Experimental Synthesis and Properties of Metastable CuNbN2 and Theoretical Extension to Other Ternary Copper Nitrides

Copper nitrides are defect-tolerant semiconductors with properties that are promising for solar energy conversion applications. Currently, there are few known ternary copper nitride materials. Here, we synthesized a previously unreported CuNbN₂ using an ion-exchange reaction and subsequently determined its properties. CuNbN₂ has a layered delafossite-type structure with NbN₆ octahedra arranged in layers separated by linear N-Cu-N bonds. Experimental measurements and theoretical calculations agree that CuNbN₂ has a 1.3-1.4 eV optical absorption threshold; theory also indicates that the lowest energy indirect band gap is 0.9 eV. The calculated CuNbN₂ electron and hole effective masses are quite isotropic (m_out /m_in = 1.3-2.1) and low (m = 0.3-0.7 m_e), as for the layered crystal structure. On the basis of these results, we propose a new lattice-matched delafossite tandem solar cell approach with Cu(Nb,Ta)N₂ absorbers, p-type CuAlO₂ contacts, and n-type ZnO contacts. Interestingly, first-principles calculations indicate that CuNbN₂ is thermodynamically unstable with respect to disproportionation, yet the successful synthesis and potentially useful photovoltaic properties of this metastable material are possible. We theoretically examine a wide range of ternary copper nitrides for thermodynamic stability and optoelectronic properties with the goal of accessing their potential for solar energy conversion. It is found that the majority of these materials are thermodynamically unstable but that some of them should have properties that are promising for solar energy conversion applications and thus are worth experimental synthesis attempts.