Enhanced efficiency of dye-sensitized solar cells by UV–O3 treatment of TiO2 layer

Solar conversion efficiency of dye-sensitized solar cells was improved by UV–O₃ treatment of TiO₂ before and/or after sintering. The enhancement was resulted from the removal of the residual organics originated from the TiO₂ precursor pastes, increased adsorption of dyes to the TiO₂, surface, and longer diffusion length and shorter electron transit time of electrons through the TiO₂ mesoscopic structure. The power conversion efficiency of the cells reaches to 7.2% with the open circuit voltage of 0.71 V, the short circuit current density of 15.2 mA/cm² and the fill factor of 0.67 under illumination with AM 1.5 (100 mW/cm²) simulated sunlight.     

Synthesis of self-light-scattering wrinkle structured ZnO photoanode by sol–gel method for dye-sensitized solar cells

A special morphological zinc oxide (ZnO) photoanode for dye-sensitized solar cell was fabricated by simple sol–gel drop casting technique. This film shows a wrinkled structure resembling the roots of banyan tree, which acts as an effective self scattering layer for harvesting more visible light and offers an easy transport path for photo-injected electrons. These ZnO electrode of low thickness (~5 μm) gained an enhanced short-circuit current density of 6.15 mA/cm², open-circuit voltage of 0.67 V, fill factor of 0.47 and overall conversion efficiency of 1.97 % under 1 sun illumination. This shows a high conversion efficiency and a superior performance than that of ZnO nanoparticle-based photoanode (η ~ 1.13 %) of high thickness (~8 μm).     

Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V₂O₅/TiO₂ system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V₂O₅/TiO₂ system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO₂ calcined at 450 °C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 °C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO₂ phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 °C was predominantly V⁴⁺. However, the presence of V⁵⁺ atoms cannot be ruled out. A qualitative analysis of extended X-ray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 °C showed that the local structure around vanadium atoms is comparable to that of VO₂ crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 °C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a V_xTi_(1−x)O₂ solid solution formation. The conditions of sample preparation used here to prepare V₂O₅/TiO₂ samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V₂O₅/TiO₂ system.     

Improved efficiency of dye-sensitized solar cells applied with nanostructured N–F doped TiO2 electrode

Dye-sensitized solar cells (DSSCs) were fabricated with N–F-doped TiO₂ electrodes. The XRD pattern of the N–F-doped TiO₂ is almost the same as that of pure TiO₂, showing that N and F doping has little influence on the formation of anatase titania. The influence of dopant N and F on band energetics and photoelectrochemical properties of nanostructured TiO₂ electrodes were investigated. Compared with pure TiO₂ electrodes, the E_fb of N–F-doped TiO₂ electrodes shifted a little in electrolytes containing LiClO₄. However the total trap densities were remarkably decreased as TiO₂ electrodes were doped with N and F. Finally the N–F-doped TiO₂ electrodes were sensitized with N3 and their photoelectrochemical properties were studied. Experimental results showed that the photoelectric conversion efficiency of N3 sensitized N–F-doped TiO₂ electrodes was 8.61% under irradiation of 100 mW cm^?2 white light, about 17.1% higher than that of a pure TiO₂ electrode.     

Characterization and tribological investigation of SiO2 and La2O3 sol–gel films

Thin films of SiO₂ and La₂O₃ were prepared on a glass substrate by a dip-coating process from specially formulated sols. The tribological properties of the resulting thin films sliding against a Si₃N₄ ball were evaluated on a one-way reciprocating friction and wear tester. The morphologies of the unworn and worn surfaces of the films were examined by an atomic force microscope (AFM) and a scanning electron microscope (SEM). La₂O₃ shows the best tribological performance. The coefficient of friction is about 0.1 and the wear life is over 5000 sliding passes both under higher (3 N) and lower load (1 N). The SiO₂ film derived from a specially formulated aqueous solution shows much better performance in resisting wear and reducing friction than the one derived from an ethanol solution. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. SEM observation of the morphologies of worn surfaces indicates that the worn surface of La₂O₃ is too slight to be observed by SEM. The wear of SiO₂ derived from TEOS solution is the characteristic of delaminating, which is responsible for the abrupt failure of the film. The wear of SiO₂ derived from aqueous solution is the characteristic of fracture. Brittle fracture and severe abrasion dominate the wear of glass substrate.     

Low-temperature photoluminescence spectra of CdO–In2O3 thin films prepared by sol–gel

(CdO)_1?x–(InO_3/2)_x thin films were deposited on glass substrates by the sol–gel method. The precursor solutions for the mixed oxide films were obtained from the mixture of the precursor solutions for CdO and In₂O₃ prepared separately. The investigated In atomic concentrations in the solution, x, were 0.0, 0.16, 0.33, 0.50, 0.67, 0.84, and 1. X-ray diffraction measurements showed that the films were mainly constituted of CdO, In₂O₃, and CdIn₂O₄. CdO and In₂O₃ were obtained for x=0 and 1, respectively. For x=0.67, which is the stoichiometric composition of the CdIn₂O₄ compound, only this oxide was formed. CdO and CdIn₂O₄ crystals were obtained in the Cd-rich region, whereas In₂O₃ and CdIn₂O₄ crystals were formed in the In-rich region. The PL spectra at 15 K for CdO showed the presence of two main emission bands at energies close to 2.2 and 3.0 eV. A blue-shift of these bands took place for increasing In concentration, which is related to the increase in the band gap energy of the mixed system in going from CdO, with a band gap energy of 2.46 eV, to CdIn₂O₄, with 3.2 eV, to In₂O₃, with 3.6 eV.     

Ion-irradiation enhanced epitaxial growth of sol–gel TiO2 films

We report the epitaxial growth of sol–gel TiO₂ films by using ion-irradiation enhanced synthesis. Our present study shows that the ion-beam process can provide highly crystalline TiO₂ even at 350°C. Nuclear energy deposition at amorphous/crystalline interface plays a dominant role in the epitaxial growth of the films at the reduced temperature via a defect-migration mechanism. In addition, the ion irradiation allows for increasing the film density by balancing the crystallization rate and the escape rate of organic components.     

Spectroscopic properties of Co2+: ZnAl2O4 nanocrystals in sol–gel derived glass–ceramics

Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl₂O₄) nanocrystals doped with tetrahedrally coordinated Co²⁺ ions were obtained by the sol–gel method for the first time. The gels of composition SiO₂–Al₂O₃–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl₂O₄ nanocrystals were precipitated. Co²⁺ ions were located in tetrahedral sites in ZnAl₂O₄ nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl₂O₄ crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co²⁺ doped glass–ceramic and Co²⁺ doped bulk crystal was analysed. The crystal field parameter D_q of 423 cm⁻¹ and the Racah parameters B of 773 cm⁻¹ and C of 3478.5 cm⁻¹ were calculated for tetrahedral Co²⁺ ions.     

Optical and transport properties of Sn-doped ZnMn2O4 prepared by sol–gel method

The paper describes the effect of ZnMn₂O₄ doping with different Sn ratios. Sn_x–ZnMn₂O₄ is prepared with Sol–Gel route at 700 °C. The structural, optical and electrical properties were studied. The X-ray diffraction patterns indicate the formation of pure tetragonal phase for the ratios lower than 0.03%, while for other compositions, the secondary phases were observed. The attenuated total reflectance (ATR) analysis confirmed the localization of Sn in octahedral sites. The optical properties showed not only the increase of E_g, but also the improvement of the optical characteristics such as extinction coefficient (k), optical conductivity (σ_opt), dissipation factor (tan δ) and the relaxation time (τ). The latter has been improved by 50%. The Hall measurements confirmed the transition of the conductivity mode, i.e, from p to n-type. The formation of Sn_Mn¹⁺ point defects is evidenced; however, the transport properties indicate that the charge carriers are mainly localized.     

Thermal analysis and infrared study of Nb2O5–TeO2 glasses

Tellurite glasses of the xNb₂O₅–(100–x) TeO₂, (3 ≤ x ≤ 20 mol%) system have been prepared and studied by IR spectroscopy and differential thermal analysis to explore the role of Nb₂O₅ on their structure. IR analysis indicates that NbO₆ transforms TeO₄ units into tellurite structural TeO₃ units, with a shift of lattice vibrations towards higher wavenumbers. The stretching force constant of the tellurite structural units increases with Nb₂O₅ content, a feature that is attributed to the higher bond strength and higher coordination number of Nb₂O₅ relative to TeO₂. The crystallization kinetics has been studied under non-isothermal conditions using the formal theory of transformations for heterogeneous nucleation. The crystallization results are analyzed, and both the activation energy of the crystallization process and the crystallization mechanism are characterized. The thermal stability of these glasses are characterized in terms of characteristic temperatures, such as the glass-transition temperature, T _g, the temperature of onset of crystallization, T _in, the temperature corresponding to the maximum crystallization rate, T _p, and two kinetic parameters, K(T _g) and K(T _p). The results reveal that thermal stability increases with increasing Nb₂O₅ content. XRD diffraction of the studied glasses indicates the presence of microcrystallites of α-tellurite, γ-telluride, Nb₂Te₄O₁₃ and an amorphous matrix.     

Crystallization and conductivity of 2CaO–La2O3–5P2O5 glass–ceramic with Al2O3 addition

Al₂O₃ was added to a 2CaO–La₂O₃–5P₂O₅ metaphosphate, to replace 10% of the Ca²⁺ ions by Al³⁺, forming a phosphate with the nominal composition 1.8CaO–0.1Al₂O₃–La₂O₃–5P₂O₅. The effect of Al₂O₃ addition and heat treatment on the microstructure and conductivity of the resulting glass–ceramics was investigated by XRD, SEM, TEM, and AC impedance spectroscopy. Upon transformation from glass to glass–ceramic, conductivities increased significantly. The glasses were isochronally transformed at 700 and at 800 °C for 1 h or 5 h, in air, following heating at 3 or 10 °C/min. With Al₂O₃ addition, after a heat treatment at 700 °C, 100–300 nm nano-domains of LaP₃O₉ crystallized from the glass matrix. Annealing at 800 °C produced a further order of magnitude conductivity increase for the Al-free glass, but less so for the Al-containing glass.     

Microstructure and varistor properties of ZnO–V2O5–MnO2 ceramics with Ta2O5 addition

The effect of Ta₂O₅ addition on microstructure, electrical properties, and dielectric characteristics of the quaternary ZnO–V₂O₅–MnO₂ vaistor ceramics was investigated. Analysis of the microstructure indicated that the quaternary ZnO–V₂O₅–MnO₂–Ta₂O₅ ceramics consisted of mainly ZnO grain and minor secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, TaVO₅, and Ta₂O₅. As the amount of Ta₂O₅ increased, the sintered density increased from 94.8 to 97.2% of the theoretical density (5.78 g/cm³ for ZnO), whereas the average grain size decreased from 7.7 to 6.0 μm. The ceramics added with 0.05 mol% Ta₂O₅ exhibited the highest breakdown field (2715 V/cm) and the highest nonlinear coefficient (20). However, further increase caused α to abruptly decrease. The Ta₂O₅ acted as a donor due to the increase of electron concentration in accordance with the amount of Ta₂O₅. The donor concentration increased from 1.97×10¹⁸ to 3.04×10¹⁸cm^?3 with increasing the amount of Ta₂O₅ and the barrier height exhibited the maximum value (0.95 eV) at 0.05 mol% Ta₂O₅.     

Effects of annealing rate and morphology of sol–gel derived ZnO on the performance of inverted polymer solar cells

The effects of annealing rate and morphology of sol–gel derived zinc oxide(ZnO)thin films on the performance of inverted polymer solar cells(IPSCs)are investigated.ZnO films with different morphologies are prepared at different annealing rates and used as the electron transport layers in IPSCs.The undulating morphologies of ZnO films fabricated at annealing rates of 10 C/min and 3 C/min each possess a rougher surface than that of the ZnO film fabricated at a fast annealing rate of 50 C/min.The ZnO films are characterized by atomic force microscopy(AFM),optical transmittance measurements,and simulation.The results indicate that the ZnO film formed at 3 C/min possesses a good-quality contact area with the active layer.Combined with a moderate light-scattering,the resulting device shows a 16%improvement in power conversion efficiency compared with that of the rapidly annealed ZnO film device.     

Study of the formation of Co3O4 thin films using sol–gel method

Sol–gel derived coatings containing cobalt have been analyzed using impedance and reflection measurements. It is found that during the thermal treatments in air at temperatures in the range of 35–400 °C, cobalt migrates to the front surface of the coating where it is oxidized by the atmospheric oxygen and forms a top layer rich in nanocrystalline Co₃O₄. In samples heated above 260 °C, the current flows entirely through this top layer because it has higher conductivity than the rest of the coating. For these samples, the impedance spectra show two semicircles, related with the electrical properties of grain and grain boundaries of the cobalt oxide layer. Using the brick model, the grain boundary volume fraction was calculated as a function of the heating temperature.     

Proton conductivity in sol–gel-derived P2O5–TiO2–SiO2 glasses

The sol–gel method was applied to prepare the P₂O₅–TiO₂–SiO₂ glasses with high proton conductivities and chemical stability. The glasses were prepared by the reaction of the Ti- and Si-alkoxides with PO(OCH₃)₃ or H₃PO₄, followed by heating at 600 °C. The obtained glasses were porous, the average pore diameter of which was <2 and 4 nm for glasses prepared using PO(OCH₃)₃ and H₃PO₄, respectively. The phosphorous ions, occurring as PO(OH)₃ in the TiO₂-free glass, were polymerized with one or two bridging-oxygen ions per PO₄ unit with the increasing TiO₂ content. Despite the P₂O₅–SiO₂ binary glasses exhibiting high conductivities of ∼10⁻² S/cm at room temperature, they also dissolved after immersing for 24 h in water. The chemical stability of these glasses increased significantly on the addition of TiO₂.     

Photoluminescent properties of Dy3+ in MgO-Ga2O3-SiO2 nano-glass-ceramic prepared by sol–gel method

The MgO-Ga₂O₃-SiO₂ (MGS) glass and glass-ceramic (GC) containing MgGa₂O₄ nanocrystals and Dy³⁺ ions were prepared by a simple sol–gel method. MgGa₂O₄ nanocrystals in MgO-Ga₂O₃-SiO₂ GC formed when the heating temperature reached 800 °C. The energy transfer from MgGa₂O₄ nanocrystals to Dy³⁺ was observed. It is important that the strong white light emission is observed corresponding to the excitation band gap of MgGa₂O₄ nanocrystals. As exciting Dy³⁺ ions, only weak yellow and blue emissions were observed.