Investigation on the energy conversion of dye-sensitized solar cells based on TiO<Subscript>2</Subscript> core/shell using metal oxide as a barrier layer

Photo-electrochemical solar cells based on a core-shell structure including ZnO shell and TiO₂ cove, have been fabricated with ruthenium bipyridyl complex (N719) as the sensitizer. Compared with the pure anatase TiO₂, the ZnO-covered TiO₂ film possesses outstanding ability to transport electrons with an overall power conversion efficiency of 3.72. Elctrochemical study shows that surface modification of TiO₂ film with ZnO can increase the concentration of free electrons in the conduction band of TiO₂. This remit implies that the charge recombination is reduced in process of electron transport through the TiO₂ porous film, which can decrease the photocurrent loss and hence improve Dye-Sensitized solar ceils (DSSCs) efficiency. This result indicates that optimization of TiO₂ porous network fabrication condition is efficient, for the improvement of TiO₂ based DSSC’s performances.     

Antireflection coatings for solar photoelectric elements based on SiO<Subscript>2</Subscript> and TiO<Subscript>2</Subscript> mixed oxides

Computer modeling of one-layer antireflection coating based on SiO₂ and TiO₂ mixed oxides was carried out for solar elements. It was concluded that a photoelectric current can be increased by varying the thickness of the covering within the limits of 55–90 nm, while the concentration of SiO₂ in TiO₂ shall not exceed 30%. A method of magnetron spattering was offered as a method for application of the coating based on a SiO₂-TiO₂ mixture.     

Effect of acidic additives on the structure and performance of TiO2 films prepared by a commercial nanopowder for dye-sensitized solar cells

This work comprises an experimental study on the effect of various dispersing agents on the morphology of mesoporous TiO₂ films prepared by the doctor blade method and on the performance of the resulting dye-sensitized solar cells. TiO₂ films were prepared using a commercial nanopowder, Degussa P25, which was ground in a mortar, with different dispersing agents and under continuous grinding, in order to break the large agglomerates (>1 μm), present in the powder. These additives can be different acids or bases and must prevent also re-agglomeration of the nanoparticles, by forming a surface charge.Two strong acids (hydrochloric, HCl and nitric, HNO₃), a weak one (acetic acid, CH₃COOH) and a ketone (acetylacetone, C₅H₈O₂) were used in turn. The properties of the films were dependent on the concentration and the kind of the acid. With increasing concentration the coagulation of the nanoparticles also increases, thus affecting the efficiency of the devices. Of all the additives used, HNO₃ gave the best results and a 40% increment in efficiency was observed, compared to the standard dispersing agent that is acetylacetone. The improved homogeneity of the nanoparticle size (24 nm for anatase and 21.5 nm for rutile) and the better connectivity between them were responsible for the improvement in efficiency.     

Antireflection coatings for solar elements based on Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> oxides

An alloy of a mixture of Al₂O₃ and SiO₂ oxides has been obtained under radiant heating. It is established that films of the Al₂O₃ and SiO₂ alloy, deposited on the surface of glass and silicon plates, have high mechanical strength and adhesion, are transparent in the SE sensitivity region, and can be used as antireflection coatings.     

Enhancement in the performance of dye-sensitized solar cells containing ZnO-covered TiO2 electrodes prepared by thermal chemical vapor deposition

A ZnO-covered TiO₂ (denoted as ZnO/TiO₂) film was prepared by incorporating a small quantity of particulate ZnO in a TiO₂ matrix by thermal chemical vapor deposition. When used in a dye-sensitized solar cell, an enhancement was observed in both short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc) by 12% and 17%, respectively, relative to those of a cell containing a bare TiO₂ film. The observed J_sc enhancement is attributed to the increase in the surface area of the ZnO/TiO₂ film, and the V_oc enhancement to the formation of a potential barrier by ZnO at TiO₂/electrolyte interface. The films were characterized by FE-SEM, EDX, and XRD.     

Possibilities of increasing the efficiency of Si and CuInSe<Subscript>2</Subscript> solar cells

The paper proposes a method of increasing the efficiency of Si and CuInSe₂ solar cells using the impact ionization and impurity photovoltaic effect in pZnTe-pSi-nSi and pZnTe-pCuInSe₂-n(CuInSe₂)_1−x (2InAs) _x structures.     

The effect of ZnO-coating on the performance of a dye-sensitized solar cell

This study investigates the effect of a ZnO-coated TiO₂ working electrode on the power conversion efficiency of a dye-sensitized solar cell (DSSC). This electrode was designed and fabricated by dipping the TiO₂ electrode with the TiCl₄ treatment in a solution of zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] and ethanol. The effects of the concentration of Zn(CH₃COO)₂·2H₂O and the duration of dipping on the band gap of a working electrode and the power conversion efficiency of a DSSC were also examined. The band gap of the working electrode increases to 3.75 eV [TiO₂ electrode dipped in 0.05 M Zn(CH₃COO)₂·2H₂O) for 3 min] from 3.22 eV (TiO₂ electrode). Interestingly, the power conversion efficiency of the DSSC with a Zn-coated TiO₂ electrode (6.7%) substantially exceeds that of the conventional DSSC with a TiO₂ electrode (5.9%), and it may be originated from an increased energy barrier between ZnO and TiO₂ that reduces the electron recombination rate.     

An analytical study of the porosity effect on dye-sensitized solar cell performance

The porosity in a dye-sensitized solar cell (DSSC) can affect light absorption and electron diffusion that govern the overall electrical current–voltage (I–V) characteristics. In this research, two methods, namely, constant overlap and variable overlap, were developed to determine the connectivity of dye-sensitized TiO₂ particles in high and low porosity levels, respectively. In turn, the light absorption coefficient α and the electron diffusion coefficient D were analytically derived in terms of the porosity P. Subsequently, the electron diffusion differential equation involving α and D was solved for the I–V output as a function of P. A parametric analysis showed that the optimal porosity was equal to 0.41 for maximum I–V output. The analytical results agree well with experimental data reported in the literature. Besides DSSC, the analytical model can be applied to predict the performance of solid-state DSSC as well as dye-sensitized photoelectrochemical cells applied to hydrogen production and water purification.     

Electronic and optical proeprties of neon-doped rutile TiO<Subscript>2</Subscript> from ab inito calculations

The electronic-structure and optical properties of neon-doped rutile TiO₂ have been investigated using density functional theory with Slater type orbitals basis set and correlation. This was done using the PBE method as implemented within the Hyper Chem 7.52 software package with Ne concentration approaching the low level may present in industrial samples of rutile TiO₂. Defect states involving substitution of an oxygen atom for a neon atom were studied along with the more stable configuration of one neon substitution. Neon change the band structure and lead to a reduce in the band gap in rutile. This make that neon doping brings the absorption edge into the visible range and therefore increase the photocatalytic activity.     

Spray deposition and compression of TiO2 nanoparticle films for dye-sensitized solar cells on plastic substrates

Spray deposition of powder suspensions followed by room temperature compression was studied as a method for preparing nanostructured TiO₂ films for dye-sensitized solar cells. The structure of the films was analyzed with optical and scanning electron microscopy and the films were applied to dye-sensitized solar cells. Continuous and fast deposition of crack-free 7–14 μm thick films was achieved by heating the substrates during the deposition. Scanning electron microscopy revealed small amount of structural imperfections in the compressed films due to the nature of the deposition method. An energy conversion efficiency of 2.8% was achieved at 100 mW/cm² light intensity.     

The influence of concentrated solar radiation on the properties of Ga<Subscript>0.7</Subscript>In<Subscript>0.3</Subscript>P/GaP photoconverters

The properties of Ga_0.7In_0.3P/GaP heterophotoconverters with a two-sided contact grid are studied in the range of solar radiation concentration K _s = 1−100 times and with natural convective heat exchange. It is found that, in the considered photoconverters, the dependences of an idle running voltage, the duty cycle of the current-voltage characteristic, and the efficiency on concentration are additionally improved due to high heat conduction of GaP and the temperature stability of the broad-band heterophotoconverter with “transparent“ structural design.     

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。     

Component exergy analysis of solar powered transcritical CO<Subscript>2</Subscript> rankine cycle system

In this paper,exergy analysis method is developed to assess a Rankine cycle system,by using supercritical CO2 as working fluid and powered by solar energy.The proposed system consists of evacuated solar collectors,throttling valve,high-temperature heat exchanger,low-temperature heat exchanger,and feed pump.The system is designed for utilize evacuated solar collectors to convert solar energy into mechanical energy and hence electricity.In order to investigate and estimate exergy performance of this system,the energy,entropy,exergy balances are developed for the components.The exergy destructions and exergy efficiency values of the system components are also determined.The results indicate that solar collector and high temperature heat exchanger which have low exergy efficiencies contribute the largest share to system irreversibility and should be the optimization design focus to improve system exergy effectiveness.Further,exergy analysis is a useful tool in this regard as it permits the performance of each process to be assessed and losses to be quantified.Exergy analysis results can be used in design,optimization,and improvement efforts.     

Fabrication of multilayer TiO2 thin films for dye-sensitized solar cells with high conversion efficiency by electrophoresis deposition

This research coats a commercial TiO₂ nanoparticle Degussa P25 with good roundness and size uniformity on an indium tin oxide (ITO) glass substrate and to be photoelectrical electrode by electrophoresis deposition. It combined with dye N719, electrolyte I^-/ and counter-electrode of Pt layer to produce dye-sensitized solar cells (DSSCs). Through the electrophoretic technique, a multilayer film of an appropriate thickness is deposited in the suspension containing TiO₂ nanoparticles and isopropanol. In this process, electric current, voltage, and the number of deposition cycles are well controlled to obtain a single TiO₂ film of around 3.3 μm thick. Stacking is then performed to obtain a multilayer-typed TiO₂ film of around 12 μm thick. As the sintering temperature reaches 400 °C, the prepared multilayer TiO₂ film with a good compactness can increase the dye adsorption capability of the thin film and enhance its adsorption percentage. In addition, the heat treatment will transfer a portion of the rutile crystalline into the anatase crystalline, resulting in better material properties for DSSCs application. DSSCs produced are exposed to metal halide lamp and their energy conversion efficiency is measured. The I-V curve of the produced DSSCs shows that it has an excellent energy conversion efficiency of 6.9%.     

Performance of dye-sensitized solar cell based on nanocrystals TiO2 film prepared with mixed template method

Highly efficient dye-sensitized solar cells were produced using high-crystalline TiO₂ nanoparticles as a thin-film semiconductor prepared with a mixed template of copolymer F127 (poly(ethylene oxide)₁₀₆-poly(propylene oxide)₇₀-poly(ethylene oxide)₁₀₆) and surfactant CTAB (cetyltrimethylammonium bromide) which allows access to larger surface area, smaller size and higher crystallinity TiO₂ particles. The light-to-electricity conversion of the TiO₂ film composed of nanocrystals with the size of 35 nm, which carry out perfect electrical contiguity between film and conducting glass and between every TiO₂ coating, was over 6% with a film of 5.5 μm thickness. Over 8% conversion efficiency has been obtained with a double-layer film composed by the TiO₂ layer and the scattering layer.     

Theoretical modeling of TiO2/TCO interfacial effect on dye-sensitized solar cell performance

A theoretical model based on an integration of both Schottky barrier model and electron diffusion differential model was developed to determine the TiO₂/TCO interfacial effect on the current–voltage (J–V) characteristics of a dye-sensitized solar cell (DSSC). The thermionic-emission theory was appropriately applied to describe the electron transfer at the TiO₂/TCO interface. A parametric analysis was conducted to study how the photoelectric outputs varied with multiple independent variables, such as Schottky barrier height (φ_b) and temperature. It was found that the variation of the maximum DSSC power output (P_max) was insignificant when φ_b varied at a low value; however, an increase in φ_b exceeding a critical value caused an apparent decrease in the maximum DSSC power output. The theoretical results were quantitatively compared and agreed very well with published theoretical results. The experimental data from literature were found to agree well with the present theoretical results, qualitatively validating the present model. The theoretical model can be applied to facilitate selection of suitable TCO material in DSSC design to avoid the adverse TiO₂/TCO interfacial effect.     

The effects of hydrothermal temperature and thickness of TiO2 film on the performance of a dye-sensitized solar cell

The effects of hydrothermal temperature on the preparation of TiO₂ colloids, and their film thickness on fluorine-doped tin oxide (FTO) glass, toward the performance of a dye-sensitized solar cell (DSSC) were investigated. Pore diameter and surface area of the TiO₂ are of paramount importance in determining the cell efficiency. With the increase of hydrothermal temperature, the pore diameter increases linearly; however, the surface area shows the reverse effect. It is found that the DSSC assembled with the TiO₂ films prepared under the hydrothermal temperature of 240 °C, and the film thickness larger than 10 μm gives optimal performance. The effect of film thickness of TiO₂ on the performance of the DSSC can be explained by the relative size of reactive species diffusing into the thin film and the lifetime of injected electrons. Electrochemical impedance spectroscopy (EIS) was also used to analyze the resistance of the cell, developed as a result of the change in the thickness of the TiO₂ thin film. The at-rest stability for over 200 days was monitored and the results show that the solar energy conversion efficiency was found to decrease from 5.0% of initial value to 3.0% at the end.     

Synthesis of pyroxene pyroceramics in large solar furnace with ZrO<Subscript>2</Subscript> crystallization nucleator

Research on the development of zirconium-containing pyroxene glass-ceramic materials with high mechanical and chemical stability has been performed.     

Thickness effect of RF sputtered TiO2 passivating layer on the performance of dye-sensitized solar cells

We report on the characteristics of a TiO₂ passivating layer grown by radio frequency (RF) magnetron sputtering on F-doped SnO₂ (FTO) electrodes as a function of its thickness. The optical transparency, surface roughness and passivation properties of the TiO₂ layer passivating the FTO electrode depend on the thickness of the TiO₂ passivating layer. In addition, it was found that the power conversion efficiency of the dye-sensitized solar cells (DSSCs) is critically dependent on the thickness of RF sputtered TiO₂ layer inserted between FTO electrode and nanoporous TiO₂ layer. The DSSC fabricated on 50 nm thick TiO₂ passivating FTO electrode showed the maximum power conversion efficiency of 4.42% due to effective prevention of the electron transfer to electrolyte. This indicates that the thickness optimization of the TiO₂ passivating layer is one of the important parameter to obtain high performance DSSCs.     

Properties of fluorine-doped SnO2 films prepared by the pyrosol deposition method

Fluorine-doped SnO₂ (SnO₂:F) films were prepared in ordinary atmosphere on heated Corning 7059 glass substrates by the pyrosol deposition method with solutions consisting of SnCl₄·5H₂O, NH₄F, CH₃OH, H₂O and HCl. It was found that the substrate temperature and the chemical composition of the solutions largely affect the deposition rate and the properties of the SnO₂:F films. Under the optimized deposition condition, a resistivity as low as ≈4.3 × 10⁻⁴ Ω cm and a specular transmittance of ≈79% could be attained for a ≈0.6 μm thick film. X-ray diffraction measurements showed that these films were polycrystalline with the tetragonal cassiterite structure and grew with a (200) preferred orientation. The surface morphology observed by scanning electron microscopy changed from round-shape to pyramidal-shape above a substrate temperature of 450°C. A similar change in the surface morphology also took place when the CH₃OH/H₂O mol ratio in solution was less than 0.1. X-ray photoelectron spectroscopy indicated that the fluorine concentrations in the films, being significantly diminished, also increase with increasing the fluorine concentrations in the starting solutions.