Cation Substitution of Solution‐Processed Cu2ZnSnS4 Thin Film Solar Cell with over 9% Efficiency

To alleviate the limitations of pure sulfide Cu₂ZnSnS₄ (CZTS) thin film, such as band gaps adjustment, antisite defects, secondary phase and microstructure, Cadmium is introduced into CZTS thin film to replace Zn partially to form Cu₂Zn_1?xCd_xSnS₄ (CZCTS) thin film by low‐cost sol–gel method. It is demonstrated that the band gaps and crystal structure of CZCTS thin films are affected by the change in Zn/Cd ratio. In addition, the ZnS secondary phase can be decreased and the grain sizes can be improved to some degree by partial replacement of Zn with Cd in CZCTS thin film. The power conversion efficiency of CZTS solar cell device is enhanced significantly from 5.30% to 9.24% (active area efficiency 9.82%) with appropriate ratio of Zn/Cd. The variation of device parameter as a function of Zn/Cd ratio may be attributed to the change in electronic structure of the bulk CZCTS thin film (i.e., phase change from kesterite to stannite), which in turn affects the band alignment at the CZCTS/buffer interface and the charge separation at this interface.     

Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (AgxCu1–x)2ZnSnSe4

The photovoltaic absorber Cu₂ZnSn(S_xSe_1–x)₄ (CZTSSe) has attracted interest in recent years due to the earth‐abundance of its constituents and the realization of high performance (12.6% efficiency). The open‐circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu/Zn antisites. By replacing Cu in CZTSSe with Ag, whose covalent radius is ≈15% larger than that of Cu and Zn, the density of I–II antisite defects is predicted to drop. The fundamental properties of the mixed Ag‐Cu kesterite compound are reported as a function of the Ag/(Ag + Cu) ratio. The extent of band tailing is shown to decrease with increasing Ag. This is verified by comparing the optical band gap extrapolated from transmission data with the position of the room‐temperature photoluminescence peak; these values converge for the pure‐Ag compound. Additionally, the pinning of the Fermi level in CZTSSe, attributed to heavy defect compensation and band tailing, is not observed in the pure‐Ag compound, offering further evidence of improved electronic structure. Finally, a device efficiency of 10.2% is reported for a device containing 10% Ag (no antireflection coating); this compares to ≈9% (avg) efficiency for the baseline pure‐Cu CZTSe.     

A Dual Role by Incorporation of Magnesium in YbZn2Sb2 Zintl Phase for Enhanced Thermoelectric Performance

1‐2‐2‐type Zintl phase compounds have promising thermoelectric properties because of their complex crystal structures and multiple valence‐band structures. In this work, a series of single phase (Yb_0.9Mg_0.1)Mg_xZn_2?xSb₂ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) compounds are prepared by alloying YbZn₂Sb₂ with 10 at% MgZn₂Sb₂ and different amounts of YbMg₂Sb₂. The incorporation of Mg at the Yb site, as well as at the Zn site, not only leads to an effective orbital alignment confirmed by the dramatically enhanced density of states effective mass and Seebeck coefficients, but also increases the point defect scattering, contributing to a low lattice thermal conductivity ≈0.54 W m^?1 K^?1 at 773 K. Combined with the optimization of the carrier concentration by Ag doping at the Zn site, a highest ZT value ≈1.5 at 773 K is achieved in (Yb_0.9Mg_0.1)Mg_0.8Zn_1.2Ag_0.002Sb₂, which is higher than that of all the previously reported 1‐2‐2‐type Zintl phase compounds.     

Suppressed Deep Traps and Bandgap Fluctuations in Cu2CdSnS4 Solar Cells with ≈8% Efficiency

The identification of performance‐limiting factors is a crucial step in the development of solar cell technologies. Cu₂ZnSn(S,Se)₄‐based solar cells have shown promising power conversion efficiencies in recent years, but their performance remains inferior compared to other thin‐film solar cells. Moreover, the fundamental material characteristics that contribute to this inferior performance are unclear. In this paper, the performance‐limiting role of deep‐trap‐level‐inducing 2Cu_Zn+Sn_Zn defect clusters is revealed by comparing the defect formation energies and optoelectronic characteristics of Cu₂ZnSnS₄ and Cu₂CdSnS₄. It is shown that these deleterious defect clusters can be suppressed by substituting Zn with Cd in a Cu‐poor compositional region. The substitution of Zn with Cd also significantly reduces the bandgap fluctuations, despite the similarity in the formation energy of the Cu_Zn+Zn_Cu and Cu_Cd+Cd_Cu antisites. Detailed investigation of the Cu₂CdSnS₄ series with varying Cu/[Cd+Sn] ratios highlights the importance of Cu‐poor composition, presumably via the presence of V_Cu, in improving the optoelectronic properties of the cation‐substituted absorber. Finally, a 7.96% efficient Cu₂CdSnS₄ solar cell is demonstrated, which shows the highest efficiency among fully cation‐substituted absorbers based on Cu₂ZnSnS₄.     

High Performance Thermoelectricity in Earth‐Abundant Compounds Based on Natural Mineral Tetrahedrites

Thermoelectric materials can convert waste heat into electricity, potentially improving the efficiency of energy usage in both industry and everyday life. Unfortunately, known good thermoelectric materials often are comprised of elements that are in low abundance and require careful doping and complex synthesis procedures. Here, we report dimensionless thermoelectric figure of merit near unity in compounds of the form Cu_12‐xM_xSb₄S₁₃, where M is a transition metal such as Zn or Fe, for wide ranges of x. The compounds investigated here span the range of compositions of the natural mineral family of tetrahedrites, the most widespread sulfosalts on Earth, and we further show that the natural mineral itself can be used directly as an inexpensive source thermoelectric material. Thermoelectrics comprised of earth‐abundant elements will pave the way to many new, low cost thermoelectric energy generation opportunities.     

An Electrochemical Study of the Influence of Marinobacter aquaeolei on the Alteration of Hydrothermal Chalcopyrite (CuFeS2) and Pyrite (FeS2) under Circumneutral Conditions

Pyrite and chalcopyrite are the two most abundant sulphides observed in seafloor hydrothermal systems. The alteration of sulphides is primarily controlled by reactions on the mineral surfaces and Fe(II)-oxidizing bacteria closely related to Marinobacter aquaeolei are thought to play a major role in iron oxidation under circumneutral conditions. We assessed the influence of M. aquaeolei on the electroactivity of FeS₂ and CuFeS₂ minerals under circumneutral conditions. Samples for the experiments were obtained from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound (field), 26 °N on the Mid-Atlantic Ridge and Ireland (CuFeS₂)]. The experimental approach relied on voltammetry and scanning electrochemical microscopy (SECM). The tip-substrate voltammetry mode of SECM was found to be particularly suitable to probe the major redox processes of those minerals and permitted an assessment of the microorganisms influence on these processes. M. aquaeolei was found to enhance FeS₂ and CuFeS₂ oxidation, particularly under suboxic conditions. M. aquaeolei also significantly enhances Fe dissolution under oxic circumneutral conditions but suppresses the dissolution of most other elements compared to abiotic conditions. Under abiotic conditions the surfaces of the minerals are rapidly passivated when oxygen is available; while addition of M. aquaeolei significantly hinders the passivation of chalcopyrite, no passivation of the pyrite surface is observed. This study demonstrates the ability of Marinobacter aquaeolei to enhance oxidation of FeS₂ and CuFeS₂ under circumneutral conditions and supports the involvement of Marinobacter species in weathering reactions on the seafloor and the control of the ultimate fate of sulphide deposits.     

Preparation and optical properties of alloyed ZnxCd1‐xS/alginate core/shell nanoparticles

Zn_xCd_1‐xS/alginate core/shell nanoparticles were synthesized via a colloidal route by reacting zinc and cadmium ions with sulfide ions, followed by coating with alginate. The crystal structure, morphology, size and optical properties of the core/shell nanoparticles were characterized by X‐ray diffraction, transmission electron microscopy, UV/vis and photoluminescent spectra, respectively. The Zn_xCd_1‐xS nanoparticles are spherical and have a cubic structure with a mean crystalline size of 2–4 nm. The band gap of Zn_xCd_1‐xS/alginate core/shell nanoparticles increases with increasing Zn/Cd molar ratio, and the UV/vis absorption blue‐shifts correspondingly. Two emissions related to zinc and sulfide ion vacancies were observed for the Zn_xCd_1‐xS/alginate core/shell nanoparticles due to the surface changes from the alginate coating. A cadmium‐related emission was observed for both the uncovered Zn_xCd_1‐xS and Zn_xCd_1‐xS/alginate core/shell nanoparticles, which has a significant blue‐shift with increasing Zn/Cd molar ratio. Copyright © 2014 John Wiley & Sons, Ltd.     

Improvement of light emission of Mn-doped Zn(2) SiO(4) phosphors with sodium

Mn‐doped willemite (Zn₂SiO₄:Mn) green phosphor were synthesized by sol–gel technology. The effect of the addition of sodium, as in the composition Zn_{(1.92 – X)} Na_XMn_0.08SiO₄, on the emission behavior was studied. FT–IR and EPR results revealed that sodium ion is incorporated into the lattice and results in the formation of isolated Mn²⁺ and Mn–Mn pairs. The maximum emission intensity of the sample under ultraviolet (UV) excitation occurred at the sodium concentration of x = ~0.03. The green emission at about 525 nm is assigned to Mn²⁺–Mn²⁺ pair centres on nearest neighbour Zn sites. The highest intensity of the green emission for x = ~0.03 is well close to the highest concentration of the Mn²⁺–Mn²⁺ pair. Copyright © 2012 John Wiley & Sons, Ltd.     

Formation of CuInSe2 and CuGaSe2 Thin‐Films Deposited by Three‐Stage Thermal Co‐Evaporation: A Real‐Time X‐Ray Diffraction and Fluorescence Study

Thin film solar cells based on co‐evaporated Cu(In,Ga)Se₂ absorber films present the highest efficiencies among current polycrystalline thin‐film technologies. Thanks to the development of a novel experimental setup for in situ growth studies, it was possible to follow the formation of the crystalline phases during such deposition processes for the first time. This synchrotron‐based energy‐dispersive X‐ray diffraction and fluorescence setup is suited for real‐time studies of thin film vapor deposition processes. Focusing on the growth of CuInSe₂ and CuGaSe₂ fabricated by three‐stage processing, we find that the phase transitions in the Cu‐In‐Se system follow the reported pseudo‐binary In₂Se₃‐Cu₂Se phase diagram. This requires a transformation of the Se sublattice during the incorporation of Cu‐Se into the In₂Se₃ precursor film from the first process stage. In the Cu‐Ga‐Se system, besides an increase in the lattice spacings, we observe no transformation of the Se sublattice. Furthermore, the structural defects of the Ga‐Se precursor film are preserved until the CuGaSe₂ stoichiometry is reached. By means of model calculations of the fluorescence signals, we confirm in both systems the segregation of Cu₂Se at the surface near a concentration of 25 at.% Cu shortly after the recrystallization of the films. The modeling also reveals that Cu₂Se penetrates into the CuInSe₂ film, whereas it remains at the surface of the CuGaSe₂ film.     

Thermoelectric Property Studies on Cu‐Doped n‐type CuxBi2Te2.7Se0.3 Nanocomposites

Combining high energy ball‐milling and hot‐pressing, significant enhancements of the thermoelectric figure‐of‐merit (ZT) have been reported for p‐type Bi_0.4Sb_1.6Te₃ nanocomposites. However, applying the same technique to n‐type Bi₂Te_2.7Se_0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n‐type Bi₂Te_2.7Se_0.3. Even though texturing was effective in improving peak ZT of Bi₂Te_2.7Se_0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi₂Te_2.7Se_0.3 to make Cu_0.01Bi₂Te_2.7Se_0.3 samples. A peak ZT value of 0.99 was achieved in Cu_0.01Bi₂Te_2.7Se_0.3 samples without texturing. With texturing by re‐pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.     

Alterations in surfaces and textures of minerals during the bacterial leaching of a complex sulfide ore

Abstract

The purpose of the work was to characterize changes in surface textures of minerals during the biological leaching of a complex sulfide ore. The ore contained pyrrhotite (Fe_I__xS), pyrite (FeS₂), sphalerite (ZnS), pentlandite [(Ni,Fe,Co)₉S₈], and chalcopyrite (CuFeS₂). Several mixed cultures were initially screened using the ore material as the sole substrate. Shake flask leaching experiments showed no major differences among test cultures, which were all derived by enrichment techniques using environmental samples collected from a mine site. Leached pyrrhotite surfaces were invariably surrounded by a dark rim of elemental S. A reaction zone was also associated with leached sphalerite grains. Chemical analyses of leach solutions indicated that the relative ranking of biological leaching of the sulfide minerals was Zn > Ni > Co > Cu. Microscopic observations were in keeping with this rankin     

Electronic structure and optical properties of Cu-doping and Zn vacancy impurities in ZnTe

The geometric structures of perfect ZnTe, that with Zn vacancy (Zn_0.875Te), and Cu-doped ZnTe (Zn_0.875Cu_0.125Te) were optimized using the pseudopotential plane wave (PP-PW) method based on the density functional theory (DFT) within generalized gradient approximation (GGA). The cohesive energy, band structure, density of states, and Mulliken populations were calculated and discussed in detail. On the other hand, an accurate calculation of linear optical functions (the dielectric function, refraction index, reflectivity, conductivity function, and energy-loss spectrum) was performed. The results demonstrated that compared to the perfect ZnTe, the lattice parameters of Zn_0.875Te and Zn_0.875Cu_0.125Te were changed and the cell volumes decreased to some extent due to the vacancy and introduction of impurity. A vacancy acceptor level and an acceptor impurity level were produced in Zn_0.875Te and Zn_0.875Cu_0.125Te, respectively. By comparison, Cu doping in the ZnTe system is relatively stable while the monovacancy system is not.     

Electronic structure and band gap engineering of ZnO-based semiconductor alloy films

We have conducted first-principles total-energy density functional calculations to study the atomic structures, band structures and electronic structures of Zn_1 ? xM_xO (M = Be, Mg, Cd, Ag, Cu) semiconductor alloys. The Heyd–Scuseria–Ernzerhof hybrid functional has been performed to yield lattice constants and band gaps of Zn_1 ? xM_xO semiconductor in much better agreement with experimental data than with the standard local exchange correlation functional. We found that the strong coupling between O 2p and Cu 3d or Ag 4d bands plays a key role in narrowing of band gaps and leading to the half-metallic behaviour interpreted with the unique spatial distribution pattern between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.     

Approaching the Minimum Thermal Conductivity in Rhenium‐Substituted Higher Manganese Silicides

Higher manganese silicides (HMS) made of earth‐abundant and non‐toxic elements are regarded as promising p‐type thermoelectric materials because their complex crystal structure results in low lattice thermal conductivity. It is shown here that the already low thermal conductivity of HMS can be reduced further to approach the minimum thermal conductivity via partial substitution of Mn with heavier rhenium (Re) to increase point defect scattering. The solubility limit of Re in the obtained Re_xMn_1‐xSi_1.8 is determined to be about x = 0.18. Elemental inhomogeneity and the formation of ReSi_1.75 inclusions with 50?200 nm size are found within the HMS matrix. It is found that the power factor does not change markedly at low Re content of x ≤ 0.04 before it drops considerably at higher Re contents. Compared to pure HMS, the reduced lattice thermal conductivity in Re_xMn_1‐xSi_1.8 results in a 25% increase of the peak figure of merit ZT to reach 0.57 ± 0.08 at 800 K for x = 0.04. The suppressed thermal conductivity in the pure Re_xMn_1‐xSi_1.8 can enable further investigations of the ZT limit of this system by exploring different impurity doping strategies to optimize the carrier concentration and power factor.     

n‐Type SnSe2 Oriented‐Nanoplate‐Based Pellets for High Thermoelectric Performance

It is reported that electron doped n‐type SnSe₂ nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe₂ shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all‐scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice thermal conductivity. As a result, an enhanced in‐plane thermoelectric figure of merit ZT_max of 0.63 is achieved for a 1.5 at% Cl doped SnSe_1.95 pellet at 673 K, which is much higher than the corresponding in‐plane ZT of pure SnSe₂ (0.08).     

Grain Boundary Scattering of Charge Transport in n‐Type (Hf,Zr)CoSb Half‐Heusler Thermoelectric Materials

High thermoelectric figure of merit zT of ≈1.0 has been reported in both n‐ and p‐type (Hf,Zr)CoSb‐based half‐Heusler compounds, and further improvement of thermoelectric performance relies on the insightful understanding of electron and phonon transport mechanisms. In this work, the thermoelectric transport features are analyzed for (Hf_0.3Zr_0.7)_1?xNb_xCoSb (x = 0.02–0.3) with a wide range of carrier concentration. It is found that, although both temperature and energy dependencies of charge transport resemble ionized impurity scattering, the grain boundary scattering is the dominant scattering mechanism near room temperature. With increasing carrier concentration and grain size, the influence of the grain boundary scattering on electron transport weakens. The dominant scattering mechanism changes from grain boundary scattering to acoustic phonon scattering as temperature rises. The lattice thermal conductivity decreases with increasing Nb doping content due to the increased strain field fluctuations. These results provide an in‐depth understanding of the transport mechanisms and guidance for further optimizing thermoelectric properties of half‐Heusler alloys and other thermoelectric systems.     

Enhancement of Thermoelectric Figure of Merit by the Insertion of MgTe Nanostructures in p‐type PbTe Doped with Na2Te

The thermoelectric properties of crystalline melt‐grown ingots of p‐type PbTe–xMgTe (x = 1–3 mol%) doped with Na₂Te (1–2 mol%) were investigated over the temperature range of 300 K to 810 K. While the powder X‐ray diffraction patterns show that all samples crystallize in the NaCl‐type structure with no MgTe or other phases present, transmission electron microscopy reveals ubiquitous MgTe nanoprecipitates in the PbTe. The very small amounts of MgTe in PbTe have only a small effect on the electrical transport properties of the system, while they have a large effect on thermal transport significantly reducing the lattice thermal conductivity. A ZT of 1.6 at 780 K is achieved for the PbTe containing 2% MgTe doped with 2% Na₂Te.     

3D Porous Cu–Zn Alloys as Alternative Anode Materials for Li‐Ion Batteries with Superior Low T Performance

Zinc is recently gaining interest in the battery community as potential alternative anode material, because of its large natural abundance and potentially larger volumetric density than graphite. Nevertheless, pure Zn anodes have shown so far very poor cycling performance. Here, the electrochemical performance of Zn‐rich porous Cu–Zn alloys electrodeposited by an environmentally friendly (aqueous) dynamic hydrogen bubble template method is reported. The lithiation/delithiation mechanism is studied in detail by both in situ and ex situ X‐ray diffraction, indicating the reversible displacement of Zn from the Cu–Zn alloy upon reaction with Li. The influence of the alloy composition on the performance of carbon‐ and binder‐free electrodes is also investigated. The optimal Cu:Zn atomic ratio is found to be 18:82, which provides impressive rate capability up to 10 A g^?1 (≈30C), and promising capacity retention upon more than 500 cycles. The high electronic conductivity provided by Cu, and the porous electrode morphology also enable superior lithium storage capability at low temperature. Cu₁₈Zn₈₂ can indeed steadily deliver ≈200 mAh g^?1 at ?20 °C, whereas an analogous commercial graphite electrode rapidly fades to only 12 mAh g^?1.     

Achieving zT > 2 in p‐Type AgSbTe2−xSex Alloys via Exploring the Extra Light Valence Band and Introducing Dense Stacking Faults

Through simultaneously enhancing the power factor by engineering the extra light band and enhancing phonon scatterings by introducing a high density of stacking faults, a record figure‐of‐merit over 2.0 is achieved in p‐type AgSbTe_2?xSe_x alloys. Density functional theory calculations confirm the presence of the light valence band with large degeneracy in AgSbTe₂, and that alloying with Se decreases the energy offset between the light valence band and the valence band maximum. Therefore, a significantly enhanced power factor is realized in p‐type AgSbTe_2?xSe_x alloys. In addition, transmission electron microscopy studies indicate the appearance of stacking faults and grain boundaries, which together with grain boundaries and point defects significantly strengthen phonon scatterings, leading to an ultralow thermal conductivity. The synergetic strategy of simultaneously enhancing power factor and strengthening phonon scattering developed in this study opens up a robust pathway to tailor thermoelectric performance.