Realizing excellent thermoelectric performance in Ag-doped Cu2SnSe3-ZnSe solid solution by symmetry modification and stacking promotion

Ternary Cu2SnSe3 exhibits an inherent low symmetry and an unfavorable Seebeck coefficient. By strategic alloying with the highly symmetrical ZnSe, the transformation of a monoclinic structure into a tetragonal one was realized, yielding a higher band degeneracy in the electronic band. Therefore, the highest power factor in Cu2SnSe3-ZnSe solid solution reached 7.39 mu W cm-1 K-2 at 773 K. Accompanied by this, the energy barrier for slip initiation during the stacking formation decreased, enabling a higher possibility of stacking faults and twins and a low lattice thermal conductivity as 0.79 W m-1 K-1 at 773 K for (Cu2SnSe3)0.6(ZnSe)0.4. Based on ZnSe alloying, the weaker covalent Ag-Se bonds are introduced to reduce the phonon velocities and facilitate mass fluctuation by Ag-substitution for Cu, resulting in a very low lattice thermal conductivity as 0.49 W m-1 K-1 at 773 K for (Ag0.2Cu1.8SnSe3)0.6(ZnSe)0.4. Eventually, an excellent ZT of 0.98 was achieved at 773 K for (Ag0.2Cu1.8SnSe3)0.6(ZnSe)0.4 through a progressive optimization on thermoelectric properties. Our work underscored the pivotal role of enhancing structural symmetry, increasing stacking nucleation probability and weakening covalent bonds in realizing excellent thermoelectric performance.