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 (alpha > 10(5) cm(-1)) occur over a few tenths of an electronvolt. First principles calculations predict CuTaS3 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 (Cu1+, d(10)) and S p characterizes states near the valence band maximum, and metal d (Ta5+, d(0)) dominates near the conduction band minimum. Optical absorption measurements on thin films confirm the abrupt onset to strong absorption alpha > 10(5) cm(-1) at E-g + 0.4 eV (E-g = 1.0 eV). Theoretical CuTaS3 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 CuInSe2. This sulfide may offer new opportunities to discover and develop a new class of mixed d-element solar absorbers.