Band gap temperature-dependence and exciton-like state in copper antimony sulphide, CuSbS2

Abstract

The temperature-dependence of the band gap of the proposed photovoltaic absorber copper antimony sulphide (CuSbS2) has been studied by Fourier-transform infrared spectroscopy. The direct gap rises from 1.608 to 1.694 eV between 300 and 4.2 K. Below 200 K an exciton-like feature develops above the absorption edge at 1.82 eV. First-principles calculations evaluate band structure, band symmetries, and dipole selection rules, suggesting distinctly enhanced absorption for certain excitonic optical transitions. Striking consistency is seen between predicted dielectric and absorption spectra and those determined by ellipsometry, which reveal rapidly strengthening absorption passing 105 cm−1 at 2.2 eV. These results suggest beneficial photovoltaic performance due to strong optical absorption arising from unusually strong electron–hole interactions in polycrystalline CuSbS2 material.

Formation of Near-IR Excitons in Low-Dimensional CuSbS2

“…Experimental detection of the changes in the excitonic peaks or lattice would be improved if the ensemble contains only monolayer platelets. We also note that the possible exciton-like peaks in the layered material may be resolved at lower temperatures, which was previously shown for bulk CuSbS 2 …”

Perovskite-inspired materials for photovoltaics and beyond—from design to devices

“…Yang et al's work also demonstrated that CuSbS2 may be defect tolerant owing to higher formation enthalpies for deep-level defects and the shallow transition level of Cu vacancies [221]. However, CuSbS2 thin films were found to have a carrier lifetime of only 0.7 ns even after the improvement on film quality via thermal treatment [223,224].…”

Formation of Near-IR Excitons in Low Dimensional CuSbS$_2$

“…The first finding (i) is consistent with recent results from diffuse reflectance spectroscopy on platelets which detected only a small decrease (0.05 eV) in the direct optical gap energy in thin (1 to 5 nm) platelets compared to the thickest platelets (25 to 50 nm). 22 The second finding, (ii), is supported by the presence of weak absorption peaks below the optical gap have been detected in the ellipsometric data of bulk CuSbS 2 56 and two-dimensional platelets. 22 The thinnest platelets had a peak at 1.04 eV and energies less than 0.85 eV were beyond the spectral range.…”