High Thermoelectric Performance in Al- and Sn-Codoped ZnO Nanosheets via Synergistic Band Structure Engineering with Low Thermal Conductivity

In this work, we report a substantial enhancement in the thermoelectric performance of ZnO achieved through codoping with Al3+ and Sn4+. The codoped compositions, Zn0.98–xAl0.02SnxO (x = 0.01, 0.02, 0.03), exhibit a synergistic improvement in both electrical conductivity and Seebeck coefficient, along with a significant reduction in lattice thermal conductivity. Band structure engineering via the hybridization of Al 3s and Zn 4s states shifts the Fermi level deeper into the conduction band, while the Sn 5s states introduce a high density of states near the Fermi level, thereby enhancing the Seebeck coefficient. Phonon scattering across all frequency ranges is achieved by interfaces, point defects, and dense dislocation arrays localized at low-angle grain boundaries (∼13.5°), leading to substantial suppression of the lattice thermal conductivity. The optimized composition, Zn0.97Sn0.01Al0.02O, achieves a high power factor of 9.8 × 10–4 W·m–1·K–2 at 1002 K and a maximum figure of merit (zT) of 0.61 at 997 K, which is the highest value reported to date for Al–Sn-codoped ZnO systems. A single-leg thermoelectric device fabricated from this composition produced an open-circuit voltage of 122 mV under a 473 K temperature gradient, underscoring its practical applicability. These findings demonstrate that dual doping is an effective approach to simultaneously optimize the electronic structure and phonon scattering for high-performance oxide-based thermoelectrics.