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.     

Thermal treatment effects in the photovoltaic conversion of spray-painted TiO2 coatings sensitized by chemically deposited CdSe thin films

We have investigated the photovoltaic conversion of spray “painted” (spray deposited) titanium dioxide coatings sensitized with chemically deposited cadmium selenide thin films. The structural, optical and photoelectrochemical characterization of the composite films indicate the importance of thermal treatments in improving the photocurrent quantum yields. Up to 400°C, the effect of air annealing is to shift the onset of absorption to longer wavelengths and to improve the photocurrent substantially. Thermal treatment above 450°C converts the heterosystem into a cadmium titanate with poor photoelectrochemical properties. Due to the thin nature of the CdSe film it is used as sensitizer, the presence of the TiO₂ matrix helps to improve the performance of the chemically deposited films. That is, higher photocurrents and a red shifted spectral response were obtained in the sensitized films, when compared to the spectra of the corresponding CdSe used as sensitizer. This behavior, along with the good photostability of the coatings, is promising for the use of the sensitized film in photocatalytic as well as photovoltaic applications.     

CdSe thin films as solar control coatings

CdSe thin films deposited by a physical vapour deposition method were investigated as solar control coatings on architectural glazings. The optical transmittance and the near-normal specular reflectance in the range 0.40−2.40 μm and spectral distribution of reflected and transmitted intesities in the same range showed that CdSe thin films have solar control characteristics comparable to commercially available metallic coatings and other materials such as PbS and Cu_ξS films. The solar control characteristics of CdSe films were found to be dependent on film parameters, including deposition rate and deposition temperature.     

Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption

CdTe thin films were electrochemically deposited using unipolar current pulses of high magnitude between 2.5 and 30 mA/cm² in an aqueous solution. Parametric study of the effect of periodic current pulse magnitude, average current and ON and OFF duration was undertaken to understand the effect of pulse variables on CdTe film properties. Increasing pulse deposition current modifies crystalline growth phase from single cubic to mixed cubic and hexagonal growth phases. In addition to the modification in CdTe growth phases, there is an increasing tendency of the oxide formation particularly CdTeO₃. Increase in pulse current density or average current yields Cd rich CdTe films. The optical absorption coefficient decreases with the decrease in pulse current density, whereas an increase is observed as the OFF time decreases. The optical energy gap is found to increase with OFF time. A systematic study on the effect of pulse variables over the structure, compositional and optical properties of CdTe film is described.     

Optical,structural and photoelectrochemical properties of CdS1−xSex semiconductor films produced by chemical bath deposition

A series of CdS_1−xSe_x thin films have been deposited on fluorine doped tin oxide (FTO) coated glass substrates by chemical bath deposition. The influences of S/Se ratio in the precursor solution and annealing treatment on the structural, morphological, compositional, optical, and photoelectrochemical properties of the films were investigated. X-ray diffraction patterns revealed that the hexagonal cadmium cyanamide and the solid solutions of CdS_1−xSe_x were formed. The morphological and compositional studies indicated that the thin film was composed of cadmium cyanamide sheets in the upper layer and CdS_1−xSe_x spherical grains in the underlying layer. The optical absorption studies revealed that the band gap of unannealed and annealed films varied from 2.4 eV to 1.94 eV and from 2.35 eV to 1.67 eV as x increased from 0 to 1, respectively. The photo responses well agreed with the optical absorption of these films. The annealed CdSe shows the best photoresponse with a photon-to-current efficiency of 1.69% at 0.27 V (versus SCE).     

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.     

Structural and optical characterization of hot wall deposited CdSexTe1−x films

CdSe_0.3Te_0.7 alloy was prepared from the individual components and its composition and structural analysis were done. Films were prepared by hot wall deposition technique using 0.15 m length tube under a vacuum of 5×10⁻⁵ Torr on well cleaned glass substrates. The composition, structural, morphological, and optical properties of hot wall deposited films were investigated. The XRD analysis revealed that the films are like amorphous in nature for lower thicknesses but with increasing thickness a more preferred orientation along (1 0 1) direction was observed. The crystallite size (D), dislocation density (δ) and strain () were evaluated. From the EDX composition analysis, the individual concentrations of Se and Te in the films were estimated. An analysis of optical measurements shows that all the films have fairly good transparency above 850 nm. The optical band gap was found to be around 1.55 eV and decreases with increasing thickness. Also comparison of band gap with corresponding values for CdSe and CdTe are made.     

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.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

Electrodeposition using non-aqueous solutions at 170 °C and characterisation of CdS, CdSxSe(1−x) and CdSe compounds for use in graded band gap solar cells

CdS, CdS_xSe_(1−x) and CdSe compounds have been grown at 170 °C using electrodeposition from an electrolyte containing ethylene glycol as the solvent. The materials were grown for x=0, 0.22, 0.50, 0.76 and 1.00, and the x values quoted here are obtained from the XRF measurements. The resulting materials were characterized by optical absorption method for determination of band gap variation, and by XRD for bulk structure variation. It has been demonstrated that the band gap could be varied from 1.7 eV for x=0 (CdSe) to 2.4 eV for x=1 (CdS) by varying the parameter x. Bulk structure remains as hexagonal, but the corresponding lattice spacing gradually increases as the smaller S⁻² ions are replaced by larger Se⁻² ions. The photoresponse shown in photoelectrochemical cell demonstrates that all compounds grown are suitable for solar cell applications.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

Characterization of ion diffusion and transient electrochromic performance in PECVD grown tungsten oxide thin films

Electrochromic tungsten oxide thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). Film density and electrochromic performance were controlled by the degree of ion bombardment. A moderate degree of ion bombardment was optimal, and the refractive index was shown to be a sensitive indicator of electrochromic performance. Chronoamperometry in concert with optical transmission was used to determine diffusion and absorption coefficients using both H⁺ and Li⁺ containing electrolytes. The absorption coefficients were similar for both ions, scaling with the degree of intercalation to 50,000 cm⁻¹ in the opaque state. The diffusion coefficients for optimized films were found to be relatively insensitive to the degree of ion intercalation, with values of 10⁻⁹ and 10⁻¹⁰ cm²/s for H⁺ and Li⁺, respectively. These values are about an order of magnitude greater than values reported for vacuum-deposited films, which was attributed to low relative density in the PECVD films. The diffusion and absorption coefficients were incorporated into a model that successfully reproduced transient optical performance.     

Synthesis and characterization of Cd0.7Pb0.3Se thin films for photoelectrochemical solar cell

Optimum composition Cd_0.7Pb_0.3Se thin films have been deposited using the chemical bath containing cadmium sulfate octahydrate, lead nitrate, tartaric acid, potassium hydroxide, ammonia, and sodium selenosulfate onto fluorine-doped tin oxide glass substrate. The various deposition parameters such as composition of reactive bath, pH of the solution, deposition temperature, deposition time, speed of rotation, etc. have been optimized for obtaining good quality film. X-ray diffraction studies revealed the polycrystalline nature of sample with the solid solution of lead (II) ions in CdSe host lattice, having a hexagonal phase structure. Scanning electron micrograph suggested that the grains were non-uniformly distributed over the substrate surface. Film composition was determined by atomic absorption spectroscopy as well as energy dispersive X-ray atomic spectroscopy. Optical absorption data showed the presence of direct transition with energy band gap 1.80 eV for the deposited thin films. The dark specific conductance of Cd_0.7Pb_0.3Se thin films was found to the order of 10⁻⁶ (Ω cm)⁻¹ having n-type semiconducting nature. Photoelectrochemical characterization was carried out using sulfide/polysulfide electrolyte with 1.401% efficiency.     

NCDP-plated CdSe film PEC cell studies

Thin films of CdSe grown by non-catalytic displacement plating (NCDP) are characterised electrochemically and photoelectrochemically in polysulphide electrolyte; using Cyclic Voltametry in dark and under intermittent illumination, band edges of NCDP-plated CdSe are located. A depletion width of 2 μm is calculated across the CdSeS²⁻/S₂²⁻ interface. Charge-transfer mechanism across this junction is discussed. Electrochemical and photoelectrochemical stabilities of NCDP-plated CdSe electrode are studied. Efficiency and fill factor of as-fabricated photoelectrochemical (PEC) cell are found to be 1.02% and 0.34, respectively.     

Photoelectrochemical studies on galvanostatically formed multiple band gap materials based on CdSe and ZnSe

The preparation of some multiple band gap semiconductor films based on CdSe and ZnSe has been carried out using galvanostatic electrochemical codeposition technique to investigate their photoelectrochemical characteristics on the basis of photoelectroconvertibility and photoaction spectral studies using I₂/I₃⁻ and [Fe (CN) ₆]³⁻/[Fe (CN) ₆]⁴⁻ redox couples. These composite systems show substantially improved photoelectrochemical properties compared to the constituent CdSe and ZnSe films prepared under comparable experimental conditions. These multiple band gap films were also found to exhibit enhanced resistance towards electrochemical corrosion.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

Facile fabrication of CdSe/CuInS2 microflowers with efficient photocatalytic hydrogen production activity

Photocatalytic water splitting to produce hydrogen has attracted extensive attention and exhibited broad development prospects. In this work, CuInS₂ microflowers were fabricated through the solvothermal method, and decorated with CdSe quantum dots on the surface. As-prepared CdSe/CuInS₂ microflowers exhibited high photocatalytic hydrogen production activity (10610.37 μmol g^?1 h^?1) and high AQE of 48.97% at 420 nm. The enhanced photocatalytic hydrogen production activity owing to the construction of p-n heterostructure improved light absorption ability, increased electrons transfer efficiency and reduced recombination of photo-induced electrons and holes. Moreover, high stability and cyclic utilization of CdSe/CuInS₂ microflowers were beneficial to photocatalytic hydrogen production application.     

Photocurrent enhancement of interlocked LB film dye layers in a p-CuSCN sensitised photoelectrochemical cell

Dye sensitised photoelectrochemical (PEC) cells based on Cu/p-CuSCN/LB films have been studied with mixed Langmuir Blodgett (LB) films as the dye layer. The effects of mixed layers were investigated in detail by observing the changes of optical absorption and photocurrent in a PEC cell configuration. Enhancements in both optical absorption and photocurrent were found when a mixture of octadecyl methylviolet–C₁₈ (M–C₁₈) and dioctadecyl rhodamine (C₁₈–R–C₁₈) were deposited using the LB technique on p-CuSCN wide band gap semiconductor. The maximum photocurrent quantum efficiency of the PEC cell reached ≈36% in KI (10⁻² M)+I₂ (10⁻⁴ M) electrolyte solution when mixed LB films were used as the dye layer. Photocurrent enhancement is believed to be the enhancement of light absorption of the dye layers due to the interlocking of M–C₁₈ between the double C₁₈ chains of rhodamine.     

Hydrogen absorption by metallic thin films detected by optical transmittance measurements

The hydrogen absorption by bilayers of Pd/Nb and Pd/Ti, grown on glass substrates, was studied by measuring changes in optical transmittance and reflectance in the visible range (wavelengths between 400 nm and 1000 nm) of the films at hydrogen pressures between 3.99 × 10² and 4.65 × 10⁴ Pa. The electrical resistance of the films was also measured during absorption to correlate with the optical data. All the films were grown by a controlled sputtering technique in high vacuum. Pd films ranging in thickness between 4 nm and 45 nm were also characterized when the films were exposed to a hydrogen pressure. The resistance and transmittance of all the Pd samples increased with the uptake of hydrogen until saturation occurred. For Pd/Ti bilayers, fast uptake of hydrogen was deduced from a transmittance increase, indicating hydrogen absorption in the Ti layer. In the case of the Pd/Nb bilayer, a decrease in transmittance was observed, indicating that hydrogen was not absorbed in the Nb layer. The transmittance decrease could be explained by a reduction of Nb native oxide by the hydrogen at the surface.     

CdSe quantum dots sensitized nanoporous hematite for photoelectrochemical generation of hydrogen

A novel system of CdSe quantum dots (QDs) sensitized porous hematite (α-Fe₂O₃) films has been investigated as a potential photoelectrode for hydrogen generation via photoelectrochemical (PEC) splitting of water. Before sensitization, nanoporous hematite thin films were prepared by spray pyrolysis. Characterizations for crystalline phase formation, crystallite size, absorption spectra, and flatband potential were carried out to analyze PEC data. Loading time of sensitizer to hematite thin films was found to be crucial in affecting its PEC properties. Film having sensitizer loading time as 42 h exhibited best photocurrent density of 550 μA cm⁻² at 1.0 V versus SCE. Current study, for the first time, explores the possibility of using low band gap QDs sensitization on a low band gap film, hematite in PEC splitting of water.