• ​Pan, Zhenyu, and Wang, Heng*, "A Descriptive Model of Thermoelectric Transport in the Resonant System of PbSe doped with Tl", Journal of Materials Chemistry A, (2019) accepted.

 

In thermoelectrics research, resonant doping is one of the few proven strategies that act on engineering the fundamental relationships between transport properties of a semiconductor. Until recently, how to utilize this strategy is largely a hit-or-miss process. no model is available for experimentalists to understand their results, or to guide rational alloy design. In this article we tried to change this: we presented a model that describes observed thermoelectric properties in a resonant system. We discussed general guidelines on how to optimize energy levels and doping for best thermoelectric performance.  

  • Ma, Hao; Ma, Yunwei; Wang, Heng; Slebodnick, Carla; Alatas, Ahmet; Urban, Jeffrey; Tian, Zhiting, "Experimental Phonon Dispersion and Lifetimes of Tetragonal CH3NH3PbI3 Perovskite Crystals", The Journal of Physical Chemistry Letters 10,1 (2019)

 

In this collaboration with scientists at LBL, Cornell and Argonne, we provided single crystal samples that enabled high quality measurement of phonon dispersion in the iconic hybrid perovskite known for its great photovoltaic performance.

 

  • Wu, Chao-Feng; Wang, Heng*, Yan, Qimin; Wei, Tianran and Li, Jing-Feng*, "Doping of thermoelectric PbSe with chemically inert secondary phase nanoparticles", Journal of Materials Chemistry C 5, 10881-10887 (2017). link

Chemically inert particles such as SiC, SiO2, or Si could "dope" the thermoelectric compound of PbSe. The free carrier density exceeds the upper bound for PbSe without other foreign atoms. This happened, at different extent, regardless of the particles'  composition, conductivity, or hardness. Although the compounds need to be made under non-equilibrium conditions, we believe these samples are doped by intrinsic point defects stablized at the PbSe - particle interface. Similar strategy might help dope other compounds that can not be doped otherwise.

  • Zhoug, Min; Gibbs, Zachary M.; Wang, Heng*; Han, Yemao; Li, Laifeng*; and Snyder, G. Jeffrey, , "Thermoelectric performance of co-doped SnTe with resonant levels", Applied Physics Letters 109, 042102 (2016). link

SnTe doped with In is arguably the first example where resonant dopant made better performing thermoelectric material that is otherwise not achievable. This work is Heng's first published effort to elucidate the influence of resonant dopants on charge transport, using co-doping of another regular dopant. There is not yet a complete theory on how resonant dopants affects either charge density of states, or charge scattering, or both. It is Heng's continued interest to find a way so these resonant dopants can be modeled in Boltzmann transport equations.   

  • Bajaj, Saurabh; Wang, Heng; Doak, Jeff W; Wolverton, Chris; and Snyder, G Jeffrey, "Calculation of dopant solubilities and phase diagrams of X–Pb–Se (X = Br, Na) limited to defects with localized charge", Journal of Materials Chemistry C 4, 1769 (2016). link

Phase diagrams are the "maps" for material synthesis. However conventional method of obtaining phase diagrams from experiments is hard to implement when doping is the concern, especially as we push the limit on how many impurities can be introduced. This work is the first part of an effort to combine DFT defect formation energy calculation with CALPHAD (calculation of phase diagram) to get detailed information on phase boundaries with typical impurity of a few percent.    

  • Wang, Heng; Cao, Xianlong; Takagiwa, Yoshiki; and Snyder, G. Jeffrey, "Higher mobility in bulk semiconductors by separating the dopants from the charge- conducting band – a case study of thermoelectric PbSe", Materials Horizons 2, 323 – 329 (2015). link

On the first order, what a (regular) dopant do to a semiconductor is shifting the chemical potential of its carriers, other than that all dopants are pretty much the same. Choice of dopant is based on convenience for processing. This notion is incorrect for heavily doped semiconductors. This work demonstrated that in polar semiconductors, dopants on cation lattice are different from those on anion lattice. One group lead to higher mobilities than the other. The difference can be explained by different scattering potentials.

 

  • Borup, Kasper A; De Boor, Johannes; Wang, Heng; Drymiotis, Fivos; Gascoin, Franck; Shi, Xun; Chen, Lidong; Fedorov, Mikhail I; Muller, Eckhard; Iversen, Bo B; and Snyder, G. Jeffrey, "Measuring thermoelectric transport properties of materials", Energy&Environmental Science 8, 423 (2015). link

A collaborative article with many experts that discusses better practices for thermoelectric properties measurement at high temperatures.

  • Wang, Heng; Gibbs, Zachary M.; Takagiwa, Yoshiki; and Snyder, G. Jeffrey,  "Tuning bands of PbSe for better thermoelectric efficiency", Energy&Environmental Science 7, 804 (2014). link

Rare-earth elements are "magic" additives to IV-VI thermoelectrics. Many successful studies have used them and achieved higher zTs. There are different explanations as of why. Some suggested they causes more phonon scattering than carrier scattering due to band energy alignment. This work, with careful experimental design, wide composition range under survey, and multiple-band transport modeling, put forward an argument that the benefit comes from favorable band structure change, whereas rare-earths indeed scatter carriers more than phonons. 

  • Chen, Cheng-Lung*; Wang, Heng*; Chen, Yang-Yuan; Day, Tristan; and Snyder, G. Jeffrey,  "Thermoelectric properties of p-type polycrystalline SnSe doped with Ag", Journal of Materials Chemistry A 2, 11171 (2014). link

We published our own study on p type SnSe in polycrystalline form about the same time a record zT of 2.6 was reported on this compound. Our conclusion was much more conservative. we found before phase transition around 800 K, the compound of SnSe does not have the "quality" to be a outstanding thermoelectric material. We also believe polycrystalline SnSe suffer from hysteresis issue through temperature cycles, which must be overcome before any discussion on zT could be meaningful.

  

  • Wang, Heng; Wang, Jianli; Cao, xianlong; and Snyder, G. Jeffrey,  "Thermoelectric alloys between PbSe and PbS with effective thermal conductivity reduction and high figure of merit", Journal of Materials Chemistry A 2, 3169 (2014). link

We quantitatively explained how alloying affects both effective masses and electron scattering in the solid solution of PbSe-PbS, with the use of only one constant fitting parameter. 

  • Wei, Tian-Ran*; Wang, Heng*; Gibbs, Zachary M; Wu, Chao-Feng; Snyder, G. Jeffrey; and Li, Jing-Feng,  "Thermoelectric properties of Sn-doped p-type Cu3SbSe4: a compound with large effective mass and small band gap", Journal of Materials Chemistry A 2, 13527 (2014). link

 

  • Xie, Hanhui; Wang, Heng; Fu, Chengguang; Liu, Yintu; Snyder, G. Jeffrey; Zhao, Xinbing; Zhu, Tiejun, The intrinsic disorder related alloy scattering in ZrNiSn Half-Heusler thermoelectric materials, Scientific Reports 4, 6888 (2014). link

The formula of half-Heusler compound ZrNiSn doesn't suggest any disorder, yet we found it contains a large number of intrinsic disorder such that its transport behavior has the signature of a disordered alloy. This contribute to its good thermoelectric performance.

 

  • Zhou, Min; Gibbs, Zachary M.; Wang Heng; Han, Yemao; Xin, Caini; Li, Laifeng; and Snyder, G. Jeffrey, "Optimization of thermoelectric efficiency in SnTe: the case for the light band" , Physical Chemistry Chemical Physics 16, 20741, (2014). link

  • Wang, H., Pei, Y., LaLonde, A. D. and Snyder, G. J., "Material Design Considerations Based on Thermoelectric Quality Factor", in Thermoelectric Nanomaterials, Springer Series in Materials Science Vol 182, 2013, pp 3-32.link

In this lead chapter of a Springer series in materials science, we discussed the different factors that influence the quality factor, which describes a compound's "quality" as thermoelectrics, given it can be optimized.

  • Wang, H., Schechtel, E., Pei, Y. and Snyder, G. J., "High Thermoelectric Efficiency of n-type PbS", Advanced Energy Materials 3, 488 (2013). link

 

  • Wang, H., LaLonde, A. D., Pei, Y. and Snyder, G. J., "The Criteria for Beneficial Disorder in Thermoelectric Solid Solutions", Advanced Functional Materials 23, 1586 (2013). link

We found the proper way to treat iso-valent atomic substitution in thermoelectric transport equations. once a constant scattering potential is known, the transport of any composition, with any doping level, at any temperature, can be predicted.

  • Xie, Hanhui; Wang, Heng; Pei, Yanzhong; Fu, Chenguang; Liu, Xiaohua; Snyder, G. Jeffrey; Zhao, Xinbing; and Zhu, Tiejun, Beneficial Contribution of Alloy Disorder to Electron and Phonon Transport in Half-Heusler Thermoelectric Materials, Advanced Functional Materials 23, 5123, (2013). link

 

  • Wang, H., Pei, Y., LaLonde, A. D. and Snyder, G. J., "Weak electron-phonon coupling contributing to high thermoelectric performance in n-type PbSe", Proceedings of the National Academy of Sciences of the United States of America 109, 9705, (2012). link 

​We explained why n type Pb chalcogenides are great thermoelectrics even though they don't have high band degeneracy as their p type counterparts. 

Selected Publications

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