Goniopolarity and where to find it

Goniopolarity is the phenomenon in which a single material exhibits p-type transport in one direction and n-type transport in the perpendicular direction. The phenomenon can potentially enable a range of thermoelectric applications without the need for a heterojunction between two materials. We consider goniopolarity theoretically, and we report a generic, quantitative set of criteria for identifying and predicting goniopolar materials. Using calculations of the thermopower for different types of band structures, we report simple inequalities for producing goniopolarity in generic classes of materials, including metals and insulators that have both electron and hole bands of carriers, as well as single-band systems that have unusually-shaped Fermi surfaces.