Optoelectronic properties n:CdS:In/p-Si heterojunction photodetector

Optoelectronic properties of CdS:In/Si anisotype heterojunction photodetector fabricated by depositing of polycrystalline CdS and indium doped CdS films used thermal resistive technique on clean monocrystalline p-type silicon substrates are presented. The effect of In diffusion temperature(T _d)in CdS layer on the optoelectronic characteristics of these devices has been studied. Two peaks situated at 650 and 800 nm with values of 0.32 and 0.43 A/W, respectively, were observed in the responsivity plot. Other optoelectronics properties such as detectivity, photovoltaic, and response time are also presented.     

Effects of copper doping on the electronic properties of CdS films sintered with CdCl2

Chlorine- and copper-doped polycrystalline CdS films were prepared by coating a slurry which consisted of CdS, CdCl₂, CuCl₂ and propylene glycol on a glass substrate and sintering in a nitrogen atmosphere, to investigate the effects of copper doping on the window properties of all-polycrystalline CdS/CdTe heterojunction solar cells. The variations of carrier concentration as a function of the amount of copper doping in CdS films which were doped with the order of 10¹⁸ cm^–3 chlorine have been explained in terms of electronic compensation of the copper impurity. The variations of optical transmission spectra as a function of copper doping have been correlated with the variations of photoconductivity spectra. Even 10 p.p.m. copper in the sintered CdS films degrades the window properties of the CdS films significantly. The degradation is caused by reduced optical transmission rather than by an increase in electrical resistivity.     

Metal chalcogenide-oxide composite coatings prepared by spray pyrolysis

The spray pyrolysis technique has been employed to deposit composite coatings of chalcogenides of cadmium, zinc, lead and cobalt with oxides of aluminium, tin, lead, zinc and cobalt. Widely varying microstructural, electronic, optical and chemical properties have been obtained for such layers by monitoring the oxide composition, its spatial distribution and profile along the thickness. The large area chalcogenide-oxide composite films prepared by this technique are eminently suited for photovoltaic energy conversion, photothermal energy conversion and voltage-dependent resistor (Varistor) applications.In this paper we report our studies on co-pyrolytically deposited CdS:Al₂O₃ and CdS:SnO₂ layers and their application to improved thin film solar cells. Each of the oxides is insoluble in CdS and is segregated at the grain boundaries in the deposited films. Small amounts (less than 10%) of oxide in CdS are found to reduce its grain size negligibly and to make the film more compact, hard, adherent and less susceptible to chemical attack. The altered microstructure modifies the surface topography of the CdS film from a pebble-like roughness to an improved void-free serpentine texture. Segregated oxide in CdS does not affect the optical band gap of the films, although the composites exhibit enhanced diffuse optical scattering.Large area CdS films with a gradient profile of oxide have been utilized to fabricate thin film CdS/Cu₂S solar cells. The growth (length and distribution) of Cu₂S fingers and/or curtains deep into the top CdS layers during the topotaxial conversion reaction of chemiplating is controlled by the presence of oxide along the grain boundaries. This has not only resulted in improved interface topography for better carrier collection and reduced shunt losses but has also enabled us to decrease drastically the CdS film thickness necessary for the solar cells. Furthermore, the subsequent degradation of the junction via the well-known mechanism of the loss of copper from the Cu₂S layer by diffusion into CdS is expected to be considerably reduced by the presence of the oxide gradient in the CdS layer.     

Storage modulus and glass transition behaviour of CdS/PMMA nanocomposites

The storage modulus and glass transition temperature (T _g) of CdS/PMMA nanocomposites have been evaluated as a function of concentration of CdS nanoparticles. CdS particles have been synthesised via chemical route using cadmium acetate, thiourea and dimethylformamide. The solution-based processing has been used to prepare PMMA composites with CdS nanoparticles at different filler concentration. Size and shape of CdS nanoparticles in PMMA have been determined with the help of small angle X-ray scattering and transmission electron microscope measurements. Dynamical mechanical analysis was carried out on the CdS/PMMA nanocomposites to study storage modulus and tan?δ. It is observed that CdS nanoparticles enhance the storage modulus and T _g for composites. The storage modulus and T _g show the maximum value of 6?wt.% of CdS nanoparticles embedded PMMA composite. The results indicate that the 6?wt.% of CdS nanoparticles in PMMA matrix provides more stability to the composite over the other composites.     

Spectral sensitivity of (Si<Subscript>2</Subscript>)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(CdS)<Subscript>x</Subscript> solid solutions

Epitaxial layers of n-type solid solutions of the (Si₂)_{1 ? x} (CdS)_x system (0 ≤ x ≤ 0.01) were grown by liquid phase epitaxy from a tin-based solution melt confined between two horizontal p-type single crystal silicon substrates. The photosensitivity spectra of p-Si/n-(Si₂)_{1 ? x} (CdS)_x structures have been measured. A photoresponse peak at E ≈ 2.35 eV (1.25 eV below the top of the valence band of silicon) has been observed, which is probably related to an impurity level due to CdS molecules.     

2.15 The stoichiometry of surfaces in cadmium sulphide thin films

The sulphur to cadmium ratio in cadmium sulphide thin films has been compared to that of a clean single crystal using Auger spectroscopy. Thermally evaporated CdS films were found to be deficient in sulphur in their outer layers, but were stoichiometric after etching. Electron beam evaporated CdS films were found to be stoichiometric.It was found that baking these films at 190°C in argon or nitrogen at atmospheric pressure increased the sulphur to cadmium ratio, probably due to sulphur diffusing to the surface and being physisorbed there.From measurements of the temperature dependence of the copper sulphide layer in CdSCu₂S cells it was found that chalcocite, djurleite and diginite copper sulphide phases are present even in efficient cells. A model is proposed for the structure of the copper sulphide layer in which the copper deficient phases are the result of excess sulphur on the CdS surfaces and vary in thickness according to the degree of non-stoichiometry of the CdS surfaces.     

Modification of the parameters of thermal converter branches based on bismuth and antimony chalcogenides by means of copper intercalation

The phenomenon of conductivity type conversion (p → n) upon copper intercalation into layered crystals of bismuth and antimony chalcogenides has been used to create a thermal converter cell proceeding from the initial p-type chalcogenide crystals. The necessary n-type conductivity branches were formed upon assembly of the cell by saturating the material with copper using the method of autoelectrochemical doping (AED). The possibility of using the AED method for the optimization of characteristics of the n-and p-type branches of thermal converters by means of copper introduction in small amounts into the materials is considered.     

Synthesis of new sulphides of transition metal ions in presence of sun light

New sulphides of transition metal ions [M⁺ⁿ = Cu⁺¹, Cu⁺² and Zn⁺²] have been synthesised in sunlight. XRD patterns show that these compounds are not MxSy but are mercaptyl, hydroxyl metal sulphides [M(SH)(OH)(H₂O)₂] which is further ascertained by I.R. spectra showing bands due to T _d-symmetry. ESCA of compound of copper in solid state shows presence of Cu¹⁺ and Cu²⁺ ion. The presence of hydroxyl, mercaptyl, aqua and S^–2 groups has finally been confirmed with TGA, DTA and ESCA. Conductivity and Seebeck coefficient measurements show that compound of copper is p-type semiconductor and compound of zinc is n-type semiconductor. The production of these low cost materials opens an interesting area of research and development for their use in solar cell devices.     

Direct bonding of silicon wafers with a diffusion layer

The possibility of solid-phase direct bonding of silicon wafers having p ⁺-or n ⁺-type diffusion layers with a high surface dopant concentration has been demonstrated for the first time. Pis’ma Zh. Tekh. Fiz. 24, 1–5 (March 26, 1998)     

Influence of absorber copper concentration on the Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS based solar cells performance

The present contribution deals with the influence of the copper concentration in Cu(In,Ga)Se₂ (CIGSe) on the solar cells based on CIGSe/(PVD)In₂S₃ and CIGSe/(CBD)CdS. We find that, depending on the buffer layer, the optimum open circuit voltage (Voc) is not reached for the same copper concentration. The values of Voc for the CIGSe/(CBD)CdS solar cells are higher when the copper content is very close to stoichiometry (25%), whereas, the Voc values for CIGSe/(PVD)In₂S₃ solar cells attain their maximum for lower copper contents. On the other hand, contrary to the case of the (CBD)CdS buffer, the Jsc is strongly hindered for the (PVD)In₂S₃ buffered cells when the copper content is lowered. The study has been made for different absorber gallium contents and the evolution is coherent with the presence of a cliff at the CIGSe/(PVD)In₂S₃ interface.     

Diffusion bonding of alumina to steel using soft copper interlayer

The diffusion bonding of a low steel to alumina has been studied in the present work. Thin foils of a soft metal (copper) were used to reduce the effects of the residual stresses produced in the joint by thermal expansion mismatch. The strength of the joint was found to be influenced by the bonding parameters, but principally by the oxygen content both on the surface and in the copper matrix. The diffusion bonds have been mechanically tested using a three-point bending test. Maximum bending strengths of 100 MPa were achieved by using a 0.1 mm copper foil, and bonding in a oxidizing atmosphere (P _{O 2}=10₄Pa). SEM and EDS investigations have shown the presence of reaction products in the copper-alumina interface which controls the mechanical properties of the joint.     

A <Emphasis Type="Italic">p-i-n</Emphasis> Model of CdTe/CdS Heterostructures

CdTe/CdS heterostructures are analyzed in terms of a p-i-n model in which the i layer consists of a CdTe_{1 − x} S_x solid solution resulting from Te and S diffusion into CdS and CdTe, respectively. The lattice parameters of the solid solution are determined. The i layer is shown to be too thick for tunneling.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 915–917.Original Russian Text Copyright © 2005 by Janabergenov, Mirsagatov, Karazhanov.     

Sputtering power and deposition pressure effects on the electrical and structural properties of copper thin films

We investigated the effects of sputtering power and deposition pressure on the electrical and structural properties of dc magnetron sputter-deposited copper films on p-type silicon grown at room temperature. Results from our experiments showed that the deposition rate of the copper films increased proportionally with the sputtering power. Sputtering power also affected the structural properties of the copper films through the surface diffusion mechanism of the adatom. From the scanning electron microscopy surface analysis, the high sputtering power favored the formation of continuous film. The poor microstructure with voided boundaries as a result of low sputtering power deposition was manifested with the high resistivity obtained. The deposition rate was found also depending on the deposition pressure. The deposition pressure had the contrary effect on structural properties of copper films in which high deposition pressure favored the formation of voided boundaries film structure due to the shadowing effect, which varied with different deposition pressure.     

Study of deep levels in Schottky/CuInSe2 single-crystal devices by deep-level transient spectroscopy measurements

Deep-level transient spectroscopy (DLTS) measurements performed on Schottky/CuInSe₂ diodes are reported. So far, Cd(Zn)S has been used as a window n-type layer to prepare CuInSe₂ diodes. The diffusion of such a layer introduced defects into Cd(Zn)S-CuInSe₂ diodes. Thus, the importance of using Schottky diodes lies in the elimination of the window n-type layer diffusion into CuInSe₂ material to reveal the intrinsic properties of the semiconductor. A comparison between previously reported defect states in Cd(Zn)S-CuInSe₂ and those found in Schottky/CuInSe₂ is made.The defect concentration is calculated as well as the capture cross-section. Some of the defect levels agree with previously published data. A common feature exhibited in all the measured samples is that the capacitance transient is non-exponential, and the DLTS spectrum is relatively broad, due to the contribution of two or more closely spaced levels.     

Kinetics of transient liquid phase diffusion bonding process for joining aluminium metal matrix composite

Abstract

The mechanism and kinetics of the transient liquid phase diffusion bonding process in a 6061–15 wt-%SiCp composite at 570°C, 0·2 MPa, with 200 μm thick copper foil interlayer, has been investigated by microstructural characterisation of the bond region using optical microscopy, scanning electron microscopy and electron probe microanalysis. The kinetics of isothermal solidification, representing the displacement of the solid/liquid interface y (in micrometres) as a function of time t (in seconds), followed a power law relationship y?=?157t^0·07. According to this kinetic equation, the effective diffusivity of copper in the composite system was found to be ～10⁶ times higher than the lattice diffusivity, indicating the dominance of short circuit diffusion through the defect rich particle/matrix interface.     

Phase control of CuxTe film and its effects on CdS/CdTe solar cell

Phase control is critical for achieving high-performance CdTe cells when Cu_xTe is used as a back-contact for CdTe cells. Cu_xTe phases are mainly controlled by the Cu/Te ratio, and they can also be affected by post-heat-treatment temperature. Although Cu₂Te has the highest conductivity, it is unstable and provides more Cu diffusion into the CdS and CdTe films. Cu diffusion into the CdS causes “cross-over”, and Cu diffusion into the CdTe film creates Cu-related defects that lower photogenerated carrier lifetime and result in voltage-dependent collection. A “recontact” experiment clearly indicated that the mechanism giving rise to “roll-over” is the formation of Cu-related oxides, rather than the loss of Cu on the back-contact.     

Synthesis and characterization of a novel poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl-CdS nanocomposites through chemical hybridization

A novel bi-functional photorefractive (PR) poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl with carbazolyl fragments and chromophoric azophenyl was synthesized by a post azo coupling reaction and characterized by ¹H NMR, elemental analysis, UV–Vis absorption and FTIR spectroscopy. Then chemical hybridized poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl-CdS nanocomposites with different molar ratio of CdS to poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl were prepared. It was confirmed that the CdS particles had a nanoscale size and quantum confinement effect adopting transmission electron microscopy and UV–Vis absorption spectroscopy. The generation of photocurrent on illumination and photoconductive properties of the nanocomposites were studied. Significant enhancement in photoconductivity induced by the chemical doping of CdS in poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl has also been demonstrated.     

Structural characterization of gel-grown neodymium copper oxalate single crystals

Sparingly soluble neodymium copper oxalate (NCO) single crystals were grown by gel method, by the diffusion of a mixture of neodymium nitrate and cupric nitrate into the set gel containing oxalic acid. Tabular crystal, revealing well-defined dissolution figures has been recorded. X-ray diffraction studies of the powdered sample reveal that NCO is crystalline. Infrared absorption spectrum confirmed the formation of oxalato complex with water of crystallization, while energy dispersive X-ray analysis established the presence of neodymium dominant over copper in the sample. X-ray photoelectron spectroscopic studies established the presence of Nd and Cu in oxide states besides (C₂O₄)²⁻ oxalate group. The intensities of Nd (3d _5/2) and Cu (2p _3/2) peaks measured in terms of maximum photoelectron count rates also revealed the presence of Nd in predominance. The inductively coupled plasma analysis supports the EDAX and XPS data by the estimation of neodymium percentage by weight to that of copper present in the NCO sample. On the basis of these findings, an empirical structure for NCO has been proposed. The implications are discussed.     

Strong quantum confinement effect in nanocrystalline CdS

CdS quantum dots have been prepared by chemical method. The X-ray diffraction results indicated the formation of CdS nanoparticles with hexagonal phase and grain size 2.5 nm. The HRTEM analysis reveals the formation of CdS quantum dots with an average grain size of ~2.5 nm. The X-ray photoelectron spectroscopy spectra exhibit the 3d _5/2 and 3d _3/2 peaks corresponding to cadmium and the S2p _3/2 peak corresponding to sulphur. Optical studies by UV–vis spectroscopy show a blue shifted absorption at 471 nm because of the quantum confined excitonic absorption. The photoluminescence spectra of CdS exhibited a broad green emission band centred at around 494 nm.     

Depth profiling of a CdS buffer layer on CuInS2 measured with X-ray photoelectron spectroscopy during removal by HCl etching

A ~ 35 nm thick CdS buffer layer has been wet-chemically deposited on a CuInS₂ thin layer solar cell absorber and subsequently removed through a stepwise HCl etch process. Sequential etching times of two and three seconds were initially performed to gradually remove the CdS top-layer and leave the CuInS₂ surface intact. After each step an investigation of the sample with X-ray photoelectron spectroscopy was used to determine the stoichiometry and elemental binding energies of the sample surface. After the removal of the CdS layer longer etching times were used to study the long-term effects of the HCl. The resulting depth profile revealed Cu diffusion from the CuInS₂ into the CdS, although the Cu atoms did not reach the surface of the buffer layer. In addition, a Cd containing layer was left on the sample surface after more than six hours of etching time and is apparently insoluble in HCl, showing that the Cd-S bonds in this layer differ from those in CdS. This method can also be used with other top-layers to create depth profiles.