Structural, compositional, and optical properties of electrochemically deposited stoichiometric CdSe thin films from non-aqueous bath

In the recent years, cadmium chalcogenide compounds have been extensively investigated because of their potential applications in solar energy conversion. Thin films of CdSe have been deposited onto the stainless steel and fluorine doped tin oxide (FTO) coated glass substrates using simple and inexpensive electrodeposition technique. The non-aqueous solvent ethylene glycol (CH₂OHCH₂OH) containing precursors of Cd and Se with ethylenediaminetetraacetic acid (EDTA) tetra sodium salt as a complexing agent is used to obtain stoichiometric deposits. Deposition potential was estimated from polarization curves and other preparative parameters such as bath temperature and concentration of solution were optimized. X-ray diffraction (XRD) analysis reveals that the films are polycrystalline with cubic structure. PEC study shows the CdSe films are photoactive. Optical absorption study shows the presence of direct transition with band gap energy 1.72 eV. The energy dispersive analysis by X-rays (EDAX) reveals that the substrate is well covered with large number of grains indicating compact structure. The average ratio of the atomic percentage of Cd:Se is 50.31:49.69, showing that the deposited films are almost stoichiometric.     

Photoelectrochemical properties of spray deposited n-CdSe thin films

Polycrystalline cadmium selenide (CdSe) thin films have been prepared by spraying a mixture of an equimolar aqueous solutions of cadmium chloride and selenourea on preheated fluorine doped tin oxide (FTO) coated glass substrates at different substrate temperatures. The cell configuration n-CdSe/1 M (NaOH + Na₂S + S)/C is used for studying the capacitance-voltage (C-V) characteristics in dark, current-voltage (I-V) characteristics in dark and under illumination, photovoltaic power output and spectral response characteristics of the as deposited films. Photoelectrochemical study shows that as deposited CdSe thin films exhibits n-type of conductivity. The spectral response characteristics of the films at room temperature show a prominent sharp peak at 725 nm. The measured values of efficiency (η) and fill factor (FF) are found to be 0.50% and 0.44 respectively for film deposited at 300 °C. Electrochemical impedance spectroscopy studies show that the CdSe film deposited at 300 °C shows better performance in PEC cell.     

Photoelectrochemical behavior of chemically deposited CdSe and coupled CdS/CdSe semiconductor films

Photoelectrochemical effects at chemically deposited CdSe thin films (2000 Å) coupled with as-prepared and air annealed (250°C) CdS films have been investigated by monitoring open-circuit voltage (V_oc) and short-circuit current density (I_sc) at varying incident light intensities and for different heat-treatments temperatures. Two consecutive chemical baths were used in the coupled system. Each bath has been optimized in earlier studies for the deposition of highly photosensitive CdS and CdSe thin films. The photoelectrochemical behavior of single and coupled films was investigated in ferricyanide redox couples. The enhanced short-circuit photocurrent of the as-deposited CdS/CdSe system, despite their lower photosensitivity, indicated that charge separation improved in the coupled system. The role of post-deposition thermal treatments in improving the photoelectrochemical cell characteristics and stability of coupled semiconductors was investigated. Excellent I–V properties were obtained for CdSe and CdS₂₅₀/CdSe photoelectrodes annealed at 280°C. For the coupled system: V_oc=960 mV; I_sc=8.6 mA/cm²; fill factor (ff)=0.53 and cell efficiency (η)=4.2%. The linearity of V_oc/ln(I_L) and I_sc/I_L plots supports the Schottky–Mott model for these interfaces. The stability of the coupled photoanode is superior to that of the CdSe only-film for the initial 3 h.     

Chemically and electrochemically deposited thin films for solar energy materials

Direct energy gap materials, e.g. CdTe, CuInSe₂, CuInGaSe₂, CdSe, ZnP₂ and Zn₃P₂, are the most interesting for thin-film solar cell applications. Among the various methods of preparation of these films, chemical bath deposition and electrodeposition deserve special attention because they have been shown to be inexpensive, low-temperature and non-polluting methods. Based on Pourbaix diagrams of CdS, CdTe, CuInSe₂, CdSe, etc., drawn from basic considerations, the best parameters for their electrodeposition are deduced. Theoretical considerations on the chemical-bath deposition of CdS, CdSe and Sb₂S₃ are also indicated. In particular, the role of the complexing agent and of the ligands in chemical bath deposition quality is discussed, as are the uniformity and stability of the films. The photoelectrochemical, Schottky barrier and heterojunction solar cell properties based on chemically and electrochemically deposited thin films with heteropolyacids are shown. Future trends for chemically and electrochemically deposited polycrystalline thin films are addressed. Results from very recent work done in the improvement of chemically and electrochemically deposited thin films are presented. Significant results obtained on advanced CdS/CdTe, CdS/CIS and CdS/CIGS solar cells developed by industry and by laboratory groups worldwide are indicated. Emerging low cost materials or/and less environmental hazards materials which may introduce solar cells into worldwide market are considered in the conclusion.     

Dielectric and conduction studies on hot-wall deposited CdSe films

CdSe films are deposited using hot-wall deposition technique on glass and ITO substrates. From the XRD analysis, the structural parameters like crystallite size, dislocation density and strain were calculated. Films had preferential orientation along (0 0 2) and the structure of the film corresponded to wurtzite nature. From the EDAX analysis a slight increase in the cadmium content is observed as thickness increases. The dielectric study has been carried out on the stoichiometric films at different frequencies and temperatures to study their effect on capacitance, dielectric constant and dielectric loss. To explore the effect of illumination on these fundamental dielectric parameters, measurements are taken in dark as well as under an illumination of 1000 lx. and observed slight variation in these parameters. The temperature coefficient of capacitance, relative permittivity and linear expansion coefficient are evaluated. From the AC conduction studies the conduction was found to be due to hopping. Variation of conductivity with temperature reveals the presence of two activation energies. The Mott–Schottky plot for the films yields the value for carrier concentration in the range 10¹⁷–10¹⁸ cm⁻³ and the conduction was found to be n-type.     

Photoelectrochemical cells based on CdSe films brush plated on high-temperature substrates

CdSe thin films were brush plated on substrates maintained at temperatures in the range 30–90 °C from the precursors. The films exhibited hexagonal crystal structure. Optical band gap of 1.65 eV was obtained. XPS measurements indicated the formation of CdSe. Capacitance–voltage measurements indicated the films to exhibit n-type behaviour. A carrier density of 10¹⁷ cm⁻³ was obtained. Photoelectrochemical cells have exhibited higher efficiency compared to earlier reports on brush-plated films. Spectral response measurements indicated a peak quantum efficiency of 75% at 1.65 eV.     

Photoelectrochemical studies of chemically deposited nanocrystalline p-type HgS thin films

Nanocrystalline mercury sulfide (HgS) thin films were deposited by chemical bath deposition (CBD) method onto the glass and fluorine doped tin oxide (FTO) coated glass substrate from an aqueous alkaline bath (pH  8) at room temperature (300 K). Mercuric acetate and thiourea were used as Hg²⁺ and S²⁻ ion sources, respectively. The photoelectrochemical (PEC) studies of HgS films were carried out, and the nanocrystalline films were found to be photoactive in polyiodide solution. The PEC cell configuration was p-HgS/0.1 M (KOH–KI–I₂)/C. From the current–voltage (I–V) characteristics, it is concluded that the HgS films are of p-type electrical conductivity. The photovoltaic output characteristics were used to calculate the fill factor (ff) and solar conversion efficiency (η). The low value of η may be due to the high value of series resistance (R_s) and interface states in the cell, which are responsible for the recombination mechanism.     

The effect of heteropolyacids and isopolyacids on the properties of chemically bath deposited CdS thin films

The deposition of CdS films on ITO/glass substrates from a chemical bath containing cadmium acetate, ammonia, ammonium acetate and thiourea has been carried out with and without small amounts of heteropolyacids (HPA) (phosphotungstic acid (PTA): H₃[PW₁₂O₄₀], silicotungstic acid (STA): H₄[SiW₁₂O₄₀], phosphomolybdic acid (PMA): H₃[PMo₁₂O₄₀]) and isopolyacids (IPA) (tungstic acid (TA): H₂WO₄ and molybdic acid (MA): H₂MoO₄) for different deposition times. The chemical, morphological, structural and optical properties of the films have been determined. The composition in sulphur and in cadmium of the films’ surface and volume was determined for various HPA and IPA used in the deposition bath. The HPA and IPA which give the thickest film with the biggest grain size were deduced. The optical transmission at 400 nm of CdS films deposited with STA at short time (20 min) (50%) is higher than those of CdS deposited at longer time (6 h) (7%). The optical transmission of CdS deposited with STA at short time is higher (50%) than that of CdS deposited without STA (20%). The performances of heterojunctions CdS/CdTe solar cells fabricated from CdS films deposited with and without STA and CdTe films deposited without STA have been determined. It was shown that the CdS/CdTe heterojunction solar cells fabricated from CdS films deposited with STA exhibited better photon collection efficiency and solar cell efficiency (η=6%) than CdS/CdTe heterojunction solar cells fabricated from CdS films deposited without STA (η=3.3%).     

Structural and optical properties of hot wall deposited CdSe thin films

Cadmium selenide (CdSe) films were prepared by hot wall deposition technique using optimized tube length under a vacuum of 6 mPa on to well-cleaned glass and ITO substrates. The X-ray diffraction analysis revealed that the films are polycrystalline in nature for lower thickness and at lower substrate temperatures, but with increasing thickness and increasing substrate temperature a more preferred orientation along (0 0 2) direction was observed. The crystallite size (D), dislocation density (δ) and strain () were calculated. An analysis of optical measurements revealed a sharp absorption around 700 nm and a direct allowed transition. The band gap was found to be around 1.7 eV. The effect of thickness and substrate temperature on the fundamental optical parameters like band gap, refractive index and extinction coefficient are studied.     

Electrochemical investigations on spray deposited tin oxide thin films

Tin oxide (SnO₂) thin films were prepared by a simple and inexpensive spray pyrolysis technique from an aqueous solution at various substrate temperatures viz. 300, 400 and 500 °C, and their electrochemical studies have been carried out. The thin films have been optically and electrochemically characterized by means of transmittance, cyclic voltammetry and chronoamperometry. The mechanism of reduction and oxidation reactions that took place during the potential cycling is presented. The samples deposited at 500 °C exhibit better performance in terms of coloration efficiency, reversibility, contrast ratio and response time.     

Photoelectrochemical properties of spray deposited n-ZnIn2Se4 thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been prepared by spraying a mixture of an equimolar aqueous solution of zinc sulphate (ZnSO₄), indium trichloride (InCl₃), and selenourea (CH₄N₂Se), onto preheated fluorine-doped tin oxide (FTO)-coated glass substrates at optimized conditions of substrate temperature and a solution concentration. The photoelectrochemical (PEC) cell configuration of n-ZnIn₂Se₄/1 M (NaOH+Na₂S+S)/C has been used for studying the current voltage (I–V), spectral response, and capacitance voltage (C–V) characteristics of the films. The PEC study shows that the ZnIn₂Se₄ thin films exhibited n-type conductivity. The junction quality factor in dark (n_d) and light (n_l), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The measured (FF) and η of the cell are, respectively, found to be 0.435% and 1.47%.     

Photoelectrochemical performances of n-CdS1−xSex thin films prepared by spray pyrolysis technique

The CdS_1−xSe_x (0.0 ? x ? 1.0) thin films of various compositions have been deposited onto the amorphous and FTO coated glass substrates using a spray pyrolysis technique. An electrode/electrolyte interface has been formed between an n-type CdS_1−xSe_x (0.0 ? x ? 1.0) alloyed/mixed type semiconductor and a sulphide/polysulphide redox electrolyte and investigated through the current-voltage, capacitance-voltage and photovoltaic power output characteristics. The dependence of the dark current through the junction and the junction capacitance on the voltage across the junction have been examined and analysed. Upon illumination of the interface with a light of 20 mW/cm², an open-circuit voltage of the order of 335 mV and a short-circuit current of 1.02 mA/cm² have been developed (for x = 0.8), which results in energy conversion efficiency and fill factor 0.79% and 0.46% respectively. The magnitudes of the barrier heights at the interfaces have been determined. The significant electrochemical properties have been observed for a cell with electrode composition x = 0.8.     

Preparation and properties of thin Pt-Ir films deposited by dc magnetron co-sputtering

A series of Pt-Ir thin films envisaged for application as fuel cell cathodic catalysts are deposited by dc co-sputtering from pure metal targets. To achieve different metal ratios, the sputtering power applied on the iridium target (P_Ir) is varied in the range 0-100 W at constant power of the Pt target (P_Pt). The influence of the sputtering power on the film composition, morphology, and surface structure is analysed by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The catalytic activity towards oxygen reduction reaction (ORR) is evaluated in sulphuric acid solutions applying the methods of cyclic voltammetry and potentiodynamic polarization curves. The performed morphological and electrochemical investigations reveal that catalytic efficiency of the co-sputtered Pt-Ir films is superior compared to pure Pt. The ORR is most intensive on the sample deposited at power ratio P_Pt:P_Ir = 100:30 W containing 11 at.% Ir that has also the most developed active surface. The ORR current density for this film achieved at 0.825 V in acid solution (4.1 mA cm⁻²) is about 6 times higher than for pure Pt (0.67 mA cm⁻²). The improved activity of the thin co-sputtered Pt-Ir over Pt allows for essential reduction of the catalyst loading at preserved performance.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.     

Physical properties of chemical bath deposited CdS thin films

Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium chloride, ammonium chloride, ammonium hydroxide and thiourea. The XRD patterns show that the films have a hexagonal phase with a preferential (0 0 2) orientation. The photoluminescence spectra show a defect structure, characteristics of the CdS films obtained by CBD. The electrical behavior in dark and under illumination, the optical properties and the band gap value reported in this work is in agreement with that reported in the literature.     

Electrodeposition and characterization of CdTe thin films on Mo foils using a two voltage technique

Polycrystalline thin film CdTe-based solar cells are one of the most promising candidates for low-cost terrestrial conversion of solar energy because of the optimum energy band gap (E_g=1.44eV) and high absorption coefficient of CdTe. In this work, a two-step electrodeposition technique has been used to prepare CdTe thin films from acidic solutions. In the first step, a thin Te rich CdTe layer was deposited at –300 mV (SCE) on a Mo foil substrate. On top of this film, a Cd rich CdTe layer was deposited at more negative voltages. The resulting films showed good adherence to the substrate and very low contact resistance between the substrate and the p+ Te rich CdTe layer. The composite film was of nearly stoichiometric composition. From X-ray diffraction results, the as-deposited films show very small grain sizes but after annealing the grain size increases considerably showing very well-defined peaks. The morphological, structural and composition results of CdTe thin films obtained by Scanning Electron microscopy, X-ray diffraction, and X-ray fluorescence will be presented. Electrical properties such as conductivity type and contact resistance values for the Mo/CdTe structures will also be presented.     

Raman and XPS characterization of vanadium oxide thin films deposited by reactive RF sputtering

In this paper we report on Raman and XPS characterization of vanadium oxide thin films deposited by RF-sputtering. The samples were deposited by using a vanadium target in different oxygen fluxes, so that the stoichiometry (O/V ratio) of the oxide was varied. Several physical parameters of the films indicate a strong structural difference between the sample deposited at lower oxygen flux (1 scc m) and those obtained with higher flux (from 1.25 to 9 scc m). The increase of O/V ratio corresponds to a lower crystallinity of the thin films as indicated by the initial lowering and the final disappearance of the characteristic Raman mode of V₂O₅ (crystal) at about 140 cm⁻¹. For the highest flux samples new broad bands develop, typical of amorphous materials, both in polarized as well as in depolarized Raman spectra.     

Influence of pulse reversal on the PEC performance of pulse-plated CdSe films

This paper embodies the first report on the pulse reversal deposition of CdSe thin films. The as-deposited and heat-treated films were characterised by XRD, optical absorption spectroscopy and electrical properties. The polycrystalline deposits of CdSe obtained indicated a hexagonal structure after heat treatment at 550°C. From the optical absorption measurements the band gap was found to be 1.71 eV. At an illumination of 70 mW cm⁻² conversion efficiencies of 3.20% and 6.57% were obtained for the photoelectrodes without and with pulse reversal.     

Characterization and application of self-assembled thin films of polyelectrolytes/TiO2/CdSe for hydrogen production

Photocatalysis of water to produce hydrogen gas over titanium dioxide or other semiconductor films, known as water splitting, is a promising alternative using solar energy to obtain a clean fuel. Self-assembled thin films (SATFs) from the physical adsorption of polyelectrolytes and inorganic semiconductor nanoparticles are created by an inexpensive and non-polluting process that gives films with high molecular organization. The aim of this work was to fabricate and characterize SATFs via the layer-by-layer technique using the polyelectrolytes polyallylamine hydrochloride (PAH), poly(acrylic acid) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in combination with titanium dioxide and CdSe nanoparticles and evaluate the application potential of these systems to produce hydrogen gas by solar radiation. The characterization of the SATFs showed that films with positive surface charge present favourable conditions for incorporation of negatively charged CdSe nanoparticles and that alkaline condition favour agglomeration of TiO₂ nanoparticles. The best system was composed of (PAH + CdSe) and (PEDOT:PSS + TiO₂ 100% anatase) in alkaline medium. With a hydrogen gas average production rate of 0.350 μmol h^?1 cm^?2, the maximum number of layers was optimized at 120 layers, beyond which there was a decrease in photocatalytic activity, reducing the average production rate to 0.07 with the SATF of 160 layers. Moreover, the presence of CdSe increased the hydrogen gas production by 75% when compared to the film containing only titanium dioxide.     

Complexing agent effect on the stoichiometric ratio of the electrochemically prepared CuInSe2 thin films

The electrodeposition of CuInSe₂ is investigated to improve the stoichiometric properties of CuInSe₂ layers on indium tin oxide (ITO)-coated glass substrates and to develop one-step electrodeposition method for solar cell applications. XPS was utilized for the characterization of the surface properties of CuInSe₂ layers. The influence of the complexing agent, e.g. benzotriazole, bulk concentration of Cu and Se and deposition potentials on the stoichiometric properties, are discussed.