Ultralow thermal conductivity in diamondoid lattices: high thermoelectric performance in chalcopyrite Cu0.8+yAg0.2In1−yTe2

Abstract

Because of its unique transport properties, CuInTe₂ has been considered as a promising p-type material for thermoelectric applications. However, its diamondoid structure gives it a high intrinsic lattice thermal conductivity that greatly limits its thermoelectric performance. In this study, we demonstrate that Ag alloying gives rise to an extremely low lattice thermal conductivity of 0.47 W m⁻¹ K⁻¹ for Cu_0.8Ag_0.2InTe₂ at 850 K. Moreover, we found Cu doping significantly improves the carrier mobility while simultaneously increasing the carrier concentration. As a result, the power factor of Cu_0.8Ag_0.2InTe₂ increases and a maximum ZT of ∼1.6 is achieved at 850 K. Both DFT calculations and low temperature heat capacity measurements suggest a strong interaction between low frequency optical phonons and heat carrying acoustic phonons, which is derived from the weak Ag–Te bonding. This strong phonon coupling decreases the Debye temperature and induces a low sound velocity.