Experimental Synthesis and Properties of Metastable CuNbN<sub>2</sub> 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 (mout /min = 1.3–2.1) and low (m = 0.3–0.7 me), 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 calculatio...