Giant Seebeck coefficient decrease in polycrystalline materials with highly anisotropic band structures: Implications in seeking high-quality thermoelectric materials

Present physical laws do not deny the existence of high-quality thermoelectric (TE) materials (with Z T > 3 – 4 or even more). But despite a fifty-year study, most of the known matters show ultra low heat-electricity conversion abilities (with Z T lower than 1). In this work, we take a canceling out of a Seebeck coefficient (thermopower, S ) of inner grains in some polycrystalline materials with highly anisotropic band structures, as one of the clues to the above contradiction. We show that an S could be drastically decreased about several hundred times (or even more) in some polycrystalline materials, compared to their single crystalline counterparts. These polycrystals can thus show low-estimated Z T (proportional to S 2 ), and should be re-examined in their single crystalline forms. Moreover, we also point out that the absence of a conductivity effective mass database at present means that there are blind spots in understanding and predicting some semiconductors’ orientation-dependent transport properties.