Characterizing temperature-dependent optical properties of (MA_0.13FA_0.87) PbI₃ single crystals using spectroscopic ellipsometry

Hsiao-Wen Chen^1*, Desman Perdamaian Gulo², Yu-Chaing Chao¹, Hsiang-Lin Liu¹

¹Department of Physics, National Taiwan Normal University, Taipei, Taiwan
²Department of Physics, Chung Yuan Christian University, Taoyuan, Taiwan

* Presenter:Hsiao-Wen Chen, email:josephmdtw@gmail.com

We present spectroscopic ellipsometry measurements of (MA_0.13FA_0.87) PbI₃ single crystals assessed at photon energies of 0.73–6.42 eV and at temperatures between 4.4 and 400 K. At room temperature, the refractive index was dispersed as a function of frequency, which is typical of a semiconductor. The absorption spectrum exhibited several electronic transitions. We estimated a room temperature direct band gap of 1.66 ± 0.02 eV. With decreasing temperature, the refractive index increased with the thermo-optic coefficient of – 1.5 x 10-4 K^-1 at a wavelength of 1200 nm. The bandgap energy and exciton peak position exhibited a redshift, which was attributed to a reverse ordering of the band structures. Additionally, an anomaly in exciton peak position and band gap occurred at approximately 100–200 K due to the structural phase transition. This phenomenon was associated with the coexistence of MA/FA-disordered and MA/FA-ordered domains.

Keywords: spectroscopic ellipsometry, halide perovskite, band gap, exciton binding energy