Band sharpening and lattice disorder: Synergistic optimization of thermoelectric properties of quaternary Cu2ZnSnSe4 via ternary Cu3SbSe4 solid solution alloying

Abstract

Research on enhancing the thermoelectric efficiency of chalcopyrite-based materials has predominantly focused on doping strategies and band convergence. Herein, we demonstrate an effective method for simultaneously increasing the power factor through band sharpening and markedly reducing the lattice thermal conductivity by alloying quaternary Cu2ZnSnSe4 (CZTSe) with ternary Cu3SbSe4, both comprising earth-abundant and low-toxic elements, making them highly attractive for commercial applications. A series of (Cu2ZnSnSe4)(1-x)(Cu3SbSe4)(x) (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, and 1) compositions were systematically investigated, while all the composition forms complete solid-solutions. The power factor was significantly enhanced owing to an increased density-of-states effective mass (m(d)(*)) and a decreased single band mass, attributed to band sharpening from reduced band curvature. Notably, at 300 K, the electrical conductivity of the x = 0.2 composition ((Cu2ZnSnSe4)(0.8)(Cu3SbSe4)(0.2), Cu2.2Zn0.8Sn0.8Sb0.2Se4) exhibited a similar to 330% increase reaching 1020 S/cm, compared to pristine CZTSe sample. Furthermore, all alloyed samples exhibited higher power factors than those of the pristine CZTSe sample across the measured temperature range. To elucidate the origin of the enhanced electrical conductivity, first-principles calculations based on density functional theory were performed, and analysis based on the single parabolic band model corroborated the observed improvements, confirming the role of enhanced m(d)(*) mass and reduced single band mass. In addition, the lattice thermal conductivity was markedly suppressed in the alloyed samples, primarily due to intensified phonon scattering induced by large amount of alloy disorder of a series of solid-solution compositions of complex Cu2+xZn1-xSn1-xSbxSe4. Debye-Callaway modeling was employed to quantitatively assess the impact of alloy scattering on phonon transport. As a result, solid-solution alloying of ternary chalcogenides in quaternary thermoelectric chalcogenides proved effective for optimizing the thermoelectric performance, leading to a maximum thermoelectric figure of merit (zT) of 0.47 at 700 K for the (Cu2ZnSnSe4)(0.2)(Cu3SbSe4)(0.8) (Cu2.8Zn0.2Sn0.2Sb0.8Se4, x = 0.8) compositions, an improvement of 2.3 times compared to pristine CZTSe, by synergistically benefiting from concurrent band sharpening and strong phonon scattering. The maximum energy conversion efficiency is also improved by 2.3 times in Cu2.8Zn0.2Sn0.2Sb0.8Se4 composition, compared to that of the pristine Cu2ZnSnSe4.