A wide-band-gap p -type thermoelectric material bad on quaternary chalcogenides of Cu 2ZnSn Q 4…Q =S,Se …
Min-Ling Liu,1Fu-Qiang Huang,1,a ͒Li-Dong Chen,1and I-Wei Chen 2
什么往开来1
CAS Key Laboratory of Materials for Energy Conversion,Shanghai Institute of Ceramics,Chine Academy of Sciences,1295DingXi Road,Shanghai 200050,People’s Republic of China 2
Department of Materials Science and Engineering,University of Pennsylvania,Philadelphia,Pennsylvania 19104-6272,USA
͑Received 14January 2009;accepted 13April 2009;published online 19May 2009͒
Chalcopyritelike quaternary chalcogenides,Cu 2ZnSn Q 4͑Q =S,Se ͒,were investigated as an alternative class of wide-band-gap p -type thermoelectric materials.Their distorted diamondlike structure and quaternary compositions are beneficial to lowering lattice thermal conductivities.Meanwhile,partial substitution of Cu for Zn creates more charge carriers and conducting pathways via the Cu Q 4network,enhancing electrical conductivity.The power factor and t
he figure of merit ͑ZT ͒increa with the temperature,making the materials suitable for high temperature applications.For Cu 2.1Zn 0.9Sn Q 4,ZT reaches about 0.4at 700K,rising to 0.9at 860K.©2009American Institute of Physics .͓DOI:10.1063/1.3130718͔
Thermoelectric ͑TE ͒materials can convert thermal en-ergy to electrical energy ͑Seebeck effect ͒or vice versa ͑Peltier effect ͒.The figure of merit of TE materials is a di-mensionless quantity,ZT =͑S 2/͒T ,where T is the absolute temperature,S is the Seebeck coefficient,and and are the electrical and thermal conductivity,respectively.Several bi-nary chalcogenides are known to have high ZT ͑Refs.1–7͒͑a summary is given in Table S1of supporting information 8͒.Within this group,a large band gap E g is correlated with a low ,a high S ,and a low ,and vice versa.In general,it is difficult to achieve a high S ,high ,and low in the same material,especially in a p -type miconductor with a large E g .
There is a similar challenge in the p -type transparent conductive material in that a large E g is usually correlated with a high transmittance ͑of visible light ͒but low ,and vice versa.Recently,we propod to meet this challenge by constructing materials with two structural/functional units,one electrically conducting and the other insulating,one ex-ample being Cu-doped chalcopyrite-type CuAlS 2that has a wide band gap ͑about 3.4eV ͒and a high .9–12Note that chalcopyrites,having a diamondlike tetrahedral framework structure,already include some low-phonon-conductive ma-terials,
such as Cu 2SnS 3and Cu 2SnSe 3,13–17which are ud for infrared transmission.Therefore,it would be interesting to investigate whether the low phonon conduction can sur-vive -enhancing doping in the large band-gap p -type miconductors,thereby providing alternative TE materials.
Specifically,following the concept of two structural/functional units,we propo a doping strategy that enhances by providing more holes in the electrically conducting pathway,and suppress by disordering the insulating pathway.Toward the latter end,a quaternary compound should be advantageous over a ternary one,so the ries of Cu 2ZnSn Q 4͑Q =S,Se ͒is chon for exploratory studies.The materials are already known as potential ͑In-free ͒pho-tovoltaic materials due to their appropriate E g ͑1.4–1.5
eV ͒.18–20Structurally,Cu 2ZnSnS 4and Cu 2ZnSnSe 4crystal-lize in the kesterite and stannite structure types ͓Fig.1͑a ͔͒,respectively.21The two structural units are ͑a ͒the ͓Cu 2Q 4͔tetrahedral slabs,which may be viewed as conducting,and ͑b ͒the ͓SnZn Q 4͔tetrahedral slabs,which may be viewed as insulating.In analogy to Cu doping of the Al site in p -type CuAlS 2,it also follows that Cu doping of the Zn site could be an efficacious strategy to enhance on the ͓Cu 2Q 4͔network and,perhaps,to suppress by disordering the ͓SnZn Q 4͔slabs.To verify this strategy and to explore their potential for high temperature TE applications,we have stud-ied their physical properties up to 8
如果时间能重来
60K.
Powders of Cu 2ZnSn Q 4and Cu 2.1Zn 0.9Sn Q 4͑Q =S,Se ͒were synthesized by solid state reaction of stoichiometric amounts of Cu ͑99.999%,SinoReag ͒,Zn ͑99.99%,Si-noReag ͒,Sn ͑99.999%,SinoReag ͒,and S/Se ͑99.999%,Si-noReag ͒powders in a aled fud silica tube evacuated to Յ1Pa argon.The samples were slowly heated to 923K at
a ͒
Author to whom correspondence should be addresd.Electronic mail:
huangfq@mail.sic.ac.
FIG.1.Crystal structure ͑a ͒of keterite ͑left ͒for Q =S and stannite ͑right ͒for Q =Se,powder XRD patterns ͑b ͒and absorption spectra ͑c ͒of Cu 2ZnSn Q 4and Cu 2.1Zn 0.9Sn Q 4͑Q =S,Se ͒.
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0.5and2K/min for sulfides and lenides,respectively,held for48h,and furnace cooled to the room temperature.The harvested powders were grounded,aled,and calcined for 96h at1123and1073K for sulfides and lenides,respec-tively.Thefinal powders were consolidated in a spark plasma sintering furnace in a graphite die͑⌽10mm͒under a pressure of60MPa at1073K͑for sulfides͒and1023K͑for lenides͒,held for5min in an argon atmosphere.The mea-sured relative densities of all the samples were about97%.
X-ray diffraction͑XRD͒patterns of the bulk samples were obtained͑Rigaku D/Max-2550V͒using Cu K␣radia-tion͑=0.15418nm͒and their compositional homogeneity was examined by electron probe mic
roanalysis͑EPMA͒͑JEOL,JXA-8100͒.Optical absorption spectra of their pul-
verized powders were measured at room temperature by a UV-visible-near IR spectrometer͑HITACHI U-3010͒equipped with an integrating sphere.For transport property measurements,bar samples about 1.5ϫ2ϫ10mm3were ud.Electrical conductivity was measured by a four-probe method.Thermoelectromotive force͑⌬E͒at the test tem-perature was measured usingfive different temperature gra-dients͑0KϽ⌬TϽ2K͒to calculate the Seebeck coefficient
from the⌬E versus⌬T plot.Thermal diffusivity coefficient was determined using a larflash method in aflowing Ar atmosphere͑Netzsch LFA427͒.The thermal conductivity was calculated from=C p,whereis the density and C p is the specific heat capacity.The above physical properties were measured up to700K.Hall coefficients were measured in an Accent HL5500Hall System at room temperature.Ad-ditional experiments at860K were also performed for one material as will be described later.
The XRD patterns͓Fig.1͑b͔͒and EPMA͑not shown͒verified that the samples are pha pure and homogeneous, as all the diffractions peaks can be indexed as Cu2ZnSnS4 and Cu2ZnSnSe4.20After Cu doping,the lattice parameters of a and c decrea͑evidenced by a slight pe
森林管理ak shift toward higher angle and a small increa of peak splitting,data not shown͒,which may be attributed to the smaller Cu2+com-pared to Zn2+,and the moreflattened tetrahedron of Cu2+Q4 compared to Zn2+Q4.At room temperature the optical data ͓Fig.1͑c͔͒plotted as͑␣h͒2against photon energy,where␣is absorption coefficient,h is the Planck constant,and v is the wave number,give an estimated E g of about1.49and 1.41eV for Cu2ZnSnS4and Cu2ZnSnSe4,respectively.The values are consistent with the reported data18–20and much larger than tho of typical binary TE materials.1–7As ex-pected,sulfide has a larger E g than lenide due to more hybridization of Se4p and Cu3d at the valence band maxi-mum͑VBM͒.
The temperature spectra offor Cu2ZnSn Q4and Cu2.1Zn0.9Sn Q4͑Q=S,Se͒in Fig.2͑a͒show a large increa from sulfide to lenide.For both,Cu doping results in a dramatic increa ofreaching2600S m−1for Cu2.1Zn0.9SnS4and86000S m−1for Cu2.1Zn0.9SnSe4, which may be attributed to creation of holes͓Cu2+3d9ver-sus Cu+3d10and conversion of insulating paths͓͑Zn Q4͔͒to conducting paths͓͑Cu Q4͔͒.Although Cu2ZnSnS4is a normal miconductor exhibiting a thermally activated behavior for ,the strong thermal activation is largely removed by Cu doping.Selenide samples show a metallic behavior in anal-ogy to the reported trend for LaCuO Q͑Q=S,Se,Te͒:the sulfide and lenide are miconducting and the telluride is metallic.23Aflat͑T͒has also been obrv
ed in Ln CuOTe ͑Ln=La,Ce,Nd͒23and is indicative of the metallic behavior with a relatively short mean free path limited by defect/ impurity scattering rather than phonon scattering.The fea-tures are all consistent with the interpretation that hole con-ductivity comes from the hybridization of Cu3d with Q n p ͑Q np=S3p,Se4p,Te5p͒near the VBM.23
Hole conduction was verified by both positive Seebeck coefficients͓Fig.2͑b͔͒and Hall coefficients for all the samples͑Hall coefficients are listed in Table S2of support-ing information8͒.The Seebeck coefficients slightly increa with the temperature.The values are comparable to tho of the well-known binary TE materials.Notably,both S and increa with the temperature in Cu2ZnSnS4,which was confirmed using duplicate samples and different measuring instruments͑data not shown͒.The enhancedin doped Cu2.1Zn0.9Sn Q4and the mostly positive temperature depen-dence of S andcau the power factors͑PF=S2͒to in-crea especially at higher temperatures,reaching0.58and 1.01mW m−1K−2at700K for Cu2.1Zn0.9SnS4and Cu2.1Zn0.9SnSe4,respectively͑the PF plots are given in Fig-ure S1in supporting information8͒.
Thevalues in Fig.2͑c͒are all very low comparable to the binary TE materials.1–7Their rapid decrea with the temperature indicates phonon conductivity is predominant. Interestingly,Cu doping lowers,suggesting a beneficial effect of disrupting the͓Zn/Sn Q4͔tetrahedral slabs and dis-torting the
财神节祝福语diamondlike structure according to the XRD analysis.The lattice thermal conductivity͑L͒may be esti-mated by subtracting the carrier thermal conductivityE from,where the Wiedemann–Franz relation with a Lorenz constant of L0=2.0ϫ10−8V2K−2is applied for estimating E͑E=L0T͒.Since the doped samples have a largerE, after subtractingE the remainder͑i.e.,L͒should reveal an even stronger suppression due to doping͑theL plots are given in Figure S2in supporting information8͒.Therefore, the strategy of doping a two-structural/functional-unit mate-rial proves to be highly effective for both electrical and ther-mal conductivity in the quaternary compounds.Lastly,an increasing PF and a decreasingespecially at higher
tem-FIG.2.Temperature dependence of electrical conductivity͑a͒,Seebeck co-efficient͑b͒,total thermal conductivity͑c͒andfigure of merit ZT͑d͒for Cu2ZnSnS4͑᭺͒,Cu2.1Zn0.9SnS4͑b͒,Cu2ZnSnSe4͑᭝͒,Cu2.1Zn0.9SnSe4͑᭡͒,and sodium silicate coated Cu2.1Zn0.9SnSe4͑ᮀ͒duringfirst heating cycle.Note in͑c͒the symbols᭡andᮀnearly coincide from300to700K.
peratures cau the dimensionlessfigure of merit ZT͓Fig.
2͑d͔͒to increa with Cu doping,with ZT at700K͑the
highest measuring temperature͒reaching0.36and0.45for
Cu2.1Zn0.9SnS4and Cu2.1Zn0.9SnSe4,respectively.
The u of Cu2ZnSnSe4is ultimately limited by a pha
transition at about883K.18In view of the rising trend of ZT
with temperature,we have attempted measurements up to
around860K using Cu2.1Zn0.9SnSe4bars side coated with a
protective glass͑sodium silicate͒to prevent oxidation,subli-
mation,or decomposition.After veral coats followed by
drying,an overall coating thickness about 1.5mm was
achieved.For thermal conductivity measurements,the disk-
shaped samples were also coated with graphite,which again
slowed the sublimation process somewhat to allow reproduc-
ible data to be obtained.The data of coated Cu2.1Zn0.9SnSe4
samples are plotted in Fig.2as open squares to compare
with tho of uncoated samples͑filled triangles͒.The two
ts of data are in agreement at low temperatures;above500
K,the coated sample has a lowerand a higher S.Such difference could be caud by a compositional change due to
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glass infiltration,and the change apparently occurred only
during thefirst heating cycle since the data from the sub-
quent cooling and heating cycles are almost indistinguishable ͑the“hysteresis”plots are shown in Figure S3of supporting information8͒.The data of coated samples give the following values at860K:=31856S m−1,S=206.7V K−1,and =1.282W m−1K−1,resulting in a PF value of 1.35mW m−1K−2and a ZT of0.91.Since the errors of un-
derestimatingand overestimating S are mutually compen-sating to some extent,the value of ZT at860K is probably accurate,which compares favorably with the well-known TE materials.
In summary,we have discovered a family of high ZT
TE materials,bad on the quaternary chalcogenides of
Cu2ZnSn Q4͑Q=S,Se͒,with an unusually large band gap ͑Ͼ1.4eV͒yet with high p-type conductivity.Their distorted diamondlike structure endows them with low lattice thermal conductivities,whereas Cu doping on the Zn site increas hole conductivity and decreas thermal conductivity.Both the power factor and ZT increa with the temperature:at 700K,Cu2.1Zn0.9SnSe4has a ZT of0.45,which increa
s to 0.91at860K.Therefore,the materials may be especially suitable for high temperature applications.
Financial support from National973Program of China ͑Grant Nos.2007CB936704and2009CB939903͒,National Science Foundation of China͑Grant No.50772123͒,Science and Technology Commission of Shanghai͑Grant No. 0752nm016͒,and Science and the Innovation Group of In-ternational Cooperation Plan͑Grant No.50821004͒are ac-knowledged.I.W.C.acknowledges support of U.S.National Science Foundation͑Grant Nos.DMR-07-05054and DMR-05-20020͒.
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