Copper Dynamics in Chalcogenides—Effects on Band Gap, Crystal Structure and Thermal Conductivity

Solar and thermoelectric energy conversions are two main approaches to generating renewable energy. Many chalcogenide materials possess superior efficiency for future energy materials: CuInGaSe2 and Cu4TiSe4 are both efficient solar absorbers, while Cu2Se offers superior zT values with Earth-abundant and non-toxic elements. The reason lies in superionic Cu vibrations, which creates the phonon-liquid electron-crystal effect, inhibiting heat transport while maintaining high electrical conductivity. However, accurate characterization of the electronic structure remains a challenge due to the strong effects of polymorphism (local disorder), electron-phonon coupling and phonon anharmonicity. Density functional theory calculations on the high-symmetry structure yield semi-metallic behavior. We address the problem by treating the effects of symmetry breaking and high-temperature anharmonic vibrations utilizing the anharmonic special displacement method (ASDM). We determined the ground-state polymorphous structure and obtained a converged bandgap of 0.9 eV, which is in excellent agreement with experimental values. Further, we applied ASDM starting from the polymorphous structure and investigated the band-gap renormalization as a function of temperature, and found that increasing temperature reduces the band gap by 0.06 eV. We lay out a framework and elucidate how Cu dynamics and anharmonicity impact the electronic properties and electronic properties of Cu2Se.<br/><br/>Moreover, by replacing Cu in the Cu2Se FCC sublattice with Zn, we synthesized single crystals of Cu2Zn3Se4 with Cu ions, Zn ions, and vacancies randomly distributed at the tetrahedral sites, according to XRD data. The unusually large 3/8 tetrahedral site vacancy fraction promotes large cation vibrations and high electrical conductivity. We perform molecular dynamics (MD) simulations and find that, while the high-symmetry structure is thermodynamically unfavorable at 0 K, Cu dynamics stabilize the structure at room temperature. Moreover, we observe local motifs that deviate significantly from the octet rule, yet the time-averaged structure supports the high-symmetry cubic picture with randomly occupied tetrahedral sites. Compared to the MD Mean Squared Displacement (MSD) in Cu2Se, Cu2Zn3Se4 exhibits similar values in Se displacements. Yet instead of a continuously increasing Cu MSD, Cu2Zn3Se4 exhibits a plateau in Cu MSD, showing large Cu vibrations without diffusion between tetrahedral sites. Thus, Cu dynamics can also explain why the measured thermal conductivity in Cu2Zn3Se4 is four times larger than Cu2Se, diminishing its thermoelectric performance.<br/><br/>This works is supported by National Science Foundation Award #2114424. M.Z. acknowledges funding by the European Union (project ULTRA-2DPK / HORIZON-MSCA-2022-PF-01 / No. 101106654). Computational resources are from Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) and National Energy Research Scientific Computing (NERSC) Center.