Effect of electron and neutron bombardment on the orange luminescence spectra of not specially doped and copper-doped cadmium sulfide single crystals

CdS single crystals which were not specially doped and which were doped with copper (N _Cu=10¹⁸ cm⁻³) have been investigated. It is concluded on the basis of an analysis of the dose dependences of the orange luminescence intensity (λ _M=605 nm) of “pure” and doped samples upon bombardment by electrons with E=1.2 MeV and by fast reactor neutrons that the centers responsible for this luminescence are complex in nature. They consist of interstitial cadmium atoms and oxygen atoms. Electron bombardment of CdS:Cu single crystals results in the formation of new centers which are responsible for luminescence with λ _M=570 and 545 nm. Fiz. Tekh. Poluprovodn. 31, 390–392 (April 1997)     

Features of subthreshold defect formation in CdS and CdS:Cu single crystals subjected to irradiation with X-ray photons

Experimental results of the study of the effect of irradiation with 230-keV electrons and with X-ray photons with energies of 8.06 and 17.5 keV and the effect of quenching on the formation and reconstruction of the centers of slow recombination of nonequilibrium charge carriers (the so-called r centers) in nominally undoped and Cu doped (N _Cu ∼ 10¹⁸ cm⁻³) CdS single crystals are reported. It is shown that defects in cadmium sublattice in the crystal (specifically, the V _Cd vacancies and the Cu_Cd defects with parameters close to those of the above vacancies) are responsible for the r centers. In the case of the X-ray irradiation of both undoped and Cu-doped CdS single crystals, subthreshold defect formation of cadmium vacancies and Cu_Cd defects takes place; this occurs at sites with distorted and weakened interatomic bonds, i.e., at “weak sites” near large structural imperfections of the lattice, of technological or other origin. Starting with a quenching temperature 170°C, the spectrum of slow-recombination centers is appreciably affected by thermally formed vacancies V _Cd and secondary defects Cu_Cd. At quenching temperatures higher than 250°C, a significant contribution to the spectrum of optical quenching of photoconductivity is made by thermally introduced free (away from structural imperfections) r centers, i.e., the V _Cd and Cu_Cd defects.     

Chlorine-doped CdS thin films from CdCl<Subscript>2</Subscript>-mixed CdS powder

The CdS:Cl thin films have been prepared using thermally evaporated, CdCl₂-mixed CdS powder at 200°C substrate temperature. The percentage of CdCl₂ in the mixture varied from 0% to 0.20%. The electrical properties and the grain size of the deposited films were investigated. The results show that light doping, resistivity, carrier concentration, and mobility follow Seto’s model for polycrystalline material. However, with heavy doping, these properties undergo a saturation trend. The saturation behavior can be understood in terms of the rapid formation of the A-center complexes in the films. The deposited films were annealed at 250°C and 300°C. The resistivity of pure and lightly doped CdS films increased with annealing temperature, whereas carrier concentration and mobility in these films decreased. However, for the higher doping concentrations, the resistivity decreased, whereas carrier concentration and mobility showed improvement. These changes in electrical properties of the deposited films with annealing and doping concentration are attributed to a reduction in the lattice defect sites in CdS upon annealing. The experimental results are interpreted in terms of a modified version of Seto’s model for polycrystalline materials.     

CdS amorphous thin films photochemical synthesis and optical characterization

Thin amorphous nanostructured CdS films were photochemically obtained via direct UV radiation (λ=254 nm) of complex Cd[(CH₃)₂CHCH₂CH₂OCS₂]₂ on Si(1 0 0) and ITO-covered glass substrate by spin coating. Thin cadmium xanthate complex films’ UV photolysis results in loss of all ligands from the coordination sphere. X-ray photoelectron spectra for as-deposited CdS thin films show the most representative signals of Cd 3d_5/2 located at 405 eV, Cd 3d_3/2 located at 412 eV and a small signal S 2p located at 162 eV. The surface morphology of the films was examined via atomic force microscopy. This can be described as a fibrous-type surface without structural order, which is characteristic of an amorphous deposit. The optical band gap value was 2.85 and 3.15±0.1 eV.     

Specific features of the effect of irradiation with electrons and neutrons on photoelectric properties of CdS single crystals nominally undoped and doped with Cu

Electrical, photoelectric, and magnetic properties of CdS single crystals undoped and doped with copper (N _Cu ≈ 10¹⁸ cm^?3) and irradiated with electrons (E = 1.2 MeV, Φ = 2 × 10¹⁷ cm^?2) and neutrons (E = 2 MeV, Φ = 10¹⁸ cm^?2) are studied. It is shown that the donor-acceptor pairs are responsible for extrinsic photoconductivity and paramagnetic properties; in particular, these pairs are represented by Cu _Cd ^? -D ⁺ complexes that are destroyed during irradiation and are formed again with time (as secondary radiation defects) in irradiated samples. It is established that the majority of paramagnetic centers and donor-acceptor pairs are located in the near-surface region of the crystal. It is confirmed that large structural defects (defect clusters) formed by irradiation with neutrons are efficient sinks for copper atoms. Specific features of isochronous annealing of paramagnetic centers and donor-acceptor pairs responsible for the variation in magnetic parameters and in the photoconductivity spectra of irradiated undoped and Cu-doped CdS samples are studied.     

Characterization of CdS Nanowires Self-Assembled in a Nanoporous Alumina Template

CdS nanowires were self-assembled in a thin film (～200 nm) anodic aluminum oxide template on an indium tin oxide-coated glass substrate via dc electrodeposition. Raman spectral studies were done to probe the vibrational properties of scattering CdS phonons. Strong 1 longitudinal optical (LO), 2 LO, and 3 LO peaks were observed at 302 cm^?1, 603 cm^?1, and 906 cm^?1 having an energy separation of 37 meV, which is in accordance with the CdS bulk values. The photoluminescence spectra showed improved intensity of emission on annealing of the CdS nanowires. Field-emission scanning microscopy confirms the growth of nanowires of diameters ranging from 10 nm to 25 nm for these templates. These diameters agreed with those extracted from the luminescence emission energies.     

Use of two low-temperature emitters to determine the cutoff wavelength of the photosensitivity of infrared photodetectors

This paper is the continuation of the analysis of a method of determining the cutoff wavelength λ _c of infrared photodetectors by irradiating the sample with radiation from two blackbodies with different temperatures. The emitters can operate at lower temperatures as the cutoff wavelength λ _c is increased. The parameters of a system employing two blackbodies, which are placed inside a liquid-nitrogen cryostat and have temperatures of 260 and 320 K, respectively, are presented. It is shown that an error of 1 K in determining the lower or higher temperature produces an error of approximately 0.3 and 0.2 μm, respectively, in λ _c if λ _c=10 μm. Measurements on photodiodes fabricated on the basis of Cd_0.24Hg_0.76Te (λ _c=8.1 μm) epitaxial layers showed that the difference in the values of λ _c obtained by this method and from spectral measurements is no more than several tenths of a micron. It is suggested that this method be used as a standard method. Fiz. Tekh. Poluprovodn. 32, 1135–1138 (September 1998)     

Minority carrier diffusion lengths in liquid phase epitaxial InGaAsP and InGaAs

Minority carrier diffusion lengths were determined for InGaAsP and InGaAs layers grown by liquid phase epitaxy on (100)-InP substrates by measuring the variation of the short circuit photocurrent as a focussed laser beam was scanned along a beveled (~1°) p-n junction. The effect of lattice-mismatch on the hole diffusion length (λ_p) for n-type unintentionally doped InGaAsP layers (λg=1.15 μm) was investigated for mismatch values from -0.25% to +0.31%, with the longest diffusion length (L_p = 1.5 μm) occurring when the epitaxial layer was lattice-matched to the substrate. As the amount of mismatch increased, L_p decreased. Electron diffusion lengths, L_n, were determined for lattice-matched quaternary and ternary layers grown from Zn doped melts over a wide range of hole concentrations. At the lowest hole concentrations, p = 3 × l0¹⁵ and 1.4 × 10¹⁶ cm⁻³, the electron diffusion lengths were 3.5 and 2.5 μm for the quaternary and ternary, respectively. As the hole concentration increased, L_n decreased and at the highest concentration (p = 5 × 10su18,cn⁻³) L_n was 0.13 μm for InGaAsP and 0.83 un for InGaAs.     

Factors Affecting the Stability of CdTe/CdS Solar Cells Deduced from Stress Tests at Elevated Temperature

CdTe/CdS solar cells were subjected to heat stress at 200 °C in the dark under different environments (in N₂ and in air), and under illumination (in N₂). We postulate that two independent mechanisms can explain degradation phenomena in these cells: i) Excessive Cu doping of CdS: Accumulation of Cu in the CdS with stress, in the presence of Cl, will increase the photoconductivity of CdS. With limited amounts of Cu in CdS, this does NOT affect the photovoltaic behavior, but explains the crossover of light/dark current–voltage (J–V) curves. Overdoping of CdS with Cu can be detrimental to cell performance by creating deep acceptor states, acting as recombination centers, and compensating donor states. Under illumination, the barrier to Cu cations at the cell junction is reduced, and, therefore, Cu accumulation in the CdS is enhanced. Recovery of light‐stress induced degradation in CdTe/CdS cells in the dark is explained by dissociation of the acceptor defects. ii) Back contact barrier: Oxidation of the CdTe back surface in O₂/H₂O‐containing environment to form an insulating oxide results in a back‐contact barrier. This barrier is expressed by a rollover in the J–V curve. Humidity is an important factor in air‐induced degradation, as it accelerates the oxide formation. Heat treatment in the dark in inert atmosphere can stabilize the cells against certain causes of degradation, by completing the back contact anneal.     

Optimization of CdS buffer layer for high‐performance Cu2ZnSnSe4 solar cells and the effects of light soaking: elimination of crossover and red kink

A spike‐like conduction band alignment of kesterite absorbers with a CdS buffer layer is one of the key factors for high‐performance solar cells using this buffer/absorber heterojunction combination. However, it can also be the origin of fill factor and current‐reducing distortions in current–voltage curves, such as light/dark curve crossover, or an s‐like curve shape for long wavelength monochromatic illumination (red kink) if light‐dependent defect states are present in the buffer layer. In this work, we show that by changing the cadmium precursor source from sulfate to nitrate salts for the chemical bath deposited cadmium sulfide for Cu₂ZnSnSnSe₄/CdS heterojunction solar cells red kink can be eliminated, and crossover greatly improved (and eliminated entirely after light soaking). These improvements lead to a decrease in series resistance and an increase in fill factor and increase power conversion efficiency from 7.0% to 8.2%. We attribute this improvement to a reduction of deep level acceptor‐like traps states inside the CdS layer, which are responsible for an increase of the conduction band spike up to a current blocking value for the sulfate precursor case. Furthermore, the effects of light soaking will be discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Three types of self-activated luminescence centers in CdS:O

The cathodoluminescence and X-ray-luminescence spectra of CdS:O crystals are studied. For these CdS:O crystals the oxygen content, the dependence of the band gap on the substitutional oxygen content [OS], and the band model calculated on the basis of band anticrossing theory are known. New data on the three types of self-activated luminescence, specifically, edge luminescence and luminescence related to SA and F ⁺ centers in CdS are obtained. The conditions of the formation of these luminescence centers under changes in the system of intrinsic point defects in CdS crystals containing oxygen are established.     

Special features of optical and photoelectric properties of nominally undoped and Cu-doped CdS single crystals

Experimental data on the effect of fast reactor neutrons on the absorption spectra, photoconductivity, and luminescence of nominally undoped and Cu-doped CdS single crystals are reported. It is shown that defect clusters formed as a result of neutron irradiation exhibit the properties of getters for easily migrating optically active impurities in the crystal lattice. In neutron-irradiated samples, the defects can be annealed in two stages. The first stage (100–150°C) involves the annealing of point defects, while the second stage (250–420°C) involves mainly the annealing of defect clusters. The degradation of the defect clusters is accompanied with the enrichment of the lattice with Cd and S vacancies.     

Optical properties of oxygen-implanted CdS:O layers in terms of band anticrossing theory

The microcathodoluminescence (MCL) and photoreflection spectra of CdS:O layers implanted with oxygen ions to 4 × 10²⁰ cm^?3 are investigated. Used method of MCL spectroscopy yields information only about the implanted-layer volume. Exciton MCL spectra, which allow one to determine the concentration of dissolved oxygen in the CdS:O layers and the influence of deviation of the substrates from stoichiometry, are recorded. The homogeneity of the ion-implanted layers is studied by cathodoluminescence (CL) scanning electron microscopy. The relationship between light-emitting areas and the luminescence band at ～630 nm is established. The reason for enhancement of this band upon radiation annealing is revealed and its nature as the luminescence of F⁺ centers in CdS is confirmed. New photoreflection spectroscopy data are obtained, which describe the specific behavioral features of oxygen on the layer surface as an isoelectronic impurity in highly mismatched alloys (HMAs). It is shown that sulfur completely bonds and removes oxygen from CdS:O. Oxygen-free CdS remains on the surface in the form of nanoparticles, the size of which depends on the oxygen concentration in the CdS:O layer bulk. The results obtained are in agreement with the predictions of band anticrossing theory.     

Voltage dependent photocurrent collection in CdTe/CdS solar cells

The voltage dependence of the photocurrent J_L(V) of CdTe/CdS solar cells has been characterized by separating the forward current from the photocurrent at several illumination intensities. J_L(V) reduces the fill factor (FF) of typical cells by 10–15 points, the open circuit voltage (V_OC) by 20–50 mV, and the efficiency by 2–4 points. Eliminating the effect of J_L(V) establishes superposition between light and dark J(V) curves for some cells. Two models for voltage dependent collection give reasonable fits to the data: (1) a single carrier Hecht model developed for drift collection in p‐i‐n solar cells in which fitting yields a parameter consistent with lifetimes of 10⁻⁹ s as measured by others; or (2) the standard depletion region and bulk diffusion length model fits almost as well. The simple Hecht‐like drift collection model for photocurrent gives very good agreement to J(V) curves measured under AM1·5 light on CdTe/CdS solar cells with FF from 53% to 70%, CdTe thickness from 1·8 to 7·0 µm, in initial and stressed states. Accelerated thermal and bias stressing increases J_L(V) losses as does insufficient Cu. This method provides a new metric for tracking device performance, characterizes transport in the high field depletion region, and quantifies a significant FF loss in CdTe solar cells. Copyright © 2007 John Wiley & Sons, Ltd.     

Inkjet printed metallizations for Cu(In1−x Ga x )Se2 photovoltaic cells

This study reports the inkjet printing of Ag front contacts on Aluminum doped Zinc Oxide (AZO)/intrinsic Zinc Oxide (i‐ZnO)/CdS/Cu(In_1−xGa_x)Se₂ (CIGS)/Mo thin film photovoltaic cells. The printed Ag contacts are being developed to replace the currently employed evaporated Ni/Al bi‐layer contacts. Inkjet deposition conditions were optimized to reduce line resistivity and reduce contact resistance to the Al:ZnO layer. Ag lines printed at a substrate temperature of 200°C showed a line resistivity of 2.06 µΩ · cm and a contact resistance to Al:ZnO of 8.2 ± 0.2 mΩ · cm² compared to 6.93 ± 0.3 mΩ · cm² for thermally evaporated contacts. These deposition conditions were used to deposit front contacts onto high quality CIGS thin film photovoltaic cells. The heating required to print the Ag contacts caused the performance to degrade compared to similar devices with evaporated Ni/Al contacts that were not heated. Devices with inkjet printed contacts showed 11.4% conversion efficiency compared to 14.8% with evaporated contacts. Strategies to minimize heating, which is detrimental for efficiency, during inkjet printing are proposed. Copyright © 2011 John Wiley & Sons, Ltd.     

CdTe/CdS thin film solar cells grown in substrate configuration

The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non‐transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl₂‐treatment procedure. For Cu‐free cells, two activation treatments were applied, whereas Cu‐containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high V_OC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO₃, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu‐free MoO₃/Te buffer material, a V_OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley & Sons, Ltd.     

Special features of the mechanism of defect formation in CdS single crystals subjected to irradiation with high doses of fast reactor neutrons

Electrical and optical properties of CdS single crystals irradiated with a dose ≥10¹⁸ cm^?2 of fast reactor neutrons have been studied. It is established that clusters of defects are formed in irradiated material, and cadmium vacancies are dominant in these clusters. In the case when the defect clusters are decomposed in the course of radiation-stimulated annealing or thermal annealing in the temperature range Φ ≈ (200–400)°C, the crystal lattice becomes enriched with Cd vacancies. It is assumed that subthreshold effects play an important role in formation of defect clusters, and that these effects are related to preferential excitation of the K shells in Cd atoms and their Coulomb ejection from the core of a cluster.     

Highly Efficient Quantum‐Dot‐Sensitized Solar Cell Based on Co‐Sensitization of CdS/CdSe

Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots (QDs) are sequentially assembled onto a nanocrystalline TiO₂ film to prepare a CdS/CdSe co‐sensitized photoelectrode for QD‐sensitized solar cell application. The results show that CdS and CdSe QDs have a complementary effect in the light harvest and the performance of a QDs co‐sensitized solar cell is strongly dependent on the order of CdS and CdSe respected to the TiO₂. In the cascade structure of TiO₂/CdS/CdSe electrode, the re‐organization of energy levels between CdS and CdSe forms a stepwise structure of band‐edge levels which is advantageous to the electron injection and hole‐recovery of CdS and CdSe QDs. An energy conversion efficiency of 4.22% is achieved using a TiO₂/CdS/CdSe/ZnS electrode, under the illumination of one sun (AM1.5,100 mW cm^?2). This efficiency is relatively higher than other QD‐sensitized solar cells previously reported in the literature.     

Cu2ZnSnS4 photovoltaic cell with improved efficiency fabricated by high‐temperature annealing after CdS buffer‐layer deposition

To improve the photovoltaic properties of Cu₂ZnSnS₄ (CZTS) cells, we investigated the effect of both the thickness of the deposited CdS layers and the post‐annealing temperature following CdS deposition on the photovoltaic properties of CZTS cells using a two‐layer CZTS structure. By depositing a thin CdS layer (40 nm) followed by high temperature annealing (603 K), we observed a remarkable increase in the short‐circuit current density because of the enhancement of the external quantum efficiency in the wavelength range of 400–800 nm. The best CZTS cell exhibited a conversion efficiency of 9.4% in the active area (9.1% in the designated area). In addition, we also fabricated a CZTS cell with open‐circuit voltage of 0.80 V by appropriately tuning the composition of the CZTS layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Signal‐On Electrochemiluminescence Biosensors Based on CdS–Carbon Nanotube Nanocomposite for the Sensitive Detection of Choline and Acetylcholine

This work describes for the first time signal‐on electrochemiluminescence (ECL) enzyme biosensors based on cadmium sulfide nanocrystals (CdS NCs) formed in situ on the surface of multi‐walled carbon nanotubes (MWCNTs). The MWCNT–CdS can react with H₂O₂ to generate strong and stable ECL emission in neutral solution. Compared with pure CdS NCs, the MWCNT–CdS can enhance the ECL intensity by 5.3‐fold and move the onset ECL potential more positively for about 400 mV, which reduces H₂O₂ decomposition at the electrode surface and increases detection sensitivity of H₂O₂. Furthermore, the ECL intensity is less influenced by the presence of oxygen in solution. Benefiting from these properties, signal‐on enzyme‐based biosensors are fabricated by cross‐linking choline oxidase and/or acetylcholine esterase with glutaraldehyde on MWCNT–CdS modified electrodes for detection of choline and acetylcholine. The resulting ECL biosensors show wide linear ranges from 1.7 to 332 µM and 3.3 to 216 µM with lower detection limit of 0.8 and 1.7 µM for choline and acetylcholine, respectively. The common interferents such as ascorbic acid and uric acid in electrochemical enzyme‐based biosensors do not interfere with the ECL detection of choline and acetylcholine. Furthermore, both ECL biosensors possess satisfying reproducibility and acceptable stability.