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

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.