•ZnO/Zn2SnO4 composite compact layer is adopted to replace the TiO2 compact layer.•ZnO/Zn2SnO4 is helpful to improve the corresponding photo-current effectively.•The optimum efficiency of perovskite solar cell reaches 12.03%.In this study, ZnO/Zn2SnO4 (ZSO) composite is synthesized as a promising alternative compact layer to TiO2 by spray pyrolysis method. Owing to the ZnO/ZSO composite possesses higher carrier mobility and suitable band gap structure, it will behave well in the application of solar cells. The characterization results show that the ZnO/ZSO composite compact layer manifests better optical transmittance, enhanced electrons collective efficiency and superior electrical conductivity. Consequently, the corresponding photo-current is improved effectively when compared with that of TiO2 compact layer. The optimum efficiency of perovskite solar cell based on the ZnO/ZSO composite compact layer reaches 12.03% at AM 1.5 solar light of 100 mW cm−2, which is 18% higher than that of perovskite solar cell based on the TiO2 compact layer.

•Two possible electronic topological transitions were uncovered in In4Se3.•The possible abnormal changes of CDW order were proposed under pressure.•The changes of thermoelectric property under pressure were discussed.•Structural changes were observed at about 7.0 and 34.2 GPa, respectively.•The compressional behaviors of the Pnnm and the Pca21 phases were all determined.High-pressure in situ angle dispersive X-ray diffraction (ADXRD) measurements were performed on the charge-density-wave (CDW) material In4Se3 up to 48.8 GPa. Pressure-induced structural changes were observed at 7.0 and 34.2 GPa, respectively. Using the CALYPSO methodology, the first high-pressure phase was solved as an exotic Pca21 structure. The compressional behaviors of the initial Pnnm and the Pca21 phases were all determined. Combined with first-principle calculations, we find that, unexpectedly, the Pnnm phase probably experiences twice electronic topological transitions (ETTs), from the initial possible CDW state to a semimetallic state at about 2.3 GPa and then back to a possible CDW state at around 3.5 GPa, which was uncovered for the first time in CDW systems. In the both possible CDW states, pressure provokes a decrease of band-gap. The observation of a bulk metallic state was ascribed to structural transition to the Pca21 phase. Besides, based on electronic band structure calculations, the thermoelectric property of the Pnnm phase under compression was discussed. Our results show that pressure play a dramatic role in tuning In4Se3's structure and transport properties.Download high-res image (528KB)Download full-size image

•A pure phase of FA1-xMAxPbI3 perovskite FA1-xMAxPbI3 was prepared.•Discussing effect of chemical composition and morphology on the photovoltaic performance in detail.•The PCE of PKSC based on FA0.7MA0.3PbI3 reaches up to 16.0%.•The long term stability and reproducibility is obtained.Preparation of high-quality and pure phase mixed-organic-cation FA1-xMAxPbI3 perovskite film is a big challenge, especially on a porous oxide scaffold for mesoscopic perovskite solar cells. In this study, we fabricated a pure phase of mixed-organic-cation perovskite FA1-xMAxPbI3 simply using a conventional two-step method. Effect of chemical composition and morphology on the photovoltaic performance and stability of the mixed-organic-cation perovskite has been studied in details. Results show that the FA1-xMAxPbI3 perovskite thin films with a FA+:MA+ molar ratio of 0.7:0.3 demonstrate the best photovoltaic performance. Mesoscopic perovskite solar cells fabricated using these FA0.7MA0.3PbI3 thin films yield a power conversion efficiency (PCE) up to 16.0%. Moreover, the devices present a superior stability and the unsealed devices exposed in ambient condition (25 °C, 50%–60% relative humidity) can maintain their photoelectric performance without apparent PCE loss for up to 20 days.Download high-res image (99KB)Download full-size image

Pristine MnTe is a p-type semiconductor with a relatively low hole concentration of 1018 cm−3, low electrical conductivity, and thus poor TE performance at room temperature owing to the broad direct band gap of 1.27 eV. In this study, Na2S was employed to be doped into the MnTe matrix to synergistically tune the electrical and thermal transport properties of the semiconductor via point defect engineering. On the one hand, Na substitution effectively improved the electrical transport properties by increasing the carrier concentration via the formation of acceptor-like defects, Na−Mn. On the other hand, thermal conductivities of the Na2S-doped samples were also sharply reduced via mass fluctuation and strain field fluctuation from the point defects introduced through Na doping and S substitution. Consequently, a maximum ZT of ∼1.09 was achieved for the 0.5 at% Na2S-doped sample at 873 K, which was the highest ZT value ever reported for p-type MnTe-based thermoelectric materials. Moreover, the high performance 0.5 at% Na2S-doped sample also exhibited good thermal stability and mechanical stability (Vickers microhardness) of ∼122 Hv, which was higher than those of other promising thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and SnTe.

As a promising mid-temperature thermoelectric (TE) material, the main obstacle to a high TE performance of the InSb compound is its high thermal conductivity. In this article, a new strategy of eutectic melting has been employed to improve the TE properties of the compound for the first time. By addition of excess Sb into the InSb matrix, an InSb–Sb eutectic structure has been introduced. When the temperature is above the melting point of the eutectic mixture, the InSb–Sb eutectic melts into a liquid phase which inhibits the propagation of transverse acoustic phonons, and the thermal conductivity is reduced drastically. Therefore, the thermoelectric performance is remarkably enhanced after the melting of the eutectic, and an unprecedented high ZT of 1.28@773 K has been achieved for the InSb1.04 sample, which is almost 3 times higher than that of the eutectic-free InSb matrix. Moreover, the Vickers hardness of the eutectic included InSb compound is higher than those of many well-established mid-temperature TE materials, and no evident hardness degradation can be detected after several melting–solidification cycles of the eutectic.

•All thermoelectric parameters of Cu3SbSe4 are optimized simultaneously by Te-doping.•The origins of improved thermoelectric parameters are analyzed in detail.•Considerable improvement in thermoelectric performance is realized after Te-doping.It is a challenge to optimize the coupled electrical and thermal transport properties of a thermoelectric material simultaneously. Here, the enhancement of overall thermoelectric properties in Cu3SbSe4 has been demonstrated through Te substitution. The results display that the simultaneous optimization of the Seebeck coefficient, electrical conductivity and thermal conductivity is realized after Te-doping and thus yielding considerable enhancements in ZT values in the whole temperature range as comparison to that of pristine Cu3SbSe4. In Te-doped Cu3SbSe4 system, the reduction of electrical resistivity is due to the narrowed bandgap; the enhanced Seebeck coefficient stems from the increased density of states near the Fermi level; and the suppressed thermal conductivity is related to the introduced phonon blocking from point defect scattering or softening of chemical bond interaction. This work can also offer a referential route to boost the overall thermoelectric properties by isovalent doping with heavy element for other thermoelectric materials.Download high-res image (384KB)Download full-size image

•The existence of multiple converged valence bands and low sound velocity are found in Cu2CoSnS4.•Single phased Cu2CoSnS4 powders with nano size have been prepared by mechanical alloying for the first time.•A highly improved power factor with an enhanced effective mass is obtained via Cu doping.•An unprecedented high ZT of 0.8 at 800 K has been achieved in Cu-Mn dual doped CCTS compound.Owing to the advantage of low thermal conductivity, large Seebeck coefficient and multiple eco-friendly components constitution, Cu2CoSnS4 (CCTS) might be a promising thermoelectric material. DFT calculations reveal that the multiple valence bands can be utilized to obtain an enhanced Seebeck coefficient and an improved power factor, and the relatively low sound velocity in multi-cations-containing CCTS is responsible for the low thermal conductivity. Here, nano-sized CCTS sulfide has been facilely synthesized through mechanical alloying and then sintered via hot-pressing. Guided by the theoretical calculations, copper substitution is firstly adopted to regulate electrical properties and results in a highly improved power factor with an enhanced effective mass. Then, transition metal M (M = Mn, Fe, Zn) and Cu are co-doped at cobalt site to further regulate the TE performance of CCTS. As a consequence, a peak ZT value about 0.8 at 800 K is achieved in the CCTS compound simply by Cu/Mn dual-doping due to the improved electrical properties by introducing metal-like second phase and the depressed lattice thermal conductivity by weakening of covalent bonding, which is also the highest ZT value reported in 700–823 K range for metal sulfides without fast ion migration or phase transformation.The existence of multiple converged valence bands and low sound velocity are found in Cu2CoSnS4. Cu/M co-substitution for Cobalt is guided to improve its thermoelectric properties. Ultimately, an unprecedented high ZT of 0.8 at 800 K had been achieved due to the improved electrical properties from enhanced effective mass and the depressed lattice thermal conductivity by weakening of covalent-bonding.Download high-res image (286KB)Download full-size image

A single phase CuSbSe2 polycrystalline chalcostibite compound has been facilely synthesized through mechanical alloying for the first time, and the phase evolution has been revealed in detail. The large Seebeck coefficient and poor electrical conductivity of this compound are ascribed to the existing Cu–Se ionic bonding and heavy band at the VBM. The active lone-pair s2 electrons in Sb3+ ions are likely to be responsible for the experimentally observed low thermal conductivity in the CuSbSe2 compound. By introducing narrow band-gap Cu3SbSe4 into the CuSbSe2 matrix, the carrier concentration and mobility can be tuned effectively; thus the power factor has been improved remarkably. As a consequence, the figure of merit (ZT) is increased by a factor of 1.6 at 623 K, from 0.25 (matrix) to 0.41.

We studied the effect of doping with Bi on the thermoelectric properties of SnTe-based materials. Doping with Bi reduced the density of holes and increased the Seebeck coefficient over a wide temperature range as a result of modulation of the carrier concentration and an increase in the density of states effective mass. The lattice thermal conductivity was also greatly reduced as a result of the wide frequency range of phonon scattering by multiscale architectures derived from Bi doping. A maximum ZT value of c. 1.1 at 873 K was obtained in Sn0.94Bi0.06Te, an enhancement of 165% compared with the undoped sample.

•A modification layer of SrTiO3 is introduced between CH3NH3PbI3 and TiO2.•Better and larger crystalline growth of perovskite is achieved.•Charge transfer is promoted from the CH3NH3PbI3 to the modified TiO2.•The perovskite solar cell efficiency has a significant improvement of 46%.By dipping the mesoporous TiO2 film in Sr2+ solutions and subsequent annealing, a modification layer of SrTiO3 has been formed on the surface of the mesoporous TiO2 film, and the effect of modification on the photovoltaic characteristics of the perovskite solar cells has been studied detailedly. The XPS and TEM results verified the existence and nature of the SrTiO3 modification layer. Benifiting from the TiO2 surface modification, the morphology of perovskite film and light absorption of the CH3NH3PbI3 absorber have been improved effectively, and a faster charge transfer has also been achieved between the CH3NH3PbI3 perovskite layer and the modified TiO2. As a result, an average conversion efficiency of 11.4% has been achieved in the perovskite solar cell based on the modified mesoporous TiO2 by 10 mM Sr2+ solution, and it increases approximately by 46% in comparison with the photovoltaic efficiency of 7.8% of the perovskite solar cell based the bare mesoporous TiO2 layer.

Skutterudites have attracted a lot of attention because of the intrinsic voids in their crystal structure. However, another important structural feature of p-type skutterudites seems to have been ignored, namely the nearly square four-membered antimony rings. To explore the influence of substitution of Te for Sb on the microstructure and thermoelectric properties, a series of p-type skutterudites with composition CeFe4Sb12−xTex with x = 0, 0.1, and 0.2 have been synthesized. The electrical resistivity decreases while the Seebeck coefficient increases with Te doping. In addition, due to disturbance of the Sb4 rings and extra phonon scattering by nanopores, the lattice thermal conductivity is reduced. The thermoelectric figure of merit for the CeFe4Sb11.9Te0.1 compound reaches 0.76 at 773 K, being about 61% higher than that of the CeFe4Sb12 sample.

Bulk Bi0.4Sb1.6Te3 materials with ZnO nanoinclusions have been fabricated by mechanical alloying and plasma-activated sintering. The effects of ZnO addition on the microstructure and thermoelectric performance of the Bi0.4Sb1.6Te3 materials have been studied in detail. Scanning electron microscopy (SEM) observation confirms that the ZnO particles distribute in the Bi0.4Sb1.6Te3 matrix homogeneously until the content is over 0.5 wt.%. The resistivity and Seebeck coefficient of the samples increase slightly with the ZnO content, while the thermal conductivity is reduced effectively by addition of ZnO nanoparticles due to extra phonon scattering at ZnO nanoinclusions, and a maximum ZT of 1.21 is achieved in the sample with 0.3 wt.% ZnO at 373 K.

Polycrystalline Mn1-xCuxTe (x = 0, 0.025, 0.05, 0.075) thermoelectric materials were prepared by a combined method of melt-quenching and hot press. The effect of Cu doping on the electrical resistivity, band gap, the Seebeck coefficient and thermal conductivity was investigated. The power factors of the Cu-doped samples increase greatly due to the decrease of electrical resistivity and the higher Seebeck coefficient in high temperatures. In addition, the thermal conductivities of the Cu-doped samples also reduce due to the extra phonon scattering from the point defects introduced by Cu doping. As a result, the thermoelectric performance of MnTe is greatly enhanced, and a maximum ZT value of ∼0.55 in the Mn0.925Cu0.075Te sample at 773 K is achieved, which is 35% greater than that of the pristine MnTe sample.Cu-substituted polycrystalline MnTe have been prepared by a melt-quenching and hot press method. The results show that both the electrical resistivity and band gap can be effectively decreased while the power factor is greatly increased by Cu doping. A maximum ZT of 0.55 was achieved in the Mn0.925Cu0.075Te sample at 773 K, which is improved by 35% in comparison to the pristine MnTe sample.Figure optionsDownload full-size imageDownload as PowerPoint slide

•A synergistic effect between photovoltaic and thermoelectric is found firstly.•The increase of non-equilibrium carrier influences the VOC and JSC.•The VOC of hybrid generator is larger than the algebraic sum of TEG and PVG.In this paper, a hybrid generator has been fabricated by connecting a dye sensitized solar cell (DSSC) with a single p–n junction thermoelectric generator (TEG) in series. Both the open-circuit voltage and the short circuit current of the hybrid generator have been enhanced obviously in comparison with the algebraic sum of those of the DSSC and TEG. The increase of non-equilibrium carrier concentration originates from the synergistic effect between the TEG and DSSC, which lifts the quasi-Fermi energy level and improve the photoelectric response rate thus enhancing the open-circuit voltage and current density of the DSSC. As a result, a conversion efficiency of 9.08% has been obtained in hybrid B, which is greatly enhanced by 20.6% and 725.5% in comparison with that of the separate DSSC and TEG, respectively.

•The microstructure of p-type Bi0.5Sb1.5Te3 was tuned successfully by regulating the cooling rate.•Coherent BST/Te and Te/BSTⅡ interfaces were observed.•An enhanced ZT of 1.23 at 323 K with a maximal flexural strength 23.2 MPa is obtained.Given the thermoelectric and mechanical performance of a given material is closely related to its microstructure, in this paper, the microstructure of p-type Bi0.5Sb1.5Te3, fabricated by a high magnetic field assisted melting-solidification (HMAMS) process, is successfully tuned by regulating the cooling rate during the solidification process, and a systematic investigation has been carried out to the effect of the cooling rate on the crystal orientation, microstructure, thermoelectric and mechanical performance of the obtained materials. By this approach, the thermal conductivity is sharply reduced due to the intensive phonon scattering by the massive BST/Te and Te/BSTⅡ interfaces, while the power factor is less affected, and the flexural strength is enhanced owing to the narrowing of eutectic strip and spacing. Eventually, a highest ZT of 1.23 at 323 K with a maximal flexural strength 23.2 MPa has been obtained in the sample prepared under a 6 T magnetic field at a cooling rate of 16 K/min.In this paper, a systematic investigation is carried out to the effect of cooling rate on the microstructure, thermoelectric and mechanical performance of p-type Bi0.5Sb1.5Te3. By this approach, the thermal conductivity is sharply reduced due to the formation of coherent BST/Te and Te/BSTⅡ interfaces, while the electrical properties are less affected. Consequently, a highest ZT of 1.23 at 323 K with a maximal flexural strength 23.2 MPa is obtained in the sample prepared under a 6 T magnetic field at a cooling rate of 16 K/min.

Since the lattice thermal conductivity of n-type multi-filled skutterudites have been reduced below 1 W (mK−1), the development of new strategies that can further enhance the power factor while maintaining the low thermal conductivity is highly desired. In this paper, we conducted a pioneering work by introducing a “core–shell” microstructure into Yb single-filled skutterudite thermoelectric materials to realise this purpose. The “core–shell” structure formed by the thermal diffusion of well dispersed Ni nanoparticles in the Yb0.2Co4Sb12 powder during hot pressing is composed of the normal “core” grains surrounded by Ni-rich nanograin “shells”. The electrical resistivity is greatly reduced due to the increase in both carrier concentration and mobility. However, the Seebeck coefficient first increases due to the increased density of states at the Fermi energy and then decreases gradually. As a consequence, the power factor is remarkably increased for the samples with the addition of Ni nanoparticles. In addition, the lattice thermal conductivity is also reduced by the extra phonon scattering introduced by the “core–shell” microstructure. The concomitant effects enable a maximum ZT of 1.07 for the 0.2 wt% Ni sample at 723 K.

In this paper, different atom combinations of Pb, I and Cu have been doped into the In4Se2.5 matrix and a systematic investigation has been carried out for the synergistic effect of multiple heteroatoms on the microstructure and thermoelectric properties of polycrystalline In4Se2.5. By this approach, the electron and phonon transport properties are rationally regulated and the electrical conductivity increases greatly due to the multiple doping, which result in a simultaneous increase of carrier concentration and mobility. The Seebeck coefficient also remains at a relatively high level in a high temperature range due to the energy-dependent electron scattering at the metal nanoparticle–matrix interfaces. In addition, the lattice thermal conductivity is also greatly reduced because of the wide frequency phonon scattering by the point defects and hierarchical metal nanoparticles combined with the phonon–phonon interactions. Consequently, an enhancement of the ZT with a maximum of 1.4 (723 K) has been achieved in the multiple doped In4Se2.5 sample.

Polycrystalline Bi2Te2.5Se0.5 materials with different amounts of added MnTe2 have been fabricated by plasma activated sintering. XRD, SEM, EDS and HRTEM techniques and measurements of the electrical and thermal transport properties have been employed to study the effect of MnTe2 addition on the microstructure and the thermoelectric performance of Bi2Te2.5Se0.5 materials. Due to the substitution of Mn at the Bi sites, the XRD peaks shift towards a higher angle with an increase in the content of MnTe2, and the electron density increases; thus, the bipolar conduction is suppressed and accordingly shifts to a higher temperature . When the content of MnTe2 is over 3.0 at%, the dissolution of MnTe2 gets saturated, and some nanoinclusions with sizes of about 40–70 nm were observed and were identified as MnTe2 by EDS and HRTEM characterization. Due to the extra phonon scattering by the MnBi′ point defects and MnTe2 nanoinclusions, the phonon thermal conductivity decreased significantly. As a result, a maximum ZT of 1.0 at 523 K was achieved in the sample with 3.0 at% MnTe2, which is about a 150 K degree shift and a 100% improvement at this temperature compared with the conventional n-type Bi2Te3-based materials.

•Imposing a high static magnetic field during the melting-solidification of Bi0.5Sb1.5Te3.•A large amount of dispersive BSTII nanorods.•Simultaneous optimization of the electrical and thermal transport properties.•An enhanced ZTmax=1.71 at 323 K in a polycrystalline Bi0.5Sb1.5Te3 sample.Imposing an intensity variable high static magnetic field during a traditional melting-solidification (MS) method has been used as a new method to prepare p-type bismuth antimony telluride thermoelectric materials in this work. On this basis, we present a systematic study of the nucleation, crystal orientation, microstructure, electrical and thermal transport properties of the obtained alloy ingots solidified under different magnetic field intensities. A c-axis alignment of bismuth antimony telluride in the direction perpendicular to the magnetic field, formation of BSTII nanorods, and a simultaneous optimization of the electrical and thermal transport properties have been observed. Consequently, an enhanced ZTmax=1.71 at 323 K has been achieved in a polycrystalline Bi0.5Sb1.5Te3 sample solidified under a 2 T magnetic field.

•A synergistic strategy of point defects and microstructure engineering is demonstrated.•Highly dispersive In2O3 nanoparticles were obained by a solid displacement reaction.•Enhanced ZTmax of 1.47 was achieved in the CuInTe2 materials.A synergistic strategy of point defects and microstructure engineering has been proposed and demonstrated in this work to regulate the electrical and thermal transport properties of chalcopyrite CuInTe2 based TE materials: on one hand, the power factor increases greatly by dissolving ZnTe into the CuInTe2 matrix, leading to an order of magnitude increase of carrier concentration due to the preferential formation of acceptor-like defects ZnIn-, meanwhile, the formation of defect pair verified by binding energy calculation insures a less reduction in mobility at high ZnTe content; on the other hand, the thermal conductivity lowers sharply by adding an appropriate content of TiO2 nanopowders, which react with the matrix and result in a wide spectrum phonon scattering by the integrated point defects, In2O3 nanoparticles, stacking faults and grain boundaries. As a consequence, a great enhancement of ZT with a maximum of 1.47 at 823 K has been achieved in the (CuInTe2)0.99(2ZnTe)0.01 with 0.1 wt% TiO2 nanofibers sample, which can be a suitable p-type candidate to couple with other high performance n-type materials for middle temperature TE power generation.

The single-filled skutterudite Yb0.2Co4Sb12 has been long known as a promising bulk thermoelectric material. In this work, we adopted a melting–milling–hot pressing procedure to prepare nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different weight percentages of AgSbTe2 inclusions were added to optimize the thermoelectric performance. We found that the addition of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent electrical conductivity, Seebeck coefficient and thermal conductivity. In particular, the significantly enhanced carrier mobility led to a ∼3-fold reduction of the electrical resistivity. Meanwhile the absolute value of Seebeck coefficient was enhanced via the energy filtering effect at the matrix–nanoinclusion interfaces. Moreover there is a topological crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6 wt% and 8 wt% of nanoinclusions. Above the crossover, further addition of nanoinclusions degraded the Seebeck coefficient and the electrical conductivity. Meanwhile, the addition of nanoinclusions generally reduced the lattice thermal conductivity. As a result, the power factor of the 6 wt% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temperature figure of merit ZT ∼ 0.51.

By means of β-Zn4Sb3 addition and thermal decomposition, dual nanoinclusions of Zn and ZnSb were introduced and Zn atoms were doped into p-type Bi0.4Sb1.6Te3 successfully. Due to the increase of hole concentration by Zn doping and band structure optimization, the bipolar conduction was suppressed and the intrinsic excitation shifts to higher temperature. The power factors of the samples were slightly improved, while the lattice thermal conductivities of the samples were greatly reduced due to the extra phonon scattering introduced by the dual nanoinclusions. As a result, the critical temperature corresponding to the maximum ZT value was greatly increased to 423 K, which is about 120 K higher when compared with the conventional Bi2Te3-based materials. A maximum ZT of 1.44 was achieved for the sample with 1.5 wt% β-Zn4Sb3 at 423 K, which is also the highest ZT value ever reported at such a high temperature for p-type Bi2Te3-based materials. This work is of importance to expand the application of Bi2Te3-based materials for low grade waste-heat recovery.

In this paper, a self-assembling double-layered film consisting of SnO2 nanosheet arrays as an underlayer and hierarchical SnO2 microspheres as an overlayer was fabricated via a facile hydrothermal process. The effect of experimental parameters, such as seed layer, acetylacetone, and NH4F on the morphology of the SnO2 hierarchical double-layered film was investigated. We disclose that these hierarchical structures are the consequence of the simultaneous processes of growing and recrystallization on the FTO seeded surface and in solution. A formation mechanism was proposed to understand the growth process. The reflectance spectra of SnO2 double-layered films were also examined.The novel SnO2-based film shows an enhanced light-to-electricity conversion efficiency compared with a simple composite of SnO2 nanosheet arrays and microspheres due to their self-assembling capability and favorable nanostructures.

Starting from elemental powder mixtures of FexIn4−xSe3 (x = 0, 0.05, 0.1, 0.15), polycrystalline In4Se3-based compounds with homogeneous microstructures were prepared by mechanical alloying (MA) and hot pressing (HP). With the increase of x from 0 to 0.15, the electrical resistivity and the absolute value of the Seebeck coefficient increased, while the thermal conductivity first decreased and then increased. The maximal dimensionless figure of merit ZT of 0.44 was obtained for the FexIn4−xSe3 (x = 0.05) sample at 723 K.

•AgSbTe2 inclusions, about 50 nm in size, are evenly distributed at the grain boundary.•The electrical conductivity and Seebeck coefficient increase with the addition of AgSbTe2.•The lattice thermal conductivity decreases with the addition of AgSbTe2.•A maximum ZT of 1.27 was obtained for the Yb0.2Co4Sb12/4wt%AgSbTe2 composite sample.Yb0.2Co4Sb12 based composites with AgSbTe2 nanoinclusion have been successfully prepared by a combined process of vacuum melting, pulverization and hot press sintering. XRD, FESEM, TEM and EDS were performed to characterize the microstructure and composition of the nanocomposites. It shows AgSbTe2 inclusions are about 50 nm in size and they distribute in the grain boundaries of Yb0.2Co4Sb12 filled skutterudite matrix. When the content of AgSbTe2 is less than 8wt%, Seebeck coefficient and electrical conductivity of the composites increase with increasing content of AgSbTe2, while the lattice thermal conductivity reduces with the increase of AgSbTe2 nanoinclusions. As a result, thermoelectric performance is improved with addition of AgSbTe2 nanoinclusion and a maximum ZT of 1.27 has been obtained for the sample of Yb0.2Co4Sb12/4wt%AgSbTe2 at about 300 °C.

High-quality CdS/TiO2/FTO nanorod-array film photoelectrodes were fabricated by electrochemical atomic layer epitaxy method (ECALE). The detailed synthesis process and the surface morphology, structure, composition, optical absorbance property of the products were characterized by electrochemical voltammetry, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy dispersion spectroscopy, ultraviolet and visible spectrophotometry, respectively. The results show that the ECALE method can significantly enhance the coverage of CdS quantum dots on TiO2 surface and control the size of CdS quantum dot availably. In comparison with a pure TiO2 nanorod array, the photovoltaic performance of the cell based on the ECALE deposited CdS/TiO2/FTO electrode achieves a maximum short circuit current density of 10.04 mA/cm2 under one sun (AM 1.5G, 100 mW/cm2), which is 5.7 times higher than that of the naked one.

Ternary oxide Zn2SnO4 was successfully introduced to SnO2 hierarchical microsphere photoanode for dye-sensitized solar cells by spin-coating method. Zn2SnO4 nanoparticles with average size of 20 nm modified the SnO2 hierarchical microsphere film uniformly and sufficiently. The shell of Zn2SnO4 nanoparticles plays an important role in resisting acid etchant and suppressing the recombination of conduction band electrons with the oxidized dye. Benefiting from the advantages of both constituents, the composite photoanode exhibits an improved chemical stability and good photoelectric properties in contrast with bare SnO2 nanostructures.Highlights► Zn2SnO4 was introduced to SnO2 photoanode by spin-coating method. ► Zn2SnO4 nanoparticles modified the SnO2 film uniformly and sufficiently. ► The shell of Zn2SnO4 plays an important role in insulating bare SnO2 with dye. ► The composite exhibits improved photoelectric properties in contrast with bare SnO2.

Thin films of p-type Bi0.52Sb1.48Te3 + 3% Te were deposited on glass substrates by flash evaporation. X-ray diffraction and field-emission scanning electron microscopy were performed to characterize the thin films, and the effects of preparation and annealing parameters on the thermoelectric properties were investigated. It was shown that the power factors of the films increased with increasing deposition temperature. Annealing the as-deposited films improved the power factors when the annealing time was less than 90 min and the annealing temperature was lower than 250°C. A maximum power factor of 10.66 μW cm−1 K−2 was obtained when the film was deposited at 200°C and annealed at 250°C for 60 min.

Starting from elemental powders of Ag, Sn, Pb, Sb, and Te, Sn-substituted single-phase LAST (AgPbmSbTem+2) materials were synthesized by a combined process of mechanical alloying and plasma-activated sintering. The effect of Sn content on thermoelectric (TE) properties of the Sn-substituted LAST materials was investigated in detail. With increase of Sn/Pb ratio, the fraction of SnTe in the (Pb,Sn)Te-based solid solution increased, and the lattice spacing decreased accordingly. The electrical conductivity, thermal conductivity, and power factor also increased with increasing substitutional Sn content. It was found that the composition AgPb9Sn9SbTe20 had superior TE properties compared with other compositions, and the maximum ZT of 0.70 was achieved at 675 K for this composition.

Polycrystalline In4Se3−x thermoelectric materials were prepared by a combined process of mechanical alloying and hot pressing (MA-HP), and the effect of Se deficiency on their thermoelectric properties was studied. Starting from elemental In and Se powders, In4Se3 compound was synthesized after mechanical alloying for 1 h. Single-phase In4Se3 compound was obtained in the sample without Se deficiency, while some In impurity was detected in Se-deficient samples. With increasing Se deficiency x, the carrier concentration n increased, and the electrical resistivity and absolute value of Seebeck coefficient of In4Se3−x decreased rapidly. All of the In4Se3−x samples (x > 0) had higher ZT than the stoichiometric In4Se3, with In4Se2.65 showing the highest ZT because of its low thermal conductivity. The maximum ZT reached 0.93 at 700 K.

A double-layer (DL) film with a TiO2 nanosheet-layer on a layer of TiO2 nanorod-array, was synthesized on a transparent conductive fluorine-doped tin oxide substrate by a two-step hydrothermal method. Starting from the precursors of NaSeSO3, CdSO4 and the complex of N(CH2COOK)3, CdSe quantum dots (QDs) were grown on the DL-TiO2 substrate by chemical bath deposition method. The samples were characterized by X-ray diffraction, Scanning electron microscopy, Energy dispersion spectroscopy, and their optical scattering property was measured by light reflection spectrometry. Some CdSe QDs sensitized DL-TiO2 films serve as the photoanodes, were assembled into solar cell devices and their photovoltaic performance were also characterized. The short circuit current and open-circuit voltage of the solar cells range from 0.75 to 4.05 mA/cm2 and 0.20 − 0.42 V under the illumination of one sun (AM1.5, 100 mW/cm2), respectively. The photocurrent density of the DL-TiO2 film is five times higher than that of a bare TiO2 nanorod array photoelectrode cell.Highlights► A two-step hydrothermal deposition method was used to deposit TiO2 films. ► Double-layer TiO2 films were synthesized on transparent FTO substrate. ► The bi-functional character of the electrode were confirmed. ► Photocurrent density of DL-film electrode was enhanced 5 times than a single film.

•Both point defects and nanoscale barrier are introduced by incorporating excess ZnX.•Three parameters have been modulated simultaneously by this simple approach.•A 90% enhanced ZT value of 1.52 has been obtained in the CuInTe2 based TE materials.Developing high thermoelectric performance CuInTe2 based materials is technologically and environmentally intriguing, in order to achieve this, nanoscale heterostructure barrier blocking is proposed and adopted in this work by directly incorporating excess ZnX (X=S, Se) to regulate the electrical and thermal transport properties of CuInTe2. The results prove that part of the ZnX dissolves into the CuInTe2 matrix during the hot press process while the residual ZnX acts as a nanoscale heterostructure barrier blocking for both the hole and phonon. As a consequence, three thermoelectric parameters of the CuInTe2 have been optimized simultaneously by this approach, owing to the formation of Zn- In point defects to improve carrier concentration, the concurrent hindering to the minority carriers resulting from the energy level difference between matrix and nano-heterostructure to enhance the Seebeck coefficient, and intensive phonon scattering by the nanoscale heterostructure barrier blocking to reduce the thermal conductivity. Eventually, a 90% enhanced ZT value of 1.52 has been obtained in the 6 wt% ZnS added CuInTe2 sample.

P-type(Bi0.26Sb0.74)2Te3+3%Te ingots were prepared by cooling at various cooling rates after vacuum melting .In this paper, the chemical composition, structure and thermoelectric properties of the crystals were evaluated by XRD, SEM, EDAX and thermoelectric measurements including Seebeck coefficient, electrical conductivity and thermal conductivity. With increase of cooling rates, Te-rich phase presents in the crystals, the electrical conductivity increases and the Seebeck coefficient decreases. While thermal conductivities of all ingots are all lower than 1.3W/mK. The maximum ZT (T=298K) was 1.12, which was obtained in the p-type(Bi0.26Sb0.74)2Te3+3%Te ingot prepared at a cooling rate of 4K/min.

•Preparing paramagnetic Mn, ferromagnetic Fe and Ni, and nonmagnetic Cu doped Bi0.5Sb1.5Te3 by a magnetic field assisted melting-solidification (HMAMS) method.•Studying the effect of different dopants on the orientation, microstructure, electrical and thermal transport properties, and thermoelectric performance of Bi0.5Sb1.5Te3 prepared by HMAMS.•Thermal conductivity was greatly reduced in the ferromagnetic ion doped samples owing to the intensive scattering of phonon by the hierarchical architecture from macroscale to atomic scale scatter centers.The present work demonstrates the effect of Mn, Fe, Ni and Cu doping on the orientation, microstructure, electrical and thermal transport properties of Bi0.5Sb1.5Te3 ingots prepared under a 2 T magnetic field. The comparative results show that the c-axis is inclined to perpendicular to magnetic field in the ferromagnetic ions (Fe and Ni) doped samples, while is inclined to parallel to magnetic field in the non-magnetic Cu and paramagnetic Mn doped samples; in addition, the growth of eutectic structure is disturbed intensively in the ferromagnetic ions doped samples, resulting in scattered arrangement of Te trips, BSTⅡ nano-rods and nano-scale Te crystals. As a consequence, the thermal conductivity in the ferromagnetic ions doped samples are sharply decreased owing to the enhanced scattering of holes and phonons by the hierarchical architecture of phase boundaries, grain boundaries, BSTⅡ nano-rods even sub-grain boundaries, and a high ZT of ∼1.6 at 323 K was obtained in the ferromagnetic ions doped samples.Figure optionsDownload full-size imageDownload high-quality image (114 K)Download as PowerPoint slide

Pristine MnTe is a p-type semiconductor with a relatively low hole concentration of 1018 cm−3, low electrical conductivity, and thus poor TE performance at room temperature owing to the broad direct band gap of 1.27 eV. In this study, Na2S was employed to be doped into the MnTe matrix to synergistically tune the electrical and thermal transport properties of the semiconductor via point defect engineering. On the one hand, Na substitution effectively improved the electrical transport properties by increasing the carrier concentration via the formation of acceptor-like defects, Na−Mn. On the other hand, thermal conductivities of the Na2S-doped samples were also sharply reduced via mass fluctuation and strain field fluctuation from the point defects introduced through Na doping and S substitution. Consequently, a maximum ZT of ∼1.09 was achieved for the 0.5 at% Na2S-doped sample at 873 K, which was the highest ZT value ever reported for p-type MnTe-based thermoelectric materials. Moreover, the high performance 0.5 at% Na2S-doped sample also exhibited good thermal stability and mechanical stability (Vickers microhardness) of ∼122 Hv, which was higher than those of other promising thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and SnTe.

Since the lattice thermal conductivity of n-type multi-filled skutterudites have been reduced below 1 W (mK−1), the development of new strategies that can further enhance the power factor while maintaining the low thermal conductivity is highly desired. In this paper, we conducted a pioneering work by introducing a “core–shell” microstructure into Yb single-filled skutterudite thermoelectric materials to realise this purpose. The “core–shell” structure formed by the thermal diffusion of well dispersed Ni nanoparticles in the Yb0.2Co4Sb12 powder during hot pressing is composed of the normal “core” grains surrounded by Ni-rich nanograin “shells”. The electrical resistivity is greatly reduced due to the increase in both carrier concentration and mobility. However, the Seebeck coefficient first increases due to the increased density of states at the Fermi energy and then decreases gradually. As a consequence, the power factor is remarkably increased for the samples with the addition of Ni nanoparticles. In addition, the lattice thermal conductivity is also reduced by the extra phonon scattering introduced by the “core–shell” microstructure. The concomitant effects enable a maximum ZT of 1.07 for the 0.2 wt% Ni sample at 723 K.

By means of β-Zn4Sb3 addition and thermal decomposition, dual nanoinclusions of Zn and ZnSb were introduced and Zn atoms were doped into p-type Bi0.4Sb1.6Te3 successfully. Due to the increase of hole concentration by Zn doping and band structure optimization, the bipolar conduction was suppressed and the intrinsic excitation shifts to higher temperature. The power factors of the samples were slightly improved, while the lattice thermal conductivities of the samples were greatly reduced due to the extra phonon scattering introduced by the dual nanoinclusions. As a result, the critical temperature corresponding to the maximum ZT value was greatly increased to 423 K, which is about 120 K higher when compared with the conventional Bi2Te3-based materials. A maximum ZT of 1.44 was achieved for the sample with 1.5 wt% β-Zn4Sb3 at 423 K, which is also the highest ZT value ever reported at such a high temperature for p-type Bi2Te3-based materials. This work is of importance to expand the application of Bi2Te3-based materials for low grade waste-heat recovery.

Polycrystalline Bi2Te2.5Se0.5 materials with different amounts of added MnTe2 have been fabricated by plasma activated sintering. XRD, SEM, EDS and HRTEM techniques and measurements of the electrical and thermal transport properties have been employed to study the effect of MnTe2 addition on the microstructure and the thermoelectric performance of Bi2Te2.5Se0.5 materials. Due to the substitution of Mn at the Bi sites, the XRD peaks shift towards a higher angle with an increase in the content of MnTe2, and the electron density increases; thus, the bipolar conduction is suppressed and accordingly shifts to a higher temperature . When the content of MnTe2 is over 3.0 at%, the dissolution of MnTe2 gets saturated, and some nanoinclusions with sizes of about 40–70 nm were observed and were identified as MnTe2 by EDS and HRTEM characterization. Due to the extra phonon scattering by the MnBi′ point defects and MnTe2 nanoinclusions, the phonon thermal conductivity decreased significantly. As a result, a maximum ZT of 1.0 at 523 K was achieved in the sample with 3.0 at% MnTe2, which is about a 150 K degree shift and a 100% improvement at this temperature compared with the conventional n-type Bi2Te3-based materials.

The single-filled skutterudite Yb0.2Co4Sb12 has been long known as a promising bulk thermoelectric material. In this work, we adopted a melting–milling–hot pressing procedure to prepare nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different weight percentages of AgSbTe2 inclusions were added to optimize the thermoelectric performance. We found that the addition of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent electrical conductivity, Seebeck coefficient and thermal conductivity. In particular, the significantly enhanced carrier mobility led to a ∼3-fold reduction of the electrical resistivity. Meanwhile the absolute value of Seebeck coefficient was enhanced via the energy filtering effect at the matrix–nanoinclusion interfaces. Moreover there is a topological crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6 wt% and 8 wt% of nanoinclusions. Above the crossover, further addition of nanoinclusions degraded the Seebeck coefficient and the electrical conductivity. Meanwhile, the addition of nanoinclusions generally reduced the lattice thermal conductivity. As a result, the power factor of the 6 wt% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temperature figure of merit ZT ∼ 0.51.

Recently, research interests in p-type skutterudites are focused on multi-filling in the intrinsic void sites of Fe4−xMxSb12. The four-membered antimony rings, another important structural feature of p-type skutterudites, seem to be overlooked. In this study, Te has been employed to substitute Sb in single-filled p-type Ce0.9Fe3.75Ni0.25Sb12 skutterudite and a systematic investigation has been carried out into the doping effect of Te on the microstructure and thermoelectric properties of this material. With an increase of Te doping, the electrical resistivity decreases due to the increase of hole concentration and the gradual decrease of mobility; while the density of state effective mass increases rapidly with the increase of hole concentration due to the intrinsic multiple bands effects, thus the Seebeck coefficient increases slightly. In addition, Te doping on Sb sites changes the valance electron balance and results in the formation of microscale γ-Ce and nanoscale CeTe2 compound. These microscale and nanoscale precipitates work together with the atomic scale distortion of Sb4 rings by the substitution of Te for Sb, could scatter phonons with a wide range of frequency thus result in a significant reduction of the lattice thermal conductivity. Therefore the thermoelectric performance has been enhanced greatly, and a maximum ZT value of 1.0 at 773 K was obtained for the Ce0.9Fe3.75Ni0.25Sb11.9Te0.1 sample.

In this paper, different atom combinations of Pb, I and Cu have been doped into the In4Se2.5 matrix and a systematic investigation has been carried out for the synergistic effect of multiple heteroatoms on the microstructure and thermoelectric properties of polycrystalline In4Se2.5. By this approach, the electron and phonon transport properties are rationally regulated and the electrical conductivity increases greatly due to the multiple doping, which result in a simultaneous increase of carrier concentration and mobility. The Seebeck coefficient also remains at a relatively high level in a high temperature range due to the energy-dependent electron scattering at the metal nanoparticle–matrix interfaces. In addition, the lattice thermal conductivity is also greatly reduced because of the wide frequency phonon scattering by the point defects and hierarchical metal nanoparticles combined with the phonon–phonon interactions. Consequently, an enhancement of the ZT with a maximum of 1.4 (723 K) has been achieved in the multiple doped In4Se2.5 sample.

We studied the effect of doping with Bi on the thermoelectric properties of SnTe-based materials. Doping with Bi reduced the density of holes and increased the Seebeck coefficient over a wide temperature range as a result of modulation of the carrier concentration and an increase in the density of states effective mass. The lattice thermal conductivity was also greatly reduced as a result of the wide frequency range of phonon scattering by multiscale architectures derived from Bi doping. A maximum ZT value of c. 1.1 at 873 K was obtained in Sn0.94Bi0.06Te, an enhancement of 165% compared with the undoped sample.

A single phase CuSbSe2 polycrystalline chalcostibite compound has been facilely synthesized through mechanical alloying for the first time, and the phase evolution has been revealed in detail. The large Seebeck coefficient and poor electrical conductivity of this compound are ascribed to the existing Cu–Se ionic bonding and heavy band at the VBM. The active lone-pair s2 electrons in Sb3+ ions are likely to be responsible for the experimentally observed low thermal conductivity in the CuSbSe2 compound. By introducing narrow band-gap Cu3SbSe4 into the CuSbSe2 matrix, the carrier concentration and mobility can be tuned effectively; thus the power factor has been improved remarkably. As a consequence, the figure of merit (ZT) is increased by a factor of 1.6 at 623 K, from 0.25 (matrix) to 0.41.

As a promising mid-temperature thermoelectric (TE) material, the main obstacle to a high TE performance of the InSb compound is its high thermal conductivity. In this article, a new strategy of eutectic melting has been employed to improve the TE properties of the compound for the first time. By addition of excess Sb into the InSb matrix, an InSb–Sb eutectic structure has been introduced. When the temperature is above the melting point of the eutectic mixture, the InSb–Sb eutectic melts into a liquid phase which inhibits the propagation of transverse acoustic phonons, and the thermal conductivity is reduced drastically. Therefore, the thermoelectric performance is remarkably enhanced after the melting of the eutectic, and an unprecedented high ZT of 1.28@773 K has been achieved for the InSb1.04 sample, which is almost 3 times higher than that of the eutectic-free InSb matrix. Moreover, the Vickers hardness of the eutectic included InSb compound is higher than those of many well-established mid-temperature TE materials, and no evident hardness degradation can be detected after several melting–solidification cycles of the eutectic.