Electron transport engineering with nanostructures for efficient thermoelectric materials

Post date: Oct 15, 2009 6:16:57 AM

About 60 % of energy generated in the society is rejected as a form of heat. Thermoelectric (TE) devices directly convert heat energy into electricity or vice versa, so they can be attached onto heat engines such as in automobiles and enhance the fuel efficiency. However, one of the main reasons that TE devices have not been largely employed in many practical applications is their low energy conversion efficiency. 

Thermoelectric energy conversion efficiency is determined by the unitless figure of merit of the TE material used, ZT = S2σT/(κelec+κlat), where S is Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κelec and κlat are, respectively, electronic and lattice thermal conductivities. Conventional TE materials have ZT~1 in their optimal temperature range. Recently, significant enhancements of ZT have been reported for various nanostructured TE materials. The breakthrough has come out by the significant reduction of the lattice thermal conductivity via effective phonon scattering at the boundaries of nanostructures.

However, enhancement of the numerator of ZT, so-called the power factor, S2σ, turns out to be even more difficult because of the trade-off relation between the Seebeck coefficient and the electrical conductivity. Since the power factor is related to charge carrier transport while the thermal conductivity is related to phonon transport, enhancement of power factor requires completely different approaches. Also, it is required that the electronic thermal conductivity remains low when the power factor is enhanced to keep the ZT high. Electronic thermal conductivity increases proportionally with increasing electrical conductivity by the Wiedemann-Franz relation. Bipolar electronic thermal conductivity becomes significant when the carrier density is relatively low.

In our group, we investigate the impact of embedded nanostructures such as nanoparticles, superlattices, and their heterostructure barriers on the electron transport in the host material, and seek the efficient and cost-effective routes towards enhancing the thermoelectric performances of TE materials.

Demonstration of the electron filtering in solution-processed SbTe

with oxide-coated Ag nanoparticles (Figure from Adv. Mater. 2014)

We also investigated nanoparticle scatterings and their impact on the power factor of the host material. With the single-phase nanoparticles on the order of a few nanometers in diameter, the power factors of III-V semiconductors can be enhanced by more than 30 % as presented in our 2011 APL paper. This is due to the replacement of ionized impurities with the ionized nanoparticles that exhibit weaker electron scattering, so that the mobility is increased.

Also, core-shell nanoparticles, when designed properly, can create quasi-bound resonant states, which scatter the electrons near the same energy level with sharp energy selectivity. This energy-sharp scattering characteristics can be utilized to enhance the Seebeck coefficient and the power factor, as highlighted in our 2012 APL paper. 

Core-shell structure nanoparticles and their resonant scattering characteristics (b)

in comparison with the non-resonant case (a). (Figure from APL 2012)

For more information, refer to the following papers and book chapters. 

•  J.-H. Bahk and A. Shakouri, “Electron transport engineering by nanostructures for efficient thermoelectrics,” Chap. 2 in Nanoscale Thermoelectrics, Ed. X. Wang and Z. Wang, a book series of Lecture Notes on Nanoscale Science and Technology vol. 16 (Springer, Nov. 2013).

• C. Kang, H. Wang, J.-H. Bahk, H. Kim, and W. Kim, “Thermoelectric materials and devices,” Chap. 6 in Hierarchical Nanostructures for Energy Devices, RSC Nanoscience and Nanotechnology Series No. 35, Ed. S. H. Ko and C. P. Grigoropoulos (RSC Publishing, 2015).

• L. Shi, C. Dames, J. R. Lukes, P. S. Reddy, J. Duda, D. G. Cahill, J. Lee, A. Marconnet, K. E. Goodson, J.-H. Bahk, Ali Shakouri, R. S. Prasher, J. Felts, W. P. King, B. Han, and J. C. Bischof, “Evaluating broader impacts of nanoscale thermal transport research,” Nanosc. and Microsc. Therm., 19, 127-165 (2015).

• H. Yang, J.-H. Bahk, T. Day, A. M. S. Mohammed, B. Min, G. J. Snyder, A. Shakouri, and Y. Wu, “Composition modulation of Ag2Te nanowires for tunable electrical and thermal properties,” Nano Lett. 14, 5398 (2014). 

• J.-H. Bahk, and A. Shakouri, “Enhancing the thermoelectric figure of merit through the reduction of bipolar thermal conductivity with heterostructure barriers,” Appl. Phys. Lett.105, 052106 (2014).

• Y. Zhang, J.-H. Bahk, J. Lee, C. Birkel, M. Snedaker, D. Liu, H. Zeng, M. Moskovits, A. Shakouri, and G. Stucky, “Hot carrier filtering in solution processed hetero-structures: a paradigm for improving thermoelectric efficiency,” Adv. Mater. 26, 2755 (2014).

• J.-H. Bahk, Z. Bian, and A. Shakouri “Electron transport modeling and energy filtering for efficient thermoelectric Mg2Si1-xSnx solid solutions,” Phys. Rev. B 89, 075204 (2014).

• J.-H. Bahk, Z. Bian, and A. Shakouri, “Electron energy filtering by a non-planar barrier to enhance the thermoelectric power factor in bulk materials,” Phys. Rev. B 87, 075204 (2013).

• J-H. Bahk, Y. Ezzahri, K. Yazawa, B. Vermeersch, G. Pernot, and A. Shakouri, "Nanoscale electrothermal energy conversion devices," Proc. THERMINIC, Vol. 18, Budapest, Hungary (2012).

• J.-H. Bahk, P. Santhanam, Z. Bian, R. Ram, and A. Shakouri, “Resonant carrier scattering by core-shell nanoparticles for thermoelectric power factor enhancement,” Appl. Phys. Lett. 100, 012102 (2012).

• J.-H. Bahk, Z. Bian, M. Zebarjadi, P. Santhanam, R. Ram, and A. Shakouri, “Thermoelectric power factor enhancement by ionized nanoparticle scattering,” Appl. Phys. Lett. 99, 072118 (2011). 

• J. M. O. Zide, J.-H. Bahk, R. Singh, M. Zebarjadi, G. Zeng, H. Lu, J. P. Feser, D. Xu, S. L. Singer, Z. X. Bian, A. Majumdar, J. E. Bowers, A. Shakouri, and A. C. Gossard, “High efficiency semimetal/semiconductor nanocomposite thermoelectric materials,” J. Appl. Phys. 108, 123702 (2010). Erratum