Cu2SnSe4 (CTS) ternary chalcogenides have potential applications in thermoelectrics for they crystallize in a high-symmetry cubic structure and consist of earth-abundant and eco-friendly elements. However, the pristine CTS does not have optimal thermoelectric (TE) performance (ZT = 0.35 at ∼700 K), so further investigation is required in this regard. In this work, we propose an incorporation of In2Te3 with a defect zinc-blende cubic structure into CTS, aiming to regulate the electronic and phonon transport mechanism simultaneously. The first-principles calculation reveals that the element In favors the residing at a vacancy site as an interstitial atom while Te at the Se site, which leads to band convergence and degeneracy, respectively. As a result, the electrical property improves with a 22% increase in the power factor (PF), and at the same time, the lattice thermal conductivity (κL) reduces to 0.31 W K–1 m–1 at 718 K. Synergistic engineering realizes a remarkable improvement in TE performance with the highest figure of merit (ZT) of 0.92 at 718 K. This value is ∼3 times that of the pristine CTS and stands among the highest in the Cu2SnSe4 family so far, which proves that the incorporation of In2Te3 into CTS is a good proposal.

Cu5Sn2Se7 (CSS) has a potential application in thermoelectrics in that it consists of affordable, non-toxic, and earth-abundant elements. However, it is less reviewed in thermoelectrics in recent years because it exhibits a metallic-like behavior with a high carrier concentration (n H) (n H ∼ 3.0 × 1021 cm–3). To improve its thermoelectric (TE) performance, an electronic structure- and entropy (ΔS)-driven design of CSS is proposed. By analyzing the electronic structures and ΔS values of CSS alloying with three different species (In, Te, or In2Te3), we determine that the In2Te3-incorporated CSS favors performance optimization. The Hall measurement reveals that the n H of (Cu5Sn2Se7)1–x (In2Te3) x (x = 0.1) reduces to the optimal value (∼8.3 × 1020 cm–3), while the mobility (μ) increases with an increase in x so that the power factor (PF) reaches 12.0 (μW/cm K2), about 20% enhancement. At the same time, the lattice thermal conductivity (κ L) reduces to 0.46 W K–1 m–1 at x = 0.1. As a consequence, the ZT value increases to 0.7 at ∼770 K, which is about 4.7 times that of the pristine Cu5Sn2Se7. The principles applied here can be used as a guidance to design other thermoelectric materials.

The carrier transport mechanism in most thermoelectric semiconductors is governed by conventional band transport, however, there is an exception in ternary Cu2SnSe4 (CTS) or its based counterparts in which the transport mechanism is dominated by a small polaron hoping model. The sluggish movement of the small polaron greatly suppresses the mobility of the carrier due to the strong coupling of the electron with the phonon, therefore, a proper decoupling is highly necessary. To achieve this goal, herein, InSe bonds are created by incorporating some indium into the cation vacancy in CTS, which effectively balances the transports between electron and phonon. As a consequence, both the electrical property (σ) and thermal conductivity (κ) are optimized in the sample Cu2Sn1−xInxSe4 (x = 0.1), and its thermoelectric performance improves with the highest figure of merit (ZT) of 0.5. This value is ≈52% higher than that of the pristine CTS, proving that incorporation of In in the cation vacancy in CTS is an alternative way to balance the transports between electron and phonon.