Strong optical absorption in CuTaN2 nitride delafossite

Abstract

We report on synthesis, stability, electronic structure and optical properties of CuTaN₂ with the delafossite crystal structure and its potential use as an absorber for solar energy conversion applications. According to theoretical first-principles calculations, the formation enthalpy of CuTaN₂ is negative (−0.66 eV per atom), but this material is metastable with respect to decomposition into Cu, Ta₃N₅ and N₂. Nevertheless, the experimental thermal stability limit of single phase CuTaN₂ powders synthesized using an ion exchange method is 250 °C in ambient atmosphere, according to combined temperature-dependent X-ray diffraction and thermo-gravimetric analyses. Electronic structure of the CuTaN₂ is different compared to that of CuAlO₂, in particular the band gap of this nitride delafossite (1.3 eV) calculated using HSE06+G₀W₀ is much smaller than the band gap of the oxide delafossite. The onset of optical absorption onset of CuTaN₂ at 1.5 eV determined from experimental diffuse reflectance measurements is consistent with the theoretical 1.4 eV optical band gap and large calculated absorption coefficient (>10⁵ cm⁻¹ above 1.5 eV) determined from time-dependent HSE06 calculations corrected by a scissors operator. The significance of our findings is that optical properties of CuTaN₂ are nearly optimal for photovoltaic energy conversion.