Energy band and charge-carrier engineering in skutterudite thermoelectric materials

Zhiyuan Liu(劉志愿) Ting Yang(楊婷) Yonggui Wang(王永貴)Ailin Xia(夏愛林) and Lianbo Ma(馬連波)

1Key Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials(Ministry of Education),Anhui University of Technology,Maanshan 243002,China

2School of Materials Science and Engineering,Anhui University of Technology,Maanshan 243002,China

Keywords: CoSb3-based skutterudite materials,energy band engineering,charge-carrier engineering,thermoelectric properties

1. Introduction

The development of environment-friendly renewable new energy and energy conversion technologies has attracted extensive attention of global researchers. Thermoelectric (TE)conversion is an environmentally friendly new energy conversion technology, which mainly uses the Seebeek, Peltier and Thomson effects of materials to realize mutual conversion of thermal and electronic energy. It has wide application prospects in the fields towards TE power generation recycling and TE refrigeration of dispersive heat sources, such as solar energy, industrial waste heat, and car exhaust waste heat.[1,2]TE properties exist in a kind of functional materials that use the movement of carriers inside to achieve mutual conversion of thermal and electrical energy. The comprehensive transport properties of TE materials can be characterized by the dimensionless TE figure of merit, i.e.,ZT.ZT=S2σT/κ, whereSis the Seebeck coefficient,σis the electrical conductivity,κis the thermal conductivity, andTis the absolute temperature. Theκis composed of electronic thermal conductivity(κE)and lattice thermal conductivity(κL).TE materials can be divided into low temperature(＜500 K),medium temperature(500–900 K) and high temperature (＞900 K) TE materials according to the service temperature.

Among many medium-temperature TE materials,[3–10]skutterudite[11–20]is one of the most promising TE materials.The binary CoSb3skutterudite TE material has high thermal conductivity and lowZTvalue due to the covalent bonding between the framework atoms.[21]Phonon engineering such as low dimensionalization,[22–24]the introduction of nano second phase,[25–28]nanointerfaces or nanopores,[29–31]phonon resonance scattering induced by filler[17,32,33]has been adopted to reduce remarkably the thermal conductivity of CoSb3materials, which significantly improves theirZTvalues. Although the thermal conductivity can be significantly reduced by adjusting the microstructure of the CoSb3material at the nanomesoscopic scale,it is also accompanied by deterioration of electrical conductivity of the material. As a result,the room for improving the electronic transport properties of CoSb3materials is limited. Therefore, it is particularly critical to explore methods that can not only optimize the thermal transport performance of CoSb3materials, but also improve the electronic transport performance. In recent years, some energy band engineering[34–41]and charge-carrier engineering optimization strategies[42–44]have been proposed. Band engineering strategies mainly include band convergence and resonance energy levels.Charge-carrier engineering strategies mainly include the optimization and improvement of carrier concentration and mobility. These strategies are effective means to improve the electronic transport performance of TE materials,and provide new research ideas of the development of highefficiency TE materials, which also provide a valuable reference for improvement of properties of CoSb3-based skutterudite TE materials.

This review mainly summarizes some methods to optimize the electronic transport properties of CoSb3materials through energy band and charge-carrier engineering,as shown in Fig. 1. These methods mainly include: (1) band engineering strategies of band convergence and resonance levels induced by doping/filling foreign elements, (2) charge-carrier engineering strategies of carrier concentration or mobility enhancement caused by alloying, element filling and the introduction of nano second phase or special structure. These optimization methods have greatly improved the TE properties of CoSb3-based skutterudite materials.

Fig. 1. Energy band and charge-carrier engineering in CoSb3-based skutterudite TE materials. Reproduced with permission from Refs.[25,45–49].

2. Energy band engineering

In recent years, applications of energy band engineering in TE materials have achieved fruitful results.[34–41]Solid solution alloys are formed by doping appropriate elements into the materials. The energy band structure of solid solution alloys will mainly show the following two changes: (i)Energy band degeneracy is improved. The multi-energy valley band degeneracyNvis related to the total density of states (DOS)effective massm*dand the single band DOS effective massm*b,as shown in Eq.(1).[36,50]The increase inNvcan cause the increase ofm*d. Then,Seebeck coefficient of the material can be significantly improved,as shown in Eq.(2).[51]

wherekBis Boltzmann’s constant,his Planck’s constant,nis the carrier concentration,andris the scattering factor.(ii)Resonant energy levels are generated.[52–54]Resonant levels will appear when the energy levels produced by the doped atom are located in the conduction band or valence band of a semiconductor matrix material. Its appearance will distort the density of states, cause narrow peaks in the density of states, and affect the transport of electrons, resulting in resonance scattering. In addition, the resonance energy level itself is a delocalized energy level with band conduction.[55]These factors affect significantly the Seebeck coefficient of the material. For CoSb3-based skutterudite TE materials, these changes of energy band structure caused by doping/filling will lead to significant increase of Seebeck coefficient. Combined with the strong phonon scattering induced by doping/filling, the decoupling of thermal and electronic transport parameters is realized, which greatly improves the TE properties of CoSb3-based materials.[56–65]Table 1 lists the optimized Seebeck coefficient andZTvalues of some typical CoSb3-based skutterudite TE materials[46,47,56–59,62–65]reported in recent years by the energy band engineering.

Table 1. The Seebeck coefficient and ZT values of some typical CoSb3-based skutterudite TE materials optimized by the energy band engineering in recent years.

2.1. Energy band convergence

Energy band convergence is a method to reduce the energy difference between light and heavy bands(valence band or conduction band) through element doping. After effective element doping/filling, the energy gap between the light and heavy energy bands becomes smaller,so that the heavy bands can participate more in the transport of carriers and improve the energy valley degeneracyNv, which leads to a significant improvement in the electronic transport properties of the material.

In recent years, energy band convergence has also been applied to CoSb3-based materials,[56–65]which has greatly improved their thermoelectric properties. Tanget al.[59]proved that the first conduction band and the high-energy second conduction band of the CoSb3and doped samples (such as Yb-filled skutterudite)were degenerated at high temperatures through theoretical calculation and experimental investigation.This is mainly due to the gradual decrease of the band gap between the first/secondconduction bands(CB1/CB2)and the valence band with increasing temperature. This result is consistent with the theoretical research results of Wanget al.[45]and Hanuset al.,[60]as shown in Fig. 2. The convergence of two conduction bands (CB1 and CB2) combined with valence band realizes the formation of three-band mode. More carriers participate in the electronic transport, which realizes the improvement of high-temperature electrical performance of the Yb0.25Co4Sb12material. The Fermi surface moves to the conduction band (Fig. 2(d)) by Yb doping. In convergence of the three-band mode and high-temperature double conduction band, more carriers are involved in the transport.The high-temperature performance of the YbxCo4Sb12material is significantly improved. A highZTvalue of 1.2 was obtained at 800 K.[59]Shiet al.[61]prepared a high-performance YbxGayCo4Sb12-y/3sample in which Ga element replaced Sb in YbxCo4Sb12skutterudite and occupied icosahedral voids at the same time (Fig. 3(a)). The first-principles calculation results show that double occupancy of Ga element destroys the symmetry of the frame Sb–Sb network, splits the deep triple degenerate conduction bands, and drives them down to the band edge, as shown in Fig. 3(b). The charge-compensation feature of the double occupancy of Ga element increases the total fill fraction limit. The Fermi level is raised to the extent that the carrier is close to these features in energy by giving this unique band structure feature and increasing the Yb filling content. The participation of these heavier energy bands in electron transport increases the Seebeck coefficient and carrier effective mass. In addition, the localized distortion caused by the Ga/Sb substitution enhances phonon scattering and effectively reduces the lattice thermal conductivity[Fig. 3(c)]. The highestZTvalue of the YbxGayCo4Sb12-y/3sample reached 1.25 at 780 K [Fig. 3(d)]. Zhanget al.[62]synthesized a Co3.75Fe0.25Sb12skutterudite TE material filled with five elements of Yb,Al,Ga,In,and Ca by using the melting spinning combined with SPS sintering technology (MSSPS)(Fig.4(a)). Five elements are filled into the icosahedral nanvoids of skutterudite,resulting in the comparable energy of multiple energy bands atΓandHpoints and the convergence of energy bands(Figs.4(c)–4(d)). Effective band convergence increases the electronic density of states of the multi-filled skutterudite near the Fermi level, resulting in the increase of the Seebeck coefficient. Specifically, the Seebeck coefficient of the multi-filled Yb0.3Ca0.1Al0.1Ga0.1In0.1Co3.75Fe0.25Sb12sample is higher than that of the Yb0.35Co3.75Fe0.25Sb12sample at similar carrier concentrations,as shown in Fig.4(b). At the same time,the microstructure shows that more nanoparticles,dislocations,and lattice strain fields are formed in multifilled skutterudite material (Fig. 4(a)). These microstructures can strongly scatter broadband phonons,resulting in a significant reduction in lattice thermal conductivity of the material.As a result,theZTvalue of the five elements-filled skutterudite material reached 1.7 at 823 K.Therefore,it is a very effective means to improve the TE properties of CoSb3-based materials by the energy band convergence induced by doping/filling.

Fig. 2. (a) Band structure of the CoSb3; (b) band structures of Yb0.25Co4Sb12 sample at 0 K, 300 K and 700 K; (c) band gaps as functions of temperature for Yb0.25Co4Sb12 sample. Reproduced with permission from Ref.[45]. (d)Band structures of the CoSb3 and Yb0.25Co4Sb12 samples. Reproduced with permission from Ref.[60].

Fig.3. (a)Scanning transmission electron microscope-high angle annular dark field(STEM-HAADF)image of YbxGayCo4Sb12-y/3 sample;(b)first-principles band structure calculation of Yb0.25Co4Sb12 and Yb0.25Ga0.25Co4Sb11.875Ga0.125 samples; (c)lattice thermal conductivity of YbxGa0.2Co4Sb11.9333 (x=0,0.05,0.10,0.15,0.26)and Yb0.26Co4Sb12;(d)ZT value of YbxGayCo4Sb12-y/3 samples with different Yb and Ga concentrations. Reproduced with permission from Ref.[61].

Fig. 4. (a) TEM image of the Yb0.3Ca0.1Al0.1Ga0.1In0.1Co3.75Fe0.25Sb12 sample; (b) the relationship between the Seebeck coefficient and the carrier concentration (nH) of the Yb0.3Ca0.1AlxGa0.1In0.1Co3.75Fe0.25Sb12 sample. Band structure of (c) Yb0.25Co3.75Fe0.25Sb12 and (d)Yb0.25Al0.125Ga0.125In0.125Co3.75Fe0.25Sb12. Reproduced with permission from Ref.[62].

2.2. Resonance levels

Resonance levels refer to the introduction of impurity levels in the matrix material through element doping/filling. The introduction of impurity levels can cause local“convexity”in the density of statesg(E)near the Fermi levelEF,as shown in Fig.5.That is to say,the introduction of impurity atoms causes the resonance energy level (impurity state) in the conduction band or valence band of the TE matrix material, resulting in the distortion of the density of states near the Fermi level and the narrow peaks (convexity) in the density of states, which ultimately affects electron transport. According to the following Mott formula,[66]the Seebeck coefficient is proportional to the density of states near the Fermi level. The steeper the density-of-states curve, the greater the slope, and the greater the Seebeck coefficient. The Seebeck coefficientS(n)can be maximized under a given carrier concentrationn,thereby maximizing the power factorS2σ.[51]

whereN(E) is the density of states;σ(E) is the differential electrical conductivity,which is used to describe the contribution of electrons with energyEto the overall electrical conductivity, as a function of the Fermi level;τis the scattering time;Lis the scattering distance (mean free path); andfis the Fermi–Dirac function. The resonant levels can also be achieved by introducing impurity atoms into the CoSb3-based TE material, which changes the density of states near the Fermi energy of the material and realizes the regulation of electronic transport performance.[46,47]

Fig.5. Schematic representation of the density of electron states(DOS)g(E)of TE semiconductors containing impurity energy levels(red line)and without impurity energy levels(dashed line). Γ is a narrow energy range,EF is the Fermi level.

Zhenget al.[46]used magnetron sputtering to prepare ntype Ag/In interstitial co-doped CoSb3-based TE films. According to the first principle calculation results,compared with the undoped CoSb3TE film (Fig. 6(a)), the resonance impurity level and the highest peak of DOS appear near the Fermi level for the interstitial Ag doped and Ag/In co-doped CoSb3TE film samples, which makes the electrons near the Fermi level more localized (Figs. 6(b) and 6(c)). The enhancement of DOS near the Fermi level caused by the impurity levels results in a higherSvalue (Fig. 6(d)). In addition, the resonant impurity levels caused by Ag and In doping make the DOS near the Fermi energy wider, resulting in the increase of electrical conductivity (Fig. 6(e)). Due to the increase inSand electrical conductivity, the power factor values of the doped samples increase significantly(Fig.6(f)). The TE properties of doped samples are significantly higher than other TE films.[67–70]Zhaoet al.[47]studied In-filled skutterudite TE materials through x-ray absorption fine structure spectrum,xray photoelectron spectroscopy,TE transport test and theoretical calculations. It was found that the electronic and thermal transport properties of materials can be regulated synergistically by the heat-carrying phonon-localized resonant scattering(Fig.7(a)),accelerated electron movement(Fig.7(b))and increase in density of states near the Fermi level (Fig. 7(c))when the In atoms are filled into Sb12icosahedral void of the CoSb3material. The doping of In atoms in the CoSb3material causes resonance impurity levels, resulting in the significant increase of the density of states near the Fermi level and Seebeck coefficient (Fig. 7(d)). Therefore, it is also a very effective means to improve TE properties of the CoSb3-based materials by doping to produce resonant energy levels and then increase the density of states near the Fermi level.

Fig.6. Calculated band structures and DOS of(a)undoped,(b)interstitially Ag-doped and(c)interstitially Ag/In Co-doped CoSb3 thin film.The temperature dependence of (d) S, (e) σ and (f) S2σ of as-fabricated CoSb3 thin films with different Ag and In doping concentrations.Reproduced with permission from Ref.[46].

Fig.7. (a)In atom fills Sb12 icosahedron voids at the 2a sites;(b)differential charge density of In0.125Co4Sb12 projected on the(111)plane;(c)total DOS and partial DOS near VBM and CBM of CoSb3 and In0.125Co4Sb12; (d)Seebeck coefficient as a function of carrier concentration for n-type filled CoSb3 at room temperature. Reproduced with permission from Ref.[47].

3. Charge-carrier engineering

In semiconductor TE materials,carrier concentration and mobility are two parameters closely related to the electronic transport properties of materials. The optimization of these two parameters is an important part of carrier engineering.Doping in a TE material matrix to form solid solution alloy usually affects the carrier concentration of the matrix.High performance solid solution TE materials can be obtained by optimizing the doping amount and adjusting the carrier concentration.[71,72]In addition,the carrier mobility of the matrix material can be significantly improved without reducing the carrier concentration by modulating doping,[44,48]forming a textured structure[73]or percolating carrier transports,[74]so that the matrix material can obtain higher electronic transport properties.

For CoSb3-based skutterudite TE materials, the regulation of carrier concentration is usually achieved by means of element filling[17,75,76]or doping to form solid solution alloys.[77,78]Element filling usually fills alkali, alkaline earth and rare earth metals and even electronegative elements[79]into the Sb12icosahedral voids of skutterudite materials to form filledskutterudite. Filling atoms are doped as donor doping. The optimal carrier concentration can be obtained by adjusting the doping amount, which further optimizes the electronic transport properties of the material. The solid solution alloy can be formed using foreign atoms to replace the framework atoms Co or Sb of the skutterudite material. The doping of foreign atoms can adjust the carrier concentration of the solid solution and improve its electronic transport performance. Recently,it is found that the introduction of magnetic nanoparticles into the CoSb3-based matrix material[16,25,80]can also regulate the carrier concentration and improve the electronic transport properties of the matrix by using the magnetic phase transition of the magnetic nanoparticles.It is worth noting that the introduction of the second phase material into the CoSb3-based skutterudite matrix material can significantly improve the carrier mobility of materials by using the modulated doping structures,[81–83]large grain size or low-energy grain boundaries[84]between the second phase and the matrix phase. In addition, the filling of foreign atoms can strongly scatter the heat-carrying phonons through the rattling of the filling atoms. The point defect and mass fluctuation of solid solution caused by replacement atoms can also strongly scatter the heat-carrying phonons. These changes caused by the filling and replacement of foreign atoms significantly reduce the lattice thermal conductivity. Combined with the regulation of the carrier transport characteristics of the material by the filling or doping of foreign atoms, the synergistic optimization of electronic and thermal transport properties of CoSb3-based materials can be finally realized. Table 2 lists the optimized carrier concentration,mobility,andZTvalues of some typical CoSb3-based skutterudite TE materials[57,63,81–91]reported in recent years. The carrier concentration or mobility of these materials has been optimized by means of filling,doping,nanocomposite or modulation-like doping.

Table 2. The carrier concentration, carrier mobility and ZT value of some typical CoSb3-based skutterudites TE materials optimized by the charge-carrier engineering in recent years.

3.1. Carrier concentration

There is a close correlation between the carrier concentration of TE materials and other TE physical parameters(except lattice thermal conductivity),as shown in Fig.8.It can be seen that the TE materials can obtain a higherZTvalue if the carrier concentration is adjusted to an appropriate value(～1019–1020cm-3). Shiet al.[17]prepared high-performance multifilled CoSb3materials.Multifilled elements(Ba,La,Yb)were introduced into a CoSb3material,and the carrier concentration of the multifilled CoSb3material was regulated by optimizing the filling fraction of these filling atoms. The sample with the highestZTvalue (1.7@850 K, Fig. 9(a)) corresponds to the optimized carrier concentration of 3.66×1020cm-3. This carrier concentration is located in the high power factor region (Fig. 9(b)). Furthermore, the sample with the optimal carrier concentration can scatter broadband phonons due to the rattling of the multi-filled atoms in the Sb12icosahedral void of CoSb3, resulting in the lowest thermal conductivity(Fig. 9(c)). Therefore, this sample has the highest TE transport properties(Fig.9(a)). Annoet al.[77]studied the effect of partial substitution of Co with different elementsM(M=Ni,Pd, Pt and Pd+Pt) on the TE properties of CoSb3materials(Co1-xMxSb3). The carrier concentration of the solid solution can be regulated by optimizing the doping amount of M element (Fig. 9(d)), and then a high-performance Co1-xMxSb3solid solution TE material can be obtained.

Fig.8. ZT and its related parameters as a function of carrier concentration. Reproduced with permission from Ref.[48].

Zhaoet al.[25]prepared magnetic nanocomposite TE materials by introducing BaFe12O19hard magnetic nanoparticles(BaM-NPs) into n-type (Ba, In) double-filled CoSb3matrix materials. BaM-NPs in the ferromagnetic state can capture electrons of matrix due to its strong micro-magnetic field when the working temperature of the magnetic nanocomposite materials is lower than the Curie temperature of BaM-NPs, which only reduces the carrier concentration (Fig. 10(a)). There is no abnormality in the relationship between the electrical conductivity/Seebeck coefficient of the composite material and the temperature. When the working temperature exceeds the Curie temperature, BaM-NPs in the paramagnetic state release trapped electrons because of the disappearance of the micro-magnetic field, resulting in the significant increase of carrier concentration(Fig.10(a)). The electrical conductivity and the Seebeck coefficient of the composite exhibited anomalous transport phenomena different from those of the matrix(Figs.10(b)and 10(c)).BaM-NPs play the role of an“electron repository” before and after the ferromagnetic-paramagnetic transition. When the curie temperature of BaM-NPs is close to the intrinsic excitation temperature (～675 K) of the filled CoSb3matrix, the “electron repository” effect of the magnetic nanoparticles in the ferromagnetic-to-paramagnetic transition can effectively regulate the carrier concentration and inhibit the deterioration of TE performance under intrinsic excitation of the matrix (Fig. 10(d)). Our research group[80]also introduced BaM-NPs into an In single-filled CoSb3matrix material and used its electron repository effect to regulate the carrier concentration of the In0.25Co4Sb12matrix material (Fig. 10(e)). The purpose of suppressing the performance degradation of the intrinsic excitation region for the filled CoSb3material is also achieved (Fig. 10(f)). Zhaoet al.[16]also introduced soft magnetic Co(Fe or Ni)nanoparticles into the(Ba,In)double-filled CoSb3matrix(Fig.10(g)).Using the different work function between the soft magnetic metal nanoparticles and the skutterudite semiconductor material, the transfer of electrons from Co nanoparticles to skutterudite semiconductor is realized (Fig. 10(h)), which regulates the carrier concentration of the matrix and significantly improves the electrical conductivity of magnetic nanocomposites(Fig.10(i)). Therefore,forming solid solution by doping,filling or introducing magnetic nano second phase is an effective means to optimize the carrier concentration of CoSb3-based materials and further to improve their TE properties.

Fig.9.(a)Temperature dependence of ZT for BauLavYbwCo4Sb12 multi-filled skutterudites;(b)carrier concentration as a function of(∑ni=1 qi×yi)for partially filled CoSb3, where qi is the effective charge state and yi is the filling fraction of the ith filler in multiple-filled skutterudites.The red lines enclose the estimated high power factor region;(c)temperature dependence of total thermal conductivity for BauLavYbwCo4Sb12 multi-filled skutterudites. Reproduced with permission from Ref. [17]. (d) Hall carrier concentration at room temperature as a function of composition x for Co1-xMxSb3 (M=Ni,Pd,Pt and Pd+Pt)samples. Reproduced with permission from Ref.[77].

3.2. Carrier mobility

Filling and alloying are an important means to regulate the carrier concentration of CoSb3-based skutterudite materials. Filled and alloyed CoSb3-based skutterudite materials are generally heavily doped semiconductors,and their carrier concentrations are mostly in the range of 1019–1021cm-3. High concentration of ionized impurity scattering in heavily doped semiconductor materials is unfavorable to the improvement of carrier mobility.[92]In order to weaken ionized impurity scattering and to obtain higher carrier mobility, the concept of modulated doping[93]have been proposed. This method is mostly used in thin-film materials.[94]For bulk materials,the heavily doped second phase can be locally embedded in the matrix phase to form the modulation structure. Combined with energy band modification, carriers of heavily doped region are spatially separated from the matrix atoms and can move freely into undoped particles(Figs.11(a)and 11(c)).[48]Therefore, the modulation doping will reduce the effect of ionized impurities on the carriers compared with conventional uniformly doping,resulting in a significant increase of the carrier mobility.[43]

Fig. 11. Schematic representation of (a) nondoped semiconductor, (b) schematic representation of modulation doping, where two types of grains consisting of nondoped and heavily doped semiconductors are spatially separated and (c) schematic representation of uniform doped semiconductor,where dopants are uniformly dispersed in the host matrix. Reproduced with permission from Ref.[48]. (d)BSE image of an A0.5B0.5 sample showing nano-crystalline grains; (e)colors(red: Yb-rich grain-A0.5; blue: Yb-deficient grain-B0.5)highlight the features of BHJ structure;(f)schematic picture showing carriers transmission in the A0.5B0.5 sample;(g)room-temperature Hall carrier mobility(μH)as function of Yb content for the A,B,C and A0.5B0.5 samples; Temperature dependences of(h)power factor(PF)and(i)ZT values for A,B,A0.5B0.5 and C samples. Reproduced with permission from Ref.[49].

Nieet al.[49]successfully prepared an n-type filled skutterudite bulk material with a heterojunction(BHJ) structure. This material with the BHJ structure is composed of two n-type filled skutterudite materials (A: Yb0.3Ca0.1Al0.1Ga0.1In0.1Fe0.25Co3.75Sb12; B:Yb0.1Ca0.1Al0.1Ga0.1In0.1Fe0.25Co3.75Sb12) with different Yb filling fractions. This BHJ material (labeled A0.5B0.5) with rich nanostructures was prepared by mixing and annealing two Yb filled skutterudites for A and B volume ratio of 1:1 (Figs. 11(d) and 11(e)). For comparison, C material with nominal composition of Yb0.2Ca0.1Al0.1Ga0.1In0.1Fe0.25Co3.75Sb12was also prepared.C material and A0.5B0.5material have similar carrier concentrations due to the similar Yb filling fraction. The structure of the A0.5B0.5sample is similar to the modulated doping structure in Fig.11(b),as shown in Fig.11(f).The C material structure is similar to the uniformly doped structure of Fig. 11(c).Under the same carrier concentration, the carrier mobility of the A0.5B0.5sample is significantly higher than that of the C sample(Fig.11(g)),which is attributed to the modulated doping structure of the A0.5B0.5sample. In this BHJ structure,the dark red region has a higher carrier concentration, and the light red region has low carrier concentration, forming a concentration gradient. In order to achieve equilibrium, the carriers will be rapidly transported from the dark red region to the light red region(Fig.11(f)),which greatly improves the mobility of the A0.5B0.5material. Due to its rich nanostructure and high mobility,the A0.5B0.5sample has high electronic transport properties (Fig. 11(h)) and low lattice thermal conductivity. Therefore,compared with other samples(A,B and C),the A0.5B0.5sample has the best TE properties(Fig.11(i)).Fuet al.[81]prepared Ni-doped Yb0.2Co4Sb12skutterudite with a special “core-shell” microstructure. The difference in Ni contents between the special core-shell nanograins and the skutterudite matrix grains also forms a similarly modulated doping structure. Ni replaces the Co sites of skutterudite in the core-shell nanograins, so that the core-shell structure has a high carrier concentration, while the matrix grains have a lower carrier concentration. Therefore, a carrier concentration gradient is generated in Ni-doped Yb0.2Co4Sb12skutterudite. In order to achieve equilibrium, the carriers are rapidly transported from the core-shell nanograins to the normal matrix grains, which greatly improve the mobility of the material (matrix sample: 23.44 cm2·V-1·S-1, 0.2 wt%Ni/Yb0.2Co4Sb12sample: 53.17 cm2·V-1·S-1). Penget al.[83]also prepared Yb0.2Co4Sb12-Bi0.4Sb1.6Te3nanocomposites with similar modulation doping structure. Compared with the Yb0.2Co4Sb12matrix material, the composite material has higher carrier mobility. This is mainly due to the fact that the Bi0.4Sb1.6Te3material with high mobility in the composite material is dispersed in the matrix with low mobility to form a structure similar to that of modulation doping.

In addition, the structure of large grain size and low energy grain boundaries can also significantly improve the carrier mobility of the material. Qinet al.[84]successfully prepared (Ba, Yb) double-filled CoSb3materials with excessive Sb by liquid-phase compaction method. Under the same carrier concentration, the carrier mobility of the BaxYb0.3Co4Sb12+20%Sb sample is significantly higher than other reported double-filled or multi-filled skutterudite materials.[75,95–97]The excellent carrier mobility is mainly attributed to two aspects:On the one hand,the samples prepared by liquid phase compaction method have large grains.According to the Ostwald ripening theory,[98]larger grains tend to gobble up smaller grains,which will accelerate the transport of carriers. On the other hand,low-energy grain boundaries with dense dislocation arrays have no negative effect on electron transport, even positive effects. This has also been reported in other material systems.[99–101]Therefore, it is an effective method to increase the carrier mobility of CoSb3-based materials by forming modulation doped structures or preparing the structures with large grains and low-energy grain boundaries.

4. Summary and perspectives

This review summarizes some main research progresses in recent years to optimize the TE transport properties of CoSb3-based materials through energy band and chargecarrier engineering. The Seebeck coefficient of CoSb3-based materials was significantly improved by the band engineering of band convergence and resonance levels induced by doping and/or filling. Combined with the strong phonon scattering caused by doping and/or filling, the thermal and electronic transport parameters are decoupled, which greatly improves the TE properties of CoSb3-based materials. In addition, the electronic transport properties of CoSb3-based materials were significantly improved by the charge-carrier engineering with the increase of carrier concentration or mobility. The optimization of carrier concentration can be obtained by forming solid solution by doping, filling or introducing magnetic nano second phase. The increase of carrier mobility can be obtained by forming modulation doped structures or preparing the structures with large grains and low-energy grain boundaries. Combined with the strong phonon scattering caused by doping,filling,or the introduction of the nanointerface, the thermal and electronic transport parameters are also decoupled, which greatly improves the TE properties of CoSb3-based materials. However, there are still some problems in research of energy band and charge-carrier engineering of CoSb3-based skutterudite TE materials, which need to be further studied: (i) It is very important to further develop new processes to prepare materials with high carrier mobility and new modulated doping structures. (ii)On the basis of theory and experiment, further screening of appropriate doping/filling elements and nano second phase can make the energy band of CoSb3based TE materials converge significantly or produce obvious resonance energy level. This is the key to improve the transport properties of CoSb3based TE materials again. It should be noted that the screened nano second phase should have appropriate energy band matching with the CoSb3based matrix at the phase interface and have stable physical and chemical properties. (iii) Further screening of appropriate doping/filling elements and nano second phase makes the carrier concentration of CoSb3based materials appear in the optimal concentration range(1019cm-3–1020cm-3)and have high carrier mobility. This is the key to improve the TE transport performance of CoSb3based materials again.

In conclusion,plenty of work remains to be carried out to further improve the TE transport properties of CoSb3-based TE materials through energy band and charge-carrier engineering. Though the challenges are pervasive,the research activities of TE materials are still hot all over the world.The field of TE materials attracts more and more researchers. Interdisciplinary exchanges between physicists,chemists,mathematicians,material scientists and device engineers are also deepening. The interdisciplinary research of TE materials and other disciplines are becoming more and more extensive,which will promote the sustained and rapid growth of TE research and future large-scale applications.

Acknowledgements

This review was supported by the National Natural Science Foundation of China(Grant No.51872006)and the Excellent Youth Project of Natural Science Foundation of Anhui Province of China(Grant No.2208085Y17).