Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate

We have studied differences in the interface between undoped and Al-doped ZnO thin films deposited on commercial Si solar cell substrates. The undoped ZnO film is significantly thicker than the Al-doped film for the same deposition time. An extended silicate-like interface is present in both samples. Transmission electron microscopy (TEM) and photoelectron spectroscopy (PES) probe the presence of a zinc silicate and several Si oxides in both cases. Although Al doping improves the conductivity of ZnO, we present evidence for Al segregation at the interface during deposition on the Si substrate and suggest the presence of considerable fixed charge near the oxidized Si interface layer. The induced distortion in the valence band, compared to that of undoped ZnO, could be responsible for considerable reduction in the solar cell performance.     

Efficiency enhancement in dye-sensitized solar cells by interfacial modification of conducting glass/mesoporous TiO2 using a novel ZnO compact blocking film

A novel and thin ZnO compact blocking film is employed at the interface of fluorine-doped tin oxide (FTO) substrate and mesoporous TiO₂, and its influence on dye-sensitized solar cells (DSSCs) is investigated. The ZnO film prepared by spin-coating method on FTO is characterized by energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and UV–vis spectrophotometer. The ZnO film is firstly employed as an energy barrier between FTO and mesoporous TiO₂ film in DSSCs, which improves open-circuit photovoltage (V_oc) and fill factor (FF) with compensation of J_sc decrease, finally increasing energy conversion efficiency from 5.85% to 6.70%. Electrochemical impedance spectra (EIS) analysis and open-circuit voltage decay (OCVD) technique reveal that the existence of the energy barrier not only resulted in the effect of suppressing back electrons transfer from FTO to electrolyte but also blocking the electrons injection from the conductive band of TiO₂ to FTO. The former effect effectively reduces the recombination which occurs in the region of FTO substrate, and the latter leads to remarkable increment of electron density in the TiO₂, thus resulting in enhanced V_oc and FF. These results suggest that the methodology of introducing the semiconductor with a more negative conduction band edge than TiO₂ as the compact blocking film in DSSCs may be feasible.     

Correlated transport and optical phenomena in Ga-doped CdO films

Series of samples of lightly Ga-doped CdO thin films (3%, 6%, and 9%) have been prepared by evaporation method on glass substrate. The prepared films were characterised by X-ray diffraction (XRD), UV–VIS–NIR absorption spectroscopy, and dc-electrical measurements. The investigation shows that Ga doping widens the energygap of CdO. The optical properties were easily explained by using Tauc et al. band-to-band transitions and classical Drude theory. The electrical behaviour of the samples shows that they are degenerate semiconductors. The 6% Ga-doped CdO sample shows increase its mobility by 3.2 times, increase its conductivity by 1.5 times, increase its intrinsic bandgap, and a slight increase its transmittance relative to undoped CdO film. Explanation was given concerning these variations. From transparent conducting oxide (TCO) point of view, Ga is not sufficiently effective for CdO doping comparing to other dopants like In, Sn, Sc, and Y.     

Preparation of transparent and conducting boron-doped ZnO electrode for its application in dye-sensitized solar cells

A simple and economic chemical spray pyrolysis method is used to prepare transparent and conducting boron-doped zinc oxide (B_nZnO) electrode having potential applications in dye-sensitized solar cells (DSSCs). The B_nZnO electrodes were critically characterized for their structural, morphological and electrical properties. The B_nZnO electrode with 2 at% boron doping showed average grain size of 20(±1) nm, surface roughness of 9 nm, ?95% transparency and resistivity of 4.5×10⁻³ Ω cm⁻¹. Furthermore, doping concentration of boron could also be easily controlled for achieving desired properties. Using this electrode as a substrate in DSSCs, the solar-to-electrical conversion efficiency with N3 dye as a sensitizer was noted to be 1.53%. This work suggests that the B_nZnO electrodes could be used as promising alternative to presently used indium- or fluorine-doped tin oxide as substrates.     

A new catalyst of AlCu@ZnO for hydrogen evolution reaction

Thin films of undoped ZnO, Al-doped ZnO, Cu-doped ZnO, and AlCu@ZnO deposited on indium tin oxide were performed by the sol-gel spin coating method. The prepared ZnO thin films were investigated for their structural and electrical properties after annealing at 500 °C for 1 h. ZnO thin films were characterized by electrochemical impedance spectroscopy, linear sweep voltammetry, scanning electron microscopy, Fourier transform infrared spectroscopy and Mott Schottky. According to the results obtained from the Nyquist diagrams of the ZnO thin films, the resistance value was found to decrease with binary doping and the resistance value was found to be lowest in AlCu@ZnO doped thin film containing 0.01 M Al and 0.1 M Cu. As ZnO thin films go to cathodic potentials, it is seen that the cathodic current value of ZnO with undoped is the lowest. It has been found that only Al and Cu doping showed less cathodic current than double doping.     

Effects of Sn doping on the manufacturing,performance and carbon deposition of Ni/ScSZ cells in solid oxide fuel cells

This work demonstrates the effect of tin (Sn) doping on the manufacturing, electrochemical performance, and carbon deposition in dry biogas-fuelled solid oxide fuel cells (SOFCs). Sn doping via blending in technique alters the rheology of tape casting slurry and increases the Ni/ScSZ anode porosity. In contrast to the undoped Ni/ScSZ cells, where open-circuit voltage (OCV) drops in biogas, Sn–Ni/ScSZ SOFC OCV increases by 3%. The maximum power densities in biogas are 0.116, 0.211, 0.263, and 0.314 W/cm² for undoped Ni/ScSZ, undoped Ni/ScSZ with 3 wt% pore former, Sn–Ni/ScSZ and Sn–NiScSZ with 1 wt% pore former, respectively. Sn–Ni/ScSZ reduces the effect of the drop in the maximum power densities by 26%–36% with the fuel switch. A 1.28–2.24-fold higher amount of carbon is detected on the Sn–Ni/ScSZ samples despite the better electrochemical performance, which may reflect an enhanced methane decomposition reaction.     

Highly efficient hematite films via mid-/ex-situ Sn-doping for photoelectrochemical water oxidation

In this study, we report the facile fabrication of Sn-doped hematite film via mid-situ and ex-situ doping methods for efficient photoelectrochemical (PEC) performances. The morphology of Sn-doped Fe₂O₃ films was varied with Sn precursor in the mid-situ doping process. The addition of SnCl₂ rendered a smooth-surfaced and well-distributed nanorod morphology, but SnCl₄ gave a deformed nanorod structure covered with layered coalescence of SnO₂ particles. The former demonstrated much higher photoelectrochemical performances as photoanodes than the latter. The photocurrent can be further improved by surface modification with SnCl₄ through spin-coating method. The effects of Sn precursors on the morphology, surface characteristics and the PEC properties of the photoanodes are investigated.     

An experimental and theoretical study of the effect of Ce doping in ZnO/CNT composite thin film with enhanced visible light photo-catalysis

Ce-doped ZnO/CNT composite thin film was fabricated successfully on soda-lime-silica glass substrate by sol-gel drop coating method. The structure and morphology of nanocrystalline Ce-doped ZnO/CNT thin film were characterized by X-ray Diffraction (XRD), X-ray photo-electron spectroscopy (XPS), Field Emission Scanning Electron Microscope (FESEM) and UV–Visible spectroscopy. The photocatalytic activity of Ce-doped ZnO/CNT thin film was evaluated by photocatalytic degradation of methylene blue (MB) in aqueous solution as a model pollutant under visible light irradiation. The synthesized Ce-doped ZnO/CNT composite thin film showed 76.71% photocatalytic efficiency whereas bare ZnO thin film showed that of only 25.30%. It has been reported that improved photocatalytic efficiency of composite is due to the synergistic effect of Ce doping and insertion of CNTs into ZnO matrix. The experimental photodegradation data were well fitted to first-order kinetics. The photocatalytic activity of the prepared thin film can be regenerated, which implies that the photocatalytic degradation process could be operated at a relatively low cost. The results suggest that Ce-doped ZnO/CNT composite thin film prepared by sol-gel drop coating method can be developed as an economically feasible and environmentally friendly method to degrade dye containing wastewater using visible light. Furthermore, atomic models for Ce doping in ZnO cluster was used to investigate the effect of doping on electronic structure of ZnO through density functional theory calculations. The computational study suggested a significant narrowing of the band gap and change of the maximum absorption bands towards higher wavelength. These all support the experimental results.     

Omnidirectional light‐harvesting enhancement of dye‐sensitized solar cells with ZnO nanorods

In this work, the chemical solution method was used to prepare one‐dimensional (1‐D) ZnO nanorod (NR) photoelectrodes, which were subsequently used in dye‐sensitized solar cells (DSSCs). The effects of ZnO NRs on the omnidirectional light‐harvesting performance of DSSCs were investigated by growing ZnO NRs with varying lengths as photoelectrodes. On the basis of field‐emission scanning electron microscopy and ultraviolet (UV)–vis‐near infrared (NIR) spectroscopy measurements on the ZnO NR photoelectrodes of varying lengths, it was observed that the dye adsorption and light‐scattering properties of NRs are affected by their length. In addition, DSSCs were prepared using ZnO NRs of varying lengths. These DSSCs were examined via electrochemical impedance spectroscopy, monochromatic incident photon‐to‐electron conversion efficiency measurements, and solar simulations to measure their photovoltaic efficiencies, carrier lifetimes, and device characteristics in omnidirectional antireflection measurements. The highest photovoltaic efficiency between these DSSCs was 0.33%. Omnidirectional antireflection measurements were performed on DSSCs with different ZnO NR lengths, and it was observed that the smallest change in efficiency between angles of incidence of 0° and 60° was 23%. Therefore, the light‐scattering properties of ZnO NR photoelectrodes improve the omnidirectional antireflection light capture characteristics of DSSCs.     

Enhanced hydrogen production from water splitting by Sn-doped ZnO/BiOCl photocatalysts and Eosin Y sensitization

The construction of semiconductor heterojunction for photocatalytic H₂ production from water splitting is an efficient and environment-friendly technology. In this work, ZnO/BiOCl (ZBC) and Sn-doped ZnO/BiOCl (ZBC-S) photocatalysts with Z-scheme heterojunction were successfully prepared by simple hydrothermal method. The photocatalytic H₂ evolution from water splitting by the as-prepared photocatalysts was investigated. The formation of ZnO/BiOCl heterojunction reduces the recombination probability of the photogenerated carriers. The impurity levels originated from Sn doping reduce the band gap width of ZnO and BiOCl to some extent, thereby enhancing the light absorption ability. The ZBC-S composite exhibits the best photocatalytic activity. In addition, the photocatalytic efficiency of H₂ production was improved by sensitization with Eosin Y (EY) dye. The H₂ production rate under simulated sunlight reaches 4146.77 μmol g^?1 h^?1, which is 27 times higher than that of pure ZnO. Finally, the Z-scheme electron transfer route in ZnO/BiOCl heterojunction was determined, and the photocatalytic H₂ production mechanism of EY sensitized ZBC-S was proposed.     

Synthesis and characterization of ZnO nanowires grown on different seed layers: The application for dye-sensitized solar cells

Zinc oxide (ZnO) nanowire electrodes which were grown on different seed layers and examination of their significant effects on the performance of dye sensitized solar cells were studied. Through chemical bath deposition process, the ZnO nanowires were grown on an indium tin oxide (ITO) coated glass using sputter-deposited aluminum doped zinc oxide (AZO) and ZnO seed layers. Afterward, main parameters such as solution concentration, growth temperature, and time were systematically investigated based on morphology of nanowires. The X-ray diffraction (XRD), field emission scanning microscopy (FESEM), and photoluminescence (PL) were applied to investigate the characteristics of the samples. The results showed ZnO nanowires, which were grown by AZO seed layer, had a high density array with hexagonal wurtzite structure distributed vertically and uniformly on ITO coated glass. The mentioned zinc-oxide nanowires grown under an optimum condition on different seed layer were used to fabricate dye solar cells afterward. The seed layer was effective on morphologic, optical, and structural features. The overall light-conversion efficiency of dye sensitized solar cell with ZnO nanowires grown on AZO seed layer was almost 2 times higher than that of those grown on ZnO seed layer. Electrochemical impedance spectroscopy analysis was measured under standard light to investigate the electron transport properties in the both ZnO-NW DSSCs. As the results showed, photoanode electron recombination rate with electrolyte was 6.02 Hz for dye solar cells of zinc oxide (ZnO-NWDSSC) produced by ZnO seed layer, which is 2.5 times faster than cells with AZO seed layer.     

Doping influence on intrinsic stress and carrier mobility of LP-MOCVD-deposited ZnO:B thin films

Textured boron-doped zinc oxide (ZnO:B) films, suitable as transparent and conductive layers in thin film silicon-based solar cells, have been obtained by low-pressure metalorganic chemical vapour deposition (LP-MOCVD) technique. The complex role of the boron doping in ZnO layers was examined and its influence on the morphological and structural properties was analysed. Furthermore, a correlation between such properties, intrinsic stress and carrier mobility in the film has been analysed. ZnO:B films have a polycrystalline structure with a columnar texture shape, they show a rough surface with pyramidal large grains. At the increase of the boron doping the pyramidal shape of the grains deteriorates and the average grain size reduces. Furthermore the a-lattice parameter decreases indicating that the boron incorporation introduces a deformation in the crystal. In addition to these large structural modifications, the decrease of the carrier mobility at the increase of the doping content is observed. At the same time, the boron content also plays a meaningful role on the intrinsic stress inside the film. Stress behaviour in ZnO films has been investigated by X-ray diffraction measurements using the sin² ψ method. Tensile stresses have been observed and the thermal and intrinsic components have been calculated. Respect to undoped ZnO films, boron incorporation on substitutional or interstitial sites increases the tensile stress by means of a lattice strain mechanism that reduces the d-spacing value.     

RF sputter deposition of the high-quality intrinsic and n-type ZnO window layers for Cu(In,Ga)Se2-based solar cell applications

Transparent ZnO films were prepared by rf magnetron sputtering, and their electrical, optical, and structural properties were investigated under various sputtering conditions. Aluminum-doped n-type(n-ZnO) and undoped intrinsic-ZnO (i-ZnO) layers were deposited on a glass substrate by incorporating different targets in the same reaction chamber. The n-ZnO films were strongly affected by argon ambient pressure and substrate temperature, and films deposited at 2 mTorr and 100°C showed superior properties in resistivity, transmission, and figure of merit (FOM). The sheet resistance of ZnO film was less dependent on film thickness when the substrate was heated during deposition. These positive effects of elevated substrate temperature are presumably attributed to the rearrangement of the sputtered atoms by the heat energy. Also, the films are electrically uniform through the 5 cm×5 cm substrate. The maximum deviation in sheet resistance is less than 10%. All of the films showed strong (0 0 2) diffraction peak near 2θ =34°. The undoped i-ZnO films deposited in the mixture of argon and oxygen gases showed high transmission properties in the visible range, independent of the Ar/O₂ ratio, while resistivity rose with increased oxygen partial pressure. The Cu(In,Ga)Se₂ solar cells, incorporating bi-layer ZnO films (n-ZnO/i-ZnO) as window layer, were finally fabricated. The fabricated solar cells showed 14.48% solar efficiency under AM 1.5 conditions (100 mW/cm²).     

Controlled synthesis of ZnO and TiO2 nanotubes by chemical method and their application in dye-sensitized solar cells

In this paper, the center hollow ZnO and TiO₂ nanotubes arrays were synthesized by chemical etching ZnO nanorods and sol-gel process assisted by ZnO nanorods templates, respectively. And the process concerning the formation of nanotubes was analyzed. Furthermore, as an application of the ZnO and TiO₂ nanotubes, dye-sensitized solar cells (DSSCs) based on them were successfully fabricated and the cell performances were characterized. The efficiency of DSSCs based on ZnO and TiO₂ nanotubes was 1.2% and 2.1%, respectively.     

Mesoporous F-doped ZnO prism arrays with significantly enhanced photovoltaic performance for dye-sensitized solar cells

A photoanode with a mesoporous F-doped ZnO prism array (F-ZnO PA) is prepared on an F-SnO₂ (FTO) glass substrate and its application in a dye-sensitized solar cell (DSSC) is investigated. A superstructure of mesoporous F-ZnO PA is obtained by thermal treatment of ZnF(OH) PA precursor, which has been previously grown directly onto the FTO substrate via chemical bath deposition. The structure and morphology of the films are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and TEM-EDS elemental mapping. The ZnO prisms are homogenously doped by F and composed of oriented nanograins and nanopores. Owing to the higher surface area and stronger light scattering, as well as longer electron lifetime and lower charge-transfer resistance, an electrical energy conversion efficiency (η) of 3.43% is achieved for a DSSC containing a 5 μm thick mesoporous F-ZnO PA photoanode, which is much higher than that for a similar cell based on a ZnO nanorod photoanode. The knowledge acquired in this work is important for the design of efficient photoanode materials for DSSCs.     

TiO2/ZnO/TiO2 sandwich multi‐layer films as a hole‐blocking layer for efficient perovskite solar cells

A continuous and compact hole‐blocking layer is crucial to prevent photocurrent recombination at the photoanode/electrode interface of high‐performance mesostructure perovskite‐based solar cells. Novel TiO₂/ZnO/TiO₂ sandwich multi‐layer compact film prepared as hole‐blocking layer for perovskite solar cell. Herein, TiO₂, ZnO, and TiO₂ layers were successfully deposited by spin‐coating onto FTO glass substrate in sequence. The fill factor and power conversion efficiency of the perovskite solar cell are remarkably improved by the employment of a TiO₂/ZnO/TiO₂ sandwich compact layer. Perovskite solar cell based on TiO₂/ZnO/TiO₂ sandwich film has been observed to exhibit maximum incident‐photon‐to‐current conversion efficiency in the visible region (400–780 nm) and reach a power conversion efficiency of 12.8% under AM1.5G illumination. Copyright © 2016 John Wiley & Sons, Ltd.     

Study of electron-beam evaporated Sn-doped In2O3 films

Electron beam evaporated Sn-doped In₂O₃ films have been prepared from the starting material with composition of (1 − x) In₂O₃ − -x SnO₂, where x = 0.0, 0.010, 0.025, 0.050, 0.090, and 0.120. X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and X-ray diffraction analysis were carried out on the films. Luminous transmittance and electrical resistivity of the films, show weak dependence on x. The composition of the film ([Sn]/[In] atomic ratio) was found to differ from that of the starting material. In fact, the atomic ratio was higher in the film than in the starting material by a factor which increases with x (ranging from 1.0 to 2.6). There is a relatively broad resistivity minimum in the layer atomic ratio range Sn/In = 0.06 − -0.09. These results compare well with those reported in the literature for Sn-doped In₂O₃ films, prepared by pyrolitic (spray) method.     

Effect of microfluidic electrolyte on the photovoltaic performance of ZnO-nanowire-based dye-sensitised solar cells

ZnO-nanowire-based dye-sensitised solar cells (DSSCs) were prepared to investigate the effect of microfluidic electrolyte on the photovoltaic performance of solar cells. At first, long and well-aligned ZnO nanowires were synthesised on FTO substrate via the improved hydrothermal method. The structure and crystallinity properties of ZnO nanowires were characterised by using SEM, TEM and XRD. The photovoltaic characteristics of the assembled DSSCs were measured under various flowrates of microfluidic electrolyte. The results showed that variations of the microfluidic electrolyte had little effect on the fill factor of the cells. However, the photocurrent and photovoltage of the solar cells exhibited systematic changes with the decrease of microfluidic electrolyte: the short-circuited current of the DSSCs decreased while the open-circuit voltage increased. The phenomenon could remain in some extent after cessation of injecting the electrolyte within a short time. The possible influencing mechanism of the microfluidic electrolyte on the photovoltaic performance was discussed.     

Cu(In,Ga)Se2 solar cells on stainless-steel substrates covered with ZnO diffusion barriers

ZnO layers were deposited as diffusion barriers by DC magnetron sputtering from a pure Zn target on stainless-steel substrates. It was found that the insertion layer of ZnO between Mo film and stainless-steel substrate had no influence on the orientation and composition of CIGS films, as identified by an X-ray diffractometer and X-ray fluorescence, respectively. However, ZnO diffusion barriers had strongly reduced the diffusion of Fe from stainless-steel substrates into the CIGS films, as investigated by secondary ion mass spectrometry. With such diffusion barriers, the efficiency, open-circuit voltage, and fill factor of CIGS solar cells all increased.     

Ge-doped ZnO nanorods grown on FTO for photoelectrochemical water splitting with exceptional photoconversion efficiency

In this study, a considerable effort has been devoted for the synthesis of Ge-doped ZnO nanorods on FTO as an efficient and robust photoanode material for solar water splitting. A unique, optimized, and ultra-rapid fabrication method to produce uniform nanorods (30–70 nm in diameter) has been demonstrated using radio frequency sputtering followed by electrochemical anodization. The effect of Ge doping on the conductivity, charge carrier concentration, optical, and photoelectrochemical properties of ZnO was investigated using scanning electron microscope (SEM), glancing angle X-ray diffraction (GAXRD), UV–Vis spectrometer, and Mott Schottky analysis. Glancing angle XRD confirmed the presence of wurtzite structure with a preferable orientation around (101) plane, which is of particular interest for many applications. As evidenced by the photoelectrochemical and transient photocurrent measurements, the fabricated Ge-doped ZnO nanorods exhibited enhanced photocurrent (12 mA/cm²) with an exceptional open circuit voltage of ?1.07 V_SCE (?0.416 V_RHE) under AM1.5G illumination, compared to the undoped ZnO based-photoanodes. Moreover, the Ge-doped ZnO nanorods showed unprecedented photoconversion efficiency of 3.6% under AM1.5G illumination. Therefore, the fabricated Ge-doped ZnO nanorods could be a promising conductive photoanode for water splitting.