Optical and structural properties of chemically deposited CdS thin films on polyethylene naphthalate substrates

CdS thin films were deposited on polyethylene naphthalate substrates by means of the chemical bath deposition technique in an ammonia-free cadmium-sodium citrate system. Three sets of CdS films were grown in precursor solutions with different contents of Cd and thiourea maintaining constant the concentration ratios [Cd]/[thiourea] and [Cd]/[sodium citrate] at 0.2 and 0.1 M/M, respectively. The concentrations of cadmium in the reaction solutions were 0.01, 7.5 × 10⁻³ and 6.8 × 10⁻³ M, respectively. The three sets of CdS films were homogeneous, hard, specularly reflecting, yellowish and adhered very well to the plastic substrates, quite similar to those deposited on glass substrates. The structural and optical properties of the CdS films were determined from X-ray diffraction, optical transmission and reflection spectroscopy and atomic force microscopy measurements. We found that the properties of the films depend on both the amount of Cd in the growth solutions and on the deposition time. The increasing of Cd concentration in the reaction solution yield to thicker CdS films with smaller grain size, shorter lattice constant, and higher energy band gap. The energy band gap of the CdS films varied in the range 2.42-2.54 eV depending on the precursor solution. The properties of the films were analyzed in terms of the growth mechanisms during the chemical deposition of CdS layers.     

An in-situ chemical reaction deposition of nanosized wurtzite CdS thin films

Nanocrystalline CdS thin films were deposited on glass substrates by an ammonia-free in-situ chemical reaction synthesis technique using cadmium cationic precursor solid films as reaction source and sodium sulfide based solutions as anionic reaction medium. Effects of ethanolamine addition to the cadmium cationic precursor solid films, deposition cycle numbers and annealing treatments in Ar atmosphere on structure, morphology, chemical composition and optical properties of the resultant films were investigated by X-ray diffraction, field emission scanning electron microscope, energy dispersive X-ray analysis and UV-Vis spectra measurements. The results show that CdS thin films deposited by the in-situ chemical reaction synthesis have wurtzite structure with (002) plane preferential orientation and crystallite size is in the range of 16 nm-19 nm. The growth of film thickness is almost constant with deposition cycle numbers and about 96 nm per cycle.     

Structural study of the initial growth of nanocrystalline CdS thin films in a chemical bath

Nanocrystalline cadmium sulfide CdS thin films with relevance for optical applications were synthesized from aqueous solutions by chemical bath deposition. Grazing incidence X-ray diffraction shows that the films formed on glass or silicon substrates are made up of nanocrystalline particles. About 80% of the particles have a diameter of 5 ± 1 nm. The nanoparticles have either sphalerite or wurtzite structure. The presence of the sphalerite phase is a signature of a highly non-equilibrium state of the nanocrystalline film. Kinetic studies show that the size of the nanocrystals and the relative fraction of the two phases do not depend on the deposition time once it exceeds a duration of 30 min. For longer times, the particle characteristics remain constant while the thickness of the film grows. Thermodynamical analysis of ionic equilibria allows to choose the reaction bath composition for the formation of cadmium hydroxide Cd(OH)₂. The experiments provide strong evidence that the beginning of the deposition of CdS is accompanied by a formation of cadmium hydroxide Cd(OH)₂.     

化学水浴沉积时间对CdS薄膜性质的影响

采用CBD法在醋酸镉溶液体系中制备CdS半导体薄膜,通过XRD、XRF、SEM和光学透过率谱等测试手段研究了沉积时间对CdS薄膜沉积过程和性质的影响.结果表明,随着沉积时间的增加,薄膜增厚;S/Cd原子比增加,但都为富Cd的CdS薄膜;XRD研究表明,薄膜结构由立方、六方混合相向立方相转变,(111)方向成为择优生长方向;SEM研究表明,随沉积时间增加,薄膜变致密,薄膜表面出现的白色附着颗粒增多,尺寸增大;沉积时间对薄膜的光学性质也有很大的影响,随着沉积时间的增加薄膜透过率减小,而禁带宽度值增大.     

Physical properties of CdS thin films grown by chemical bath deposition with directly immersed ultrasonication module

Ultrasonical colloid chemistry deposition (UCCD) has been widely used to fabricate columnar-structured cadmium sulfide (CdS) thin films with ultra-fine particles. In conventional UCCD, the ultrasonic source is installed outside the reaction bath. In this study, an ultrasonic homogenizer was used as the ultrasonic source and was immersed directly in the bulk solution. The advantages of the ultrasonic homogenizer include homogeneous deposition, fine mixing, and enhancement of the ion-by-ion reaction. We compared the physical properties of CdS thin films prepared with and without ultrasound by using an X-ray diffraction, scanning electron microscope, atomic force microscope, and 3D surface analyzer and UV-Vis-NIR measurements.     

Study of CdS-sensitized solar cells, prepared by ammonia-free chemical bath technique

In this study, cadmium sulfide (CdS)-sensitized solar cells have been fabricated, where nanoporous titanium oxide (TiO₂) photoelectrode of equal thickness has been prepared on SnO₂:F coated glass substrate using TiO₂ paste. Different amounts of CdS have been deposited by an ammonia-free chemical bath deposition technique with various deposition times. The CdS-sensitized TiO₂ photoelectrodes show polycrystalline nature. The optical measurement reveals that absorbance edge of the CdS-sensitized TiO₂ photoelectrode extends up to 540 nm and the amount of absorbance increases with the enhancement of CdS-deposition time. The CdS solar cell, with deposition time of 12 min, shows impressive photocurrent and moderate solar cell efficiency.     

Photoelectrochemical behaviour of pulse plated CdS films

Cadmium sulphide (CdS) films were deposited by the pulse plating technique at room temperature and at different duty cycles in the range of 6–50% using AR grade 0.25 M cadmium sulphate and 0.30 M sodium thiosulphate at a deposition potential of −0.75 V (SCE). The total deposition time was kept constant at 1 h. The thickness of the films were around 2.0 μm. X-ray diffraction (XRD) studies indicate the formation of polycrystalline films with the cubic structure. The crystallite size increased from 23.0 to 27.5 nm as the duty cycle increased from 10 to 50%. Optical absorption studies indicated a direct band gap in the range of 2.40–2.80 eV as the duty cycle is decreased. XPS studies indicated the formation of CdS. Photoelectrochemical (PEC) cell measurements made with the photoelectrodes deposited at 50% duty cycle have exhibited higher conversion efficiency compared to earlier reports.     

Analysis of sulphur deficiency defect prevalent in SILAR-CdS films

When cadmium sulphide (CdS) films are deposited using successive ionic layer adsorption and reaction (SILAR) process, a sulphur deficiency defect is always observed. The reason for this defect is addressed in the present work, by systematically analyzing the increasing pattern of Cd²⁺ions available in the anionic solution bath during the SILAR dipping cycles. The variation of the Cd²⁺ ion concentration in the anionic solution bath with respect to the number of cycles is analyzed in detail using inductive coupled plasma (ICP) analysis. Moreover, the corresponding sulphur deficiency generated in the CdS thin films is also analyzed using the energy dispersive X-ray analysis profiles, appropriately correlated with the ICP results. The optical transmittance and the thickness measurements are carried out as supplementary studies to support the analysis on the sulphur deficiency. The obtained results may be useful for rectifying this sulfur deficiency defect which is commonly occurred in CdS films deposited using chemical bath deposition as well as SILAR methods.     

Characterization of cadmium sulphide thin films prepared by successive ionic layers adsorption and reaction method

Cadmium sulfide (CdS) thin films were deposited on glass substrate at room temperature by successive ionic layer adsorption and reaction method (SILAR). The deposition parameters such as rinsing time, rinsing cycle and concentration of precursor solution were varied during the preparation of the samples. The structural characterization and optical characterization were carried out. The deposited films by lower growth rate and lower precursor concentration solutions were having mixed hexagonal and cubic phases. Thickness dependence of the optical band gap energy was evaluated and it varies from 2.46 to 2.32 eV in the thickness range 38–330 nm.     

Preparation of II–VI and IV–VI semiconductor films for solar cells by the isovalent substitution technique with a CBD-made substrate

II–VI and IV–VI semiconductor films for solar cell applications, namely, CdTe, CdS, CdSe, PbS, PbSe and PbTe, can be prepared in a two-stage deposition process. In this work we illustrate the two-stage process to obtain PbTe and CdSe films from precursor oxide or hydroxide films deposited by chemical bath deposition (CBD). At the first stage, plumbonacrite Pb₁₀(CO₃)₆O(OH)₆ or cadmium oxide/hydroxide CdO₂/Cd(O₂)_0.88(OH)_0.24 films were deposited onto a glass substrate by CBD, using an ammonia-free low-temperature process in an alkaline aqueous solution with corresponding ion sources. Then, at the second stage, the obtained film was placed in a chemical vapor deposition (CVD) Hot Wall reactor with gas transportation, where it acted as a substrate in the reaction of isovalent substitution of Te or Se for the nonmetallic film component, thus forming PbTe and CdSe films. A nitrogen flux of 0.25 L/min was used as the transporting gas. The source temperature was adjusted between boiling (Tb) and melting point (Tm) to control the flux gas of the source. The substrate temperature was adjusted to improve film quality. Structural and optical investigation of the films proved their high quality, which determines the possibility of using them as solar cell elements, in particular, in multijunction cells.     

Chemically deposited CdS films in an ammonia-free cadmium-sodium citrate system

In this work, we report the properties of chemically deposited CdS thin films in a cadmium-sodium citrate system. This chemical bath deposition process does not employ ammonia. We deposited four series of films at different cadmium content in the chemical bath process and determined their properties. The obtained information can be very useful for the optimization of the deposition process in order to reduce the amount of toxic chemical waste, mainly Cd-containing waste. The structural and optical properties of the CdS films were determined from X-ray diffraction, optical transmission and reflection spectroscopy and scanning electron microscopy measurements. We found that the properties of the films are very sensitive to the amount of cadmium in the deposition process. The process allows the deposition of good quality CdS thin films using 1.12, 0.84 and 0.76 mg of cadmium per milliliter of reaction solution.     

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.     

Preparation of CdTe films by electrodeposition

Single-phase CdTe thin films have been prepared by depositing sequentially a layer of tellurium and a layer of cadmium on a molybdenum substrate followed by a short thermal treatment. Deposition of tellurium films was done in an aqueous solution containing TeO₂ at a current density of ≈ 1 mA/cm². An aqueous solution containing cadmium sulfate was used for cadmium deposition with a current density of ≈1 mA/cm². Solution temperature was ≈ 95°C for tellurium film deposition and was 50°C for cadmium deposition. It was found that after a heat treatment at ≈ 370°C for 10 min the deposited Te/Cd layers were converted to CdTe thin films with a cubic structure. Compositional uniformity of the films was also investigated by electron probe microanalysis.     

X-ray photoelectron spectroscopic studies of sprayed CdS films

The chemical composition of sprayed CdS films has been evaluated using X-ray photoelectron spectroscopy. The general impurity content in the film is discussed, throwing light on the pyrolysis reaction involved in CdS deposition. Further, the stoichiometry of these films is studied as a function of process parameters such as pyrolysis temperature, Cd/S ratio in the solution, deposition rate and film thickness. A definite correlation is observed between composition and process parameters. The compositional variation appears to be related to the structure of CdS films as well as the growth mechanism. The effects induced by annealing in nitrogen, hydrogen and ambient air are also discussed. Hydrogen and nitrogen annealing is responsible for oxygen desorption from CdS. On the other hand air annealing induces stoichiometric variations along with oxygen intake in the films.     

Comparative study of CdS/CdTe cells fabricated with and without evaporated Te-layer

Thin film CdS/CdTe solar cells were fabricated by Close space Sublimation (CSS) at the Moldova State University using a Te layer at the back contact deposited by thermal evaporation. The influence of the Te layer thickness and a cadmium chloride solution treatment of the CdTe layer before and after Te layer deposition are studied. A proper comparison of the two CdS/CdTe solar cells shows that the cell with the Te layer has a higher current because of the higher blue photosensitivity, but has a lower open circuit voltage in relation to the absorber band gap.     

Growth technology,X-ray and optical properties of CdSe thin films

An ammonia-free chemical-bath deposition was used to obtain CdSe thin films on glass substrate. The materials used in the chemical bath were cadmium chloride complexed with sodium citrate and sodium selenosulphate. The preparation conditions, especially the starting solution characteristics, such as concentration of dissolved materials, temperature, pH value as well as deposition time and immersion cycles were optimized to obtain homogeneous stoichiometric films with good adherence to the glass substrate. The films thickness was in the range of 400–500 nm with a growing time of 4 h. The material obtained was characterized by optical absorption, SEM with the energy dispersive X-ray analysis (EDS) and X-ray diffraction. The films obtained at bath temperatures of 70 and 80 °C had the hexagonal structure (of wurtzite type), with crystallite size of about 20 nm. Room temperature deposition results in films with the cubic structure and crystallite size of about 4 nm. From optical transmission data, an energy gap equal to 1.88 eV was found. The material is interesting for applications in hybrid systems for solar energy conversion.     

Solution precursor plasma deposition of nanostructured CdS thin films

Cadmium sulfide (CdS) films are used in solar cells, sensors and microelectronics. A variety of techniques, such as vapor based techniques, wet chemical methods and spray pyrolysis are frequently employed to develop adherent CdS films. In the present study, rapid deposition of CdS thin films via plasma spray route using a solution precursor was investigated, for the first time. Solution precursor comprising cadmium chloride, thiourea and distilled water was fed into a DC plasma jet via an axial atomizer to create ultrafine droplets for instantaneous and accelerated thermal decomposition in the plasma plume. The resulting molten/semi-molten ultrafine/nanoparticles of CdS eventually propel toward the substrate to form continuous CdS films. The chemistry of the solution precursor was found to be critical in plasma pyrolysis to control the stoichiometry and composition of the films. X-ray diffraction studies confirmed hexagonal α-CdS structure. Surface morphology and microstructures were investigated to compare with other synthesis techniques in terms of process mechanism and structural features. Transmission electron microscopy studies revealed nanostructures in the atomized particulates. Optical measurements indicated a decreasing transmittance in the visible light with increasing the film thickness and band gap was calculated to be ～2.5 eV. The electrical resistivity of the films (0.243 ± 0.188 × 10⁵ Ω cm) was comparable with the literature values. These nanostructured polycrystalline CdS films could be useful in sensing and solar applications.     

Formation and properties of cadmium sulfide buffer layer for CIGS solar cells grown using hot plate bath deposition

In the present study, cadmium sulfide (CdS) thin films were deposited on different substrates [soda glass, fluoride doped tin oxide, and tin doped indium oxide (ITO) coated glass] by a hot plate method. To control the thickness and the reproducibility of the sample production, the thin films were coated at different temperatures and deposition times. The CdS thin films were heated at 400 °C in air and forming gas (FG) atmosphere to investigate the effect of the annealing temperatures. The thickness of the samples, measured by ellipsometry, could be controlled by the deposition time and temperature of the hot plate. The phase formation and structural properties of CdS thin films were studied by X-ray diffraction and scanning electron microscopy, whereas the optical properties were obtained by UV–vis spectroscopy. A hexagonal crystal structure was observed for CdS thin films and the crystallinity improved upon annealing. The structural and optical properties of CdS thin films were also enhanced by annealing at 400 °C in FG atmosphere (95 % N₂, 5 % H₂). The optical band gap was changed from 2.25 to 2.40 eV at different annealing temperatures and gas atmospheres. A higher electrical conductivity, for the sample annealed at FG, was noticed. The samples deposited on ITO and annealed in FG atmosphere showed the best structural and electrical properties compared to the other samples. CdS thin films can be widely used for application as a buffer layer for copper–indium–gallium–selenide solar cells.     

Effects of deposition time and temperature on the optical properties of air-annealed chemical bath deposited CdS films

CdS films of over 1-μm thickness were deposited onto glass substrates by chemical bath deposition (CBD). Deposition temperature and time were varied from 40 °C to 60 °C and from 30 min to 4 h, respectively. The highest deposition rate, 6.39 nm/min, was obtained with samples deposited for 90 min at 60 °C. The films deposited at 60 °C for 4 h were found to have the best adhesion and without defects. The optical properties, in particular the optical band gap, depended on film thickness, the deposition and annealing temperatures. Annealing in air resulted in a shift of the absorption edge towards higher wavelengths, i.e., a decrease in the gap value from 2.45 eV to 2.38 eV. The optical band edges of the films were not constant but depended on the annealing temperature. The refractive index, calculated by applying the envelope method on the transmission of the films in wavelengths from 550 nm to 850 nm, was in the range 1.95-2.26. The resistivity determined from dark conductivity measurement, as a function of the annealing temperature, was found to be in the order of 10⁵ Ω cm for samples annealed in air at 250 °C, 3 h, and the activation energy was about 0.22 eV.     

Influence of the molar concentration and substrate temperature on fluorine-doped zinc oxide thin films chemically sprayed

The effect of both the molar concentration of the starting solution and the substrate temperature on the electrical, morphological, structural and optical properties of chemically sprayed fluorine-doped zinc oxide (ZnO:F) thin films deposited on glass substrates is analyzed in this work. All the starting solutions employed were aged for 10 days before the deposition. The results show that as the molar concentration increases, a decrease in the electrical resistivity values is obtained, reaching the minimum resistivity in films ZnO:F deposited from a 0.4 M solution at 500 °C. A further increase in the molar concentration leads to a very slight increase in the resistivity. On the other hand, as the substrate temperature is increased, the resistivity decreases and a tendency towards to minimum value is evidenced; taking the molar concentration as parameter, minimum values are reached at 500 °C. The obtaining of ZnO:F thin films, with a resistivity as low as 7.8 × 10^− 3 Ω cm (sheet resistance of 130 Ω/□ and film thickness of 600 nm) measured in as-deposited films is reported here for the first time. The concurrent effect of the high molar concentration of the starting solution, the substrate temperature values used, and the ageing of the starting solution, which might cause polymerization of the zinc ions with the fluorine species, enhance the electrical properties. The structure of the films is polycrystalline, with a (002) preferential growth. Molar concentration rules the surface morphology as at low concentration an hexagonal and porous structure is developed changing to a uniform compact and small grain size surface in the films deposited with the high molar concentrations.