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

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, Cu_2.2Zn_0.8Sn_0.8Sb_0.2Se₄) exhibited a ∼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 Cu_2+xZn_1-xSn_1-xSb_xSe₄. 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 (Cu_2.8Zn_0.2Sn_0.2Sb_0.8Se₄, 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 Cu_2.8Zn_0.2Sn_0.2Sb_0.8Se₄ composition, compared to that of the pristine Cu₂ZnSnSe_4.