Impact of contact resistances on the low-dimensional scaling of thermoelectric energy conversion devices

Thermoelectric energy conversion devices have several advantages relative to alternative heat removal and energy harvesting technologies-they are free of moving parts, acoustically silent, highly reliable and compatible with on-chip integration. Yet their historically limited performance and low efficiency in comparison with alternative technologies have restricted their use from more widespread applications. Previous work has made significant improvements in the low-dimensional figure of merit for several thermoelectric materials, such as superlattice, quantum dot, and skutterite thin-films. Electrical and thermal contact resistances at these length scales, however, have limited the impacts of these advances in material properties so severely that practical devices with high efficiency have yet to be developed. Whereas past work has focused extensively on the enhancement of material rather than interface properties, this work aims to quantify the impact of electrical and thermal contact resistances on device performance and efficiency at low-dimensions. Effective expressions for thermal conductivity, electrical conductivity, and figure of merit of the thermoelectric pellet are presented to account for electrical and thermal interfacial effects at the pellet-interconnect interfaces. Contact resistances are determined for various semiconductor-metal interfaces. Implications of the results are illustrated for the performance and efficiency of two classes of representative thermoelectric devices-bulk and on-chip thermoelectric cooling and energy harvesting devices. For practically achievable values of modern Bi₂Te₃s heat exchangers, the impact of modern parasitic resistances results in a 50% reduction in the figure of merit at length scales less than ~0.5 mum. The furthered understanding and mitigation of interfacial resistances will enable the potential of high figure of merit materials to create thermoelectric energy conversion devices that are - - competitive with alternative technologies.