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 ZT>3–4ZT>3–4 or even more). But despite a fifty-year study, most of the known matters show ultra low heat-electricity conversion abilities (with ZTZT lower than 1). In this work, we take a canceling out of a Seebeck coefficient (thermopower, SS) 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 SS 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  ZTZT (proportional to S2S2), 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.