CuTaS₃: Intermetal d–d Transitions Enable High Solar Absorption

To realize the fundamental limits of photovoltaic device efficiency, solar absorbers must exhibit strong absorption and abrupt absorption onsets. Ideally, onsets to maximum absorption (α > 10 5 cm –1 ) occur over a few tenths of an electronvolt. First-principles calculations predict CuTaS 3 represents a potentially new class of materials with such absorption characteristics. Narrow metallic d bands in both the initial and final states present high joint densities of states and, therefore, strong absorption. Specifically, a mixture of metal d (Cu 1+, d 10 ) and S p characterizes states near the valence band maximum, and metal d (Ta 5+, d 0 ) dominates near the conduction band minimum. Optical absorption measurements on thin films confirm the abrupt onset to strong absorption α > 10 5 cm –1 at E g + 0.4 eV ( E g = 1.0 eV). Theoretical CuTaS 3 solar cell efficiency is predicted to be 28% for a 300 nm film based on the metric of spectroscopic limited maximum efficiency, which exceeds that of CuInSe 2 . This sulfide may offer new opportunities to discover and develop a new class of mixed d -element solar absorbers.