Unconventional Thin-Film Thermoelectric Converters: Structure, Simulation, and Comparative Study

Bi2Te3 or Sb2Te3 are the materials most widely used in thermoelectric generators (TEG) operating near room temperature. These materials are, however, environmentally harmful, expensive, and incompatible with complementary metal-oxide semiconductor technology, in contrast to silicon (Si), germanium (Ge), or silicon-germanium (SiGe). Although the thermopower (S) and electrical conductivity (σ) of Si and Ge are high, use in thermoelectricity is severely hindered by their high thermal conductivity (κ). By altering the phonon band structure of this Si films by use of an artificial phononic pattern, spectacular reduction of κ by two orders of magnitude has been demonstrated. To take full advantage of phonon band modification and scattering in thin films, converter structure based on thin-film membranes is proposed for κ reduction. To consolidate the position of Si-based materials, coupled charge and heat-transport simulations have been conducted to demonstrate the potential of the materials for thermoelectric conversion compared with such widespread materials as Bi2Te3. The effect of contact resistance on generator performance has been carefully taken into consideration to reflect integration constraints at the TEG level. For a temperature difference ΔT = 30 K, the maximum electrical power density reaches approximately 6 W/cm2 for Si and Ge, and approximately 3 W/cm2 for Si0.7Ge0.3, values which are similar to those for Bi2Te3. Finally, it is emphasized that the proposed approach is compatible with conventional Si technology and naturally provides augmented mechanical flexibility that substantially widens the field of application of thermal harvesting