Visible light-induced degradation of blue textile azo dye on TiO2/CdO–ZnO coupled nanoporous films

The photodegradation of a typical textile blue azo dye, followed by UV–VIS spectra analysis, has been carried out successfully under white light illumination on TiO₂/CdO–ZnO nanoporous coupled thin films. A relatively fast degradation occurs in dye solutions with concentrations of 100 mg/l (pH=3), at temperatures of 85°C, and with the aid of 400 mg/l hydrogen peroxide. Photodegradation also occurs on nanoporous TiO₂ films but with significant lower efficiency than on TiO₂/CdO–ZnO coupled nanoporous films. Dye photodegradation does not occur on TiO₂/CdO or TiO₂/ZnO nanoporous films, suggesting that both CdO and ZnO components are required on the sensitization of TiO₂ nanoporous films. A combined effect of new sensitizing interband states (response to white illumination) and/or rectification phenomena (improved charge separation) may be responsible of the higher photocatalytic activity of the TiO₂/CdO–ZnO nanoporous films. Similarly, the alternative route for visible degradation, the photosensitized degradation mechanism, could also benefit from the coupled nanoporous films due to a higher driving force for electron injection (dye oxidation).     

Photovoltaic devices based on PPHT: ZnO and dye-sensitized PPHT: ZnO thin films

We have investigated the optical and photovoltaic properties of hybrid organic solar cells based on the blend of poly-3-phenylhydrazone thiophene (PPHT):ZnO and PPHT:dye:ZnO. In this architecture, ZnO and PPHT were the electron acceptor and donor, respectively, and dye was used both as acceptor as well as sensitizer to enhance the photon absorption in visible region. These results showed that on incorporation of dye in PPHT: ZnO composite the light absorption, exciton separation and photocurrent under white light dramatically enhanced. The dependence of photovoltaic parameters on the weight fraction of ZnO in PPHT: ZnO was also investigated. It is found that the device with 45% of ZnO in both composites exhibits the best photovoltaic performance. The thermal annealing of PPHT: ZnO-based device gives rise to a significant increase in power conversion efficiency as evident from the measurements of incident photon to charge carrier efficiency (IPCE) spectra and current–voltage characteristics under illumination. The absorption band of PPHT: ZnO blend becomes stronger and the absorption peak ascribed to PPHT is shifted towards the longer wavelength region (red shift) upon thermal annealing. The effect of solvent used for thin-film fabrication was also studied. It is also observed that the fluorescence (FL) quenching and solar cell efficiency were found to be strongly dependent on the solvent used for spin coating.     

Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO₂. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO₂ catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO₂, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO₂. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH₄⁺, NO₃⁻, Cl⁻ and SO₄²⁻.     

Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers

Selective solar absorber coatings of carbon embedded in ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings. The surface morphology of the samples was studied by scanning electron microscopy (SEM). Cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Chemical composition analysis was done by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The crystal structure of ZnO and NiO samples was also investigated with an X-ray diffraction (XRD) technique. Samples were subjected to an accelerated ageing test for 95 h, with condensation at relative humidity of 95% and at a climate chamber temperature of 45 °C. The thermal emittances of the samples were 6% for the ZnO and 4% for the NiO matrix materials. The solar absorptances were 71% and 84% for ZnO and NiO samples, respectively. The SEM revealed a smooth featureless surface for both C–ZnO and C–NiO samples. Some C–NiO samples showed dentritic features. X-HRTEM, EDS and EELS studies revealed a nanometric grain size for both types of samples. The C–ZnO and C–NiO coatings contained amorphous carbon embedded in nanocrystalline ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The accelerated ageing tests produced performance criterion (PC) values of 0.15 and 0.054 at 95 h for the C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have better solar selectivity behaviour than the C–ZnO counterparts.     

Effect of electrolytes on the photovoltaic performance of a hybrid dye sensitized ZnO solar cell

The efficiency of dye sensitized solar cell depends on the number of factors such as impedance due to anions in the electrolytes, oxidation–reduction process of anions and size of cations of the electrolyte. This paper reports the effect of electrolytes on the photovoltaic performance of hybrid dye sensitized ZnO solar cells based on Eosin Y dye. The size of the cations has been varied by choosing different electrolytes such as LiBr+Br₂, LiI+I₂, tetrapropylammonium iodide +I₂ in mixed solvent of acetronitrile and ethylene carbonate. The impedance of anions has been determined by electrochemical impedance spectra. It is observed that Br⁻/Br₃⁻ offers high impedance as compared to I⁻/I₃⁻ couple. The oxidation–reduction reactions of electrolytes are measured by linear sweep voltammogram. It is found that Br⁻/Br₃⁻ is more suitable than an I⁻/I₃⁻ couple in dye sensitized solar cell (DSSC) in terms of higher open-circuit photovoltage production and higher overall energy conversion efficiency. This is attributed to more positive potential of the dye sensitizer than that of Br⁻/Br₃⁻. The gain in V_oc was due to the enlarged energy level difference between the redox potential of the electrolyte and the Fermi level (E_f) of ZnO and the suppressed charge recombination as well.     

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO₂, Nb₂O₅, ZnO, SnO₂, and In₂O₃ with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400–600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochrome-sensitized ZnO solar cell with an I⁻/I₃⁻ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99 mW cm⁻²) reached 2.5% with a short-circuit photocurrent density (J_sc) of 7.44 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.52 V, and a fill factor (ff) of 0.64. The J_sc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the V_oc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.     

Optical properties of silicon-based thin-film solar cells in substrate and superstrate configuration

The optical properties of microcrystalline silicon substrate-type solar cells with a front ZnO thickness of 500–1000 nm as required for monolithic series connection in PV-modules are investigated. The surface texture of the front ZnO was varied to study the possibility to reduce reflection losses and to improve light trapping. Before encapsulation, certain textures of the front ZnO exhibit improved light coupling for substrate solar cells. For substrate solar cells encapsulated with EVA and a glass cover, however, additional texture of the front ZnO has hardly any effect. Encapsulation also removes the antireflection conditions in case that the front ZnO was originally designed as ARC. So far, the quantum efficiency of encapsulated substrate solar cells does not show the high level as observed in superstrate solar cells possibly due to parasitic absorption in the front and back contact and the not optimized texture of the reflector (substrate).     

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.     

Sunlight/ZnO-mediated photocatalytic degradation of reactive red 22 using thin film flat bed flow photoreactor

Photocatalytic degradation of one of the most widely used cotton dyes, namely reactive red 22 (RR 22), was investigated in the presence of a thin film of ZnO photocatalyst using a thin film flat bed flow photoreactor under solar radiation. The effects of reaction parameters such as pH, amount of ZnO coating, flow rate and concentration of the dye solution on the percentage removal of dye were examined. In a single pass mode at 30 ml/min flow rate, 52.7% decrease in concentration was achieved for 200μM dye solution (pH 10). It has been demonstrated that in continuous circulation mode the time required to decompose half the concentration of the dye in 500 ml of 200 μM was 15.8 min. Complete removal of 200 μM dye solution (pH 10) was achieved at about 100 min.     

Antireflective coating fabricated by chemical deposition of ZnO for spherical Si solar cells

ZnO thin films as an antireflective (AR) coating have been successfully fabricated on spherical Si solar cells by chemical deposition, which enables uniform film formation. ZnO films were prepared chemically by immersing the cell in an aqueous solution of zinc nitrate and dimethylamineborane maintained at 80 °C. The current–voltage measurements of the solar cells confirmed the increase in short circuit current induced by the AR effect. The open circuit voltage and fill factor were improved by surface passivation. As a result, the conversion efficiency of cells without an AR coating (9.45%) increased to 11.8%, which represents a 25% (relative) increase. The results indicate that the chemical deposition of ZnO is effective for the AR coating of spherical Si solar cells.     

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.     

Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates

ZnO films prepared by magnetron sputtering on glass substrates and textured by post-deposition chemical etching are applied as substrates for p–i–n solar cells. Using both rf and dc sputtering, similar surface textures can be achieved upon etching. Excellent light trapping is demonstrated by high quantum efficiencies at long wavelengths for microcrystalline silicon solar cells. Applying an optimized microcrystalline/amorphous p-layer design, stacked solar cells with amorphous silicon top cells yield similarly high stabilized efficiencies on ZnO as on state-of-the-art SnO₂ (9.2% for a-Si/a-Si). The efficiencies are significantly higher than on SnO₂-coated float glass as used for module production.     

Rough ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells

Doped ZnO layers deposited by low-pressure chemical vapour deposition technique have been studied for their use as transparent contact layers for thin-film silicon solar cells.Surface roughness of these ZnO layers is related to their light-scattering capability; this is shown to be of prime importance to enhance the current generation in thin-film silicon solar cells. Surface roughness has been tuned over a large range of values, by varying thickness and/or doping concentration of the ZnO layers.A method is proposed to optimize the light-scattering capacity of ZnO layers, and the incorporation of these layers as front transparent conductive oxides for p–i–n thin-film microcrystalline silicon solar cells is studied.     

Chemical studies of solar cell structures based on electrodeposited CuInSe2

ZnO/CdS/CIS solar cell structures have been made by using different deposition techniques for each layer: standard rf-magnetron sputtering and chemical bath deposition for ZnO and CdS, respectively, and direct electrodeposition for CIS as a low-cost alternative to the co-evaporated absorber. Chemical studies of ZnO, CdS and CIS films, ZnO–CdS and CdS–CIS bilayers, and ZnO–CdS–CIS cells have been performed by utilising XPS. No chemical reactions were detected in the interfaces. Photovoltaic quality was evaluated from the spectral response data.     

Simplifying the construction of dye-sensitized solar cells to increase their accessibility for community education

Simple dye-sensitized solar cells were developed using blackboard chalk as a substrate for mixed ZnO and SnO₂ films that were sensitized with Mercurochrome (Merbromine) dye. Graphite pencil “leads” were used as counter electrodes for the cells and the electrolyte consisted of an aqueous solution of iodine and potassium iodide that was gelled with a disinfectant containing quaternary ammonium compounds and cyanoacrylate adhesive (Superglue^®). The open circuit potential of constructed cells was typically 0.50–0.64 V and the short circuit current varied between 0.5 and 2.0 mA cm⁻². The cells were developed as an educational resource that could be simply and safely constructed in a home or school environment with readily accessible materials.     

Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques

A larger number of dye sensitization solar cells based on cis-Ru^II(LH₂)₂(NCS)₂ with LH₂=2,2′-bipyridyl-4,4′-dicarboxylic acid with an electrolyte consisting of 0.5 M LiI, 50 mM I₂, 0.2 M tert.-butyl pyridine in acetonitrile have been studied, using spatially resolved photocurrent imaging techniques. Measurements have been made after preparation and periodically during a longer period of simulated solar light illumination. The observed phenomena have been grouped into five categories. The first one concerns significant inhomogeneities reflecting the TiO₂-layer preparation technique used. The second category concerns an inhomogeneous deterioration of the dye sensitization cell during illumination. The third phenomenon involves photodegradation itself, which can be visualized by selectively illuminating the dye sensitization solar cell. Changes observed in the composition of the electrolyte, typically indicated by a bleaching of the iodide/iodine solution were also observed. Finally, the fifth category to be considered deals with a loss of electrolyte and the parallel appearance of gas bubbles in the solar cell. All these phenomena may coexist, being responsible for the overall process of degradation. The different mechanisms are discussed and analyzed in an effort to determine parameters critical for increasing efficiency and stability of dye sensitization solar cells.     

Decolourization of reactive dyes by thin film immobilized surface photoreactor using solar irradiation

The photocatalytic degradation of four reactive dyes using TiO₂ was investigated in suspended and immobilized systems under solar irradiation. Batch degradation experiments were carried out at initial concentrations ranging from 25 to 100 mg l⁻¹ and at a catalyst loading of 0.5–1 g l⁻¹. The studies on batch photocatalytic degradation of four dyes, showed about 30–70% colour removal depending on the initial dye concentration, dye structure (functional group and reactivity of dyes) and the amount of catalyst. The thin film immobilized surface photoreactor was able to give nearly 90–98% colour removal depending on the initial concentration and exposure time. Flow rate has noticeable effect on colour removal particularly at higher concentration (100 mg l⁻¹). High colour removals obtained with solar radiation indicated effectiveness of this process and its potential for practical application.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

ZnO与TiO2的质量比对染料敏化太阳能电池性能的影响

采用低温水溶液法制备ZnO微米棒;ZnO微米棒与TiO2纳米粉以不同比例混合,制备复合浆料;采用刮涂法把复合浆料涂敷在透明导电玻璃上,制备ZnO/TiO2复合薄膜光阳极。通过电池的I-U特性和电化学阻抗谱测试,研究ZnO微米棒与TiO2纳米粉的比例对电池性能的影响。结果表明:当ZnO与TiO2的质量比为1∶1时,DSSC的效率最高,此时的光电转换效率比纯TiO2电池的效率提高了31%。这主要得益于ZnO微米棒更高的光利用率和良好的电子转移特性。     

Field and laboratory studies of the stability of amorphous silicon solar cells and modules

If photovoltaic solar cells and modules are to be used as a major source of power generation it is important to have a good knowledge and understanding of their long-term performance under different climatic and operating conditions. A number of studies of the long-term performance of commercially available photovoltaic modules manufactured using different technologies have now been reported in the literature. These have shown clear differences in the seasonal and long term performance and stability of different solar cell techniques. In addition to general module engineering factors that result in a loss of performance in all modules some types of solar cells, such as those made from thin film amorphous silicon (a-Si:H), also suffer specific losses in performance due to fundamental material changes, such as photodegradation or the Staebler–Wronski effect (SWE). A field evaluation of the long term performance of state-of-the-art crystalline and amorphous silicon photovoltaic modules in Australian conditions is currently being undertaken at Murdoch University. The initial results from this monitoring program are reported. This paper also reports on laboratory and field studies being undertaken on the nature of the Staebler–Wronski effect in amorphous silicon solar cells and how the stability of these cells is affected by different operating conditions. Based on a mechanism for the SWE in a-Si:H solar cells developed as a result of our research we propose a number of possible ways to reduce the Staebler–Wronski effect in a-Si:H solar cells.