Design of Next Generation Thermoelectrics

It is remarkable that almost 2/3 of utilized energy in the world is lost; most of it in the form of waste heat. In this context, thermoelectric materials present a unique opportunity to convert even a small fraction of such massive lost thermal energy to useful electricity. The thermoelectric figure of merit, which is the ratio of power factor and thermal conductivity, represents a fascinating dichotomy of “contraindicated” behavior. The project team has been at the vanguard of this field and has made significant and sustained contributions towards the fundamental understanding of the complex interplay among the thermoelectric parameters. Our collaborative work has resulted in world-record breaking ZT; accomplished by innovative strategies to significantly reduce the thermal conductivity through panoscopic all-length-scale architecturing of nanostructured thermoelectrics, which enables efficient scattering of heat carrying phonons across relevant length-scales. While further reduction in thermal conductivity may be possible, we propose that the biggest gains in thermoelectrics must now come from enhancement in power factor, primarily through advances in our scientific understanding in significantly increasing the Seebeck coefficient. We hypothesize that alloying lead, tin and germanium chalcogenides will significantly modify the nature of both of the valence and conduction bands to achieve band convergence in the electronic structure. These systems are likely to feature multiple pockets of electrons (n-type) and holes (p-type) at or near the Fermi level through appropriate doping for large enhancements in the power factor. Coupled with the control over nanostructured phases and interfaces, these ideas provide fertile ground for innovations through a combined theoretical and experimental undertaking. In the proposed research, we will tackle the following formidable scientific objectives for the enhancement of the power factor in chalcogenides: (i) Band-engineering by leveraging multi-band strategies to enhance Seebeck coefficient: By manipulating the band structure of thermoelectric chalcogenides via alloying, we will systematically examine the possibility of bringing the multiple bands (near the Fermi level) closer in energy to achieve band degeneracy, thereby enhancing the Seebeck coefficients. Particularly promising is the ability to control the valence band in p-type lead and other chalcogenides. Our proposed work will identify broad guiding principles in increasing the band degeneracy by controlling the energy levels of the thermoelectric matrix. (ii) Band-alignment between the second phases and matrix: Charge scattering across the matrix-precipitate interfaces deteriorates carrier mobilities and power factors in nanostructured systems. We hypothesize that controlling and minimizing the band offsets across these interfaces can minimize this scattering and hence retain high mobilities. Understanding the physical factors that affect band alignment and charge mobilities is critical in predicting and selecting the optimal second phases. Our proposed work will generate such intellectual insights, enable us to test our hypothesis, and ultimately allow us to design improved thermoelectric materials; based on a rational alloying approach for both nanostructures and matrix. Embedded in these objectives is the need to simultaneously understand and optimize the doping mechanism as well as tailor the nanostructure phase to maintain high mobility of the charge carriers. The proposed research will also invoke all-length-scale hierarch