbBr₄ perovskite, and robust photo-generated excitons lead exciton-polaritons and exciton-polarons as the important phenomena near the band edge for application in the lighting aspect. In this work, a convenient methodology is proposed based on the polariton resonant modes in temperature-dependent (77 K to RT) spectroscopy, and investigate the effect of these quasi-particles on refractive index dispersion. The large binding energy (≈335 meV) of quasi-2D excitons is obtained by the reflectance measurements at 77 K. Stable exciton-polaritons and exciton-polarons are confirmed by energy dispersions and the observation of self-trapped exciton-polaron state, respectively. Furthermore, the large negative thermal-optic coefficient due to damping effect of exciton-phonon scattering is observed. The phenomenon is opposite to those observed in conventional semiconductors (e.g., Si, Ge, GaN, AlN, GaAs, AlAs, and ZnO etc.). The observed stable negative thermal-optic coefficients from 160 K to RT indicate that the quasi-2D perovskite can be used as a phase compensator for conventional semiconductor materials.