Optical Study of Csx (MA0.13FA0.87) 100-xPbI3 Single Crystal
Hsiao-Wen Chen (陳孝文)1*, Desman Perdamaian Gulo2, Yu-Chaing Chao (趙宇強)1, Hsiang-Lin Liu (劉祥麟)1
1Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan
2Department of Physics, Chung Yuan Christian University, Taoyuan 32023, Taiwan
* Presenter:Hsiao-Wen Chen (陳孝文)
We present the spectroscopic ellipsometry measurements of Csx (MA0.13FA0.87) 100-xPbI3 single crystal over the energy range of 0.73 ~ 6.42 eV and at the temperatures between 4.5 and 300 K. These organic-inorganic metal halide perovskites have been demonstrated as great potential as light-emitting devices and solar cells [1]. At room temperature, the dispersion of refractive index as a function of frequency is typical of a semiconductor. The absorption spectrum exhibits eleven electronic transitions at approximately 1.51, 2.64, 2.97, 3.17, 3.30, 3.49, 3.71, 4.06, 4.74, 5.51, and 5.95 eV. We estimate the room temperature direct band gap of 1.51 ± 0.01 eV. With decreasing temperature, the value of refractive index increases from 2.10 to 2.16 in the near-infrared frequency region. The band gap and exciton peak position show a redshift. This unusual redshift can be attributed to a reverse ordering of the band structures. Furthermore, anomalies of band gap and exciton peak position are observed around 130 ~ 160 K due to the structural phase transition. This phenomenon is associated with the coexistence of MA-disordered and MA-ordered orthorhombic domains [2].

[1] M. A. Green et al., Nat. Photonics 8, 506 (2014).
[2] M. I. Dar et al., Sci. Adv. 2, e1601156 (2016).

Keywords: bandgap, spectroscopic ellipsometry