Spray deposition of CuInS2 on electrodeposited ZnO for low-cost solar cells

CuInS₂ (CIS) thin films are deposited using chemical spray pyrolysis on top of a single (compact) and a double (compact + porous) ZnO film obtained by electrodeposition by changing the composition, pH and temperature of the bath. Conductive glass has been used as primary substrate. CIS films are deposited at 300 °C and using N₂ as carrier. A buffer layer of TiO₂ is incorporated by spray to protect the ZnO layer against dissolution during the subsequent spraying of CIS. Relevant properties, such as morphology, composition and thickness are evaluated using SEM, EDX and XRD. Also, UV-Vis analyses are carried out to assess the band gap value of CIS, resulting in 1.42 eV. Carrier densities and flat band potentials (V_fb) are calculated from Mott-Schottky plots. The values of V_fb are 0.70, 1.10 and 0.42 V for TiO₂, ZnO and CIS respectively. I-V curves in the dark and under illumination prove that the materials can be combined to obtain solar cells. The dark response for the two devices built with single and double layers of ZnO is very similar, showing diode behavior with good rectification ratios. Under illumination, the presence of the porous ZnO improves the performance of the cell, as reflected by the higher values of photocurrent and open circuit potential.     

Preparation of porous flower-like ZnO nanostructures and their gas-sensing property

Porous flower-like ZnO nanostructures have been synthesized by a template-free, economical hydrothermal method combined with subsequent calcination. Calcination of the precursors produced flower-like ZnO nanostructures, composed of interconnected porous ZnO nanosheets with high porosity resulting from the thermal decomposition of the as-prepared precursors, i.e., flower-like basic zinc carbonate (BZC). Moreover, the nanostructures have been characterized through X-ray diffraction, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. Compared with ZnO nanorods, the as-prepared porous flower-like ZnO nanostructures exhibit a good response and reversibility to some organic gas, such as ethanol and acetone. The sensor responses to 100 ppm ethanol and acetone are 21.8 and 16.8, respectively, at a working temperature of 320 °C. In addition, the sensors also exhibited a good response to 2-propanol and methanol, which indicate that these porous flower-like ZnO nanostructures are highly promising for applications of gas sensors.     

The synthesis of novel ZnO microcrystals through a simple solvothermal method and their optical properties

ZnO microcrystals with novel structures have been synthesized by a solvothermal method that is facile, low-cost and environment-friendly. Zn(NO₃)₂·6H₂O is the only precursor and absolute ethanol is the solvent. By controlling the reaction time, temperature and molarity of zinc nitrate, ZnO entities with the shape of flower, nut, hexagon-pillar, popcorn, brush and sphere can be synthesized in high selectivity. The ZnO micronuts (length ∼8 μm and width ∼5 μm) are uniform in morphology, displaying an open gap on the surface that divides the body into two. The investigation on the optical properties of the ZnO microcrystals reveals that all the ZnO samples exhibit an excitonic absorption edge around 376 nm, and compared to bulk ZnO, there is a modest red shift of ∼6 nm that can be ascribed to size effect as well as the unique morphologies of the ZnO microcrystals.     

High-quality vertically aligned ZnO nanorods synthesized by microwave-assisted CBD with ZnO-PVA complex seed layer on Si substrates

We successfully synthesized vertically aligned zinc oxide (ZnO) nanorods on seeded silicon substrates using chemical bath deposition assisted by microwave heating. ZnO nanorods were grown on seed layers of ZnO-polyvinyl alcohol (PVA) nanocomposites spin-coated on p-type Si (1 1 1). The nanorod's diameter was found to be dependent on the annealing temperature of the ZnO-PVA seed layer. We produced ZnO nanorods with diameters in the range of 50-300 nm from five groups of seed layers annealed at 250 °C, 350 °C, 380 °C, 450 °C, and 550 °C. The nanorods were examined with X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy, which revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of (0 0 2). Photoluminescence measurements revealed high UV emission at a high I_UV/I_vis ratio of 175. We also conducted Raman scattering studies on the ZnO nanorods to estimate the lattice vibration modes.     

Textured zinc oxide prepared by liquid phase deposition (LPD) method and its application in improvement of extraction efficiency for 650 nm resonant-cavity light-emitting diode (RCLED)

In this article, we report on the textured zinc oxide (ZnO) prepared by liquid-phase deposition (LPD) method and apply it as a window layer of 650 nm resonant-cavity light-emitting diode to enhance the extraction efficiency. The treatment solution for LPD ZnO (LPD-ZnO) growth consists of ZnO powder saturated with hydrochloric acid (HCl) and hydrogen peroxide (H₂O₂). Temperature-controlled water bath system was used to maintain a constant temperature of 40 °C in LPD system. The experimental results indicate that the deposition rate was determined by the concentration of H₂O₂ and growth temperature, and the average roughness of LPD-ZnO is dominated by the concentrations of HCl. In order to perform the practicability of LPD-ZnO, the textured LPD-ZnO is used as a window layer of 650 nm AlGaInP/GaInP resonant-cavity light-emitting diode (RCLED) to enhance the light output power. In addition, the calculated results indicate that the optimum roughness for enhancing the light output power of RCLED is in the range of 80-100 nm, which are close to the experimental results. As compared to the conventional RCLED, the RCLED with textured LPD-ZnO, which has the optimum average roughness of 82 nm, performs a high light output power, a high external quantum efficiency, a narrow linewidth of electroluminescence spectrum and the same far-field angle.     

Filtered vacuum arc deposition of undoped and doped ZnO thin films: Electrical, optical, and structural properties

Filtered vacuum (cathodic) arc deposition (FVAD, FCVD) of metallic and ceramic thin films at low substrate temperature (50-400 °C) is realized by magnetically directing vacuum arc produced, highly ionized, and energetic plasma beam onto substrates, obtaining high quality coatings at high deposition rates. The plasma beam is magnetically filtered to remove macroparticles that are also produced by the arc. The deposited films are usually characterized by their good optical quality and high adhesion to the substrate. Transparent and electrically conducting (TCO) thin films of ZnO, SnO₂, In₂O₃:Sn (ITO), ZnO:Al (AZO), ZnO:Ga, ZnO:Sb, ZnO:Mg and several types of zinc-stannate oxides (ZnSnO₃, Zn₂SnO₄), which could be used in solar cells, optoelectronic devices, and as gas sensors, have been successfully deposited by FVAD using pure or alloyed zinc cathodes. The oxides are obtained by operating the system with oxygen background at low pressure. Post-deposition treatment has also been applied to improve the properties of TCO films.The deposition rate of FVAD ZnO and ZnO:M thin films, where M is a doping or alloying metal, is in the range of 0.2-15 nm/s. The films are generally nonstoichiometric, polycrystalline n-type semiconductors. In most cases, ZnO films have a wurtzite structure. FVAD of p-type ZnO has also been achieved by Sb doping. The electrical conductivity of as-deposited n-type thin ZnO film is in the range 0.2-6 × 10^− 5 Ω m, carrier electron density is 10²³-2 × 10²⁶ m^− 3, and electron mobility is in the range 10-40 cm²/V s, depending on the deposition parameters: arc current, oxygen pressure, substrate bias, and substrate temperature. As the energy band gap of FVAD ZnO films is ∼ 3.3 eV and its extinction coefficient (k) in the visible and near-IR range is smaller than 0.02, the optical transmission of 500 nm thick ZnO film is ∼ 0.90.     

Formation of ZnO nanorod arrays on polytetraflouroethylene (PTFE) via a seeded growth low temperature hydrothermal reaction

ZnO nanorod arrays were formed by a low temperature hydrothermal process on seeded polytetraflouroethylene (PTFE) sheets. The seed layer was formed using thermal oxidation of a thin evaporated Zn film on the PTFE sheet at 300 °C in air for 10 min. The formation of ZnO nanorod arrays in the hydrothermal reactive bath consisting of hexamethylamine (HMT) and Zn ions occurred via the reaction of hydroxyl ions released during the thermal degradation of HMT with the Zn ions. The seed layer provided a template for the nucleation of the ZnO and HMT which also acted as a chelating agent that promoted growth of the ZnO along the c-axis, leading to the formation of exclusively (0 0 2) ZnO nanorods. The effect of exposure time of the seeded PTFE to the reactive solution on the formation of the nanorods was investigated. Well aligned, relatively uniform tapered 300 nm long nanorods can be formed after 8 h of exposure. Longer exposure times to 24 h resulted in the formation of more uniform nanorods with base diameter averaged of ∼100 nm and the tip diameter of ∼50 nm. XRD analysis showed that the ZnO nanorod array had a hexagonal wurtzite structure. This result is in agreement with HR-TEM observations and Raman scattering analysis. Photoluminescence study showed that a strong UV emission peak was obtained at 380 nm and a small peak at 560 nm, which is associated with green emission. The optical band gap measured from these plots was at 3.2 eV on average.     

Novel bilayer structure ZnO based photoanode for enhancing conversion efficiency in dye-sensitized solar cells

ZnO film with a novel bilayer structure, which consists of ZnO nanowire (ZnO NW) arrays as underlayer and polydisperse ZnO nanocrystallite aggregates (ZnO NCAs) as overlayer, is fabricated and studied as dye-sensitized solar-cell (DSSC) photoanode. Results indicate that such a configuration of the ZnO nanocrystallite aggregates on the ZnO nanowire arrays (ZnO-(NCAs/NWs)) can significantly improve the efficiency of the DSSC due to its fast electron transport, relatively high surface area and enhanced light-scattering capability. The short-circuit current density (J_sc) and the energy-conversion efficiency (η) of the DSSC based on the ZnO-(NCAs/NWs) photoanode are estimated and the values are 9.19 mA cm⁻² and 3.02%, respectively, which are much better than those of the cells formed only by the ZnO NWs (J_sc = 4.02 mA cm⁻², η = 1.04%) or the ZnO NCAs (J_sc = 7.14 mA cm⁻², η = 2.56%) photoanode. Moreover, the electron transport properties of the DSSC based on the ZnO-(NCAs/NWs) photoanode are also discussed.     

Influence of metal ions on the formation of artificial zinc rusts

The artificial rust particles were prepared from ZnCl₂ solutions dissolving Al(III), Fe(III), Fe(II), Ni(II), Co(II) and Mg(II) at different atomic ratios from 0 to 0.3 in metal/Zn. With increasing metal/Zn the crystal phases of the products turned following as ZnO → a mixture of ZnO and Zn₅(OH)₈Cl₂ · H₂O (ZHC) → ZHC. Al(III) most facilitated the formation of ZHC but Mg(II) and Fe(III) produced no ZHC. The morphology of the formed particles varied following as agglomerate → fine → rod → sheet → irregular with the increase of metal/Zn. The sheet and irregularly shaped particles were identified as ZHC and the other particles as ZnO.     

Temperature impact on morphological evolution of ZnO and its consequent effect on physico-chemical properties

The temperature influenced morphology evolution and its effect on physico-chemical properties of ZnO thin films deposited onto glass substrates from alkaline environment, complexed via EDTA chelant are systematically studied. Temperature dependent growth mechanism model for change in microstructure is proposed. The physico-chemical properties of deposited films are studied by the analysis of structural, morphological, surface wettabillity, optical and electrical properties. Nanocrystalline ZnO thin films with hexagonal structure having mari-gold flowers and tetra pods like morphologies with optical band gaps 3.1 and 2.96 eV showed drastic surface wettabillity transformation from highly hydrophobic (142°) to superhydrophilic (<5°) behavior for bath placed at room temperature (300 K) and 333 K, respectively. The room temperature photoluminescence spectrum in the visible light region showed decreasing in intensity and electric resistivity measurement showed reduction in electrical resistivity from 10⁶ to 10⁴ Ω cm as consequence of increment in deposition temperature. The morphology evolution as impact of bath temperature can provide wide scope with significant change in physico-chemical properties of smart ZnO, which can be potentially tuned in many functional applications with feasibility.     

Chemical deposition of CuInSe2 thin films by photoelectrochemical applications

Copper indium diselenide films have been synthesized by chemical bath deposition method. The configuration of fabricated cell is n-CuInSe₂|NaOH (1 M) + S (1 M) + Na₂S (1 M)|C_(graphite). The photoelectrochemical cell characterization of the films is carried out by studying current-voltage characteristics in dark, capacitance-voltage in dark, barrier height measurements, power output, photoresponse and spectral response. The study shows that CuInSe₂ thin films are n-type conductivity. The junction ideality factor is found to be 3.81. The flat band potential is found to be 0.763 V. The barrier height value is found to be 0.232 eV. The study of power output characteristic shows open circuit voltage, short circuit current, fill factor and efficiency are found to be 310 mV, 20 μA, 42.12% and 0.82%, respectively. Photoresponse shows lighted ideality factor which is 2.92. Spectral response shows the maximum current observed at 650 nm.     

Direct synthesis and characterization of mesoporous Fe3O4 through pyrolysis of ferric nitrate-ethylene glycol gel

Mesoporous magnetite (Fe₃O₄) was successfully synthesized on a large scale by direct pyrolysis of ferric nitrate-EG (EG = ethylene glycol) gel in a one-end closed horizontal tube furnace in the air without using any template, additions, and carrier gas. The as-synthesized mesoporous Fe₃O₄ were characterized by powder X-ray diffraction (XRD), infrared spectra (IR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and thermal gravimetric analysis (TGA). Results from TEM showed that the as-obtained Fe₃O₄ has mesoporous structure formed by the loose agglomeration of nanoparticles with diameter of about 6 nm, which was also confirmed by small-angle XRD and nitrogen adsorption analysis. Furthermore, vibrating sample magnetometer (VSM) measurements indicated that the saturated magnetization of the as-obtained mesoporous Fe₃O₄ was ferromagnetic with the saturation magnetization (Ms) and coercivity (Hc) of 46 emu/g and 136 Oe, respectively. In addition, a possible growth mechanism of mesoporous Fe₃O₄ was also discussed.     

Sensing properties of sprayed antimony doped tin oxide thin films: Solution molarity

Transparent conductive thin films of nanocrystalline Sb:SnO₂ have been deposited onto preheated glass substrates by using spray pyrolysis technique. The effect of the solution molarity on structural, morphological, optoelectronic properties of Sb:SnO₂ films has been investigated. XRD study reveals that films are polycrystalline with tetragonal crystal structure having average crystallite size about 20 nm. The compact and homogeneous grains are seen in FESEM images. The BEs of Sn 3d_5/2 for all samples show the Sn⁴⁺ bonding state for SnO₂. The BEs of Sb 3d_5/2 are in the range of 530.6-530.9 eV, indicating that all antimony detected is in a pentavalent state (Sb⁵⁺). Transparency of films in the visible region decreases with increase in precursor concentration. Photoluminescence study shows the strong violet and weak orange emission. The sensing properties of the Sb:SnO₂ films for acetone, ethanol and LPG with operating temperature and gas concentration have been investigated.     

Effects of oxygen partial pressure on doping properties of Ga-doped ZnO films prepared by ion-plating with traveling substrate

Polycrystalline Ga-doped ZnO (GZO) thin films were prepared by ion-plating on a traveling glass substrate at 200 °C. Effects of O₂ gas flow rate and Ga₂O₃ content in source on the electrical and structural properties of GZO films were investigated. GZO films having a low resistivity of 210^− 4 Ω cm order were obtained under the conditions of Ga₂O₃ contents of 3-5 wt.% and O₂ gas flow rates below 10 sccm. In particular, for GZO films prepared with a Ga₂O₃ content of 4 wt.% at an O₂ gas flow rate of 2.5 sccm, the lowest resistivity of 2.23 × 10^− 4 Ω cm was obtained; the carrier concentration and Hall mobility were 1.17 × 10²¹ cm^− 3 and 23.9 cm²/Vs, respectively. Excess Ga₂O₃ content in source (> 6 wt.%) cause deterioration both in crystallinity and in electric property most probably due to the solubility limit for Ga doping in ZnO at the glass substrate temperature of 200 °C. Excess O₂ gas flow rates (> 10 sccm) during the film growth also lower the electric properties of the GZO films.     

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150 nm-thick RuO₂-coated SiO₂/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO₂/SiO₂/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5 nm-thick ZnO buffer layer on the RuO₂/SiO₂/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300 °C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5 nm-thick ZnO buffered RuO₂/SiO₂/Si substrates grown in 0.02 M electrolyte at −1.5 mA with a subsequent annealing in vacuum at 300 °C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.     

ZnO thin films growth by pulsed vacuum arc discharge

A study of structural and morphologic features of ZnO coatings grown by pulsed vacuum arc discharge is presented in this work. The characteristics of the plasma generated during the production process are determined by using the Optical Emission Spectroscopy technique. The system employed for growing the films consists of a vacuum chamber with two opposite electrodes (the cathode is the target of zinc and the anode is the substrate of glass). The gas used for filling the reactor was high purity oxygen. The discharge was generated at a pressure of 200 Pa and a voltage of 200 V. Coatings were characterized by using X-ray diffraction and scanning probe microscopy (SPM) techniques. XRD patterns showed a hexagonal crystalline structure with peaks produced by planes (002) and (103). The roughness observed was of 98.6 nm. No great quantity of nanodroppers was observed on the coatings surface. Plasma spectroscopic studies revealed many lines belonging to different species like Zn (328.2, 330.3, 334.5 nm), Zn⁺ (491.3 and 492.5 nm), O (372 nm) and O⁺ (336.1, 337.25 and 518 nm) and weak bands of ZnO (526.47, 531.2, 536.94 nm). The presence of ZnO is an important finding because, for coatings produced by arc techniques, authors have reported ZnO as being exclusively produced on the substrate surface and not generated into the plasma. The electron temperature (T_e) and density (n_e) were determined by means of spectroscopic methods such as the relation between lines of the same ionization state for T_e and the calculation of Stark Broadening for n_e.     

X-ray photoelectron spectroscopic study of catalyst based zinc oxide thin films

X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface and interface analysis, providing an elemental composition of surfaces and the local chemical environment of adsorbed species. The surface composition and chemical states of the F/ZnO and In/ZnO catalysts deposited using spray technique have been studied by high resolution and high sensitivity X-ray photoelectron spectroscopy. A hybrid multiline method is proposed for quantitative XPS analysis that combines the first principles approach with the experimental determination of overall response function. The chemical shifts of XPS core lines for Zn (2P_3/2, F 1s and In 3d) and Auger parameter for zinc (β_Zn = 2012.6, 2011.48 eV for F/ZnO and In/ZnO, respectively) have been calculated. The results have been used to determine the bond iconicity (0.55).     

Phase transformation behavior of nanocrystalline Ni-W-P alloys containing various W and P contents

In the present investigation, electroless (EL) ternary Ni-W-P coatings were prepared using hypophosphite based alkaline bath by varying sodium tungstate as tungsten source (5-80 g/L). Maximum amount of W incorporation (8.2 ± 1 wt.%) was obtained when the bath contained about 20 g/L of tungsten source. At very high concentrations of W source in the bath the deposit contained about 4 wt.% W and 2 wt.% P. All the as-deposited ternary coatings exhibited nodular surface morphology. X-ray diffractograms (XRD) obtained for as-deposited EL NiWP alloys indicated that crystallinity of the coatings increased with decrease in phosphorus content. Calculated grain size for the deposits varied from 1.2 to 12.7 nm when the tungsten source varied from 5 to 80 g/L in the bath. Higher crystallization temperatures were obtained due to W codeposition in NiP matrix. Presence of metastable phases such as Ni₅P₂ and NiP apart from stable Ni and Ni₃P was identified for the heat treated deposits (400 °C/1 h) containing lower amount of W and higher amount of P. Whereas other ternary deposits after the heat treatment predominantly revealed face centered cubic (f.c.c.) Ni (111) peak. Activation energy for the crystallization of all the alloys has been carried out by modified Kissinger method. Microhardness measurements were carried out on all the deposits isothermally heat treated at 600 °C for 1 h.     

Solvothermally synthesized tungsten oxide nanowires/nanorods for NO2 gas sensor applications

One-dimensional nanorods or nanowires of W₁₈O₄₉ were synthesized by solvothermal method at 200 °C with tungsten hexachloride (WCl₆) as precursor and cyclohexanol or 1-propanol as reaction solvent. Their morphology and structure properties were systematically characterized. The NO₂-sensing properties of the sensors based on nanowires and nanorods were investigated at 100 °C up to 250 °C over NO₂ concentration ranging from 1 ppm to 20 ppm. The results indicate that both nanowires and nanorods exhibit reversible response to different concentrations of NO₂, and the highest gas response is achieved at 150 °C. In comparison with nanorods, nanowires showed a much quicker response characteristic and a relative higher response value to the same concentration of NO₂ gas due to the smaller diameter and larger specific surface area.     

Organic additive-copper(II) complexes as plating precursors

Cu(II) ions previously coordinated with typical electroplating organic additives were investigated as an alternative source of metal for plating bath. The coordination complexes were isolated from reaction between CuSO₄ and organic additives as ligands (oxalate ion, ethylenediamine or imidazole). Deposits over 1010 steel were successfully obtained from electroplated baths using the complexes without any addition of free additives, at pH = 4.5 (H₂SO₄/Na₂SO₄). These deposits showed better morphologies than deposits obtained from CuSO₄ solution either in the absence or presence of oxalate ion as additive (40 mmol L^− 1), at pH = 4.5 (H₂SO₄/Na₂SO₄). It is suggestive that the starting metal plating coordinated with additives influences the electrodeposition processes, providing deposits with corrosion potentials shifted over + 200 mV in 0.5 mol L^− 1 NaCl (1 mV s^− 1). The resistance against corrosion is sensitive to the type of additive-complex used as precursor. The complex with ethylenediamine presented the best deposit results with the lowest pitting potential (− 0.27 V vs 3.0 mol L^− 1 CE). It was concluded that the addition of free additives to the electrodeposition baths is not necessary when working with previously coordinated additives. Thus, the complexes generated in ex-situ are good alternatives as plating precursors for electrodeposition bath.