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

Abstract

The ternary zinc nitrides CaZn 2 N 2 and SrZn 2 N 2 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 SrZn 2 N 2 . Our calculations reveal that although the N vacancy has a relatively low formation energy among the native defects in SrZn 2 N 2 and shows deep levels within the band gap, its concentration can be sufficiently reduced by controlled crystal growth and extrinsic doping. The SrZn 2 N 2 powder was synthesized by NH 3 nitridation of the SrZn 2 alloy at 600 °C and atmospheric pressure. We experimentally determined the direct band gap of SrZn 2 N 2 to be 1.6 eV, consistent with our theoretical prediction.

Theoretical Prediction and Thin-Film Growth of the Defect-Tolerant Nitride Semiconductor YZn3N3

“…Figure S1 indicates that the deep nature is attributable to polaronic hole trapping by neighboring N atoms. This behavior is likely to be partly related to the particular chemical environment in which the Zn atom is coordinated with only three N atoms, unlike the fourfold coordination at the Zn1 site in YZn 3 N 3 and the Zn sites in CaZn 2 N 2 and SrZn 2 N 2 …”

“…Calculations were conducted using the projector augmented-wave (PAW) method as implemented in the Vienna Ab initio Simulation Package (VASP) code. , We used the Perdew–Burke–Ernzerhof functional tuned for solids in combination with Hubbard U corrections − for the Zn 3d states ( U effective = 5 eV) in the search for the ground-state crystal structure of YZn 3 N 3 , which includes geometry optimization and phonon calculations at the Γ point based on density functional perturbation theory. , The procedures for the crystal structure search follow our previous study, which is based on element substitution in prototype crystal structures . The details of the computational settings are found in the Supporting Information.…”

“…35,36 The procedures for the crystal structure search follow our previous study, which is based on element substitution in prototype crystal structures. 24 The details of the computational settings are found in the Supporting Information. The resultant ground-state crystal structure was further optimized using the Heyd−Scuseria− Ernzerhof (HSE06) hybrid functional, 38−40 followed by the calculations of its band structure, effective masses, density of states (DOS), dielectric function, and point defects.…”

“…In the lead iodide perovskites, the antibonding character of the Pb 6s and I 5p states is present at the valence band maximum (VBM) and therefore the dangling-bond states of anions tend to lie in the vicinity of the VBM . The same electronic behavior can be expected due to the Zn 3d and N 2p states in the ternary zinc nitrides. , Studies of the properties of CaZn 2 N 2 and SrZn 2 N 2 have revealed that the defect levels of the cation vacancies are shallow or buried in the VBM. , Ternary zinc nitrides have thus been of interest in terms of their defect behavior as well as the elements they contain and their fundamental properties.…”

“…15,16 Studies of the properties of CaZn 2 N 2 and SrZn 2 N 2 have revealed that the defect levels of the cation vacancies are shallow or buried in the VBM. 16,24 Ternary zinc nitrides have thus been of interest in terms of their defect behavior as well as the elements they contain and their fundamental properties.…”