An assessment of silver copper sulfides for photovoltaic applications: theoretical and experimental insights

Abstract

As the worldwide demand for energy increases, low-cost solar cells are being looked to as a solution for the future. To attain this, non-toxic earth-abundant materials are crucial, however cell efficiencies for current materials are limited in many cases. In this article, we examine the two silver copper sulfides AgCuS and Ag₃CuS₂ as possible solar absorbers using hybrid density functional theory, diffuse reflectance spectroscopy, XPS and Hall effect measurements. We show that both compounds demonstrate promising electronic structures and band gaps for high theoretical efficiency solar cells, based on Shockley-Queisser limits. Detailed analysis of their optical properties, however, indicates that only AgCuS should be of interest for PV applications, with a high theoretical efficiency. From this, we also calculate the band alignment of AgCuS against various buffer layers to aid in future device construction.

Fig. 1. Crystal structures of (a) RT-AgCuS and (b) RT-Ag₃CuS₂. A single unit cell is marked in each, and the following atom labels are used: Ag atoms in grey, Cu in blue and S in yellow.

Fig. 2. Total and partial density of states (DoS) of AgCuS, with HSE06; individual partial DoS are labelled in legend, valence band maximum (VBM) set to 0 eV.

Fig. 3. Band structure diagram of AgCuS using the HSE06 functional, showing a direct 1.27 eV band gap; valence band marked in blue, conduction band marked in orange, VBM set to 0 eV.

Fig. 4. Band structure diagrams of I4₁/amd Ag₃CuS₂ using HSE06, demonstrating a direct 1.05 eV band gap; valence band marked in blue, conduction band marked in orange, VBM set to 0 eV.

Fig. 5. Kubelka–Munk plot from diffuse reflectance measurement of (a) AgCuS and (b) Ag₃CuS₂. Intersections of background and absorption marked, giving the optical band gaps, in red.

Fig. 6. Comparison of valence band XPS (black) and HSE06 density of states (red) for (a) AgCuS and (b) Ag₃CuS₂. Valence band edge has been normalized to 0 eV.

Fig. 7. Calculated optical absorption of AgCuS and Ag₃CuS₂, with: absorption coefficients marked as bold lines; fundamental band gaps marked by vertical lines; (αhν)², representative of the predicted optical band gap, is marked as alternating dot-and-dash lines.

Fig. 8. Valence band alignment of AgCuS with a number of other materials. Ionisation potentials and workfunctions of other materials have been taken from the literature.^,–