Dye sensitized solar cell of TiO2 nanoparticle/nanorod composites prepared via low-temperature synthesis in oleic acid

Titania (TiO₂) nanorods (NRs) and nanoparticles (NPs) were synthesized using oleic acid as a surfactant and employed as photoanodes for dye sensitized solar cell (DSSC) fabrication. The synthesized NRs and NPs were characterized using transmission electron microscopy and X-ray diffraction. The photovoltaic performances were compared between NRs, NPs, and their composites. The results showed that the power conversion efficiencies (η) of the composites depend on the relative compositions of NRs and NPs in photoanodes, reaching the greatest at 10% NR content. η of the pure NRs DSSC was lower than that of the NPs DSSC. Electrochemical impedance spectroscopy revealed that the highest η at 10% NRs is mainly due to reduced charge transport resistance at the TiO₂/dye/electrolyte interface and electrolyte diffusion resistance, overcoming the reduction of the number of adsorbed dye molecules.     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

Dye Adsorption Characteristics of Anatase TiO2 Film Prepared in an Aqueous Solution

Anatase TiO₂ film was deposited on SnO₂: F substrate in aqueous solution. The film had an assembly of acicular TiO₂ nanocrystals on the surface. The crystals grew along the c-axis, i.e. perpendicular to the substrate. Dye adsorption increased with film thickness. Intensity of photoluminescence originating from the dye adsorbed on the nanostructured film after annealing was 3 times higher than that of thicker particulate film constructed of TiO₂ nanoparticles (P25). Additionally, dye adsorption property of the film without annealing was two times higher than the film with annealing. Consequently, the as-deposited film had high dye adsorption property which is about 6 times higher that that of thicker particulate film constructed of TiO₂ nanoparticles (P25). Assemblies of acicular crystals on the surface increased the surface area and amount of dye adsorption. The film may be useful for biomolecule sensors and dye-sensitized solar cells.     

The optical properties of nanoporous structured titanium dioxide and the photovoltaic efficiency on DSSC

This study examined the characterization of nanoporous structured titanium dioxide and its application to dye-sensitized solar cells (DSSCs). TEM revealed nanopore sizes of 10.0 nm with a regular hexagonal form. When nanoporous structured TiO₂ was applied to DSSC, the energy conversion efficiency was enhanced considerably compared with that using nanometer sized TiO₂ prepared using a hydrothermal method. The energy conversion efficiency of the DSSC prepared from nanoporous structured TiO₂ was approximately 8.71% with the N719 dye under 100 mW cm⁻² simulated light. FT-IR spectroscopy showed that the dye molecules were attached perfectly to the surface and more dye molecules were absorbed on the nanoporous structured TiO₂ than on the nano-sized TiO₂ particles prepared using a conventional hydrothermal method. Electrostatic force microscopy (EFM) showed that the electrons were transferred rapidly to the surface of the nanoporous structured TiO₂ film.     

Charge recombination reduction in dye-sensitized solar cells by depositing ultrapure TiO2 nanoparticles on “inert” BaTiO3 films

Ultrapure TiO₂ nanoparticles (∼5 nm in size) were supported on “inert” BaTiO₃ films by TiCl₄ treatment, which was used to fabricate dye-sensitized solar cells (DSSCs). The optimized electrode, designated as BaTiO₃/TiO₂(4), was obtained upon four cycles of TiCl₄ treatment. DSSC with BaTiO₃/TiO₂(4) electrode exhibits superior power conversion efficiency (PCE) compared to that with conventional anatase TiO₂ (∼25 nm in size) electrode. The interfacial charge recombination kinetics was investigated by electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). In contrast to DSSC with anatase TiO₂ electrode, the dramatically enhanced electron lifetime for DSSC with BaTiO₃/TiO₂(4) electrode could be attributed to the decrease of recombination reaction at the TiO₂ photoelectrode/electrolyte interface. It is proposed that the lower interfacial charge recombination can be related to the relatively shallower trap distributions in DSSC with BaTiO₃/TiO₂(4) electrode.     

Composite electrolytes of polyethylene glycol methyl ether and TiO2 for dye-sensitized solar cells—Effect of heat treatment

For solid-state dye-sensitized solar cells (DSSCs), a composite electrolyte of polyethylene glycol methyl ether (PEGME) and titania (TiO₂) nanoparticles was prepared and characterized by Fourier transform-infra red (FT-IR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy. The heat treatment on PEGME and TiO₂ composite was found to be an essential step to improve morphology, amorphicity and ionic conductivity of PEGME–TiO₂ composite electrolytes. It was attributed to the increased bond strength of OC–O–Ti between PEGME and TiO₂ and increased surface roughness of composite materials, which may help to absorb a large amount of iodide couple and effective generation of I₃⁻ ions. A DSSC fabricated with heat treated PEGME–TiO₂ composite electrolyte showed significantly enhanced overall conversion efficiency of 3.1%, which was 20% higher than that of the DSSC fabricated with bare PEGME–TiO₂ composite electrolyte.     

Effect of atomic layer deposited ultra thin HfO2 and Al2O3 interfacial layers on the performance of dye sensitized solar cells

The objective of this work was to investigate the improvement in performance of dye sensitized solar cells (DSSCs) by depositing ultra thin metal oxides (hafnium oxide (HfO₂) and aluminum oxide (Al₂O₃)) on mesoporous TiO₂ photoelectrode using atomic layer deposition (ALD) method. Different thicknesses of HfO₂ and Al₂O₃ layers (5, 10 and 20 ALD cycles) were deposited on the mesoporous TiO₂ surface prior to dye loading process used for fabrication of DSSCs. It was observed that the ALD deposition of ultrathin oxides significantly improved the performance of DSSCs and that the improvement in the DSSC performance depends on the thickness of the deposited HfO₂ and Al₂O₃ films. Compared to a reference DSSC the incorporation of a HfO₂ layer resulted in 69% improvement (from 4.2 to 7.1%) in the efficiency of the cell and incorporation of Al₂O₃ (20 cycles) resulted in 19% improvement (from 4.2 to 5.0%) in the efficiency of the cell. These results suggest that ultrathin metal oxide layers affect the density and the distribution of interface states at the TiO₂/organic dye and TiO₂/liquid electrolyte interfaces and hence can be utilized to treat these interfaces in DSSCs.     

TiO2/PEN表面改性对柔性DSSC性能的影响

对于柔性DSSC的透明光阳极而言,在低温下制备出结晶性高、颗粒间结合性能良好的TiO₂薄膜是其获得优异光电性能的关键。本文通过对比三种不同浆料添加剂(盐酸、乙二醇乙醚、小颗粒凝胶)对DSSC光电性能的影响,发现小颗粒凝胶的加入可以显著提高器件的光电性能。在此基础上,优化了小颗粒凝胶添加量及退火工艺,得出了最佳制备工艺条件,最终获得短路电流密度为8.14 mA/cm²、填充因子为67.9%、光电转化效率为4.1%的DSSC。进一步研究表明,小颗粒凝胶的加入一方面可以增强TiO₂颗粒之间的连接,从而提高TiO₂的成膜质量;另一方面可以增加TiO₂薄膜对光的散射,从而提高电池的吸光率。     

Synthesis of micro-sized hierarchical TiO2 particles of nano-scale effectiveness and their photocatalytic activities at various surface hydroxyl concentrations

The 3-dimensional hierarchical TiO₂ particles of micro-sized diameter were synthesized through modified sol–gel process with polyethylene glycol (PEG) as a structure-controlling agent. The anatase crystal structure was obtained after calcination at 450 °C. The size and specific surface area of particles were 1.0–1.8 μm and 96.85 m² g⁻¹, respectively. The specific surface area of the TiO₂ particles corresponded to that of the spherical nanoparticles with average size of 15.9 nm. Although the size of synthesized TiO₂ was micro-scale, they had the specific surface area similar to that of nano-scale particles due to the effect of PEG on the formation of particles. Subsequently, the surface modification with various concentration of ammonia solution was carried out for the preparation of hydroxyl-rich TiO₂ particles at surfaces. As the concentration of ammonia solution was increased, the amount of chemically adsorbed hydroxyl groups on the TiO₂ surface was increased. As an application of prepared TiO₂ for water treatment, their catalytic performances for the degradation of methylene blue (MB) were examined by using UV–Vis spectrophotometer with the assistance of UV lamp. After hydroxylation treatment, the micro-sized TiO₂ particles showed the higher performance of MB degradation than that of nano-sized P25 particles because of their large specific surface area and hydroxyl-rich surface.     

Visible light enhanced TiO2 thin film bilayer dye sensitized solar cells

Visible light enhanced nitrogen-sulfur (N-S) doped titanium dioxide (TiO₂) thin films were prepared by the sol-gel method using thiourea as a dopant. The physical and chemical properties of the TiO₂ thin films were greatly influenced by the amount of thiourea added to the sol-gel solution. The greatest shift to longer wavelengths for visible light absorption was observed with 0.6 g of thiourea in the precursor solution, while 0.4 g yielded the largest particle sizes. These single-cycle dip-deposited N-S doped TiO₂ thin films were used as visible light harvesters as well as blocking layers in dye sensitized solar cells. When deposited directly on conducting fluorine doped tin oxide electrodes, photo-conversion efficiencies were reduced. However, the opposite configuration, with N-S doped thin films on top of nanoporous TiO₂, yielded an increased open-circuit voltage of 0.84 V, a short-circuit current density of 9.86 mA cm⁻², and an overall conversion efficiency of 5.88% greater than that of a standard cell. The effectiveness of the blocking layer on the cell efficiencies was analyzed by electrochemical impedance spectroscopy.     

Properties of TiO2 prepared by acid treatment of BaTiO3

TiO₃ powders were prepared by acid treatment of BaTiO₃ and their properties were investigated. The BaTiO₃ powder was subjected to HNO₃ in concentrations ranging from 10⁻³ to 8 M at 90 °C for 0.5-6 h. Dissolution of BaTiO₃ and precipitation of TiO₂ occurred at acid concentrations of 2-5 M. BaTiO₃ dissolves completely to form a clear solution at reaction times of 0.5-1 h, but a rutile precipitate is formed after 2 h of acid treatment. By contrast, anatase is precipitated by adjusting the pH of the clear solution to 2-3 using NaOH or NH₄OH solution. The rutile crystals were small and rod-shaped, consisting of many small coherent domains connected by grain boundaries with small inclination angles and edge dislocations, giving them a high specific surface area (S_BET). With increasing HNO₃ concentration, the S_BET value increased from 100 to 170 m²/g while the crystallite size decreased from 25 to 11 nm. The anatase crystals obtained here were very small equi-axial particles with a smaller crystallite size than the rutile and S_BET values of about 270 m²/g (higher than the rutile samples). The photocatalytic activity of these TiO₂ was determined from the decomposition rate of Methylene Blue under ultraviolet irradiation. Higher decomposition rates were obtained with larger crystallite sizes resulting from heat treatment. The maximum decomposition rates were obtained in samples heated at 500-600 °C. The photocatalytic activity of the TiO₂ was found to depend more strongly on the sample crystallite size than on S_BET.     

Effect of lead on formation and dielectric tunability of (Pbx,Sr1? x )0.85Bi0.1TiO3 thin films

(Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin films with the perovskite phase structure were prepared on an ITO glass substrate by sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and an impedance analyzer were respectively used in order to characterize the phase status, morphology and dielectric properties of the thin films. The results show that during the formation process of (Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin films, the nucleus of the perovskite phase are initially formed and then congregated. These aggregated nucleus are then transformed as the perovskite-phase crystalline in the thin film. Finally, the crystalline phase grows and separates gradually to form the perfect crystalline-phase structure. The content of the perovskite phase formed in the thin film under rapid thermal process (RTP) is more than that formed under traditional heat treatment with kinetic equilibrium. This is due to the high active decomposed ions that form the perovskite phase directly when heat-treated by RTP. The formation of the perovskite phase therefore overcomes a much lower barrier under RTP than that under traditional calcinations. The structure of the perovskite phase has a close relation to the ratio of Pb/Sr in the system because of the radius difference between Pb²⁺ and Sr²⁺. The transformation temperature between the cubic and the tetragonal structures of the perovskite phase increases with increasing Pb²⁺ content because the radius of Pb²⁺ is larger than that of Sr²⁺. It appears at room temperature when the content of Pb²⁺/Sr²⁺ is about 40/60 in the thin film. Meanwhile, the tetragonality of the perovskite phase is increased when Pb²⁺ ions increase due to its high polarization. The higher tunability of the (Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin film is exhibited when the film composition is close to the transformation point between the paraelectric and ferroelectric phases. Pb²⁺ ions show a dominant factor to affect the Curie point of the system and then changing tunability. Translated from Journal of Inorganic Materials, 2006, 21(2): 466–472 [译自: 无机材料学报]     

Iron promotion of the TiO2 photosensitization process towards the photocatalytic oxidation of azo dyes under solar-simulated light irradiation

The photocatalytic oxidation of the azo dye Orange-II (Or-II) using Fe loaded TiO₂ (Fe–TiO₂) was studied under ultraviolet (UV), visible (vis) and simultaneous UV–vis irradiations using a solar light simulator. Photocatalysts were characterized by means of XRD, SEM-EDX, FTIR and DRS. Fe³⁺ species, identified in XPS analyses, were responsible of the increased absorption of visible light. Moreover, DRS analyses showed a decrease in the bandgap due to Fe³⁺ loading. Photocatalystic tests proved that Fe modification enhanced the TiO₂ photocatalytic activity towards Or-II photodegradation under simultaneous UV–vis irradiation. Even so, the performance of the Fe–TiO₂ samples towards the photodegradation of phenol, under UV irradiation, was lower than TiO₂ suggesting the recombination of the UV photogenerated electron–hole pair. Therefore, results evidence a Fe³⁺ promotion of the electron caption in the photosensitization process of TiO₂ by Or-II acting as a sensitizer. Such process leads to the Or-II photooxidation under UV–vis irradiation by losing energy in electron transferring processes to sensitize TiO₂, and, the formation of reactive oxygen species promoted by the injected electron to the TiO₂ conduction band.     

Investigation on solar photocatalytic activity of TiO2 loaded composite: TiO2/Eggshell, TiO2/Clamshell and TiO2/CaCO3

The TiO₂/Eggshell, TiO₂/Clamshell and TiO₂/CaCO₃ loaded composites were prepared by sol-gel method and characterized by XRD and SEM. Their photocatalytic activities were measured through the degradation of Acid Red B under solar light irradiation. The influences of TiO₂ loaded content, heat-treated temperature and time on the photocatalytic activities were reviewed. The effects of irradiation time and dye initial concentration on the photocatalytic degradation were also investigated. The results showed that the photocatalytic activity can be greatly enhanced by appropriate TiO₂ loaded content.     

Nitrogen and europium doped TiO2 anodized films with applications in photocatalysis

Micro-arc oxidation method is a useful process for mesoporous titanium dioxide films. In order to improve the photocatalytic activity of the TiO₂ film, N-Eu co-doped titania catalyst was synthesized by micro-arc oxidation in the H₂SO₄/Eu(NO₃)₃ mixture solution.The specific surface area and the roughness of the anodic titania film fabricated in the H₂SO₄/Eu(NO₃)₃ electrolyte, were increased compared to that of the anodic TiO₂ film prepared in H₂SO₄ solution. The absorbance response of N-Eu titania film shows a higher adsorption onset toward visible light region, and the incorporated N and Eu ions during anodization as a dopant in the anodic TiO₂ film significantly enhanced the photocatalytic activity for dye degradation. After dye decomposition test for 3 h, dye removal rates for the anodic TiO₂ film were 60.7% and 90.1% for the N-Eu doped titania film. The improvement of the photocatalytic activity was ascribed to the synergistic effects of the surface enlargement and the new electronic state of the TiO₂ band gap by N and Eu co-doping.     

Dye-sensitized solar cells with TiO2 nanocrystalline films prepared by conventional and rapid thermal annealing processes

Titanium oxide (TiO₂) thin films are prepared by the sol-gel method and annealed at 600 °C by conventional (CTA) and rapid thermal annealing (RTA) processes on fluorine-doped tin oxide -coated glass substrates for application as the work electrode for the dye-sensitized solar cells (DSSC). TiO₂ thin films are crystallized using a conventional furnace and the proposed RTA process at annealing rates of 5 °Cmin⁻¹ and 600 °Cmin⁻¹, respectively. The TiO₂ thin films are characterized by X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller analysis. Based on the results, the TiO₂ films crystallized by RTA show better crystallization, higher porosity and larger surface area than those of CTA. The short-circuit photocurrent and open-circuit voltage values increased from 5.2 mAcm⁻² and 0.6 V for the DSSC with the CTA-derived TiO₂ films to 8.3 mAcm⁻² and 0.68 V, respectively, for the DSSC containing RTA-derived TiO₂ films.     

Effect of the SiO2/TiO2 ratio on the solid acidity of SiO2-TiO2/montmorillonite composites

SiO₂-TiO₂/montmorillonite composites with varying SiO₂/TiO₂ molar ratios were synthesized and the effect of the SiO₂/TiO₂ ratio on the solid acidity of the resulting composites was investigated. Four composites with SiO₂/TiO₂ molar ratios of 0, 0.1, 1 and 10 were synthesized by the reaction of colloidal SiO₂-TiO₂ particles prepared from alkoxides with sodium-montmorillonite at room temperature. The composites showed slight expansion and broadening of the XRD basal reflection, corresponding to the intercalation of fine colloidal SiO₂-TiO₂ particles into the montmorillonite sheets and incomplete intercalation to form disordered stacking of exfoliated montmorillonite and colloidal SiO₂-TiO₂ particles. The colloidal particles crystallized to anatase in the low SiO₂/TiO₂ composites but remained amorphous in the high SiO₂/TiO₂ composites. The specific surface areas (S_BET) of the composites measured by N₂ adsorption ranged from 250 to 370 m²/g, considerably greater than in montmorillonite (6 m²/g). The pore size increased with decreasing SiO₂/TiO₂ molar ratio of the composites. The NH₃-TPD spectra of the composites consisted of overlapping peaks, corresponded to temperatures of about 190 and 290 °C. The amounts of solid acid obtained from NH₃-TPD were 186-338 μmol/g in the composites; these values are higher than in the commercial catalyst K10 (85 μmol/g), which is synthesized by acid-treatment of montmorillonite. The present sample with SiO₂/TiO₂ = 0.1 showed the highest amount of acid, about four times higher than K10.     

Multi-layered TiO2 nanostructured films for dye-sensitized solar cells

Multi-layered TiO₂ nanostructured films were fabricated to improve the light harvest efficiency of the dye-adsorbed TiO₂ electrode in dye-sensitized solar cells (DSSCs) by light scattering. Three different structures of TiO₂ electrodes, with layers consisting of TiO₂ pastes with average diameters of 9, 20, and 300 nm, respectively, were fabricated and their photovoltaic effects on the DSSC devices were investigated. By utilizing the multi-layered TiO₂ electrode constructed using the three different TiO₂ pastes, the overall power conversion efficiency of the DSSC devices in the PEG-based electrolyte was increased to 5.24% under irradiation of 100 mW/cm² at AM 1.5.     

Luminescence enhancement of Y2O3:Eu and Y2SiO5:Ce,Tb core particles with SiO2 shells

This paper reports on the luminescence and microstructural features of oxide nano-crystalline (Y₂O₃:Eu³⁺) and submicron-sized (Y₂SiO₅:Ce³⁺,Tb³⁺) phosphor cores, produced by two different synthesis techniques, and subsequently coated by an inert shell of SiO₂ using a sol-gel process. The shells mitigate the detrimental effect of the phosphor particle surfaces on the photoluminescence emission properties, thereby increasing luminous output by 20-90%, depending on the core composition and shell thickness. For Y₂O₃:Eu³⁺, uniformly shaped, narrow particle size distribution core/shell particles were successfully fabricated. The photoluminescence emission intensity of core nanoparticles increased with increasing Eu³⁺ activator concentration and the luminescence emission intensity of the core/shell particles was 20-50% higher than that of the core particles alone. For Y₂SiO₅:Ce³⁺,Tb³⁺, the core/shell particles showed enhancement of the luminescence emission intensity of 35-90% that of the core particles, depending on the SiO₂ shell thickness.     

Low-cost quasi-solid-state dye-sensitized solar cells based on a metal-free organic dye and a carbon aerogel counter electrode

Low-cost quasi-solid-state dye-sensitized solar cells (DSSCs) are designed and fabricated by using a metal-free organic dye and a mesoporous carbon aerogel instead of expensive ruthenium-based sensitizers and Pt electrode. The electrospun TiO₂ nanorods are added into a polyvinylidene fluoride (PVDF) solution to form a 3D network nanocomposite gel electrolyte. The presence of TiO₂ nanorods in the gel electrolyte obviously increases the ionic conductivity and decreases charge-transfer resistance of the DSSC. The effects of the gel electrolyte and the carbon aerogel counter electrode on electrochemical and photovoltaic properties have been investigated in detail. Particularly, an optimized DSSC with a nanocomposite gel electrolyte and a carbon aerogel counter electrode affords a power conversion efficiency (PCE) of 6.20% at a light intensity of 100 mW cm⁻².