Analysis of a Low T Phase Transition in Cs₂AgBiBr₆ via Resonance Raman Spectroscopy and Photoluminescence

Halide double perovskites are of interest due to their potential as high efficiency solar cell materials while eliminating the toxicity and stability issues of the current high efficiency perovskites[1]. Out of this family, Cs₂AgBiBr₆ has advantageous characteristics as a solar cell material including strong absorption and long carrier lifetimes[2]. While the structural phase transition at 120 K has been well-documented there appears to be another characteristic temperature near 38 K. Through polarized resonance enhanced Raman spectroscopy, the temperature dependence of the photoluminescence can be analyzed by fitting measured spectra to a series of Voight oscillators. These oscillator fitting parameters show a strong blue shift in the bandgap seen in the photoluminescence, deviating from the typical semiconducting Varshni trend. In addition, the A_1g/A_g phonon displays a hardening of the center frequency below 38 K. The investigation into this low temperature phase will aid in further developing the understanding of the optoelectronic properties of Cs₂AgBiBr₆ and similar halide double perovskites.

[1] F. Igbari et al. Advanced energy materials 2019. 9 (12), 1803150.

[2] A. H. Slavney et.al. Journal of the American Chemical Society 2016. 138 (7), 2138-2141.