Photoelectrochemical performance of Cu-doped ZnIn2S4 electrodes created using chemical bath deposition

A solution growth method for the deposition of Cu-doped zinc-indium-sulfide (ZnIn₂S₄) semiconductor film electrodes is presented. The structural, optical, and photoelectrochemical (PEC) properties of samples were studied as a function of Cu content in samples. The X-ray diffraction pattern of the cubic ZnIn₂S₄ phase of an undoped sample was obtained. No Cu alloys or other binary compounds that included the Cu element were present in Cu-doped ZnIn₂S₄ samples. Images from a scanning electron microscope and atomic ratios of elements in samples obtained from the energy dispersion analysis of X-ray reveal a change in surface morphology and composition for Cu-doped ZnIn₂S₄ samples. The direct energy band gaps, indirect energy band gaps, and thicknesses of samples prepared in this study varied in the ranges 2.07-2.58 eV, 1.60-2.06 eV, and 521-879 nm, respectively. The maximum photoelectrochemical response of samples in 0.5 M K₂SO₄ aqueous solution reached 1.15 mA cm⁻² at an external potential of +1.0 V vs. an Ag/AgCl reference electrode under illumination using a 300-W Xe lamp with light intensity kept at 100 mW cm⁻². The experimental results show that Cu doping with Cu/(Cu+Zn) atomic ratio of 0.08 in samples improves the performance of the ZnIn₂S₄ photoabsorber for PEC applications.     

Influence of chelating agents on the growth and photoelectrochemical responses of chemical bath-synthesized AgIn5S8 film electrodes

An aqueous method for the deposition of silver-indium-sulfide ternary semiconductor film electrodes is presented. Various deposition parameters, such as reaction temperature and molar ratios of different chelating agents, were changed in order to grow uniform and adherent films on indium-tin-oxide glass substrates. With a reaction temperature higher than 65 °C, a film composed of AgIn₅S₈ was grown on the substrates in our experimental conditions. The direct and indirect energy band gaps of samples prepared in this study varied from 1.70 to 1.97 eV and 1.61 to 1.72 eV, respectively. The maximum photocurrent density of samples in this study reached 3.0 mA/cm² at an external potential of +1.0 V vs. a Pt electrode under illumination using a 300 W Xe lamp system with the light intensity set at 100 mW/cm².     

The preparation and characterization of CdS1−xTex semiconductor films for hydrogen production by the chemical bath deposition method

Te-doped CdS semiconductor films were fabricated on indium-tin-oxide (ITO) substrates by chemical bath deposition. The chemical composition, morphology, crystalline, and optical properties of the Te-doped CdS films were characterized by XPS, SEM, XRD, UV-vis, and Ellipsometer. Additionally, the carrier density and flat-band potential of the semiconductor electrodes were measured by Hall and potentiostat. Results show that the electrical property of Te-doped CdS films was changed from n-type to p-type semiconductors, when the molar ratios of Te in the bath solution were higher than 0.4. Besides, energy band-gap and carrier densities of the Te-doped samples were found in the range of 1.85-2.33 eV and 9.68×10¹⁵-1.56×10¹⁷ cm⁻³, respectively. Furthermore, the maximum photocurrent density of the samples was found to be −0.81 mA/cm² (under the external potential of −1.0 V) with the largest hydrogen production capability of 1.29 ml/cm², when illuminated under a 150 W Xe lamp.     

Surface modification of ZnO microrod arrays films by ion-exchange approach and their photoelectrochemical performances

ZnO microrod arrays films with the surface modification by two steps ion-exchange approach have been investigated as photoanodes in photoelectrochemical (PEC) cells. X-ray diffraction, Raman, scanning electron microscope, energy dispersive X-ray detector, UV–vis techniques and PEC measurement have been used in the pristine and surface modified ZnO microrod films. The results show that ZnS and CdS layer can be deposited on ZnO microrod surface through a two steps ion-exchange procedure. What's more, it is found that ion-exchange method is a simple approach to adjust CdS content on the samples surface via changing experimental temperature. Consequently, the PEC property of films can be improved through optimizing CdS content on the ZnO microrods surface. In this experiment, it is found that the optimized condition for preparing film is 70 °C (first step) and 100 °C (second step). These results suggest that surface tuning via ion-exchange method should represent a viable strategy to further improve the efficiency of ZnO microrods photoanodes.     

Photoelectrochemical performances of AgInS2 film electrodes fabricated using the sulfurization of Ag-In metal precursors

Ternary silver-indium-sulfide samples were deposited on various substrates using the sulfurization of Ag-In metal precursors. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in metal precursors on the structural, morphological, and photoelectrochemical properties of the samples was examined. X-ray diffraction patterns of samples show that the films are in the polycrystalline AgInS₂ phase. The thickness and direct band gap of the films were in the ranges of 1.1-1.2 μm and 1.92-1.94 eV, respectively. The conduction type of all samples was n-type. The carrier concentration, mobility, and resistivity of samples were in the ranges of 1.5×10¹³-7.0×10¹³ cm⁻³, 2.6-14.8 cm²V⁻¹s⁻¹, and 2.6×10⁴-3.5×10⁴ Ωcm, respectively. It was found that the samples with an [Ag]/[In] molar ratio of 0.89 in Ag-In metal precursors had a maximum photo-enhancement current density of 2.43 mAcm⁻² at an applied bias of +0.5 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.5 M K₂SO₄. The results show that high-quality AgInS₂ films can be obtained using the sulfurization of Ag-In metal precursors for photoelectrochemical (PEC) applications.     

Photoelectrochemical studies on polycrystalline CdS (chemical bath deposition) and CdSe (chemical bath and electro deposition) thin film electrodes

The photoelectrochemical behaviour of polycrystalline CdS and CdSe thin film electrodes prepared by a chemical bath deposition (CBD) method has been examined in an aqueous sulfide—polysulphide electrolyte. The open circuit voltage, short circuit current, flat band potentials, and efficiencies are reported. The CBD technique is simple and promising for the preparation of polycrystalline metal chalcogenides. The behaviour of CdSe prepared by electrodeposition has also been examined.     

Electrodeposition and characterization of ZnSe semiconductor thin films

In this work, results on the preparation and characterization of ZnSe thin films obtained by electrodeposition are presented. Voltammetric curves were recorded in order to characterize the electrochemical behavior of the Zn⁺²/SeO₂ system on different substrates. Thin films were deposited potentiostatically from an unstirred, deareated aqueous solution onto titanium, glass substrates coated with fluorine doped tin oxide and ITO glass substrates. The effect of main parameters such as the deposition potential, SeO₂ concentration and annealing on film composition and structure were analyzed. The as-grown and treated layers were characterized by X-ray energy dispersive analysis, X-ray diffraction, scanning electron microscopy and photoelectrochemical studies. Optical measurements were done on these samples which gave a clear band edge near 2.6 eV quite close to the accepted room temperature value of 2.7 eV for ZnSe.     

Semiconductor thin films by chemical bath deposition for solar energy related applications

In this paper we present the basic concepts underlying the chemical bath deposition technique and the recipes developed in our laboratory during the past ten years for the deposition of good-quality thin films of CdS, CdSe, ZnS, ZnSe, PbS, SnS, Bi₂S₃, Bi₂Se₃, Sb₂S₃, CuS, CuSe, etc. Typical growth curves, and optical and electrical properties of these films are presented. The effect of annealing the films in air on their structure and composition and on the electrical properties is notable: CdS and ZnS films become conductive through a partial conversion to oxide phase; CdSe becomes photosensitive, SnS converts to SnO₂, etc. The use of precipitates formed during deposition for screen printing and sintering, in polymer composites and as a source for vapor-phase deposition is presented. Some examples of the application of the films in solar energy related work are presented.     

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.     

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 properties of hematite films grown by plasma enhanced chemical vapor deposition

Nanostructured α-Fe₂O₃ thin films were grown by plasma-enhanced chemical vapor deposition (PE-CVD) using iron pentacarbonyl (Fe(CO)₅) as precursor. Influence of the plasma parameters on photoelectrochemical (PEC) properties of the resulting hematite thin films toward solar oxidation of water was investigated under one sun illumination in a basic (1 M NaOH) electrolyte. PEC data analyzed in conjunction with the data obtained by scanning electron microscopy, X-ray diffraction and Mott–Schottky analysis showed 100 W plasma power to be an optimal RF-power value for achieving a high photocurrent density of ∼1098 μA/cm² at 0.9 V/SCE external applied potential. The donor density, flat band potential, grain size and porosity of the films were observed to be highly affected by RF-power, which in turn resulted in enhanced photoresponse.     

A suitable deposition method of CdS for high performance CdS-sensitized ZnO electrodes: Sequential chemical bath deposition

A suitable deposition method of CdS is necessary for the high performance CdS-sensitized ZnO electrodes. In this paper, chemical bath deposition (CBD) and sequential chemical bath deposition (S-CBD) methods were used to deposit CdS on ZnO mesoporous films for ZnO/CdS electrodes. The analysis results of XRD patterns and UV-vis spectroscopy indicated that CBD deposition method leaded to the dissolving of ZnO mesoporous films in deposition solution and thickness reduction of ZnO/CdS electrodes. Absorption in visible region by the ZnO/CdS electrodes with CdS deposition by S-CBD was enhanced as deposition cycles increased due to the stability of ZnO mesoporous films in the S-CBD deposition solutions. The results of photocurrent-voltage (I-V) measurement showed that the performance of ZnO/CdS electrodes with CdS deposition by CBD first increased and then decreased as deposition time increased, and the greatest short-circuit current (J_sc) was obtained at the deposition time of 4 min. The performance of ZnO/CdS electrodes with CdS deposition by S-CBD increased as deposition cycles increased, and both open-circuit voltage (V_oc) and J_sc were greater than those electrodes with CdS deposition by CBD when the deposition cycles of S-CBD were 10 or greater. These results indicated that S-CBD is a more suitable method for high performance ZnO/CdS electrodes.     

Conductivity controlling of Cu2O film photoelectrode for water splitting by a novel electrochemical approach - Differential potentiostatic deposition

Cuprous oxide (Cu₂O) is a kind of low-cost and promising material for water splitting to produce hydrogen (p-type Cu₂O) and oxygen (n-type Cu₂O). However, the reason of conductivity transforming from p-type to n-type for Cu₂O films during potentiostatic deposition is waiting to be revealed. In this work, a novel electrochemical technology, differential potentiostatic deposition (DPD), is developed by coupling a 3-electrode setup with a resistor connected in series with the counter electrode circuit through a potentiostat. By this approach, deposition current density is adjusted in a short period to simulate different stages in a traditional potentiostatic deposition (TPD). The result shows that semiconducting conductivity of Cu₂O film changes from p-type to n-type with time during a long-term TPD in basic CuSO₄ solution. Employing the DPD method, conductivity of Cu₂O film transforms from p-type to n-type with current density decreasing. Through characterizing thickness, composition and photoelectrochemical performance of Cu₂O films, the mechanism of semiconducting conductivity transformation for Cu₂O films is proposed. Besides, the results indicate that the DPD is an effective method to tune the conductivity of metal oxide photoelectrodes for water splitting.     

PbS nanocrystal solar cells fabricated using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cell was fabricated using microwave-assisted Chemical Bath Deposition (CBD). The CdS window layer (340 nm thickness) was deposited on ITO-glass. The PbS absorber layer (685–1250 nm thickness) with different molar concentration (0.02, 0.05, 0.075 and 0.1 M) was then grown on ITO/CdS to fabricate the p–n junction. X-ray diffraction analysis confirms the formation of pure and nanocrystalline CdS and PbS phases with a preferred orientation depending on molarity; (111) or (200). Scanning electron microscopy observations show a uniform surface morphology with gatherings. UV–Vis spectrophotometer and FTIR was used to estimate the optical properties. Optical measurements gave an energy gap of 2.6 eV for CdS whereas that for PbS thin films were found to vary in a narrow range 0.40–0.47 eV, depending on the molar concentration. The photovoltaic properties under 30 mW/cm² solar radiation including J–V characteristics, short-circuit current (I_sc), open-circuit voltage (V_oc), fill factor (ff), efficiency (η) of CdS/PbS heterojunction cells have been as well examined. The results show that changing the molar concentration improved the performances of the fabricated photovoltaic cells; a high efficiency was observed at 0.1 M. However, high series resistance and poor crystallinity of PbS lead to low efficiency at lower molarity.     

Electrodeposition of ZnO thin films by using molecular oxygen and hydrogen peroxide as oxygen precursors: Structural and optical properties

Zinc oxide thin films were potentiostatically electrodeposited from a ZnCl₂+LiCl bath using two different oxygen precursors: molecular oxygen and hydrogen peroxide. X-ray diffraction (XRD) studies confirmed the presence of the ZnO wurtzite structure with marked preferential orientation along the (0 0 2) axis. The optical transmittance shows a clear absorption edge in the ultraviolet (UV) region which corresponds to an energy band gap of 3.41±0.03 eV. As a general rule the higher band gap energies are related to the more transparent films.     

Electrochromic properties of porous NiO thin films prepared by a chemical bath deposition

Highly porous nickel oxide thin films were prepared on ITO glass by a simple chemical bath deposition (CBD) method in combination with a following heat-treatment process. XRD analysis revealed that the as-deposited precursor film contained β-Ni(OH)₂ and γ-NiOOH, and they changed to cubic polycrystalline NiO after annealing. The FTIR results showed presence of free hydroxyl ion and water in the NiO thin films. The electrochromic properties of NiO thin films were investigated in an aqueous alkaline electrolyte (1 M KOH) by means of transmittance, cyclic voltammetry (CV) and chronoamperometry (CA) measurements. The NiO thin film annealed at 300 °C exhibited a noticeable electrochromism and good memory effect. The coloration efficiency was calculated to be 42 cm² C⁻¹ at 550 nm, with a variation of transmittance up to 82%. The porous NiO thin films also showed good reaction kinetics with fast switching speed, and the coloration and bleaching time were 8 and 10 s, respectively.     

Photoelectrochemistry of semiconductor electrodes made of solid solutions in the system Fe2O3–Nb2O5

Ceramic semiconductor photoelectrodes made from solid solutions in the system Fe₂O₃–Nb₂O₅ were synthesized. The spectral and capacitance–voltage characteristics of the photoelectrodes were determined, and the dynamic polarization with chopped light was investigated. The anodic photocurrent onset potential, the flat band potential and the shallow and deep donor density of these materials were determined. The threshold photon energies corresponding to the inter-band optical transitions near the edge of the fundamental absorption of the semiconductor photoelectrode were calculated. Analysis of the frequency dispersion of the real and imaginary parts of the complex impedance of photoelectrochemical cell was carried out. On the basis of this analysis, equivalent circuits describing the structure of the double electrical layer of the semiconductor–electrolyte interface were proposed and their parameters calculated. The main limiting steps of the electrode processes, which determine the electrode polarization and current, are defined.     

High-performance vertically aligned Bi2O3 nanosheet arrays for water splitting applications by controlling the chemical bath deposition method parameters (precursor concentration and pH)

In this work, vertically aligned β-Bi₂O₃ nanosheet arrays are deposited on FTO using a simple, cost-effective, low-temperature, and easy-tunable technique called chemical bath deposition. Coatings were deposited through selective correlation of varying bismuth ion concentrations at fixed pH and, also, a fixed bismuth ion concentration at different pH values to optimize their structure, morphology, and optical properties. With an increase in bismuth precursor concentration from 0.008 M to 0.5 M, a more crystallized and compact coating with finer nanosheets was formed. Low pH values tended to result in either no coating or a coating composed of discrete particles. As the pH increased to the optimal level, a thicker and more compact coating with a morphology made of thicker and wider nanosheets was formed. Further increase in pH led to a non-uniform coating composed of small and large nanosheets that could not cover the entire surface of the substrate. The optimized photoelectrode exhibited a maximum photocurrent density of 470 μA/cm² at 1.23 V_RHE under 100 mW/cm² simulated sunlight, which is among the top recorded values of Bi₂O₃ photoelectrodes.     

Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

Calculation of the photocurrent-potential characteristic for regenerative, sensitized semiconductor electrodes

The steady state anodic photocurrent for sensitized semiconductor electrodes, where the sensitizer is regenerated by a redox electrolyte, has been modeled taking into account the rate of light absorption by the sensitizer S, the rate of electron injection from the excited state S^* of the sensitizer to the conduction band of the semiconductor, the rate of decay of S^* (radiative and non-radiative) and the rate of reductive regeneration of the sensitizer by the redox electrolyte. In this model the rate of recombination between the conduction band electron and the oxidized sensitizer, S⁺, and the reactions between S^* and the redox couple have been assumed to be negligible. The rate constant for injection, k_inj, the injection efficiency, φ_inj, the photocurrent density, J_P, and the steady state concentrations of S^* and S⁺, have been calculated as a function of light intensity, Helmholtz potential, λ_max and halfwidth, ΔE_0.5, of the sensitizer absorption spectrum, and the semiconductor band gap and electron affinity both for monochromatic light and for AM1.5 sunlight simulated by radiation from a 5600K black body. For the calculation of k_inj as a function of Helmholtz potential, the Gurney -Gerischer-Marcus (GGM) model has been used. Allowance for the distribution of electrode potential between the semiconductor and the electrolyte has been introduced in principle. The steady state concentrations of S^* and S⁺ were used in the Nernst equation to calculate the S^*/S⁺ quasi-reversible potential. It is shown that the short-circuit current density of the cell is a maximum for a sensitizer with a λ_max of about 1550 nm. Inter-relationships between variables involving the sensitizer have been used to show that only four sensitizer parameters are needed when considering the effects of changing the sensitizer. These parameters are the reorganization energies and the standard reduction potentials for the couples S⁺/S^* and S⁺/S. For a related series of sensitizers, such as obtained by changing a substituent group, only the two standard reduction potentials are required to predict the effects of changing the sensitizer.