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%.     

Studies on photoelectrochemical (PEC) cell formed with SILAR deposited Bi2Se3–Sb2Se3 multilayer thin films

Nanocrystalline Bi₂Se₃–Sb₂Se₃ multilayer thin films were deposited by simple and less investigated successive ionic layer adsorption and reaction (SILAR) method onto glass- and fluorine-doped tin oxide (FTO)-coated glass substrate from aqueous solution. Characterizations such as XRD, surface morphology and optical absorption have been carried out for Bi₂Se₃–Sb₂Se₃ thin films onto glass substrates. The films deposited onto FTO-coated glass substrates were used to study photoelectrochemical behaviour in 0.1 M (NaOH–Na₂S–S) electrolyte and results are reported.     

Preparation and photoelectrochemical study of BiVO4 thin films deposited by ultrasonic spray pyrolysis

Thin films of BiVO₄ with monoclinic structure were deposited onto indium-doped tin oxide (ITO)-coated glass substrates by ultrasonic spray pyrolysis. The effects of tungsten doping and hydrogen reducing were investigated. The films were characterized with XRD, Raman spectra, SEM, UV–Vis transmittance spectra. Furthermore, the films were investigated by electrochemical and photoelectrochemical measurements with regard to splitting water for solar energy conversion. The films possessed a scheelite-monoclinic structure with good absorption to visible light. The optical band gaps were evaluated to be about 2.65 eV. The flat band potentials were estimated to be about −0.61 V vs. saturated calomel electrode (SCE) in 0.5 M Na₂SO₄ solution from Mott–Schottky plots. For non-doping samples, the incident photon to current conversion efficiency (IPCE) was relative low because of low density and activity of carriers. When treated with hydrogen reducing, the carrier density increased due to more oxygen vacancies, resulting in the increase of IPCE. In addition, substituting 1% vanadium with equal mole tungsten can increase IPCE remarkably, which achieved about 10% at 0.3 V vs. SCE potential under 400–450 nm wavelength photo irradiation.     

Electrophoretic behavior of solvothermal synthesized anion replaced Cu2ZnSn(SxSe1-x)4 films for photoelectrochemical water splitting

Quaternary Cu₂ZnSn(S_xSe_1-x)₄ (CZT(S_xSe_1-x)₄) compounds have drawn a great deal of attention for being used in the fabrication of optoelectronic devices such as solar cells, photocatalysts, and photoelectrochemical water splitting. However, one major challenge facing the utilization of this material is to reduce the production cost of synthesis and fabrication of high quality CZT(S_xSe_1-x)₄ films. In the present study, a facile and beneficial solvothermal route has been reported for synthesis of CZT(S_xSe_1-x)₄ compounds. The process of electrophoretic deposition (EPD) of synthesized CZT(S_xSe_1-x)₄ nanoparticles, is systematically compared with each other in order to obtain high quality films with appropriate porosity. The XRD patterns, EDS and Raman spectra confirm the formation of CZT(S_xSe_1-x)₄ phases with no trace of impurities and appreciable crystallinity and also with near stoichiometry composition in all the samples. The obtained particle size for CZTS, CZTSSe and CZTSe samples was in the range of 50–100 nm and also for some agglomerate particles was in the range of 500 nm to 2 μm. Based on the obtained results for thin films prepared using EPD in the present study, the best EPD parameters for each CZTS, CZTSe and CZTSSe samples with 120 V and 5 min as applied voltage and deposition time were reported as the best samples. The obtained photocurrent-potential and current-time curves of CZT(S_xSe_1-x)₄ thin film samples demonstrate that the photocurrents of each CZTS, CZTSe, CZTSSe thin films, are different in the range of ?2.1 to ?6 mA/cm² and also the CZTS and CZTSe samples show a detectable current under the exposure of sunlight that can have an appropriate stability for 3000 s but the CZTSSe sample showed a stable photocurrent just for 2000 s. According to the mentioned results in this study, the CZTS and CZTSe samples can potentially be suitable candidates for further applications.     

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.     

Development of thin film solar cell based on Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (hereafter CZTS) thin films were successfully formed by vapor-phase sulfurization of precursors on a soda lime glass substrate (hereafter SLG) and a Mo-coated one (hereafter Mo-SLG). From the optical properties, we estimate the band-gap energy of this thin film as 1.45–1.6 eV which is quite close to the optimum value for a solar cell. By using this thin film as an absorber layer, we could fabricate a new type of thin film solar cell, which was composed of Al/ZnO:Al/CdS/CZTS/Mo-SLG. The best conversion efficiency achieved in our study was 2.62% and the highest open-circuit voltage was 735 mV. These device results are the best reported so far for CZTS.     

Inkjet printed chalcopyrite CuInxGa1−xSe2 thin film solar cells

In this paper, we report a novel approach for the fabrication of chalcopyrite CuIn_xGa_1−xSe₂ thin film solar cells by inkjet printing. The short circuit current (J_sc), open circuit voltage (V_oc), fill factor (FF), and total area power conversion efficiency (η) of the device are 29.78 mA/cm², 386 mV, 0.44%, and 5.04%, respectively. Inkjet printing at atmospheric environment offers an opportunity for the direct patterning of absorber materials at large scale. This provides a potential cost advantage over conventional fabrication process that involves sequential deposition, patterning, and etching of selected materials. In addition, inkjet printing increases the raw material utilization ratio compared to more wasteful vacuum-based deposition techniques.     

Characterization of Cu(InxGa1−x)2Se3.5 thin films prepared by rf sputtering

Cu(In_xGa_1−x)₂Se_3.5 thin films were fabricated by rf sputtering from CuIn_xGa_1−xSe₂ and Na mixture target by controlling the mixture ratio. X-ray diffraction analyses show that the structure of Cu(In_xGa_1−x)₂Se_3.5, thin films is different from chalcopyrite structure: especially, CuIn₂Se_3.5 thin films have a defect chalcopyrite structure. The lattice parameters for Cu(In_xGa_1−x)₂Se_3.5 thin film are slightly smaller than those for CuIn_xGa_1−xSe₂ thin film and linearly decreased with increasing Ga content. The optical absorption coefficients for Cu(In_xGa_1−x)₂Se_3.5, thin films exceed 2 × 10⁴ cm⁻¹ in energy region above the fundamental band edge. The band gap for Cu(In_xGa_1−x)₂Se_3.5 thin films is larger than that for CuIn.Ga_1−x2Se₂ with the same Ga content and increased with increasing Ga content.     

CuInS2/In2S3 thin film solar cell using spray pyrolysis technique having 9.5% efficiency

Copper indium sulfide (CuInS₂)/In₂S₃ solar cells were fabricated using spray pyrolysis method and high short circuit current density and moderate open circuit voltage were obtained by adjusting the condition of deposition and thickness of both the layers. Consequently, a relatively high efficiency of 9.5% (active area) was obtained without any anti-reflection coating. The cell structure was ITO/CuInS₂/In₂S₃/Ag. We avoided the usual cyanide etching and CdS buffer layer, both toxic, for the fabrication of the cell.     

(Photo)electrochemical studies on p-Sm2S3 films

p-Sm₂S₃ films were electrosynthesised from aqueous 0.05 M Sm₂O₃−0.05 M Na₂S₂O₃−0.25 M tartaric acid bath onto Ti and stainless steel coated substrates. The films were characterized using optical absorption, X-ray diffraction, optical microscopy and chemical analysis. X-ray diffraction showed that films are microcrystalline and free from elemental Sm andS. Optical absorption studies showed that the bandgap is 2.08 eV. (Photo)electrochemical studies of the films were carried out by forming a p-Sm₂S₃/KI---I₂---₂KCl/C cell configuration.The current-voltage characteristics were studied in the dark and under white light illumination and at different temperatures. The capacitance-voltage characteristics were studied at 1 kHz frequency. The type of material, junction quality factor, barrier height, flat band potential, etc., were estimated.     

Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol-gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency

The properties of Cu₂ZnSnS₄ (CZTS) thin films deposited by sol-gel sulfurization were investigated as a function of the chemical composition of the sol-gel solutions used. The chemical composition ratio Cu/(Zn+Sn) of the sol-gel solution was varied from 0.73 to 1.00, while the ratio Zn/Sn was kept constant at 1.15. CZTS films deposited using sol-gel solutions with Cu/(Zn+Sn)<0.80 exhibited large grains. In addition, the band gaps of these Cu-poor CZTS thin films were blue shifted. Solar cells with the structure Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass were fabricated under non-vacuum conditions. The solar cell with the CZTS layer deposited using the sol-gel solution with Cu/(Zn+Sn)=0.80 exhibited the highest conversion efficiency of 2.03%.     

Structural,electronic, dynamical and thermodynamic properties of Ca10(PO4)6(OH)2 and Sr10(PO4)6(OH)2: First-principles study

Density functional theory (DFT) with optPBE-vdW functional is used to simulate the structural, electronic, dynamical and thermodynamic properties of Ca₁₀(PO₄)₆(OH)₂(Ca-HA) and Sr₁₀(PO₄)₆(OH)₂(Sr-HA). The calculated structural properties within optPBE-vdW functional is found to yield better agreement with the experimental results, which indirectly suggests the important role of weak hydrogen bond in this crystal. The calculated electronic properties indicate that Ca-HA and Sr-HA are insulator materials with indirect band gap of 5.52 eV and 5.10 eV, respectively. The detailed dynamical properties of two apatites are obtained by the linear-response approach. With replacement of Ca by Sr, the librational mode of OH group decreases from 612 cm^?1 to 569 cm^?1, the stretching mode of OH group increases from 3614.5 cm^?1–3649.9 cm^?1, which is consistent with the experimental results. Finally, some phonon related thermodynamic properties, such as Helmholtz free energy F, internal energy E, entropy S and heat capacity C_V of Sr-HA and Ca-HA are studied according to the phonon calculations within the harmonic approximation. The present calculation results of two apatites with optPBE-vdW functional are in good agreement with the existing experimental.     

Fabrication and photoelectrochemical study of Ag@TiO2 nanoparticle thin film electrode

Ag@TiO₂ nanoparticle thin film was fabricated for photoelectrochemical water splitting in the visible light region. Under the irradiation of UV light, positive photocurrent was enhanced in both electrolytes of 0.1 M HNO₃ and 0.1 M NaOH owing to the excitation of photoelectrons within the TiO₂ shells. However, under the irradiation of visible light, the enhancement of positive photocurrent was observed only in 0.1 M HNO₃ because of the formation of a Schottky barrier band bending at the Ag-TiO₂ core-shell interface and the generation of photoelectrons resulted from the surface plasmon resonance of Ag cores. In 0.1 M NaOH, significant negative photocurrent was enhanced due to the influences of higher pH on the surface state and energy level of TiO₂ shells. Such a visible light-induced photoresponse enhancement and photocurrent direction switching made the Ag@TiO₂ nanoparticle thin film useful not only as a photoelectrode for water splitting but also as a photo-switch in a basic electrolyte.     

Kinetics and modeling study of a Mg(BH4)2/Ca(BH4)2 destabilized system

Borohydrides such as Mg(BH₄)₂ and Ca(BH₄)₂ continue to attract attention as potential hydrogen storage materials because of their high hydrogen content. In this study the desorption kinetics of Mg(BH₄)₂, Ca(BH₄)₂ and a 5Mg(BH₄)₂/Ca(BH₄)₂ mixture of the two were compared in the two-phase region at the same temperature and at a constant pressure thermodynamic driving force. The rate of hydrogen desorption from the two-phase region in the 5Mg(BH₄)₂/Ca(BH₄)₂ mixture was faster than that from either of the constituents. This indicated that Ca(BH₄)₂ was able to serve as a destabilizing agent for Mg(BH₄)₂. The results also showed that the desorption rates from the two-phase region were much faster than those from the single phase region. Modeling studies showed that the rate of hydrogen release from Mg(BH₄)₂, during the first 80% of the reaction, is diffusion controlled while in Ca(BH₄)₂ the reaction rate is phase boundary controlled. In the mixture the rate appears to be under the mixed control of both processes.     

Real-time investigations on the formation of CuInSe2 thin film solar cell absorbers from electrodeposited precursors

In this article, we present results of a detailed real-time X-ray diffraction (XRD) study on the formation of CuInSe₂ from electroplated precursors. The solid-state reactions observed during the selenisation of three different types of precursors are presented. The first type of precursors (I) consists of the nanocrystalline phases Cu_2−xSe and InSe at room temperature, which react to CuInSe₂ starting at 470 K. The second type of precursor (II) shows an inhibited CuInSe₂ formation out of the initial phases Cu_2−xSe and γ-In₂Se₃ starting at 400 K. The third precursor type (III) shows completely different selenisation behaviour. Starting from the intermetallic compound Cu₁₁In₉ and amorphous selenium, the formation of the binary selenides In₄Se₃ and CuSe is observed after the melting point of selenium at 494 K. After selenium transfer reactions, the compound semiconductor CuInSe₂ is formed out of Cu_2−xSe and InSe. This type (III) reaction path is well known for the selenisation of SEL precursors (stacked elemental layers of sputtered copper and indium and thermally evaporated selenium).     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

Investigations on electron beam evaporated Cu(In0.85Ga0.15)Se2 thin film solar cells

CIGS bulk with composition of CuIn_0.85Ga_0.15Se₂ was synthesized by direct reaction of elemental copper, indium, gallium and selenium. CIGS thin films were then deposited onto well-cleaned glass substrates using the prepared bulk alloy by electron beam deposition method. The structural properties of the deposited films were studied using X-ray diffraction technique. The as-deposited CIGS films were found to be amorphous. On annealing, the films crystallized with a tetragonal chalcopyrite structure. An intermediate Cu-rich phase precipitated at 200 °C and dissociated at higher annealing temperatures. Average grain size calculated from the XRD spectra indicated that the films had a nano-crystalline structure and was further corroborated by AFM analysis of the sample surface. The chemical constituents present in the deposited CIGS films were identified using energy dispersive X-ray analysis. CIGS based solar cells were then fabricated on molybdenum and ITO coated glass substrates and the efficiencies have been evaluated.     

Preparation of Culn(SxSe1−x)2 thin films by sulfurization and selenization

A CuIn(S_xSe_1−x)₂ alloy thin-film was prepared by selenization of CuInS₂: its composition ratio x can be controlled by the number of selenization cycles implemented. Crystallinity of the films was improved by annealing in vacuum. The resistivity of the film was about 1 Ω cm and increased by one to two orders of magnitude after KCN treatment. An 8.1 % efficiency solar cell was obtained by using this annealed alloy thin-film.     

Behavior of the monophosphate tungsten bronzes (PO2)4(WO3)2m (m = 4 and 6) in electrochemical lithium insertion

The electrochemical lithium insertion process has been studied in the family of monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m, where m = 4 and 6. Structural changes in the pristine oxides were followed as lithium insertion proceeded. Through potentiostatic intermittent technique, the different processes which take place in the cathode during the discharge of the cell were analysed. The nature of the bronzes Li_x(PO₂)₄(WO₃)_2m formed was determined by in situ X-ray diffraction experiments. These results have allowed establishment of a correlation with the reversible/irreversible processes detected during the electrochemical lithium insertion. Measurements of resistivity showed that upon lithium insertion, the metallic pristine oxides become insulating.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.