Conventional CdS Research Articles

Spray-ILGAR ZnS nanodots/In2S3 as defect passivation/point contact bilayer buffer for Cu(In,Ga)(S,Se)2 solar cells

The sequential and cyclic spray ion layer gas reaction (spray-ILGAR) technique allows the deposition of metal chalcogenide films controllable from monodispersed nanodots to homogeneous compact layer. With access to this technique, a structured buffer layer for Cu(In,Ga)(S,Se)2 cells, named “defect passivation/point contact bilayer buffer” is introduced at the heterogeneous interfaces to replace the conventional toxic CdS buffeProd. Type: FTPr material and to improve the pure In2S3 buffer. Here, the spray-ILGAR ZnS nanodots serve as a passivation layer with a reduced interface recombination, while the compact spray-ILGAR In2S3 film on top and in-between the nanodots acts as point contact layer for the charge carrier transport. The optimal ZnS dot density and In2S3 thickness are determined and discussed in detail. As yet, the solar cell efficiency with ZnS/In2S3 buffer layer can be improved by about 1.5% absolutely as compared to a pure In2S3 buffered cell. Apart from the electronic properties of the absorber/buffer interface, also the chemical and diffusion processes during junction formation, which may influence the properties of the completed solar cell, are investigated and discussed.

Selective Photo-Reduction of <I>p</I>-Nitrophenol to <I>p</I>-Aminophenol by Au Deposited CdS Nanostructures of Different Shapes Having Large Surface Area

The preparation and photocatalytic reductive ability of a new spongy flower or cotton bud like amorphous CdS microstructures (size = 200-300 nm) having intermediate crystal phase between cubic and hexagonal structure and possesses much larger surface area ca. 119 m2 g(-1) is demonstrated here. Structural analysis has been done by TEM, XRD, BET surface area, UV-Vis absorption, luminescence analysis, and photocatalytic study. The photoreactivity of as-prepared CdS for p-nitrophenol reduction to p-aminophenol conversion under visible light (150 W halogen lamp) irradiation has been found to be greatly enhanced from 62% to 94% (yield) and 81% to 100% (selectivity) after 1 wt% Au deposition. The obtained p-aminophenol yield is always better than 32% for CdS nanosphere (size 10-12 nm) and 40% for CdS nanorod (length/width in nm = 126/6). Surface passivation with a thin layer of SiO2 coating over this flower like spongy CdS appreciably increased the BET surface area to 158 m2 g(-1) and displayed stable photocatalytic activity for p-nitrophenol reduction up to 30 h of light exposure as compared to bare CdS of low activity because of its easy photodissolution. A good co-relation between the BET surface area and surface photoactivity of as-prepared CdS with the conventional cubic and hexagonal CdS nanoparticles of different dimensions is established here.

Spectroscopic ellipsometry studies of In2S3 top window and Mo back contacts in chalcopyrite photovoltaics technology

AbstractSpectroscopic ellipsometry (SE) was used to characterize the conventional back contact layer (Mo) as well as a novel top window layer (In2S3) for use in chalcopyrite photovoltaics technology. For the Mo back contact, in‐situ SE measurements of thin films magnetron sputtered onto glass enabled determination of ε at different thicknesses during growth. The observed strong variations could be understood on the basis of a Drude relaxation time that varies with the Mo film thickness. For the In2S3 window layer, ex‐situ SE measurements showed critical point structures at 2.77±0.08 eV, 4.92±0.005 eV, and 5.64 ± 0.005 eV, as well as an absorption tail with an onset near 1.9 eV. A comparison of the solar cell performance in simulations using either In2S3 or conventional CdS revealed similar quantum efficiencies irrespective of the selected window layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoconductive Characteristics of Single-Crystal CdS Nanoribbons

The photoconductive characteristics of CdS single nanoribbons were investigated. The device characteristics, including spectral response, light intensity response, and time response, were studied systematically. It is found that CdS nanoribbon has the response speed substantively faster than those ever reported for conventional film and bulk CdS materials and the size of nanoribbons has a significant influence on the response speed with smaller CdS nanoribbons showing higher response speed. The high photosensitivity and high photoresponse speed are attributable to the large surface-to-volume ratio and high single-crystal quality of CdS nanoribbons and the reduction of recombination barrier in nanostructures. Measurements in a different atmosphere demonstrate that the absorption of ambient gas (mainly oxygen) can significantly change the photosensitivity of CdS nanoribbons through trapping electrons from the nanoribbons.

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

AbstractRecent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (Eg = 3·6–3·8 eV) than the conventional buffer material CdS (Eg = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS2 (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm2 in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm2 (total area: 22·5 mA/cm2). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm2 substrates have been prepared and already reach an efficiency (active area: 17·2 cm2) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.

Synthesis of Single-Crystal CdS Microbelts Using a Modified Thermal Evaporation Method and Their Photoluminescence

Large-scale synthesis of single-crystal CdS microbelts was achieved by a modified thermal evaporation method, in which an adiabatic layer was used to provide abrupt temperature decrease and high gas concentration for the microbelt growth. The synthesized CdS microbelts were usually several micrometers in width, 70 nm in thickness, and tens to several hundred micrometers in length. Studies found that the adiabatic layer has great influence on the formation of CdS microbelts. Room-temperature photoluminescence indicated the disappearance of the emission at 1.77 eV, the appearance of an emission at 2.07 eV, and also the enhancement of the emission band at 2.38 eV, whose position shifted toward the blue region compared with conventional narrow CdS nanobelts. A possible growth mechanism of the CdS microbelts was briefly discussed.

Fabrication and characterization of In 2S 3 thin films deposited by thermal evaporation technique

Indium sulphide, In 2S 3, thin films present an alternative to conventional CdS films as buffer layer for CIS-based thin film solar cells. The objective is to eliminate toxic cadmium for environmental reasons. Indium sulphide is synthesized and deposited by single source vacuum thermal evaporation method on glass substrates. The films are analyzed by X-ray diffraction (XRD) and spectrophotometric measurements. They have a good crystallinity, homogeneity and adhesion. The X-ray diffraction analysis confirmed the initial amorphous nature of the deposited InS film and phase transition into crystalline In 2S 3 formed upon annealing at free air for 250 °C substrate temperature for 2 h. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflectance spectral data over the wavelength range of 300–1800 nm. We note that the films annealed at 250 °C for 2 h show a good homogeneity with 80% transmission. An analysis of the optical absorption data of the deposited films revealed an optical direct band gap energy in the range of 2.0–2.2 eV.

Growth of an In<sub>x</sub>(OOH,S)<sub>y</sub> Buffer Layer and Its Application to Cu(In,Ga)(Se,S)<sub>2</sub> Solar Cells

As an alternative to a CdS buffer layer for Cu(In,Ga)Se2-based solar cells, we prepared In-based buffer layers using a chemical bath deposition method. XPS and XRD analyses revealed that the In-based buffer layers contained In2S3 and InOOH phases. Compared with CdS film, the In-based film, Inx(OOH,S)y, had higher optical transmittance and a shorter absorption edge. The Cu(In,Ga)(Se,S)2 solar cell with the Inx(OOH,S)y buffer layer had better photovoltaic properties than that with a conventional CdS buffer layer. The conversion efficiency of the best Cu(In,Ga)(Se,S)2 solar cell with Inx(OOH,S)y buffer layer was 12.55 % for an active area of 0.19 cm2.

MOCVD as a dry deposition method of ZnSe buffers for Cu(In,Ga)(S,Se)2 solar cells

AbstractZnSe prepared by metal organic chemical vapor deposition is used as a buffer layer in Cu(In,Ga)(S,Se)2 solar cells without any utilization of wet chemistry. Cell efficiencies are as good as cells with the conventional CdS buffer. Stability of unencapsulated cells under damp heat conditions is somewhat lower for the alternative buffer. The first stages of photoassisted growth are studied. X‐ray photoemission spectroscopy shows that a continuous layer is formed. Copyright © 2004 John Wiley & Sons, Ltd.

Recovery Swaps

We derive an arbitrage free relationship between recovery swap rates, digital default swap spreads and conventional CDS spreads, and argue that the fair forward recovery rate used in recovery swaps must contain a convexity premium over the expected recovery value.

Digital Premium

Credit default swaps cover two different risks: the event risk of a default and the recovery risk arising from the fact that the recovery percentage is unknown. A digital default swap makes a fixed payment in case of default, and is therefore unaffected by the recovery rate. However, as this article demonstrates, recovery risk does affect digital default swap valuation when calibration is done against market spreads from conventional CDS. This is because uncertainty about the recovery rate in a CDS interacts with the nonlinearity of the price-yield relationship to produce a Jensen9s Inequality effect. That is, if the expected value of the recovery rate is used to back out an implied expected default probability from a market CDS spread, the result will understate the true probability of default due to Jensen9s Inequality. The market price for a digital default swap therefore includes a kind of premium, in that it will reflect the market9s estimate of the true probability of a default event, and possibly a risk premium for the uncertainty around that expectation, rather than the biased probability that would be implied out from the CDS spread.

Amplitude-coded color doppler ultrasonography in pediatrics

The goal of this study was to evaluate the use of amplitude coded colour Doppler sonography (aCDS) in infants and children. We analysed our own experiences in 270 patients (mean age: 6.3 years, range: newborn--16 years) with various paediatric aCDS applications. The aCDS results of 460 individual investigations were correlated with conventional CDS results, clinical findings and the results of other imaging modalities. Additionally, relevant literature is reviewed and discussed. ACDS is a valuable tool in paediatric sonography. It enriches the diagnostic field of ultrasound, particularly in regard to renal, abdominal, oncological and small part applications. ACDS could successfully be performed in 92.4% of investigations. Overall accuracy was 86.6% (in regard to the total number), sensitivity was 92% (calculated for the number of successful investigations), additional diagnostic information was gained in 18.6% to 48% of patients--depending on the field of application. ACDS is applicable to the majority of infants and children, in spite of the risk of motion artefacts. It is a valuable addition to conventional paediatric ultrasound and colour Doppler sonography. In conjunction with new modalities like three-dimensional sonography, harmonic imaging and ultrasound contrast agents it promises to continuously widen sonographic diagnostic potential.

Size-quantized CdS films in thin film CuInS2 solar cells

Semiconductor quantum dots and wires are the subject of great interest, mainly due to their size-dependent electronic structures, in particular increased band gap and therefore optoelectronic properties. We have electrodeposited films of size-quantized CdS (∼4 to 5 nm cross section by 15 nm height) as a buffer layer on CuInS2. The resulting CuInS2/CdS thin-film solar cells gave increased photocurrents and higher light-to-electricity conversion efficiencies (>11%) than those made with conventional nonquantized CdS films. This was due mainly to the increased band gap of the quantized CdS, allowing more light to reach the active CuInS2 layer.

Digital Video Disk/Compact Disk (DVD/CD)-Compatible Pickup Head with Dual Lens Rotating Actuator

Recently, a high-density optical disk system (DVD) which has about 7 times higher recording density than the conventional CD has been proposed and many manufacturers have announced plans to market DVD players in 1996. The DVD is also expected to be used as a large-capacity peripheral memory for personal computers and it is assumed that a DVD drive will be compatible with conventional CDs, i.e., the DVD-ROM drive will be able to read CD-ROMs, as well as DVD-ROMs. We have developed a DVD/CD-compatible pickup head which permits reading of data from both DVDs and CDs by means of exchanging objective lenses. These two types of lenses are set on a common table, where two sets of flat coils are situated in order to exchange lenses. Experimental data show that our dual lens actuator head satisfies our preliminary design targets and performs as well as a single lens actuator.

CDROMs: at the heart of multimedia

CDROMs are not just an important tool for archiving reference materials. Now, personal computers equipped with CDROM drives can be used to display and play back audio recordings (on conventional CDs), drawings and photographic materials as well. Multimedia has become the latest catchword for personal computer vendors and the CDROM drive is at the heart of current audio‐photo‐computer applications. Connect a CDROM drive to your computer, plug in a sound board, attach loudspeakers and you have become a vanguard personal computer user.

The structure of the Cu2S layer of Cu2S/CdS thin-film solar cells

Techniques have been developed for separating the Cu2S layer from Cu2S/CdS thin-film solar cells by slowly dissolving away the CdS. Free-standing Cu2S layers, cemented-down Cu2S layers, Cu2S layers exposed after potting and cross sectioning, and replicas of Cu2S layers have been prepared and examined in the scanning electron microscope. The Cu2S morphology so disclosed is a complex composite of a conformal coating of the upper surface of the texture-etched CdS film and more or less vertical penetrations into the CdS film along grain boundaries. The texture etch of the CdS film opens deep [about (10–15) ×10−4 cm] etch pits from which a substantial portion of the Cu2S penetration into grain boundaries occurs. The thickness of the Cu2S layer of conventional CdS thin-film solar cells is about 1000 Å, though the thickness of some of the deeper penetrations may be less than 500 Å.