Improved Thermoelectric Performance of Cu₂SnSe₄ by Proper Decoupling Between Electron and Phonon Through Replacement of Sn with In

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 Cu₂Sn_1−xIn_xSe₄ (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.