Strong optical absorption in CuTaN2 nitride delafossite†
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.