CdS annealing treatments in various atmospheres and effects on performances of CdTe/CdS solar cells

In this work, a systematic research on CdS annealing treatments under various atmospheres had been done to understand their effects on CdS/CdTe solar cells. CdS films were prepared by a standard CBD method and annealed under various atmospheres, including Ar, Ar+H₂, O₂, Ar+S and Ar+CdCl₂. Morphological, structural, optical and chemical properties were investigated using Atom force microscope (AFM), X-ray diffraction (XRD), UV–VIS spectroscopy and X-ray photoelectron spectroscopy (XPS). Annealing treatments enhanced modifications of morphology, structure and electrical properties of CdS films. AFM showed different surface morphologies and roughnesses of CdS films annealed under various atmospheres. XRD indicated the transition of CdS films from metastable cubic structure to stable hexagonal structure after annealing treatment, especially annealed in Ar+CdCl₂. From XPS analysis, Fermi levels of CdS films shifted closer to conduction band after annealing under O₂ and Ar+CdCl₂, while the levels shifted away from conduction band under Ar+H₂ and Ar+S. The relationships between those modifications by annealing treatments and effects on the performance of solar cells were discussed. Solar cell based on CdS annealed with Ar+CdCl₂ had the best performance due to the high n-doping of CdS layer introduced by annealing process.     

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.     

Optical properties of nano-silicon

We investigated the optical properties of silicon clusters and Si nanocrystallites using photolumine-scence (PL) and Raman scattering technique. Broad luminescence band in the red region was observed from Si-doped SiO₂ thin films deposited by co-sputtering of Si and SiO₂ onp-type Si (100) substrates, annealed in Ar and O₂ atmosphere. Nanocrystalline Si particles fabricated by pulsed plasma processing technique showed infrared luminescence from as grown film at room temperature. Raman spectra from these films consisted of broad band superimposed on a sharp line near 516 cm⁻¹ whose intensity, frequency, and width depend on the particle sizes arising from the phonon confinement in the nanocrystalline silicon. We also performed PL, Raman and resonantly excited PL measurements on porous silicon film to compare the optical properties of Si nanostructures grown by different techniques. An extensive computer simulation using empirical pseudo-potential method was carried out for 5–18 atoms Si clusters and the calculated gap energies were close to our PL data. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

ZrO2:CdS薄膜的光物理性质

利用吸收和荧光光谱研究了ＺｒＯ２：ＣｄＳ薄膜的光物理特性实验观察到了随颗粒尺寸的减小ＣｄＳ的吸收带边的蓝移现象,研究了不同激发条件下的荧光光谱。发现了薄膜中ＣｄＳ的微弱的荧光发射。并且分析了介质效应对ＣｄＳ光学特性的影响。     

The role of the surface ligand in the optical properties of CdS quantum dots in poly(vinyl alcohol) matrix

The synthesis and characterization of CdS nanoparticles prepared in poly(vinyl alcohol) (PVA) in situ, to produce a series of CdS/PVA nanocomposite films, is described in this paper. The role of 2-mercaptoethanol as the surface ligand for the nanoparticles has been investigated. Different molar concentrations of the cadmium precursor have also been evaluated, aiming at the preparation of stable aqueous colloidal systems and polymeric films. UV–visible (UV–Vis) and photoluminescence spectroscopies together with scanning electron microscopy and transmission electron microscopy have been used for characterization of the growth kinetics and the relative stability of CdS nanocrystals in the polymeric matrix. The results clearly indicate the formation of hexagonal CdS nanoparticles embedded in the PVA matrix. PVA was not effective in stabilizing colloidal CdS nanoparticles against aggregation. However, it leads to a displacement of the first optical transition of CdS due to compressive deformation effect. The combination of PVA with 2-mercaptoethanol as surface ligands had a strong effect on the optical properties of the resulting embedded CdS nanoparticles.     

CdS nanoparticles incorporated onion-like mesoporous silica films: Ageing-induced large stokes shifted intense PL emission

CdS nanoparticles (NPs) were generated in onion-like ordered mesoporous SiO₂ films through a modified sol–gel process using P123 as a structure directing agent. Initially Cd²⁺ doped (12 equivalent mol% with respect to the SiO₂) mesoporous SiO₂ films were prepared on glass substrate. These films after heat-treatment at 350 °C in air yielded transparent mesoporous SiO₂ films having hexagonally ordered onion-like pore channels embedded with uniformly dispersed CdO NPs. The generated CdO NPs were transformed into CdS NPs after exposing the films in H₂S gas at 200 °C for 2 h. The as-prepared CdS NPs incorporated mesoporous SiO₂ films (transparent and bright yellow in color) showed a band-edge emission at 485 nm and a weak surface defect related emission at 530 nm. During ageing of the films in ambient condition the band-edge emission gradually weakened with time and almost disappeared after about 15 days with concomitant increase of defect related strong surface state emission band near 615 nm. This transformation was related to the decay of initially formed well crystalline CdS to relatively smaller and weakly crystalline CdS NPs with surface defects due to gradual oxidation of surface sulfide. At this condition the embedded CdS NPs show large Stokes shifted (∼180 nm) intense broad emission which could be useful for luminescent solar concentrators. The detailed process was monitored by UV–Visible, FTIR and Raman spectroscopy, XPS, XRD and TEM studies. The evolution of photoluminescence (PL) and life times of CdS/SiO₂ films were monitored with respect to the ageing time.     

ZrO2:CdS薄膜的光物理性质

利用吸收和荧光光谱研究了ＺｒＯ_２：ＣｄＳ薄膜的光物理特性．实验观察到了随颗粒尺寸的减小ＣｄＳ的吸收带边的蓝移现象．研究了不同激发条件下的荧光光谱,发现了薄膜中ＣｄＳ的微弱的荧光发射．并且分析了介质效应对ＣｄＳ光学特性的影响．     

Effects of CdCl2 treatment and annealing on CdS/SnO2/glass heterostructures for solar cells

Effects of CdCl₂ post-growth treatments and annealing under different conditions on the surface and interface properties of CdS/SnO₂/glass heterostructure were studied. CdS thin films were grown on SnO₂-coated glass substrates for CdS/CdTe heterojunction solar cells by the solution growth technique. It was found that CdCl₂ post-growth treatments and annealing enhanced the CdS-related XRD peaks, narrowed the CdS characteristic Raman bands, removed or depressed the disorder related Raman features, and improved the CdS film crystalline quality significantly, which are advantageous to the application in solar cells as a window layer material.     

Chemical bath deposition of CdS thin films doped with Zn and Cu

Zn- and Cu-doped CdS thin films were deposited onto glass substrates by the chemical bath technique. ZnCl₂ and CuCl₂ were incorporated as dopant agents into the conventional CdS chemical bath in order to promote the CdS doping process. The effect of the deposition time and the doping concentration on the physical properties of CdS films were investigated. The morphology, thickness, bandgap energy, crystalline structure and elemental composition of Zn- and Cu-doped CdS films were investigated and compared to the undoped CdS films properties. Both Zn- and Cu-doped CdS films presented a cubic crystalline structure with (1 1 1) as the preferential orientation. Lower values of the bandgap energy were observed for the doped CdS films as compared to those of the undoped CdS films. Zn-doped CdS films presented higher thickness and roughness values than those of Cu-doped CdS films. From the photoluminescence results, it is suggested that the inclusion of Zn and Cu into CdS crystalline structure promotes the formation of acceptor levels above CdS valence band, resulting in lower bandgap energy values for the doped CdS films.     

Morphology and properties of electrodeposited CdS films in non-aqueous solvents

Semiconducting films of CdS are produced by cathodic deposition of Cd on stainless steel electrodes in the presence of dissolved S₈ in dimethylsulfoxide and diethylene glycol solutions. Uniform CdS films have been obtained by controlling growth parameters like current density and electrodeposition times. The transmittance spectra have been recorded and the band gap determined. The deposited phase has been examined by XRD analysis.     

Raman,photoluminescence analysis on Cu(InAl)Se2 thin films and the fabrication of Cu(InAl)Se2 based thin film solar cells

Cu(InAl)Se₂ (CIAS) thin films have been prepared by chemical bath deposition technique. Thickness of the prepared films has been measured by gravimetric technique. The structure, composition and optical transition as well as bandgap have been estimated by X-ray diffraction, energy dispersive X-ray analysis and spectrophotometer analysis. Raman analysis has been made on the prepared CIAS thin films to assign the fundamental lattice mode and to confirm the films crystallinity and stoichiometry. PL analysis has been carried out to find the effective mass of holes and electron, dielectric constant, the involved defects and their activation energy. Cu(InAl)Se₂-based solar cells with different types of buffer layers such as CdS, CdS:Cu, CdS:In were fabricated. The current and voltage were measured using an optical power meter and an electrometer and the fabricated solar cells were illuminated using 100 mW/cm² white light under AM1 conditions.     

Effect of the substrate temperature on the structural, optical and electrical properties of spray-deposited CdS:B films

Boron doped CdS films have been deposited by spray pyrolysis method onto glass substrate temperature in the range of 350–450 °C. And the effect of substrate temperature (T _s) on the structural, electrical and optical properties of the films were studied. The structural properties of boron doped CdS films have been investigated by (XRD) X-ray diffraction techniques. The X-ray diffraction spectra showed that boron doped CdS films are polycrystalline and have a hexagonal (wurtzite) structure. By using SEM analysis, the surface morphology of the films was observed as an effect of the variation of substrate temperature. The substrate temperature is directly related with the shift detected in the band gap values derived from optical of parameters and the direct band gap values were found to be in the region of 2.08–2.44 eV. The electrical studies showed that the film deposited at the substrate temperature 400 °C had high carrier concentration and Hall mobility and minimum resistivity. This resistivity value decreased with increase in temperature up to 400 °C indicating the semiconducting nature of B- doped CdS films. The lattice parameter, grain size, microstrain and dislocation densities were calculated and correlated with the substrate temperature (T _s ).     

Studies on polycrystalline Cd0.96Zn0.04Te thin films prepared by vacuum evaporation

Polycrystalline Cd_0.96Zn_0.04Te thin films are deposited onto glass substrates (Corning 7059) kept at room temperature by vacuum evaporation. The films exhibit zinc blende structure with predominant (1 1 1) orientation. The rms roughness of the films evaluated by atomic force microscope is 3.7 nm. The band gap energy of the films measured by optical transmittance measurement is 1.539 eV. The photoluminescence (PL) spectrum of the films shows intense emission due to free and bound exciton recombination and no emission associated with crystal imperfection and PL line shapes give indications of the high quality of the layers. These films have been implanted with properly mass analyzed Boron ions (¹⁰B⁺) and the effect of implantation has been analyzed by X-ray diffraction, Raman scattering and optical transmittance measurements and the results are explained on the basis of the implantation induced surface roughness and lattice disorder.     

Structural,optical and magnetic properties of Ba and Ni doped CdS thin films prepared by spray pyrolysis method

Pure, Barium and Nickel doped cadmium sulphide (CdS) thin films have been coated on glass substrates at 400?°C by spray pyrolysis technique. The prepared CdS and doped CdS thin films were analysed by various measurements such as X-ray diffraction (XRD), SEM, optical and Vibrating Sample Magnetometer (VSM). X-ray diffraction measurements show that the coated pure, Ba and Ni-doped CdS thin films belong to the cubic crystal structure with orientation preferentially along (111) direction. The average crystallite size of pure, Ba and Ni doped CdS thin films were determined as 31, 33 and 45 nm, respectively. The average dislocation density (δ) and stacking fault (SF) of pure, Ba and Ni doped CdS thin films were also determined. The surface morphology and elemental analysis of the thin films were determined by scanning electron microscopy and energy dispersive X-ray spectrum (SEM with EDAX). It is observed that the optical energy bandgap has been decreased from 2.43 to 2.1 eV due to the doping Ba. The luminescence spectrum shows a strong emission peak at 517 nm in the case of pure CdS thin film and a meager red shift has been observed due to the doping. VSM studies were employed to study the magnetic behaviour of Ba and Ni doped CdS thin films.     

Structural and optical characterization of CdS and CdHgTe thin films for the fabrication of CdHgTe/CdS structure

This article presents the deposition and characterization of CdS and CdHgTe thin films for the fabrication of CdHgTe/CdS structure. The growth of CdS and CdHgTe thin films on FTO-coated conducting glass substrates have been performed by chemical bath deposition (CBD) and electrodeposition methods, respectively. The deposition conditions have been optimized for getting better quality layers of CdS and CdHgTe. The grown layers of both CdS and CdHgTe have been characterized by photoelectrochemical cell (PEC) measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis spectrophotometer. Annealing effect of the deposited films has also been investigated. Finally the fabrication of CdHgTe/CdS structure has been performed and investigated by I–V characteristics. PEC, XRD, SEM and UV–vis spectrophotometer studies reveal that chemically deposited CdS layers are n-type with band gap values vary from 2.29 to 2.41 eV and cubic with (111) preferential orientation, and have spherical grain distributed over the surface. However, electrodeposited CdHgTe layers are p-type with band gap values varying from 1.50 to 1.53 eV and cubic with highly oriented CdHgTe crystallites with the (111) planes parallel to the substrate, and have uniform distribution of granular grains over the surface. The fabricated CdHgTe/CdS structure gave an open-circuit photovoltage and a short-circuit photocurrent of 510 mV and 13 mA/cm² respectively, under AM 1.5 illumination.     

Chemical deposition of CdS films: structure,RBS, PIXE,optical and photoelectrochemical solar cell studies

CdS semiconductor films have been chemically prepared from a basic solution containing CdSO₄, thiourea and NH₄OH. X-ray diffraction studies have revealed the presence of polycrystalline mixed cubic and hexagonal phases of CdS in the deposit. The surface topography has been identified by SEM analysis and found to be non-uniform and spongy. Rutherford Backscattering Spectrometry (RBS) analysis gave the Cd to S ratio as 1.016 whereas that of Proton Induced X-ray Emission (PIXE) analysis gave the Cd to S ratio as 0.905. The band gap of the CdS film has been estimated from optical absorption studies and found to be 2.46 eV. The CdS/S^2–, S₂ ^2– interface has been characterized in order to identify the charge transfer process. Finally, Photoelectrochemical Solar Cells (PESCs) have been fabricated with cell configuration Ti/CdS/S^2–, S₂ ^2–/Pt and the results have been discussed.     

Physical properties and flux effect of printed CdS thick films

Screen printed CdS films has been studied with various techniques. Although the thick films preserved the feature of powder crystals, i.e., lack of orientation, it is found that more amounts of (112) wurtzite phase might be good for the electrical property of CdS films. The flux CdCl₂, which was added to the CdS films, resulted in two trapping levels, 0.4 ev and 0.68 ev. The mechanism and the effect of flux on the CdS films are discussed. Besides, the effect of vacuum annealing is also described.     

Effects of annealing atmosphere and substrate on the photoluminescence and Raman scattering from Si nanocrystals in a SiO2 matrix

Thin films of SiO _x have been prepared on quartz or c-Si substrates by thermal evaporation of SiO in vacuum and post-annealed at 1373 K in an argon or hydrogen atmosphere. High-resolution electron microscopy has shown the existence of silicon nanocrystals in the annealed films, and this result has been confirmed by Raman scattering. Photoluminescence has been observed from annealed films and attributed to radiative recombination in Si nanocrystals. Its intensity is appreciably higher upon annealing in Ar than in H₂. It is shown that substrates strongly affect the Raman scattering from Si nanocrystals in nc-Si–SiO₂ thin films with low filling factors.     

A structural property study on the role of Sm ions in nano-textured Zn(1−x)Sm x O thin films for green emission

Sm doped ZnO nanocrystalline textures were deposited through pyrolytic reactions on preheated glass substrates to tune the green emissions from the wurtzite matrix. The textures were found to be well-defined, possessing grain like morphology; with their dimensional parameters depending on the level of Sm substitution. The films were then characterized using several analytic techniques to establish the origin of their green emissions. The structural data obtained using the 2θ plots revealed the polycrystalline nature of the species and their preferred orientation along (002) plane. The morphology studies revealed a gradual decrease in grain size, while increasing the Sm composition. Their evolution has been correlated with the heterogeneous nucleation taking place on the surface of the substrate. The role of defects and crystallinity of the Zn_1?xSm_xO has been explained using Raman spectroscopy. Taucs plot revealed a red shift in the optical energy band gap of the species, while the luminescence spectra illustrated the enhancement of green emissions.     

Understanding spectroscopic phonon-assisted defect features in CVD grown 3C-SiC/Si(1 0 0) by modeling and simulation

New phonon-assisted defect features are observed using photoluminescence (PL) and Raman scattering spectroscopy on 3C-SiC/Si(1 0 0) films grown by chemical vapor deposition (CVD) technique. The ultraviolet excitation room-temperature (RT) PL-Raman spectra show a luminescence band near 2.3 eV due to RT recombination over the 3C-SiC indirect band gap. In addition to the strong Raman lines characteristic of Si substrate and 3C-SiC we also observed weaker impurity modes near 620, 743 and 833 cm⁻¹. These frequencies are compared with the results of Green's function simulations of impurity modes with plausible defect structures to best support the observed Raman features as well as modes of some prototypical defect center.