Site-Selective Doping Chemistry on Quaternary Diamondoid Thermoelectrics Cu2ZnSnSe4

Abstract The intrinsic trade-off between carrier concentration and mobility remains a fundamental hurdle in enhancing the thermoelectric efficiency of diamondoid Cu2ZnSnSe4. Here, we systematically compare the doping behaviors of In, Cd, and Zn at the Sn site and elucidate their distinct impact on structural symmetry and charge transport. Our results reveal that mismatches in atomic radius and electronegativity between the dopants and the Sn host induce different lattice evolution pathways. While all three dopants effectively increase the hole concentration, Cd and Zn substitutions uniquely enable a simultaneous enhancement in carrier mobility. This counterintuitive phenomenon originates from the suppression of cation disorder and the evolution toward an ideal lattice symmetry (η ≈ 1), which collectively reduce carrier scattering. Consequently, high room-temperature electrical conductivities of 1200 and 900 S cm–1 are achieved for Cd and Zn-doped samples, respectively. To further decouple the electronic and thermal transport, Ag alloying was introduced at the Cu site in the Zn-doped matrix. The introduction of Ag induces intense mass-fluctuation scattering and lattice softening due to weakened chemical bonding, leading to a substantial reduction in lattice thermal conductivity. Driven by these synergistic optimizations, a peak ZT of 0.54 at 800 K is achieved in Cu1.8Ag0.2Zn1.08Sn0.92Se4. This work provides critical insights into site-selective doping chemistry and phonon engineering, offering a viable strategy to overcome the intrinsic transport trade-offs in quaternary diamondoid thermoelectrics.