Cu2-xS Phases Research Articles

Pre-phase transition of a Cu2-xS template enables polymorph selective synthesis of MS (M = Zn, Cd, Mn) nanocrystals via cation exchange reactions.

Utilization of copper-deficient Cu2-xS nanocrystals (NCs) with diverse crystal phases and stoichiometries as cation exchange (CE) templates is a potential route to overcome the current limitations in the polymorph selective synthesis of desired nanomaterials. Among the Cu2-xS NCs, covellite CuS is emerging as an attractive CE template to produce complicated and metastable metal sulfide NCs. The presence of a reducing agent is essential to induce a phase transition of CuS into other Cu2-xS phases prior to the CE reactions. Nevertheless, the effect of the reducing agent on the phase transition of CuS, especially into the hexagonal close packing (hcp) phase and the cubic close packing (ccp) phase, has been scarcely exploited, but it is highly important for the polymorphic production of metal sulfides with the wurtzite phase and zinc blende phase. Herein, we report a reducing agent dependent pre-phase transition of CuS nanodisks (NDs) into hcp and ccp Cu2-xS NCs. 1-Dodecanethiol molecules and oleylamine molecules selectively reduced CuS NDs into hcp djurleite Cu1.94S NDs and ccp digenite Cu1.8S NCs. Afterward, the hcp Cu1.94S NDs and ccp Cu1.8S NCs were exchanged by Zn2+/Cd2+/Mn2+, and the wurtzite phase and the zinc blende phase of ZnS, CdS, and MnS NCs were produced. Without the pre-phase transition, direct CE reactions of CuS NDs are incapable of synthesizing the above wurtzite and zinc blende metal sulfide NCs. Therefore, our findings suggest the importance of the pre-phase transition of the CE template in polymorphic syntheses, holding great promise in the fabrication of other polymorphic nanomaterials with novel physical and chemical properties.

Resin-derived carbon to in-situ support Cu-Cu2-xS heteroparticles for efficient photocatalytic reduction of Cr(VI)

Compared with heterojunction composite, the heteroparticle theoretically provide the larger interfacial area between components and higher photoactivity. Nonetheless, two or more substances are very hard to synchronously form on an individual particle under the same experimental conditions, due to their different thermodynamical property and stability. Herein, sulfonic resins are utilized to in-situ prepare the ternary Cu-Cu2-xS/C catalyst via a simple synthetic strategy, of which the supported Cu-Cu2-xS monoparticles are proved to include both metallic Cu and Cu2-xS phases. This unique heterostructure significantly accelerates the electron transfer between Cu and Cu2-xS. Under the irradiation of visible light, the Cu-Cu2-xS/C product exhibits a superior photocatalytic activity for the reduction of Cr(VI), which is ∼5.2 times as high as the physically mixed Cu+Cu2-xS+C sample. This methodology will inspire the development and study of more heteroparticle catalysts with the high activity to apply in the possible industrial production.

Effect of sulfur concentration on structural, optical and electrical properties of Cu2CoSnS4 absorber film for photovoltaic devices

In this study, Cu2CoSnS4 (CCTS) films were grown on glass substrates by dipping the sol-gel method. The effect of the thiourea molarity in solution on film properties was investigated. X-ray diffraction (XRD) and Raman Scattering were used to determine the structure of the prepared films. Raman spectroscopic results showed that films with low thiourea molarity have two phases, Cu2-xS and Cu2CoSnS4. As the thiourea molarity increases, this Cu2-xS phase disappears. Optical band gap values found with the help of optical transmittance measurements of the films also decreased from 1.7eV to 1.4eV with increasing thiourea molarity. Morphological properties were examined by Scanning Electron Microscope (SEM) and Atomic Force Microscopy (AFM). The electrical resistivity of the prepared films is also studied by the two-contact method at room temperature. In general, structural, optical, morphological, and electrical properties of the films were affected by thiourea molarity in solution.

Seeded Growth Combined with Cation Exchange for the Synthesis of Anisotropic Cu2-x S/ZnS, Cu2-x S, and CuInS2 Nanorods.

Colloidal copper(I) sulfide (Cu2–xS) nanocrystals (NCs) have attracted much attention for a wide range of applications because of their unique optoelectronic properties, driving scientists to explore the potential of using Cu2–xS NCs as seeds in the synthesis of heteronanocrystals to achieve new multifunctional materials. Herein, we developed a multistep synthesis strategy toward Cu2–xS/ZnS heteronanorods. The Janus-type Cu2–xS/ZnS heteronanorods are obtained by the injection of hexagonal high-chalcocite Cu2–xS seed NCs in a hot zinc oleate solution in the presence of suitable surfactants, 20 s after the injection of sulfur precursors. The Cu2–xS seed NCs undergo rapid aggregation and coalescence in the first few seconds after the injection, forming larger NCs that act as the effective seeds for heteronucleation and growth of ZnS. The ZnS heteronucleation occurs on a single (100) facet of the Cu2–xS seed NCs and is followed by fast anisotropic growth along a direction that is perpendicular to the c-axis, thus leading to Cu2–xS/ZnS Janus-type heteronanorods with a sharp heterointerface. Interestingly, the high-chalcocite crystal structure of the injected Cu2–xS seed NCs is preserved in the Cu2–xS segments of the heteronanorods because of the high-thermodynamic stability of this Cu2–xS phase. The Cu2–xS/ZnS heteronanorods are subsequently converted into single-component Cu2–xS and CuInS2 nanorods by postsynthetic topotactic cation exchange. This work expands the possibilities for the rational synthesis of colloidal multicomponent heteronanorods by allowing the design principles of postsynthetic heteroepitaxial seeded growth and nanoscale cation exchange to be combined, yielding access to a plethora of multicomponent heteronanorods with diameters in the quantum confinement regime.

Investigation of Zn/Sn ratio for improving the material quality of CZTS thin films with the reduction of Cu2-xS secondary phase

In this study, a sputtering system was adopted for the preparation of Cu2ZnSnS4 (CZTS) thin films with high material quality. The system precisely controlled the elemental ratio in a copper-zinc-tin (CZT) precursor for contributing to the grain growth of CZTS crystallites, and reduced the detrimental formation of the Cu2-xS secondary phase during the high-temperature sulfurization process.The CZT precursor was prepared in a layer sequence of Zn/Sn/Cu/Zn, as the Cu-capping layer could protect the underlying Zn/Sn bilayer from volatilization during the high-temperature sulfurization. The CZT precursors were subjected to the sulfurization process at varied temperatures ranging from 480 to 560 °C for 20 min to form the CZTS thin film. In this study, the Cu/(Zn + Sn) and Zn/Sn ratios in the CZTS film were studied by energy dispersive X-ray spectroscopy measurements, and both ratios revealed elevations with an increasing sulfurization temperature, owing to the thermal volatilization of Zn and Sn. Thus, the formation of the undesirable Cu2-xS secondary phase was enhanced, and the surface roughness and density of voids on the CZTS thin films were increased. For a reduced Zn/Sn ratio with a reduced sulfurization temperature, the improved CZTS thin-film quality was demonstrated by the increased X-ray diffraction (XRD) and Raman intensity, along with the reduction of the XRD linewidth of the (112) CZTS crystal phase. A CZTS thin film with a sulfurization condition of 500 °C for 20 min showed a low Zn/Sn ratio (at 1.1), along with the features of a dense and smooth thin-film surface. Moreover, a reduced photoluminescence spectra full width at half maximum (FWHM) value of 92.7 meV was observed.In addition, for further improving the grain growth of CZTS, a pre-annealing process of a CZT precursor was conducted at a temperature of 350 °C for 30 min. An enhancement of the CZTS crystallite size with the stoichiometric formation of a CZTS phase and a reduced Cu2-XS phase was observed, indicating the superior material quality of the CZTS thin films.

Persistent Double-Layer Formation in Kesterite Solar Cells: A Critical Review

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered - or double-layered - CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics and its effect on the performance of devices. In this work, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a large statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence, and find that the double-layer limits the efficiency of our devices to below 7%. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. Finally, we comment on the generality of the mechanism proposed, by comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide- and oxide-based targets. We find that although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double-layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix. We suggest that understanding the limitations imposed by the double layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

Valence Selectivity of Cation Incorporation into Covellite CuS Nanoplatelets

Synthesis of copper sulfide-based nanomaterials by cation incorporation into copper deficient copper sulfide (Cu2–xS) is of interest as a powerful means to obtain nanostructures with otherwise inaccessible combinations of size, shape, composition, and crystal phase. Incorporation of a heterocation (M) may produce heterogeneous Cu2–xS-MS nanocrystals (NCs) or homogeneous Cu-M-S alloys. However, the factors determining whether heterogeneous NCs or homogeneous alloy NCs are produced have not been fully elucidated. In this report, we incorporate diverse cations into covellite CuS nanoplatelet (NPl) templates in the presence of dodecanethiol (DDT). These cations are categorized by their valencies. We demonstrate that trivalent and tetravalent cations can be incorporated into reduced CuS NPls to produce homogeneous ternary alloy NPls, while the divalent cations cannot coexist with Cu+ ions in the Cu2–xS phase. In turn, the incorporation of divalent cations leads to formation of heterogeneous NPls and finally pr...

Critical role of Zn/Sn ratio to enhance Cu-Zn-Sn-S solar cell efficiency by suppressing detrimental Cu2-xS secondary phase

The co-existing secondary phases, in off-stoichiometric Cu-Zn-Sn-S (CZTS) solar cells, are one of the main causes of performance gap between earth abundant CZTS and matured Cu-In-Ga-Se (CIGS) solar cells technology. Highly metallic Cu-S based secondary phases provide shunting paths to the current flow and critically affect the device performance. Several chemical etchants have been developed and used to remove conductive Cu2-xS phase from CZTS absorber surface before device fabrication. Present work is mainly focused to tune the Zn/Sn composition ratio in co-sputtered CZTS active layer to suppress the formation of Cu2-xS phase during the growth itself to eliminate the additional step of etching in device fabrication. Significant reduction of Cu2-xS secondary phase was observed with decreased Zn/Sn ratio, which resulted in more than 3 times efficiency enhancement for the final composition. The fabricated solar cell in the stacking order of Ag/ITO/i-ZnO/CdS/CZTS/Mo/SLG with optimum composition showed an efficiency of 6.11% without any antireflection coating. Controlling the Zn/Sn ratio resulted in an effective and virtuous solution to avoid the detrimental secondary phases like Cu2-xS, which conventionally required a chemical etching treatment.

Effects of annealing on elemental composition and quality of CZTSSe thin films obtained by spray pyrolysis

The spray pyrolysis was used for the deposition of Cu2ZnSn(S, Se)4 (CZTSSe) kesterite thin films. The basic spray pyrolysis solution was prepared from two precursor solutions containing thiourea and cooled to a temperature near 1°C, which leads to minimizing the number of insoluble hydrates of copper chloride. The optimal substrate temperature was 350°C and the distance from the sprayer nozzle 30 cm. The as-deposited Cu2ZnSnS4 layers were annealed in S2 atmosphere for the compensation of the sulfur deficiency and with the addition of Sn in order to avoid tin loss. Cu2ZnSn(S, Se)4 thin films were obtained after the annealing of as-deposited films in the (S2 + Se2) atmosphere. The surface morphology and composition of obtained thin films were investigated using the energy dispersive X-ray (EDX) microanalysis and Raman spectroscopy measurements. The structural characterization by the grazing incidence X-ray diffraction (GIXRD) showed the presence of Cu2–xS phases in all of the annealed thin films. For the Se/(S + Se) ratio of the thin films annealed in the (S + Se) atmosphere was established from EDX measurements and analysis of GIXRD data, the results are in satisfactory agreement.

Cu2SnS3 absorber thin films prepared via successive ionic layer adsorption and reaction method

Abstract In this paper, we report the production of Cu2SnS3 thin films with high phase purity via successive ionic layer adsorption and reaction method on soda lime glass substrates. Structural, morphological, compositional, optical and electrical investigations were carried out. The X-ray diffraction patterns of the samples matched very well with the reference pattern and proved the polycrystalline nature of the films. As a secondary phase, one weak peak indicating covallite Cu2–xS phase was observed in the pattern of the sample deposited by using equimolar Cu and Sn. The surface morphologies of the films were found to be continuous and composed of homogeneously distributed large grains. From the reflectance and transmittance data, the optical absorption coefficient values of the films were found to be about 104 cm−1 and the films were found to be almost opaque in the wavelengths from 200 to 600 nm with a small reflectivity of about 10%. Band gap values of the films decreased from 1.45 to 1.35 eV with decreasing Cu content. Electrical characterization showed that the films were p-type semiconductor. Two different impurity levels for each film were found via resistivity-temperature characteristics.

Precursors' order effect on the properties of sulfurized Cu2ZnSnS4 thin films

A dc magnetron sputtering-based method to grow high-quality Cu2ZnSnS4 (CZTS) thin films, to be used as an absorber layer in solar cells, is being developed. This method combines dc sputtering of metallic precursors with sulfurization in S vapour and with post-growth KCN treatment for removal of possible undesired Cu2−xS phases. In this work, we report the results of a study of the effects of changing the precursors' deposition order on the final CZTS films' morphological and structural properties. The effect of KCN treatment on the optical properties was also analysed through diffuse reflectance measurements. Morphological, compositional and structural analyses of the various stages of the growth have been performed using stylus profilometry, SEM/EDS analysis, XRD and Raman Spectroscopy. Diffuse reflectance studies have been done in order to estimate the band gap energy of the CZTS films. We tested two different deposition orders for the copper precursor, namely Mo/Zn/Cu/Sn and Mo/Zn/Sn/Cu. The stylus profilometry analysis shows high average surface roughness in the ranges 300–550 nm and 230–250 nm before and after KCN treatment, respectively. All XRD spectra show preferential growth orientation along (1 1 2) at 28.45°. Raman spectroscopy shows main peaks at 338 cm−1 and 287 cm−1 which are attributed to Cu2ZnSnS4. These measurements also confirm the effectiveness of KCN treatment in removing Cu2-xS phases. From the analysis of the diffuse reflectance measurements the band gap energy for both precursors' sequences is estimated to be close to 1.43 eV. The KCN-treated films show a better defined absorption edge; however, the band gap values are not significantly affected. Hot point probe measurements confirmed that CZTS had p-type semiconductor behaviour and C–V analysis was used to estimate the majority carrier density giving a value of 3.3 × 1018 cm−3.

Copper Sulfide Assisted Recrystallization of Cu-poor CuInS2 Observed in-situ by Polychromatic X-ray Diffraction

AbstractThe microstructural changes during heating of bi-layers of Cu-poor CuInS2 and CuS under different sulfur excess conditions were studied. This was done by means of energy dispersive X-ray diffraction of polychromatic synchrotron radiation in a vacuum setup where the sulfur pressure conditions can be controlled. Understood as the formation of a new microstructure, the recrystallization of the Cu-poor CuInS2 phase was characterized by a change in the reflection profile (from Cauchy-type to Gauss-type), the reduction of the breadth and a subsequent normalized-intensity increase of the 112 reflection. The Cauchy component of the breadth was used to monitor the recrystallization under different sulfur and heating rate conditions. The main results are: a) Cu availability for the consumption of the CuIn5S8 phase is a pre-requisite for recrystallization, b) in presence of the Cu2-xS phase, increased sulfur pressure enhances recrystallization.