Public Research Seminar by Advanced Materials Thrust, Function Hub, HKUST(GZ) - Space-charge effects in solid dielectrics: Conduction, diffusion, and Seebeck effect  

General expressions for electrical currents of conduction and diffusion in solid dielectrics have been derived [J. Appl. Phys. 95, 2665 (2004); Book chapter (2011)], using the mass-action relation for p-type and n-type carriers in a semiconductor. The electrical conductivity is counterintuitively not an algebraic sum of Ohmic and space-charge contributions, but rather a nonlinear function of the two types of contributions with a dependence on the mobility difference between p-type and n-type carriers. For both conduction and diffusion, the expressions obtained are consistent with known results for the limiting cases of either Ohmic or space-charge contributions. Based on an electrostatic zero-current condition derived from these nonlinear expressions, we predict that in the presence of space charge the Seebeck coefficient of a homogeneous plane-parallel capacitor could go beyond the corresponding upper limit of the linear-response regime [J. Appl. Phys. 126, 125106 (2019)]: If only Ohmic conductivity is considered or if the distribution of space charge is taken to be uniform, the Seebeck coefficient, which is the same as that given by Onsager’s reciprocal relations, takes on a magnitude between 0 and kB/e, where kB is the Boltzmann constant and e is the electronic charge. But in any space-charge-limited scenario with a nonuniform spatial distribution of space charge, Onsager’s reciprocal relations break down and the magnitude of the Seebeck coefficient takes on a value between kB/e and 2kB/e, exceeding the upper limit of the linear-response regime.