High Thermoelectric Figure of Merit (zT) in β‐Ag₂Se via Aliovalent Doping

High‐performance thermoelectric materials are essential for efficient low‐temperature (300–400 K) heat energy harvesting, with n ‐type Ag 2 Se being a promising candidate. To further enhance the thermoelectric figure of merit (zT) of Ag 2 Se, aliovalent doping has emerged as a key strategy. However, achieving wet‐chemical aliovalent doping of Ag 2 Se at ambient temperature has proven challenging. In this work, a high zT max of 1.57 at 398 K is reported for an optimally Cd(II)‐doped Ag 2 Se sample, specifically in the structurally phase‐pure Ag 1.98 Cd 0.02 Se, which is successfully synthesized via an aqueous‐based method at room‐temperature (300 K). The Ag 1.98 Cd 0.02 Se sample also exhibits an impressive average zT avg of 1.12 over the temperature range of 315–400 K. Density functional theory (DFT) calculations for both the pristine and doped samples reveal significant changes in the electronic band structures, including notable modulations in the density of states near the Fermi energy, particularly for the Ag‐3 d states. The remarkable thermoelectric performance of Ag 1.98 Cd 0.02 Se is attributed to an optimization of charge carrier induced by the Cd(II)‐doping.