Dynamical Stabilization in Delafossite Nitrides for Solar Energy Conversion*

Delafossite structured ternary nitrides, ABN2, have been of recent experimental investigation for applications such as tandem solar and photoelectrochemical cells. We present a thorough computational investigation of the stability, electronic structure, and optical properties of nine compounds, where A = Cu, Ag, Au and B = Ta, Nb, V. Phonon density of states indicate all compounds to be dynamically unstable at low temperatures. Including finite-temperature anharmonic effects stabilizes all compounds at 300 K, with the exception of AgVN2. Crystal Orbital Hamiltonian Populations (COHP) provide insight into bonding and antibonding characters of A-N and B-N pairs. Instability at low temperatures can be attributed to strong A-N antibonding character. CuTaN2, CuNbN2, AgTaN2, and AgNbN2 are predicted to exhibit band gaps and large light absorption in the range of 1.0 to 1.7 eV, making these materials good candidates for solar-energy conversion applications. AuTaN2 and AuNbN2 have band gaps and absorption onsets near the ideal range for obtaining high solar-cell conversion efficiency, suggesting these compounds could become candidates as absorber materials in tandem solar cells or for band-gap engineering by alloying.