SrZn₂N₂ as a Solar Absorber: Theoretical Defect Chemistry and Synthesis by Metal Alloy Nitridation

Abstract

The ternary zinc nitrides CaZn2N2 and SrZn2N2 are promising materials for solar energy conversion because their direct band gaps are tunable to optimal values, and they contain only earth-abundant elements. We report first-principles calculations with a focus on defect chemistry and propose a method of synthesis for SrZn2N2. Our calculations reveal that although the N vacancy has a relatively low formation energy among the native defects in SrZn2N2 and shows deep levels within the band gap, its concentration can be sufficiently reduced by controlled crystal growth and extrinsic doping. The SrZn2N2 powder was synthesized by NH3 nitridation of the SrZn2 alloy at 600 °C and atmospheric pressure. We experimentally determined the direct band gap of SrZn2N2 to be 1.6 eV, consistent with our theoretical prediction.