Boundary Effects in Thin-Film Thermoelectrics

Abstract

AbstractEquations describing the resistances to the transport of heat and electricity through boundaries of thin-film thermoelectrics are derived. We show that these boundary resistances obey a boundary form of the Wiedemann-Franz law and cause a new type of thermal instability for short thermoelectric devices. We consider boundary thermal resistances both for phonons (Kapitza resistance) and for electrons, the contact electrical resistance at the junctions, and the boundary thermoelectric effects. It is shown that the Kapitza resistance causes reduction of the effective thermal conductivity of the system only if the electron and phonon subsystems are out of equilibrium. In this case, the thermoelectric figure of merit Z can be increased by reducing the thickness of the film. The electrical contact resistance at the junctions is shown to degrade the performance of the device. However, according to the boundary Wiedemann-Franz law, electrical contact resistance is accompanied by a thermal boundary resistance for the electron subsystem, which can cause an additional enhancement of Z. In some cases, this can lead to a device with ZT as high as 3 at room temperatures.