Enhanced thermoelectric performance through grain boundary engineering in quaternary chalcogenide Cu₂ZnSnSe₄

Quaternary chalcogenide Cu₂ZnSnSe₄ (CZTSe) is a promising wide band-gap p-type thermoelectric material. The structure and thermoelectric properties of lead substituted Cu₂ZnSn_1-xPb_xSe₄ are investigated. Lead primarily exists in the framework of PbSe as demonstrated by x-ray diffraction and calculation of x-ray absorption near-edge structure spectroscopy. The second phase distributes at the boundaries of CZTSe with thickness in several hundreds of nanometer. With appropriate grain boundary engineering, the enhancement of power factor and a decrease of thermal conductivity can be achieved simultaneously. As a result, a maximum figure of merit zT of 0.45 is obtained for the sample with x=0.02 at 723K.