Some physical investigations on AgInS2 sprayed thin films

AgInS₂ thin films have been prepared on glass substrates by the spray pyrolysis process using an aqueous solution which contains silver acetate (AgCH₃CO₂), thiourea (SC(NH₂)₂) and indium chloride (InCl₃) as precursors. The depositions were carried out in the range of the substrate temperature from 260 to 420 °C. The value of the concentration ratio in the spray solution of indium and silver elements x=[Ag⁺]/[In³⁺] was varied from 1 to 1.5 with [In³⁺]=10⁻² M and [S²⁻]/[In³⁺] was taken constant, equal to 4. The structural study shows that AgInS₂ thin film, prepared at 420 °C using optimal concentration ratio x=1.3 crystallizes in the chalcopyrite phase with a strong (1 1 2) X-ray diffraction line. Moreover, microprobe analysis (EPMA) shows that a nearly stoichiometric composition is obtained for these experimental conditions. Indeed, the atomic percentage of elements were. 24.5, 25.0, 49.5 for Ag, In and S, respectively. On the other hand from transmission and reflectance spectra, the obtained band gap energy is 1.83 eV for such film.     

Effect of S/In concentration ratio on the physical properties of AgInS2-sprayed thin films

 AgInS₂ thin films have been prepared on glass substrates by the spray pyrolysis process using an aqueous solution, containing silver acetate (AgCH₃CO₂), thiourea (SC(NH₂)₂) and indium chloride (InCl₃) as precursors. The depositions were carried out at the substrate temperature of 420 °C. The value of the concentration ratio in the spray solution of indium and silver elements x=[Ag⁺]/[In³⁺] was equal to 1.3, whereas y=[S²⁻]/[In³⁺] varied between 4 and 7. The structural study (XRD, EPMA and AFM ) shows that all films obtained using y=4 with a nearly stoichiometric composition consist essentially of AgInS₂ chalcopyrite compound and they exhibit in the as-deposited state, the best crystallinity with a (1 1 2) preferential orientation. On the other hand, films obtained using y higher than y=4 exhibit p-type character. Moreover, the optical analysis via the transmittance, reflectance reveals that the band-gap energy E_g increases slightly as a function of y composition (E_g varies from 1.87 to 2.07 eV).     

Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

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.     

Preparation of In2O3:F thin films grown by spray pyrolysis technique

Transparent conducting fluorine doped indium oxide (In₂O₃:F) thin films have been deposited on Corning 7059 glass substrates by the spray pyrolysis technique. The structural, electrical, and optical properties of these films were investigated as a function of substrate temperature. The X-ray diffraction pattern of the films deposited at lower substrate temperature (T_s=300 °C) showed no peaks of In₂O₃:F. In the useful range for deposition (i.e. 425–600 °C), the orientation of the films was predominantly [400]. For the 4500 Å thick In₂O₃:F deposited with an F content of 10-wt%, the minimum sheet resistance was 120 Ω and average transmission in the visible wavelength rang (400–700 nm) was 88%.     

In2O3:Sn/TiO2/CdS heterojunction nanowire array photoanode in photoelectrochemical cells

The design of photoanode with highly efficient light harvesting and charge collection properties is important in photoelectrochemical (PEC) cell performance for hydrogen production. Here, we report the hierarchical In₂O₃:Sn/TiO₂/CdS heterojunction nanowire array photoanode (ITO/TiO₂/CdS-nanowire array photoanode) as it provides a short travel distance for charge carrier and long light absorption pathway by scattering effect. In addition, optical properties and device performance of the ITO/TiO₂/CdS-nanowire array photoanode were compared with the TiO₂ nanoparticle/CdS photoanode. The photocatalytic properties for water splitting were measured in the presence of sacrificial agent such as SO₃²⁻ and S²⁻ ions. Under illumination (AM 1.5G, 100 mW/cm²), ITO/TiO₂/CdS-nanowire array photoanode exhibits a photocurrent density of 8.36 mA/cm² at 0 V versus Ag/AgCl, which is four times higher than the TiO₂ nanoparticle/CdS photoanode. The maximum applied bias photon-to-current efficiency for the ITO/TiO₂/CdS-nanowire array and the TiO₂ nanoparticle/CdS photoanode were 3.33% and 2.09%, respectively. The improved light harvesting and the charge collection properties due to the increased light absorption pathway and reduced electron travel distance by ITO nanowire lead to enhancement of PEC performance.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

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 performance of sprayed n-CdIn2Se4 photoanodes

Cadmium indium selenide thin films have been synthesized by spraying mixture of equimolar solution concentrations of cadmium chloride, indium trichloride and selenourea in aqueous media onto preheated FTO coated glass substrates at optimized substrate temperature and solution concentration. The photoelectrochemical cell configuration of n-CdIn₂Se₄/(1 M NaOH + 1 M Na₂S + 1 M S)/C has been used for investigate the current-voltage characteristics under dark and white light illumination, photovoltaic output, spectral response, photovoltaic rise and decay characteristics. It reveals the film of CdIn₂Se₄ exhibits 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. Gartner’s model was used to calculate minority carrier diffusion length and donor concentration (n_D). The observed efficiency and FF of PEC solar cell is 1.95% and 0.37% respectively. Mott-Schottky plot shows the flat-band potential (V_fb) of CdIn₂Se₄ films is −0.655 V/SCE.     

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.     

Influence of substrates on photoelectrochemical performance of sprayed n-CdIn2S4 electrodes

Cadmium indium sulphide (CdIn₂S₄) electrodes have been prepared onto the preheated fluorine doped tin oxide (FTO) coated glass and stainless steel (SS) substrates at optimized deposition conditions by using spray pyrolysis. Influence of substrates on the photoelectrochemical (PEC) performance has been carried out using cell configuration n-CdIn₂S₄/1 M (NaOH + Na₂S + S)/C for studying the current-voltage (I-V), photovoltaic output, photovoltaic rise and decay, photo and spectral responses and capacitance-voltage (C-V) characteristics. The junction ideality factor in dark (n_D) and light (n_L), series and shunt resistances (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The measured fill factor (FF) and cell efficiency (η) of the cells are found to be 0.47%, 0.38%, and 1.06%, 0.38% for FTO and SS substrates respectively. The Energy band diagram of band bending has been constructed using the physical parameters estimated from Mott-Schottky plots. Mott-Schottky plots shows the flat-band potential (V_fb) of CdIn₂S₄ films to be −1.15 V/SCE and −0.90 V/SCE on FTO and SS substrates respectively.     

Photoluminescence properties of sodium incorporation in CuInSe2 and CuIn3Se5 thin films

CuInSe₂ and CuIn₃Se₅ thin films have been deposited using sodium compounds such as Na₂Se and Na₂S onto Corning 7059 glass substrates by the two-stage co-evaporation method. Enhanced grain growth and preferred (1 1 2) grain orientation as well as a decrease in resistivity with respect to undoped films were observed with sodium incorporation. A clear correlation between the photoluminescence spectra and the resistivity of the films was found by comparing the properties of films with and without Na incorporation. These observations suggest that compensation is reduced due to the suppression of donor-type defects by the presence of Na.     

Photoluminescence studies of CdS thin films annealed in CdCl2 atmosphere

We have grown CdS films by the Close Spaced Vapor Transport technique under specific conditions: substrate temperature (T_s): 450 °C, source temperature (T_so): 725 °C, argon pressure in the chamber (P_Ar): 100, 200 and 500 mT, deposition time (t_d): 100 s. The films were studied by measuring the luminescence properties at different temperatures in the range 10–300 K. The room-temperature PL spectrum of the as-grown CdS films showed a very broad band centered at 2.26 eV and a shoulder in the low-energy side at 1.80 eV. After CdCl₂ thermal annealing at 300 K, the spectrum showed better PL characteristics: a strong band in the low-energy side at 1.67 eV and a band in the high-energy side at 2.47 eV. The analysis at lower temperatures showed that the high-energy band becomes most intense and shifts to higher energies reaching a value of 2.54 eV, very close to the energy band gap at 10 K. The low-energy band becomes broader and centered around 1.9 eV. Analysis of the PL intensity as a function of temperature in an Arrhenius representation, allows applying a theoretical model for the quenching of the PL intensity.     

Photoluminescence properties of polycrystalline AgGaTe2

Due to its high absorption coefficient and close to optimal bandgap energy, AgGaTe₂ is a promising material for solar energy conversion. In order to avoid recombination losses, the study of the defect structure of solar cell materials is very important. This paper reports the results of photoluminescence experiments on polycrystalline AgGaTe₂. Two emission regions centred at 1.32 and 0.8 eV were found. The first region appears near the bandgap energy and comprises three bands that are identified by the theory of heavily doped semiconductors as the band-to-band (1.337 eV), the band-to-tail (1.317 eV) and the band-to-impurity (1.287 eV) recombination. The second deep PL region consists of two bands with the peak energies of 0.835 and 0.75 eV. Both these deep bands have rather low thermal activation energy; 18.5 and 20.8 meV, respectively. The possible origins of these bands are discussed.     

Effects of deposition variables on spray-deposited MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

Undoped MnO₂ thin films have been prepared by a modified spray pyrolysis technique under various deposition conditions and the effects of different variables on electrical and optical properties have been studied in detail. It is found that substrate temperature, spray rate, solution concentration, carrier air pressure and post-deposition heat-treatment, spray outlet to substrate distance play important role in obtaining optimum films.Electrical conductivity study shows an anomaly in conductivity at a temperature 323 K and its thickness dependent resistivity follows Fuchs–Sondheimer theory. The Hall effect and thermoelectric studies indicate that the deposited sample is an n-type semiconductor. Optical study in the entire wavelength 0.3–2.5 μm range exhibits a high transmittance in the visible as well as in the near infrared. Calculation from optical data, the sample exhibits a band gap at 0.28 eV, which also supports the value obtained from the Hall effect study. These studies may be of importance for the applications of this material in energy efficient surface coating devices.     

Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

The effect of CO₂ reactivity on CH₄ oxidation and H₂ formation in fuel-rich O₂/CO₂ combustion where the concentrations of reactants were high was studied by a CH₄ flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH₄ flat flame experiment, the residual CH₄ and formed H₂ in fuel-rich O₂/CO₂ combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O₂/CO₂ combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO₂ reactivity, mainly expressed by the reaction CO₂ + H → CO + OH (R1), caused the differences between air and O₂/CO₂ combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO₂ was to advance CH₄ oxidation during fuel-rich O₂/CO₂ combustion. Under fuel-rich combustion, H₂ was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H₂O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H₂ formed more easily; however, CO₂ reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H₂, whereas the H radicals did not reduce H₂. It was shown that OH radicals formed by CO₂ reactivity were not suitable for H₂ formation. As for pulverized coal combustion, the tendencies of CH₄, CO, and H₂ formation in pulverized coal combustion were almost the same as those in the CH₄ flat flame.     

HDMR correlations for the laminar burning velocity of premixed CH4/H2/O2/N2 mixtures

Correlations for the laminar burning velocity of premixed CH₄/H₂/O₂/N₂ mixtures were developed using the method of High Dimensional Model Representation (HDMR). Based on experiment data over a wide range of conditions reported in the literature, two types of HDMR correlation (i.e. global and piecewise HDMR correlations) were obtained. The performance of these correlations was assessed through comparison with experimental results and the correlation reported in the literature. The laminar burning velocity predicted by the piecewise HDMR correlations was shown to agree very well with those from experiments. Therefore, the piecewise HDMR correlations can be used as an effective replacement for the full chemical mechanism when the prediction of the laminar burning velocity is needed in certain combustion modeling.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis

The specific contact resistivity (ρ_C) for aluminum (Al), silver (Ag) and indium (In) metallic contacts on CuInS₂ thin films was determined from I-V measurements, with the purpose of having the most appropriate ohmic contact for TCO/CdS/CuInS₂ solar cells; ρ_C was measured using the transmission line method (TLM) for the metallic contacts evaporated on CuInS₂ thin films deposited by spray pyrolysis with ratios x=[Cu]/[In]=1.0, 1.1, 1.3 and 1.5 in the spray solution. The results show that In contacts have the lowest ρ_C values for CuInS₂ samples grown with x=1.5. The minimum ρ_C was 0.26 Ω cm² for the In contacts. This value, although not very low, will allow the fabrication of CuInS₂ solar cells with a small series resistance.