There exist multiple classes of energy conversion materials and devices - including electrochemical cells, photovoltaics, and thermoelectrics. In all systems, heat dissipation can be a limiting factor in determining overall efficiency. In this talk, I will discuss thermoelectric materials (TEs), which are materials that interconvert thermal gradients and electric fields for power generation or for refrigeration. TEs find only niche applications because of their limited efficiency, which is measured by the dimensionless parameter ZT=S2σT/κ. Here S is the Seebeck coefficient, or thermoelectric power (measured in Volts-K−1), and σ and κ are the electrical and thermal conductivities, respectively. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another - for example, the Weidemann-Franz law limits the ratio of σ/κ to be a constant for metallic systems. Several groups have achieved significant improvements in ZT through multi-component nanostructured TEs (5-7), such as Bi2Te3/Sb2Te3 thin film superlattices, or embedded PbSeTe quantum dot superlattices. We recently reported efficient TE performance from the single component system of silicon nanowires (SiNWs) for cross-sectional areas of 10nm × 20nm and 20nm × 20nm. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk Si are achieved over a broad temperature range, including a ZT ~ 1 at 200K. Independent measurements of S, a, and K, combined with theory, indicate that the improved efficiency originates from phonon effects. For the smallest width nanowires, the thermal conductivity was observed to be below the Slack-limit for silicon, implying that fundamentally new physics is being observed in these materials. In addition, phonon drag appears to make important, positive contributions to the nanowires thermopower. These results are expected to apply to other classes of semiconductor nanomaterials, as well as nanostructured bulk materials, and I will discuss some of our recent work that is aimed toward exploring these ideas. Finally, from a practical point of view, thermoelectric or thermocooling applications require both p- and n-type conductors. For Si NWs, both p- and n-type NWs can exhibit a high thermoelectric efficiency, although there are differences between the two materials.
© 2008 Optical Society of AmericaPDF Article