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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

On Recombination Processes in CdS–PbS Films

The transverse and longitudinal photoconductivity, photoluminescence, and cathodoluminescence of sublimated (CdS){sub 0.9}–(PbS){sub 0.1} films at room temperature and upon cooling are studied. The role of inclusions of the narrow-gap phase in the processes is shown. The films are excited over the entire active surface and pointwise (within one crystallite). The surface recombination rate and the lifetime of majority charge carriers at different generation rates and characters of excitation are estimated. A comparative table of recombination parameters of CdS and CdS–PbS films is presented.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Heat treatment effects in Cu2S-CdS heterojunction photovoltaic cells

The dependence of the short-circuit current on photon energy, temperature, and the state of optical degradation (or enhancement) is determined in a study of the photovoltaic properties of Cu2S-CdS single-crystal heterojunctions. A coherent formulation is proposed for the relationship between enhancement and optical degradation and for their effects on the transport of a short-circuit photoexcited current and dark forward-bias current in a photovoltaic cell. Optical degradation in a Cu2S-CdS cell is shown to be identical to the optical degradation of lifetime in a homogeneous CdS:Cd:Cu crystal.

Fahrenbruch, A. L.↗
DOE OSTI2017

Violet-to-Blue Gain and Lasing from Colloidal CdS Nanoplatelets: Low-Threshold Stimulated Emission Despite Low Photoluminescence Quantum Yield

Amplified spontaneous emission (ASE) and lasing from solution-processed materials are demonstrated in the challenging violet-to-blue (430–490 nm) spectral region for colloidal nanoplatelets of CdS and newly synthesized core/shell CdS/ZnS nanoplatelets. Despite modest band-edge photoluminescence quantum yields of 2% or less for single excitons, which we show results from hole trapping, the samples exhibit low ASE thresholds. Furthermore, four-monolayer CdS samples show ASE at shorter wavelengths than any reported film of colloidal quantum-confined material. This work underlines that low quantum yields for single excitons do not necessarily lead to a poor gain medium. The low ASE thresholds originate from negligible dispersion in thickness, large absorption cross sections of 2.8 × 10–14 cm–2, and rather slow (150 to 300 ps) biexciton recombination. We show that under higher-fluence excitation, ASE can kinetically outcompete hole trapping. Using nanoplatelets as the gain medium, lasing is observed in a linear optical cavity. This work confirms the fundamental advantages of colloidal quantum well structures as gain media, even in the absence of high photoluminescence efficiency.

amplified spontaneous emission↗
DOE OSTI2020

Materials Data on CdS by Materials Project

CdS is Boron Nitride-like structured and crystallizes in the trigonal P3m1 space group. The structure is two-dimensional and consists of one CdS sheet oriented in the (0, 0, 1) direction. Cd2+ is bonded in a trigonal planar geometry to three equivalent S2- atoms. All Cd–S bond lengths are 2.46 Å. S2- is bonded in a trigonal planar geometry to three equivalent Cd2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Quantum Efficiency of Charge Transfer Competing against Nonexponential Processes: The Case of Electron Transfer from CdS Nanorods to Hydrogenase

Photoexcited charge transfer from semiconductor nanocrystals to charge acceptors is a key step for photon energy conversion in semiconductor nanocrystal-based light-harvesting systems. Charge transfer competes against relaxation processes within the nanocrystals, and this competition determines the quantum efficiency of charge transfer. The quantum efficiency is a critical design element in photochemistry, but in nanocrystal-acceptor systems its extraction from experimental data is complicated by sample heterogeneity and intrinsically nonexponential excited-state decay pathways. In this manuscript, we systematically explore these complexities using TA spectroscopy over a broad range of timescales to probe electron transfer from CdS nanorods to the redox enzyme hydrogenase. To analyze the experimental data, we build a model that quantifies the quantum efficiency of charge transfer in the presence of competing, potentially nonexponential, relaxation processes. Our approach can be applied to calculate the efficiency of charge or energy transfer in any donor-acceptor system that exhibits nonexponential donor decay and any ensemble distribution in the number of acceptors, provided that donor relaxation and charge transfer can be described as independent, parallel decay pathways. We apply this analysis to our experimental system and unveil the connections between particle morphology and quantum efficiency. Our model predicts a finite quantum efficiency even when the mean recombination time diverges, as it does in CdS nanostructures with spatially separated electron-hole pairs that recombine with power-law dynamics. We contrast our approach to the widely used expressions for the quantum efficiency based on average lifetimes, which for our system overestimate the quantum efficiency. Furthermore, the approach developed here is straightforward to implement and should be applicable to a wide range of systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Engineering of Exciton Spatial Distribution in CdS Nanoplatelets

Zinc-blende CdS nanoplatelets with atomically flat and very large {100} basal planes terminated solely by one type of element (either Cd or S atoms) are synthesized. Optical spectroscopy, X-ray diffraction, X-ray absorption, and transmission electron microscopy confirm that the surface structures of newly developed S-terminated CdS nanoplatelets are at least as well-defined as the original Cd-terminated nanoplatelets. Band gaps of the nanoplatelets are found to depend on not only the quantum-confined dimension (thickness) but also the nature of the surface termination. The facet structure dictates the packing of the ligands (carboxylate for Cd-terminated nanoplatelets and alkyl for S-terminated nanoplatelets), which causes a difference in the lattice strain and significantly affects the optical spectral width. Experimental and theoretical results reveal that engineering the exciton spatial distribution by the tailored synthesis of semiconductor nanocrystals with a precisely controlled surface structure is fully possible, which should open a new door for delivering the long-promised potential of semiconductor nanocrystals.

Cadmium sulfide↗
DOE OSTI2023

High Affinity Electrostatic Interactions Support the Formation of CdS Quantum Dot:Nitrogenase MoFe Protein Complexes

Nitrogenase MoFe protein can be coupled with CdS nanocrystals (NCs) to enable photocatalytic N 2 reduction. The nature of interactions that support complex formation is of paramount importance in intermolecular electron transfer that supports catalysis. In this work we have employed microscale thermophoresis to examine binding interactions between 3-mercaptopropionate capped CdS quantum dots (QDs) and MoFe protein over a range of QD diameters (3.4-4.3 nm). The results indicate that the interactions are largely electrostatic, with the strength of interactions similar to that observed for the physiological electron donor. In addition, the strength of interactions is sensitive to the QD diameter, and the binding interactions are significantly stronger for QDs with smaller diameters. Furthermore, the ability to quantitatively assess NC protein interactions in biohybrid systems supports strategies for understanding properties and reaction parameters that are important for obtaining optimal rates of catalysis in biohybrid systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Intrinsically Slow Cooling of Hot Electrons in CdSe Nanocrystals Compared to CdS

The utilization of excited charge carriers in semiconductor nanocrystals (NCs) for optoelectronic technologies has been a long-standing goal in the field of nanoscience. Experimental efforts to extend the lifetime of excited carriers have therefore been a principal focus. To understand the limits of these lifetimes, in this work, we theoretically study the time scales of pure electron relaxation in negatively charged NCs composed of two prototypical materials: CdSe and CdS. We find that hot electrons in CdSe have lifetimes that are 5 to 6 orders of magnitude longer than in CdS when the relaxation is governed only by the intrinsic properties of the materials. Although these two materials are known to have somewhat different electronic structure, we elucidate how this enormous difference in lifetimes arises from relatively small quantitative differences in electronic energy gaps and phonon frequencies, as well as the crucial role of Fröhlich-type electron–phonon couplings.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Revealing the Phonon Bottleneck Limit in Negatively Charged CdS Quantum Dots

The capture of photoexcited hot electrons in semiconductors before they lose their excess energy to cooling is a long-standing goal in photon energy conversion. Semiconductor nanocrystals have large electron energy spacings that are expected to slow down electron relaxation by phonon emission, but hot electrons in photoexcited nanocrystals, nevertheless, cool rapidly by energy transfer to holes. This makes the intrinsic phonon-bottleneck-limited hot electron lifetime in nanocrystals elusive. For this work, we used a combination of theory and experiments to probe the hot-electron dynamics of negatively charged cadmium sulfide (CdS) colloidal quantum dots (QDs) in the absence of holes. Experiments found that these hot electrons cooled on a 100 ps time scale. Theoretical simulations predicted that pure phonon-bottleneck-limited hot electron cooling occurs on a similar time scale. This similarity suggests that the experimental measurements reflect the upper limit on the hot-electron lifetimes in these CdS QDs and the lower limit on the rates of processes that can harvest those hot electrons.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Optimizing photovoltaic performance in CuInS 2 and CdS quantum dot-sensitized solar cells by using an agar-based gel polymer electrolyte

Quantum dot-sensitized solar cells (QDSSCs) offer new opportunities to address the clean energy challenge, being one of the top candidates for third generation photovoltaics. Like dye-sensitized solar cells (DSSCs), QDSSCs normally use liquid electrolytes that suffer from issues such as evaporation or leakage. In this study a gel polysulfide electrolyte was prepared containing a natural polymer, agar, and was used as a quasi-solid-state electrolyte in solar cells to replace the conventional liquid electrolytes. This gel electrolyte shows almost the same conductivity as the liquid one. The solar cells were fabricated using CuInS 2 quantum dots (QDs), previously synthesized, deposited on TiO 2 photoanodes by electrophoretic deposition (EPD). CdS was deposited on TiO 2 by successive ionic layer adsorption and reaction (SILAR). Reduced graphene oxide (RGO)–Cu 2 S, brass, and thin film CuxS were used as counter electrodes. Compared to a liquid polysulfide water based electrolyte, solar cells based on CuInS 2 and CdS using gel polymer electrolyte (GPE) exhibit greater incident photon to current conversion efficiency (IPCE = 51.7% at 520 nm and 72.7% at 440 nm), photocurrent density (J sc = 10.75 and 13.51 mA cm -2 ), and power conversion efficiency (η = 2.97 and 2.98%) while exhibiting significantly enhanced stability. The solar cells employing the agar-based gel polymeric electrolyte are about a factor of 0.20 more stable than using a liquid electrolyte. The higher photovoltaic performance is due to the good conductivity and high wettability as well as the superior permeation capability of the gel electrolyte into the mesoporous matrix of a TiO 2 film

14 SOLAR ENERGY↗
DOE OSTI2020

Scale and rate in CdS pressure-induced phase transition

Here, we describe recent efforts to improve our predictive modeling of rate-dependent behavior at, or near, a phase transition using molecular dynamics simulations. Cadmium sulfide (CdS) is a well-studied material that undergoes a solid-solid phase transition from wurtzite to rock salt structures between 3 and 9 GPa. Atomistic simulations are used to investigate the dominant transition mechanisms as a function of orientation, size and rate. We found that the final rock salt orientations were determined relative to the initial wurtzite orientation, and that these orientations were different for the two orientations and two pressure regimes studied. The CdS solid-solid phase transition is studied, for both a bulk single crystal and for polymer-encapsulated spherical nanoparticles of various sizes.

42 ENGINEERING↗
DOE OSTI2024

A Comparison of CdS and Zn(O,S) Buffer Layers in (Ag,Cu)(In,Ga)Se2 Solar Cells

Cu(In,Ga)Se 2 -based solar cells typically use a C dS buffer layer, even though its relatively low bandgap causes parasitic absorption. While alternatives to CdS have been explored in other studies, limited results have been shown for Ag-alloyed CIGS (ACIGS). In this study, ACIGS solar cells with both CdS and Zn(O,S) buffer layers were fabricated and compared for solar cell performance. The Zn(O,S) buffer slightly improved J sc through reduced absorption, although surface optimization is necessary. The Zn(O,S) ACIGS also had improved FF through reduced series resistance and ideality factor, which could be improved further through optimization. However, the Voc in the Zn(O,S) samples was reduced, likely due to increased bulk and front interface recombination. This study shows the promise of Zn(O,S) as a buffer layer in ACIGS solar cells and suggests pathways for further improvement.

absorption↗
DOE OSTI2018

Photoinduced Degradation of the Optical Properties of Colloidal Ag{sub 2}S and CdS Quantum Dots Passivated by Thioglycolic Acid

The results of studying degradation of the optical properties of colloidal Ag{sub 2}S and CdS quantum dots (QDs) 2.6–3.2 nm in size passivated by thioglycolic acid (TGA) are presented. The photoluminescence intensity of colloidal Ag{sub 2}S QDs has been found to decrease under laser irradiation at a wavelength of 445 nm, beginning with the effective power of 10 mW. The observed effect is interpreted as a photochemical reaction of formation of new nonradiative-recombination channels in Ag{sub 2}S QDs upon excitation. It is established for colloidal CdS QDs passivated by TGA that a decrease in the optical density in the entire absorption spectrum and the luminescence intensity is accompanied by precipitation of the colloidal particles in a cell and related to photodegradation of the passivating shell.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Electronic Excitation Energy Transfer in an Array of CdS Quantum Dots on a Quasi-Two-Dimensional Surface

The optical spectra of films composed of spherical silicon-dioxide particles coated with small-radius CdS quantum dots are recorded and analyzed. Large shifts of the absorption and photoluminescence bands are detected and studied in relation to the concentration of quantum dots and to the pumping density and wavelength. Analysis of the experimental data shows that the effects are due to electronic excitation energy transfer by particles through the mechanism of tunneling induced by a strong interaction between quantum dots. The results obtained at low pumping densities and different excitation wavelengths make it possible to describe the size distribution of CdS quantum dots. This distribution can be adequately approximated with a Gaussian function.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1971

Thermally restorable optical degradation effects in heat-treated Cu2S-CdS crystal heterojunctions.

We have shown that heat treatment alone does not produce appreciable degradation of the short-circuit current in Cu2S-CdS single-crystal photovoltaic heterojunctions. The degradation, which has been commonly observed, is the result of optically induced effects in the heat-treated cell, subsequent to the heat treatment. These optically induced effects can be correlated with photochemical changes induced in single crystals of CdS:Cd:Cu in which the density of Cu acceptors exceeds the density of excess-Cd donors. Major conclusions about the mechanism of the photovoltaic effect and heat-treatment processes can be drawn.

Kanev, S. K.↗