Co-reporter:Binqiang Zhou, Shuai Li, Wen Li, Juan Li, Xinyue Zhang, Siqi Lin, Zhiwei Chen, and Yanzhong Pei
ACS Applied Materials & Interfaces October 4, 2017 Volume 9(Issue 39) pp:34033-34033
Publication Date(Web):September 12, 2017
DOI:10.1021/acsami.7b08770
Tin sulfide (SnS), a low-cost compound from the IV–VI semiconductors, has attracted particular attention due to its great potential for large-scale thermoelectric applications. However, pristine SnS shows a low carrier concentration, which leads to a low thermoelectric performance. In this work, sodium is utilized to substitute Sn to increase the hole concentration and consequently improve the thermoelectric power factor. The resultant Hall carrier concentration up to ∼1019 cm–3 is the highest concentration reported so far for this compound. This further leads to the highest thermoelectric figure of merit, zT of 0.65, reported so far in polycrystalline SnS. The temperature-dependent Hall mobility shows a transition of carrier-scattering source from a grain boundary potential below 400 K to acoustic phonons at higher temperatures. The electronic transport properties can be well understood by a single parabolic band (SPB) model, enabling a quantitative guidance for maximizing the thermoelectric power factor. Using the experimental lattice thermal conductivity, a maximal zT of 0.8 at 850 K is expected when the carrier concentration is further increased to ∼1 × 1020 cm–3, according to the SPB model. This work not only demonstrates SnS as a promising low-cost thermoelectric material but also details the material parameters that fundamentally determine the thermoelectric properties.Keywords: carrier concentration; SnS; SPB model; thermoelectric; zT;
Co-reporter:Xinyue Zhang, Zhiwei Chen, Siqi Lin, Binqiang Zhou, Bo Gao, and Yanzhong Pei
ACS Energy Letters October 13, 2017 Volume 2(Issue 10) pp:2470-2470
Publication Date(Web):September 26, 2017
DOI:10.1021/acsenergylett.7b00813
New materials have been playing a continuously more important role in advancing thermoelectric technology. Many known novel thermoelectric materials share the similarity of an intrinsic low lattice thermal conductivity (κL) due to various mechanisms. Because heat is generally conducted by acoustic phonons because of the much higher velocities as compared to those of optical phonons, many known low-κL thermoelectrics rely on the complexity of crystal structure to lead the fraction of acoustic phonons to be small. In addition to structural complexity, an overall low sound velocity is found to be helpful for realizing an extremely low κL. In this work, a new thermoelectric compound Ag5−δTe3, having both a complex crystal structure and a low sound velocity (∼1300 m/s), is shown to be one of the least thermally conductive dense solids (κκL ∼ 0.2 W/m·K). The resultant high thermoelectric figure of merit, zT, of unity, with the help of its large band gap of ∼0.6 eV, leads this material to be superior to known silver tellurides. These preliminary results demonstrate Ag5−δTe3 as a promising thermoelectric material, with possibilities for further improvements particularly through enhancements focusing on electronic properties.
Co-reporter:Wen Li, Yixuan Wu, Siqi Lin, Zhiwei Chen, Juan Li, Xinyue Zhang, Linglang Zheng, and Yanzhong Pei
ACS Energy Letters October 13, 2017 Volume 2(Issue 10) pp:2349-2349
Publication Date(Web):September 6, 2017
DOI:10.1021/acsenergylett.7b00658
PbTe has been leading the advancements in the field of thermoelectricity due to its capability for demonstrating and integrating various new concepts. However, the toxicity of Pb is always a concern for terrestrial applications, which inspired great advancement to be achieved very recently in its alternative analogue SnTe. Challenges making p-type SnTe as thermoelectrically efficient as PbTe rely on a reduction of its carrier concentration, valence band offset, and lattice thermal conductivity. Utilization of newly developed concepts including both band and defect engineering amazingly increases the thermoelectric figure of merit, zT, from 0.4 up to 1.6 while remaining a nontoxic composition. The corresponding conceptual route diagram is surveyed, and future considerations on composition, crystal structure, and microstructure for further advancements are discussed in this Perspective. Concepts discussed here not only have promoted SnTe as a highly efficient environment-friendly thermoelectric material but also guided advancements in many other thermoelectrics.
Co-reporter:Juan Li, Xinyue Zhang, Siqi Lin, Zhiwei Chen, and Yanzhong Pei
Chemistry of Materials 2017 Volume 29(Issue 2) pp:
Publication Date(Web):December 5, 2016
DOI:10.1021/acs.chemmater.6b04066
GeTe-based alloys have been intensively considered as p-type thermoelectrics for about 50 years, yet existing literature barely discussed the thermoelectric properties of pristine GeTe at high temperatures (300–800 K). This work first backs to a fundamental understanding on the thermoelectric transport properties inherent to p-type GeTe, based on more than 50 samples synthesized with expected carrier concentrations ranging from 1 × 1020 to 3 × 1021 cm–3. A thermoelectric figure of merit zT as high as ∼1.7 is found inherent to this compound when it is optimally doped with a Hall carrier concentration of 2.2 ± 10% × 1020 cm–3, offering a reference substance to expose the origins for the high zT in historical GeTe-based alloys. Guided by the above knowledge, further alloying Te with Se in samples with an optimal carrier concentration enables a reduction on the lattice thermal conductivity by ∼40% and eventually leads to a further enhancement on zT (up to 2.0) by ∼20%. This work demonstrates not only GeTe as an inherently high performance thermoelectric matrix compound but also its availability for further improvements by additional strategies.
Co-reporter:Yidong Xu;Wen Li;Chen Wang;Juan Li;Zhiwei Chen;Siqi Lin;Yue Chen
Journal of Materials Chemistry A 2017 vol. 5(Issue 36) pp:19143-19150
Publication Date(Web):2017/09/19
DOI:10.1039/C7TA04842D
Semiconducting MnTe has long been considered a potential thermoelectric material for p-type conduction. However, its low carrier concentration limits its peak thermoelectric performance, zT, which is only ∼0.6. In this study, Na and Ag are found to enable a significant increase in hole concentration from ∼1019 cm−3 in pristine MnTe to ∼1021 cm−3 in the doped samples, leading to an effective enhancement in power factor in the entire temperature range investigated. Moreover, doping simultaneously introduces additional phonon scattering by point defects and secondary phases, leading to a reduction in lattice thermal conductivity, as low as ∼0.6 W m−1 K−1, approaching the amorphous limit. The synergic effects of carrier concentration optimization and lattice thermal conductivity reduction realize a peak zT as high as 1.0, which is one of the highest reported thus far for this thermoelectric material. The broad carrier concentration achieved in this study enables a reasonable assessment of the electronic transport properties of MnTe, which reveals effective single parabolic band behavior with dominant carrier scattering by acoustic phonons. It is further expected, according to the model, that a peak zT up to 1.1 could be achieved once the lattice thermal conductivity is further reduced. Moreover, the valence band structure strongly suggests the probability of a well-improved zT through a further band engineering approach due to the existence of low-lying bands with large valley degeneracies. This study not only demonstrates that MnTe is a promising thermoelectric material, but also provides guidance for its further optimization.
Co-reporter:Xiao Wang;Wen Li;Chen Wang;Juan Li;Xinyue Zhang;Binqiang Zhou;Yue Chen
Journal of Materials Chemistry A 2017 vol. 5(Issue 46) pp:24185-24192
Publication Date(Web):2017/11/28
DOI:10.1039/C7TA08869H
Zintl compounds are usually rich in composition and thus enable a large degree of manipulation in both electronic and phononic properties for potential thermoelectric applications. This is typified by the AB2C2 compounds (A = Eu, Yb, Ba, Ca; B = Zn, Cd, Mg and C = As, Sb, Bi) that have attracted extensive attention. Among this class of compounds, a few existing works indicate that the high thermoelectric performance in p-type EuZn2Sb2 relies on its relatively high mobility and the reported figures of merit are scattered. This has motivated this study to focus on the thermoelectric transport properties of p-EuZn2−xAgxSb2 (Ag-doped) in a broad carrier concentration range (3.5–14 × 1019 cm−3), which reveals a single parabolic band (SPB) behavior in this material and enables insights into the fundamental material parameters determining the thermoelectric performance. This finding has guided a further enhancement to be achieved by a reduction in the lattice thermal conductivity, which is realized by a strong phonon scattering through the Ca/Eu isovalent substitutional defects. The achieved peak figure of merit, zT, was as high as unity, demonstrating EuZn2Sb2 as a promising thermoelectric material.
Co-reporter:Wen Li;Linglang Zheng;Binghui Ge;Siqi Lin;Xinyue Zhang;Zhiwei Chen;Yunjie Chang
Advanced Materials 2017 Volume 29(Issue 17) pp:
Publication Date(Web):2017/05/01
DOI:10.1002/adma.201605887
Compared to commercially available p-type PbTe thermoelectrics, SnTe has a much bigger band offset between its two valence bands and a much higher lattice thermal conductivity, both of which limit its peak thermoelectric figure of merit, zT of only 0.4. Converging its valence bands or introducing resonant states is found to enhance the electronic properties, while nanostructuring or more recently introducing interstitial defects is found to reduce the lattice thermal conductivity. Even with an integration of some of the strategies above, existing efforts do not enable a peak zT exceeding 1.4 and usually involve Cd or Hg. In this work, a combination of band convergence and interstitial defects, each of which enables a ≈150% increase in the peak zT, successfully accumulates the zT enhancements to be ≈300% (zT up to 1.6) without involving any toxic elements. This opens new possibilities for further improvements and promotes SnTe as an environment-friendly solution for conventional p-PbTe thermoelectrics.
Co-reporter:Juan Li;Zhiwei Chen;Xinyue Zhang;Hulei Yu;Zihua Wu;Huaqing Xie;Yue Chen
Advanced Science 2017 Volume 4(Issue 12) pp:
Publication Date(Web):2017/12/01
DOI:10.1002/advs.201700341
AbstractIn order to locate the optimal carrier concentrations for peaking the thermoelectric performance in p-type group IV monotellurides, existing efforts focus on aliovalent doping, either to increase (in PbTe) or to decrease (in SnTe and GeTe) the hole concentration. The limited solubility of aliovalent dopants usually introduces insufficient phonon scattering for thermoelectric performance maximization. With a decrease in the size of cation, the concentration of holes, induced by cation vacancies in intrinsic compounds, increases rapidly from ≈1018 cm−3 in PbTe to ≈1020 cm−3 in SnTe and then to ≈1021 cm−3 in GeTe. This motivates a strategy here for reducing the carrier concentration in GeTe, by increasing the mean size of cations and vice-versa decreasing the average size of anions through isovalent substitutions for increased formation energy of cation vacancy. A combination of the simultaneously resulting strong phonon scattering due to the high solubility of isovalent impurities, an ultrahigh thermoelectric figure of merit, zT of 2.2 is achieved in GeTe–PbSe alloys. This corresponds to a 300% enhancement in average zT as compared to pristine GeTe. This work not only demonstrates GeTe as a promising thermoelectric material but also paves the way for enhancing the thermoelectric performance in similar materials.
Co-reporter:Zihang Liu, Jun Mao, Shengyuan Peng, Binqiang Zhou, Weihong Gao, Jiehe Sui, Yanzhong Pei, Zhifeng Ren
Materials Today Physics 2017 Volume 2(Volume 2) pp:
Publication Date(Web):1 September 2017
DOI:10.1016/j.mtphys.2017.08.002
•Te doping on the Sb site, as an n-type strong donor, could significantly suppress the high-temperature bipolar effect in the nanostructured Zintl Zr3Ni3Sb4.•Both high majority-carrier concentration and enlarged band gap lead to the significant suppression of high-temperature bipolar effect upon Te doping.•Both carrier mobility and power factor by Te doping, due to the detrimental effect of ionized scattering, are lower than that of Cu doping in which the mixed acoustic phonon and ionized impurity scattering dominates.Thermoelectric figure of merit ZT has been greatly improved in the past decade via band engineering to enhance power factor or nanostructuring to reduce thermal conductivity, but less attention has been paid to other significant factors, e.g., carrier scattering mechanism, bipolar effect, etc. Here we show that Te doping on the Sb site, as an n-type strong donor, could significantly suppress the high-temperature bipolar effect in the nanostructured Zintl Zr3Ni3Sb4, which can be ascribed to the combination of high majority-carrier concentration and enlarged band gap. A relatively good ZT of ∼0.6 at 773 K for Te doping can be achieved and that is almost double of the previous reported ZT by Cu doping. In addition, the role of carrier scattering mechanism on the low-temperature electrical transport properties is also pointed out, where both carrier mobility and power factor of Te doping, due to the detrimental effect of ionized impurity scattering, are lower than that of Cu doping in which the mixed acoustic phonon and ionized impurity scattering dominates.Download full-size image
Co-reporter:Yixuan Wu, Wen Li, Alireza Faghaninia, Zhiwei Chen, Juan Li, Xinyue Zhang, Bo Gao, Siqi Lin, Binqiang Zhou, Anubhav Jain, Yanzhong Pei
Materials Today Physics 2017 Volume 3(Volume 3) pp:
Publication Date(Web):1 December 2017
DOI:10.1016/j.mtphys.2017.10.001
•Large inter-layer distance is shown to be helpful for a low lattice thermal conductivity in layered compounds.•Br is found to be an effective donor for SnSe2 to tune the carrier concentration.•Fundamental transport-property parameters of SnSe2 are determined by a single parabolic band approximation.•SnSe2 show a comparable zT to that of polycrystalline SnSe with optimized carrier concentration.SnSe as a lead-free IV–VI semiconductor, has attracted intensive attention for its potential thermoelectric applications, since it is less toxic and much cheaper than conventional PbTe and PbSe thermoelectrics. Here we focus on its sister layered compound SnSe2 in n-type showing a thermoelectric performance to be similarly promising as SnSe in the polycrystalline form. This is enabled by its favorable electronic structure according to first principle calculations, its capability to be effectively doped by bromine on selenium site to optimize the carrier concentration, as well as its intrinsic lattice thermal conductivity as low as 0.4 W/m-K due to the weak van der Waals force between layers. The broad carrier concentration ranging from 0.5 to 6 × 1019 cm−3 realized in this work, further leads to a fundamental understanding on the material parameters determining the thermoelectric transport properties, based on a single parabolic band (SPB) model with acoustic scattering. The layered crystal structure leads to a texture in hot-pressed polycrystalline materials and therefore anisotropic transport properties, which can be well understood by the SPB model. This work not only demonstrates SnSe2 as a promising thermoelectric material but also guides the further improvements particularly by band engineering and texturing approaches.Download full-size image
Co-reporter:Jiawen Shen;Xinyue Zhang;Zhiwei Chen;Siqi Lin;Juan Li;Wen Li;Shasha Li;Yue Chen
Journal of Materials Chemistry A 2017 vol. 5(Issue 11) pp:5314-5320
Publication Date(Web):2017/03/14
DOI:10.1039/C6TA10770B
It is known that phonon scattering by point defects is effective for reducing the lattice thermal conductivity due to the mass and strain fluctuations between the host and guest atoms. Therefore a high concentration of defects having big mass and strain fluctuations is desired. Based on this strategy, this work focuses on the effect of Ag/Cu substitution on reducing the lattice thermal conductivity in CuGaTe2. It is seen that the lattice thermal conductivity can be significantly reduced by a factor of 4 when >30% Cu is substituted by isovalent Ag, which further leads to a great enhancement in the thermoelectric figure of merit, zT in the entire temperature range. The peak zT of ∼1.0 at 750 K is obtained in the samples with an optimal carrier concentration, which is one of the highest reported so far for this material in a single phase at the same temperature. This work demonstrates CuGaTe2 as a promising thermoelectric material and the point defect scattering as an effective strategy for enhancing its zT.
Co-reporter:Siqi Lin;Wen Li;Xinyue Zhang;Juan Li;Zhiwei Chen
Inorganic Chemistry Frontiers 2017 vol. 4(Issue 6) pp:1066-1072
Publication Date(Web):2017/06/13
DOI:10.1039/C7QI00138J
As one of the focus areas, phonon scattering by boundary interfaces enables an effective reduction in the lattice thermal conductivity for improving thermoelectrics. Introduction of boundary interfaces is expected through precipitation by low-temperature processing of over-saturated solids or melts. In this study, a single step of both doping and precipitation is achieved in elemental tellurium with a few percent of Sb, leading to a high thermoelectric figure of merit, zT, of 0.9. This is quite comparable with the As-doped Te reported previously, but without involving any toxic elements. Evolutionarily, Sb-substitution of Te within 0.5% sufficiently increases the hole concentration leading to an optimized thermoelectric power factor, while higher concentration of Sb introduces Sb2Te3 precipitates, enabling an effective phonon scattering for a reduced lattice thermal conductivity by ∼25%. This study further demonstrates tellurium as a promising elemental thermoelectric material from 300 to 700 K.
Co-reporter:Linglang Zheng, Wen Li, Siqi Lin, Juan Li, Zhiwei Chen, and Yanzhong Pei
ACS Energy Letters - New in 2016 2017 Volume 2(Issue 3) pp:
Publication Date(Web):February 2, 2017
DOI:10.1021/acsenergylett.6b00671
SnTe, as a top Pb-free alternative to PbTe, attracts extensive attention for thermoelectric applications. For thermoelectric performance enhancement, successful electronic strategies are typified by band convergence and resonant doping, while effective thermal strategies include nanostructuring and, recently, interstitial defects. This work demonstrates that phonon scattering by interstitial defects, as a nearly immune strategy integratable to band convergence, independently reduces the lattice thermal conductivity to the amorphous limit. This leads to a zT as high as 1.4 in Sn1–yMgyTe(Cu2Te)x, where Cu2Te acts as the source of interstitial defects while MgTe converges the valence bands. Evolutionarily, either Cu2Te or MgTe enables a ∼100% zT-enhancement as compared with that of pristine SnTe, while an overall ∼200% enhancement is successfully realized when both exist. This work not only improves SnTe as an eco-friendly alterative to thermoelecric PbTe but also demonstrates an approach potentially applicable for improving thermoelectrics.
Co-reporter:Wen Li, Siqi Lin, Xinyue Zhang, Zhiwei Chen, Xiangfan Xu, and Yanzhong Pei
Chemistry of Materials 2016 Volume 28(Issue 17) pp:6227
Publication Date(Web):August 22, 2016
DOI:10.1021/acs.chemmater.6b02416
Phonon scattering by point defects has been proven as an effective strategy for thermoelectric performance enhancements through reducing lattice thermal conductivity. This type of scattering largely relies on the mass and strain fluctuations between host and guest atoms, both of which can be maximized by vacancies as demonstrated in a few thermoelectric solid solutions showing a significant reduction in the lattice thermal conductivity. Here we show Cu2SnSe4, a new compound with intrinsic vacancies on the cation site, as a promising thermoelectric material due to the low lattice thermal conductivity of 0.6 W m–1 K–1 intrinsically resulting from the vacancies. A peak thermoelectric figure of merit (zT) of ∼0.6 is achievable in this compound without relying on additional approaches such as nanostructuring or band engineering. This result demonstrates the existence of intrinsic vacancies as an important guidance for exploring new thermoelectric materials.
Co-reporter:Jiawen Shen, Zhiwei Chen, Siqi lin, Linglang Zheng, Wen Li and Yanzhong Pei
Journal of Materials Chemistry A 2016 vol. 4(Issue 1) pp:209-214
Publication Date(Web):16 Nov 2015
DOI:10.1039/C5TC03325J
The existence of a noticeable discrepancy in thermoelectric properties reported in the literature motivates the current work on the transport properties of CuGaTe2. Taking Zn- and Mn-doping at the Ga site as an example, the hole concentration can be effectively tuned within 1018–1020 cm−3 that enables a reliable assessment of the transport properties. It is evident that both temperature and carrier concentration dependent transport properties follow well within the framework of a single parabolic band approximation with a dominant carrier scattering by acoustic phonons. This work helps distinguish the effects that contribute to the high thermoelectric figure of merit zT in CuGaTe2. The modeling further suggests that this compound can show a thermoelectric figure of merit of unity or higher, when further strategies are taken for reducing the lattice thermal conductivity and engineering the band structure.
Co-reporter:Yanzhong Pei;Linglang Zheng;Wen Li;Siqi Lin;Zhiwei Chen;Yanying Wang;Xiangfan Xu;Hulei Yu;Yue Chen;Binghui Ge
Advanced Electronic Materials 2016 Volume 2( Issue 6) pp:
Publication Date(Web):
DOI:10.1002/aelm.201600019
Due to point defect phonon scattering, formation of solid solutions has long been considered as an effective approach for enhancing thermoelectric performance through reducing the lattice thermal conductivity. The scattering of phonons by point defects mainly comes from the mass and strain fluctuations between the guest and the host atoms. Both the fluctuations can be maximized by point defects of interstitial atoms and/or vacancies in a crystal. Here, a demonstration of phonon scattering by interstitial Cu atoms is shown, leading to an extremely low lattice thermal conductivity of 0.5 W m−1 K−1 in SnTe-Cu2Te solid solutions. This is the lowest lattice thermal conductivity reported in SnTe-based materials so far, which is actually approaching the amorphous limit of SnTe. As a result, a peak thermoelectric figure of merit, zT, higher than 1 is achieved in Sn0.94Cu0.12Te at 850 K, without relying on other approaches for electrical performance enhancements. The strategy used here is believed to be equally applicable in thermoelectrics with interstitial point defects.
Co-reporter:Zhengzhong Jian, Zhiwei Chen, Wen Li, Jiong Yang, Wenqing Zhang and Yanzhong Pei
Journal of Materials Chemistry A 2015 vol. 3(Issue 48) pp:12410-12417
Publication Date(Web):10 Nov 2015
DOI:10.1039/C5TC03068D
Alloying, to minimize the energy offset between the two valence bands of PbTe, has been well demonstrated to yield thermoelectric performance enhancements. Distinct from the currently known alloying species such as MnTe, MgTe, CdTe, PbSe and PbS, where the high performance partially comes from the alloy phonon scattering as well as because of the high alloying content of ≥3%, YbTe, even with a concentration as low as 1%, is found here to have an extraordinary role in engineering the band structure to achieve a thermoelectric figure of merit, zT, approaching 1.8, without relying on strong phonon scattering. This work demonstrates new insight into the further improvement of thermoelectric materials, because this strategy leaves the feasibility of lattice thermal conductivity reduction nearly unaffected, which can be obtained by other well-demonstrated approaches such as nanostructuring.
Co-reporter:Wen Li, Zhiwei Chen, Siqi Lin, Yunjie Chang, Binghui Ge, Yue Chen, Yanzhong Pei
Journal of Materiomics 2015 Volume 1(Issue 4) pp:307-315
Publication Date(Web):December 2015
DOI:10.1016/j.jmat.2015.09.001
The thermoelectric figure of merit zT of SnTe, an analogue to PbTe, has long been known to be about 0.6, mainly due to its single band transport behavior. Similar to what has been found in PbTe, alloying with group II monotellurides such as CdTe, HgTe, MgTe enables a reduced energy separation between the two valence bands of SnTe, leading to converged bands for a significantly increased zT to ∼1.1 with demonstrated further improvements by other independent strategies such as nanostructuring. Here we show alloying with highly soluble MnTe not only tunes the band structure but also reduces the lattice thermal conductivity, leading to a record zT of ∼1.3 at 900 K in the alloy form that does not rely on additional mechanisms for lattice thermal conductivity reduction. This work demonstrates Sn1−xMnxTe as a potential alternative for PbTe with toxic Pb.According to our previous experiences on successful manipulation of band structure by alloying, this work is motivated by a heavily alloying strategy for aligning the bands from the well-separated ones in pristine SnTe. The resulting well-aligned conducting channels (bands) for charge carriers and high-concentration blocking centers for heat carriers (phonons) propagation, lead to a record thermoelectric figure of merit, zT = 1.3 in SnTe alloys. Most importantly, the obtained high performance does not rely on any other independent strategies than alloying for even lower thermal conductivity, enabling a bright future for a further improvement.Figure optionsDownload full-size imageDownload as PowerPoint slide
Co-reporter:Yanzhong Pei;Zachary M. Gibbs;Andrei Gloskovskii;Benjamin Balke;Wolfgang G. Zeier;G. Jeffrey Snyder
Advanced Energy Materials 2014 Volume 4( Issue 13) pp:
Publication Date(Web):
DOI:10.1002/aenm.201400486
Taking La- and I-doped PbTe as an example, the current work shows the effects of optimizing the thermoelectric figure of merit, zT, by controlling the doping level. The high doping effectiveness allows the carrier concentration to be precisely designed and prepared to control the Fermi level. In addition to the Fermi energy tuning, La-doping modifies the conduction band, leading to an increase in the density of states effective mass that is confirmed by transport, infrared reflectance and hard X-ray photoelectron spectroscopy measurements. Taking such a band structure modification effect into account, the electrical transport properties can then be well-described by a self-consistent single non-parabolic Kane band model that yields an approximate (m*T)1.5 dependence of the optimal carrier concentration for a peak power factor in both doping cases. Such a simple temperature dependence also provides an effective approximation of carrier concentration for a peak zT and helps to explain, the effects of other strategies such as lowering the lattice thermal conductivity by nanostructuring or alloying in n-PbTe, which demonstrates a practical guide for fully optimizing thermoelectric materials in the entire temperature range. The principles used here should be equally applicable to other thermoelectric materials.
Co-reporter:Xinyue Zhang, Jiawen Shen, Siqi Lin, Juan Li, Zhiwei Chen, Wen Li, Yanzhong Pei
Journal of Materiomics (December 2016) Volume 2(Issue 4) pp:
Publication Date(Web):December 2016
DOI:10.1016/j.jmat.2016.09.001
The recently reported superior thermoelectric performance of SnSe, motivates the current work on the thermoelectric properties of polycrystalline GeSe, an analog compound with the same crystal structure. Due to the extremely low carrier concentration in intrinsic GeSe, various dopants are utilized to substitute either Ge or Se for increasing the carrier concentration and therefore for optimizing the thermoelectric power factor. It is shown that Ag-substitution on Ge site is the most effective, which enables a hole concentration up to ∼1018 cm−3. A further isovalent substitution by Pb and Sn leads to an effective reduction in the lattice thermal conductivity. A peak figure of merit, zT of ∼0.2 at 700 K can be achieved in Ag0.01Ge0.79Sn0.2Se, a composition with the highest carrier concentration. The transport properties can be well described by a single parabolic band model with a dominant carrier scattering by acoustic phonons at high temperatures (>500 K). This further enables a prediction on the maximal zT of ∼0.6 at 700 K and the corresponding carrier concentration of ∼5 × 1019 cm−3.A peak figure of merit, zT of ∼0.2 at 700 K is achieved in Ag0.01Ge0.79Sn0.2Se, a composition with the highest carrier concentration, through a combination method of doping and alloying. The transport properties can be well described by a single parabolic band model which further enables a prediction on the maximal zT of ∼0.6 at 700 K and the corresponding carrier concentration of ∼5 × 1019 cm−3, indicating that GeSe shows a potentially high thermoelectric figure of merit but requires a much higher carrier concentration.Figure optionsDownload full-size imageDownload as PowerPoint slide
Co-reporter:Jiawen Shen, Xinyue Zhang, Siqi Lin, Juan Li, Zhiwei Chen, Wen Li and Yanzhong Pei
Journal of Materials Chemistry A 2016 - vol. 4(Issue 40) pp:NaN15470-15470
Publication Date(Web):2016/09/02
DOI:10.1039/C6TA06033A
Enhancing the thermoelectric performance through an effective phonon scattering by point defects has long been proven to be successful in many materials. This type of phonon scattering relies on the mass and strain fluctuations between the host and guest atoms, both of which can be maximized when the dominant point defects are vacancies. Besides the intrinsic vacancies in some compounds by nature, the formation of solid solutions with a solvent having a smaller cation-to-anion ratio as compared to the matrix is expected to create vacancies because the crystal structure needs to be stabilized in that of the matrix compound. In this work, In2Te3 and Ga2Te3i, compounds with a smaller cation-to-anion ratio as compared to the CuGaTe2 matrix, are chosen as molecular solvents to form solid solutions. The resulting high concentration vacancies on the cation sites, which can act as the phonon scattering centers, significantly reduce the lattice thermal conductivity and therefore enhance the thermoelectric performance by up to ∼75% in the entire temperature range. This work demonstrates a useful strategy for enhancing thermoelectric performance by vacancy creation in solid solutions.
Co-reporter:Jiawen Shen, Zhiwei Chen, Siqi lin, Linglang Zheng, Wen Li and Yanzhong Pei
Journal of Materials Chemistry A 2016 - vol. 4(Issue 1) pp:NaN214-214
Publication Date(Web):2015/11/16
DOI:10.1039/C5TC03325J
The existence of a noticeable discrepancy in thermoelectric properties reported in the literature motivates the current work on the transport properties of CuGaTe2. Taking Zn- and Mn-doping at the Ga site as an example, the hole concentration can be effectively tuned within 1018–1020 cm−3 that enables a reliable assessment of the transport properties. It is evident that both temperature and carrier concentration dependent transport properties follow well within the framework of a single parabolic band approximation with a dominant carrier scattering by acoustic phonons. This work helps distinguish the effects that contribute to the high thermoelectric figure of merit zT in CuGaTe2. The modeling further suggests that this compound can show a thermoelectric figure of merit of unity or higher, when further strategies are taken for reducing the lattice thermal conductivity and engineering the band structure.
Co-reporter:Siqi Lin, Wen Li, Xinyue Zhang, Juan Li, Zhiwei Chen and Yanzhong Pei
Inorganic Chemistry Frontiers 2017 - vol. 4(Issue 6) pp:NaN1072-1072
Publication Date(Web):2017/04/27
DOI:10.1039/C7QI00138J
As one of the focus areas, phonon scattering by boundary interfaces enables an effective reduction in the lattice thermal conductivity for improving thermoelectrics. Introduction of boundary interfaces is expected through precipitation by low-temperature processing of over-saturated solids or melts. In this study, a single step of both doping and precipitation is achieved in elemental tellurium with a few percent of Sb, leading to a high thermoelectric figure of merit, zT, of 0.9. This is quite comparable with the As-doped Te reported previously, but without involving any toxic elements. Evolutionarily, Sb-substitution of Te within 0.5% sufficiently increases the hole concentration leading to an optimized thermoelectric power factor, while higher concentration of Sb introduces Sb2Te3 precipitates, enabling an effective phonon scattering for a reduced lattice thermal conductivity by ∼25%. This study further demonstrates tellurium as a promising elemental thermoelectric material from 300 to 700 K.
Co-reporter:Jiawen Shen, Xinyue Zhang, Zhiwei Chen, Siqi Lin, Juan Li, Wen Li, Shasha Li, Yue Chen and Yanzhong Pei
Journal of Materials Chemistry A 2017 - vol. 5(Issue 11) pp:NaN5320-5320
Publication Date(Web):2017/02/08
DOI:10.1039/C6TA10770B
It is known that phonon scattering by point defects is effective for reducing the lattice thermal conductivity due to the mass and strain fluctuations between the host and guest atoms. Therefore a high concentration of defects having big mass and strain fluctuations is desired. Based on this strategy, this work focuses on the effect of Ag/Cu substitution on reducing the lattice thermal conductivity in CuGaTe2. It is seen that the lattice thermal conductivity can be significantly reduced by a factor of 4 when >30% Cu is substituted by isovalent Ag, which further leads to a great enhancement in the thermoelectric figure of merit, zT in the entire temperature range. The peak zT of ∼1.0 at 750 K is obtained in the samples with an optimal carrier concentration, which is one of the highest reported so far for this material in a single phase at the same temperature. This work demonstrates CuGaTe2 as a promising thermoelectric material and the point defect scattering as an effective strategy for enhancing its zT.
Co-reporter:Zhengzhong Jian, Zhiwei Chen, Wen Li, Jiong Yang, Wenqing Zhang and Yanzhong Pei
Journal of Materials Chemistry A 2015 - vol. 3(Issue 48) pp:NaN12417-12417
Publication Date(Web):2015/11/10
DOI:10.1039/C5TC03068D
Alloying, to minimize the energy offset between the two valence bands of PbTe, has been well demonstrated to yield thermoelectric performance enhancements. Distinct from the currently known alloying species such as MnTe, MgTe, CdTe, PbSe and PbS, where the high performance partially comes from the alloy phonon scattering as well as because of the high alloying content of ≥3%, YbTe, even with a concentration as low as 1%, is found here to have an extraordinary role in engineering the band structure to achieve a thermoelectric figure of merit, zT, approaching 1.8, without relying on strong phonon scattering. This work demonstrates new insight into the further improvement of thermoelectric materials, because this strategy leaves the feasibility of lattice thermal conductivity reduction nearly unaffected, which can be obtained by other well-demonstrated approaches such as nanostructuring.
