Boosting the power factor and thermoelectric performance in eco-friendly Cu3SbS4 by twin boundary and grain boundary phase

Te/Se-free Cu3SbS4 is one of the most promising thermoelectric materials because of its earth-abundant, low-cost and environment-friendly characteristics. However, its thermoelectric performance is relatively low due to the high electrical resistivity. In this work, dense twin boundaries and grain boundary phases were constructed in the Ge-doped Cu3SbS4 materials by the melting reaction, sulfuration process, and plasma activated sintering. Density Functional Theory (DFT) calculations demonstrate that the Ge doping makes the Fermi level (EF) move from the forbidden-band into the valence band, thus significantly increasing carrier concentration. These twin boundaries don't degrade the carrier mobility, leading to the enhanced electrical conductivity, thus obtaining a higher power factor of -14.4 & mu;W cm-1 K-2 at 623 K. As a result, a peak ZT value of -0.78 at 623 K is achieved in the Cu3Sb0.975Ge0.025S4 sample, which is much higher than those of most metal sulfides. Moreover, high-density twin boundaries and grain boundary phases can inhibit the propagation of fatigue crack and enhance the fracture toughness. This modified approach can efficiently construct dense nanotwin boundaries in metal sulfides for high-performance thermoelectrics.