Concept: Cadmium sulfide
Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technology. With laboratory efficiencies approaching 20 per cent, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per watt (ref. 2) and to minimize the environmental impact. A central part of the manufacturing process involves doping the polycrystalline thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface and to passivate the grain boundaries, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per gram) and a water-soluble source of toxic cadmium ions, presenting a risk to both operators and the environment during manufacture. Here we demonstrate that solar cells prepared using MgCl2, which is non-toxic and costs less than a cent per gram, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9 × 10(14) cm(-3)) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concentrations of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of electrically active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell production.
Over the past years a number of studies have described the instability of the pigment cadmium yellow (CdS). In a previous paper we have shown how cadmium sulfide on paintings by James Ensor oxidizes to CdSO(4)·H(2)O. The degradation process gives rise to the fading of the bright yellow color and the formation of disfiguring white crystals that are present on the paint surface in approximately 50 μm sized globular agglomerations. Here, we study cadmium yellow in the painting “Flowers in a blue vase” by Vincent van Gogh. This painting differs from the Ensor case in the fact that (a) a varnish was superimposed onto the degraded paint surface and (b) the CdS paint area is entirely covered with an opaque crust. The latter obscures the yellow color completely and thus presents a seemingly more advanced state of degradation. Analysis of a cross-sectioned and a crushed sample by combining scanning microscopic X-ray diffraction (μ-XRD), microscopic X-ray absorption near-edge spectroscopy (μ-XANES), microscopic X-ray fluorescence (μ-XRF) based chemical state mapping and scanning microscopic Fourier transform infrared (μ-FT-IR) spectrometry allowed unravelling the complex alteration pathway. Although no crystalline CdSO(4) compounds were identified on the Van Gogh paint samples, we conclude that the observed degradation was initially caused by oxidation of the original CdS pigment, similar as for the previous Ensor case. However, due to the presence of an overlying varnish containing lead-based driers and oxalate ions, secondary reactions took place. In particular, it appears that upon the photoinduced oxidation of its sulfidic counterion, the Cd(2+) ions reprecipitated at the paint/varnish interface after having formed a complex with oxalate ions that themselves are considered to be degradation products of the resin and/or oil in the varnish. The SO(4)(2-) anions, for their part, found a suitable reaction partner in Pb(2+) ions stemming from a dissolved lead-based siccative that was added to the varnish to promote its drying. The resulting opaque anglesite compound in the varnish, in combination with the underlying CdC(2)O(4) layer at the paint/varnish interface, account for the orange-gray crust that is disfiguring the painting on a macroscopic level. In this way, the results presented in this paper demonstrate how, through a judicious combined use of several microanalytical methods with speciation capabilities, many new insights can be obtained from two minute, but highly complex and heterogeneous paint samples.
Well-aligned CdS nanorod arrays (CdS NRs) with ∼100nm in diameter and ∼700nm in length were fabricated on FTO (fluorine-doped tin oxide) substrate by using glutathione as capping agents. The growth of CdS NRs was studied in details by exploring the roles of each active binding group in glutathione. The thiol group in glutathione plays an important role in forming a compact CdS nanocrystal film, upon which the nanorods grow subsequently via the synergetic effect of thiol and dicarboxyl groups in glutathione. The influence of surface passivation with glutathione on the photoelectrical property of CdS NRs was also tested. The results revealed that glutathione ligands encapsulated in the surfaces of CdS NRs act as insulating barriers between CdS NRs and solution, hindering charge transport. Hybrid photovoltaic cells of FTO/CdS NRs/P3HT (poly(3-hexylthiophene))/Au were then assembled. The performance of the photovoltaic devices was increased with increasing the length of the as-prepared CdS nanorods and further enhanced to the highest efficiency of 0.373% after the thermal sulfuration treatment.
A facile electrodeposition technique was utilized to deposit single-walled carbon nanotubes (SWNTs) with cadmium telluride (CdTe) with well-controlled size, density, surface morphology, and composition. By controlling the applied charge, the morphology of these hybrid nanostructures was altered from CdTe nanoparticles on SWNTs to SWNT/CdTe core/shell nanostructures and the composition of the CdTe nanoparticles was altered from Te-rich (29 at% Cd) to Cd-rich (79 at% Cd) CdTe by adjusting the deposition potential. The electrical and optoelectrical properties of these hybrid nanostructures showed that photo-induced current can be tuned by tailoring the conductivity type (n-type or p-type), morphology, and size of the CdTe nanostructures, with a maximum photosensitivity (ΔI/I(0)) of about 30% for SWNT/Cd-rich CdTe (n-type) core/shell nanostructures. This work demonstrates a novel approach for synthesizing metal chalcogenide/SWNT hybrid nanostructures for various electrical and optoelectrical applications.
The optical properties of cadmium sulfide (CdS) nanoparticles in suspension are affected by morphology and suspending solvent. Time dependent stability of these properties is solvent dependent and is affected by illumination conditions under which the suspension is stored. Moreover, minute amounts of dissolved oxygen are sufficient in order to facilitate photodegradation.
- Proceedings of the National Academy of Sciences of the United States of America
- Published about 2 years ago
Nature has evolved several unique biomineralization strategies to direct the synthesis and growth of inorganic materials. These natural systems are complex, involving the interaction of multiple biomolecules to catalyze biomineralization and template growth. Herein we describe the first report to our knowledge of a single enzyme capable of both catalyzing mineralization in otherwise unreactive solution and of templating nanocrystal growth. A recombinant putative cystathionine γ-lyase (smCSE) mineralizes CdS from an aqueous cadmium acetate solution via reactive H2S generation from l-cysteine and controls nanocrystal growth within the quantum confined size range. The role of enzymatic nanocrystal templating is demonstrated by substituting reactive Na2S as the sulfur source. Whereas bulk CdS is formed in the absence of the enzyme or other capping agents, nanocrystal formation is observed when smCSE is present to control the growth. This dual-function, single-enzyme, aerobic, and aqueous route to functional material synthesis demonstrates the powerful potential of engineered functional material biomineralization.
The development of new methods for the direct transformation of methanol into two or multi-carbon compounds via controlled carbon-carbon coupling is a highly attractive but challenging goal. Here, we report the first visible-light-driven dehydrogenative coupling of methanol into ethylene glycol, an important chemical currently produced from petroleum. Ethylene glycol is formed with 90% selectivity and high efficiency, together with hydrogen over a molybdenum disulfide nanofoam-modified cadmium sulfide nanorod catalyst. Mechanistic studies reveal a preferential activation of C-H bond instead of O-H bond in methanol by photoexcited holes on CdS via a concerted proton-electron transfer mechanism, forming a hydroxymethyl radical (⋅CH2OH) that can readily desorb from catalyst surfaces for subsequent coupling. This work not only offers an alternative nonpetroleum route for the synthesis of EG but also presents a unique visible-light-driven catalytic C-H activation with the hydroxyl group in the same molecule keeping intact.
Dispersible cadmium sulfide (CdS) nanoparticles are synthesized by a facile solvothermal reaction and are used for the first time as an electron transport layer (ETL) in inverted planar perovskite solar cells. The CdS ETL has superb electron extraction and transport properties, leading to a solar cell with light hysteresis and a high efficiency of 13.36%.
The emerging field of biofabrication capitalizes on nature’s ability to create materials with a wide range of well-defined physical and electronic properties. Particularly, there is a current push to utilize programmed, self-organization of living cells for material fabrication. However, much research is still necessary at the interface of synthetic biology and materials engineering to make biofabrication a viable technique to develop functional devices. Here, the authors exploit the ability of Escherichia coli to contribute to material fabrication by designing and optimizing growth platforms to direct inorganic nanoparticle (NP) synthesis, specifically cadmium sulfide (CdS) NPs, onto porous polycarbonate membranes. Additionally, current, nonbiological, chemical synthesis methods for CdS NPs are typically energy intensive and use high concentrations of hazardous cadmium precursors. Using biosynthesis methods through microorganisms could potentially alleviate these issues by precipitating NPs with less energy and lower concentrations of toxic precursors. The authors adopted extracellular precipitation strategies to form CdS NPs on the membranes as bacterial/membrane composites and characterized them by spectroscopic and imaging methods, including energy dispersive spectroscopy, and scanning and transmission electron microscopy. This method allowed us to control the localization of NP precipitation throughout the layered bacterial/membrane composite, by varying the timing of the cadmium precursor addition. Additionally, the authors demonstrated the photodegradation of methyl orange using the CdS functionalized porous membranes, thus confirming the photocatalytic properties of these composites for eventual translation to device development. If combined with the genetically programmed self-organization of cells, this approach promises to directly pattern CdS nanostructures on solid supports.
ZnS as an inorganic passivation agent has been proven to be effective in suppressing charge recombination and enhancing power conversion efficiency (PCE) in quantum dot-sensitized solar cells (QDSCs). In the present study, we constructed a novel TiO2/ZnS/CdS/ZnCdS/CdSe/ZnS photoelectrode via successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD). The complementary effects of multi-ZnS layers on the optical and electrochemical performance of the QDSCs were systematically investigated. The multi-ZnS can not only facilitate the growth and distribution of QDs, but also suppress the different interface charge recombination effectively. We suggest that the formation of the ZnCdS intermediate layers via ion-exchange route during CBD process contributes to the higher photoelectrochemical cell performance of the QDSCs with the middle ZnS layer. When the three-layer ZnS was employed in CdS/CdSe co-sensitized system, the PCE reached 6.05%, which is much higher than that observed for solar cells with the conventional single ZnS treatments and the two layers inserted at the interface between CdS QDs and CdSe QDs and electrolyte (4.80% and 5.42%, respectively).