TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact

In traditional solar cells, metal-semiconductor contacts used to extract photogenerated carriers are very important. In dye-sensitized solar cells (DSSC) not much attention has been given to contact between the TiO₂ and the transparent conducting glass (TCO), which is used instead of a metal contact to extract electrons. TiO₂ layers obtained by microwave-activated chemical-bath deposition (MW-CBD) are proposed to improve TiO₂ contact to conducting glass. Spectra of incident photon to current conversion efficiency (IPCE) are obtained for two-photoelectrode TiO₂ photoelectrochemical cells. IPCE spectra show higher values when TiO₂ double layer photoelectrodes are used. In these, the first layer or contacting layer is made by MW-CBD. Best results are obtained for double layer photoelectrodes on FTO (SnO₂:F) as conducting oxide substrate. Modeling of IPCE spectra reveals the importance of electrical contact and electron extraction rate at the TiO₂/TCO interface.     

Unique TiO2 paste for high efficiency dye-sensitized solar cells

A novel titanium oxide paste based on Pechini sol-gel method and nanocrystalline titanium oxide powder have been successfully developed. Titanium oxide layers possess high inner surface area assuring high dye loading and well-connected nanocrystalline grains assuring good electron transport within the layer. The dye-sensitized layers have been used to assemble dye-sensitized solar cells with acetonitrile- and ionic liquid-based electrolyte. Overall conversion efficiencies of dye-sensitized solar cells (DSSCs) determined under standard test conditions (100 mW/cm², 25 °C and AM 1.5 G) are 10.2% for acetonitrile and 7.3% for ionic liquid-based electrolyte.     

Dye-sensitized solar cell based on Rose Bengal dye and nanocrystalline TiO2

Dye-sensitized solar cell is fabricated using Rose Bengal dye (RB) for sensitization of nanocrystalline TiO₂ and that imparts extension in spectral response towards visible region by modifying the semiconductor surface. Further, the photoresponse of the cell was evaluated by analyzing its J–V and impedance characteristics under illumination with metal halide light source of 400 W with an incident light of 73 mW/cm². Various photovoltaic parameters like J_sc, V_oc, FF were evaluated and found to be 3.22 mA, 890 mV, 0.53, respectively, resulting conversion efficiency (η) of 2.09%. Impedance analysis of the cell was carried out to investigate the internal resistance of the cell by recording Cole–Cole plots in between real and imaginary impedance in dark and with illumination under variable biasing, i.e. from 0 to 3 V.     

A comparison of microwave and conventional heat treatments of nanocrystalline TiO2

TiO₂ thin films are an important component of dye-sensitized solar cells. For these solar cells, the TiO₂ film must be sintered to achieve crystallization and good interparticle connections. Microwave processing may allow a reduction in the required temperature and time for this heat treatment. Therefore, microwave heating of nanocrystalline TiO₂ has been investigated. No significant difference was found between microwave and conventional heating in the sintering of TiO₂, but microwave heating promotes the phase transformation from anatase to rutile. Microwave heating improved the solar cell performance when a surface treatment of the TiO₂ film with titanium isopropoxide was applied.     

Formation of anatase TiO2 by microwave processing

Dye-sensitized solar cells, in which nanoparticles of anatase titanium dioxide play an important role, offer an attractive alternative to conventional photovoltaic cells. The possibility of reducing the required sintering time and temperature for production of these solar cells is investigated. Following synthesis by a sol–gel method, anatase TiO₂ was produced by heat treatment at only 225°C. Using microwave processing, crystallization to the anatase phase was achieved with a shorter heat treatment and a lower temperature than for conventional furnace treatments.     

Spectral response and IV-characterization of dye-sensitized nanocrystalline TiO2 solar cells

Dye-sensitized nanocrystalline TiO₂ solar cells (nc-DSCs) are based on a fundamentally different working principle than solar cells based on semiconductors. This could have implications for the characterization of nc-DSCs. In this study a comparison is made between two methods for determination of the spectral response of nc-DSCs. The standard method for determination of the spectral response according to the ASTM E1021-84 norm appears to be valid for the nc-DSC. The response of the solar cell to pulsed irradiation plays an important role in this determination, since pulsed illumination of the solar cell is involved. The response time of the nc-DSC is related to electron trapping in the TiO₂ and depends on illumination conditions and also on chemical composition of the cell. For this reason, prior to measurements of spectral response of nc-DSCs, the response time of the cell should be measured under the same illumination conditions that are applied during spectral response measurements.     

Nanocrystalline TiO2 film with textural channels: Exhibiting enhanced performance in quasi-solid/solid-state dye-sensitized solar cells

Novel nanocrystalline TiO₂ films with the textural channels are obtained for dye-sensitized solar cells (DSSCs). The textural channels consisting of the cracks on the surface and the nanopores with average diameter of about 41 nm are produced by packaging ZnO nanowires with diameter of 30–50 nm into TiO₂ films and subsequently etching ZnO nanowires by hydrochloric acid. The performances of DSSCs based on novel TiO₂ films (with the textural channels) and traditional TiO₂ films (without the textural channels) are investigated, respectively. When two kinds of typical quasi-solid-state electrolytes and one kind of solid-state electrolyte are used, the energy conversion efficiencies of DSSCs from novel TiO₂ films are improved by 20–30% compared to that from traditional TiO₂ films. The reasons for the great improvement are investigated chiefly by UV–vis absorption spectra, field emission-scanning electron microscope (FE-SEM) and electrochemical impedance spectroscopy (EIS) technique. The results show that the introduction of the textural channels facilitates better penetration of quasi-solid/solid-state electrolytes into the nanopores of novel TiO₂ films and thus results in better interfacial/electrical contact and faster interfacial reaction.     

Electrodeposition of TiO2/SiO2 nanocomposite for dye-sensitized solar cell

For the working electrode of dye-sensitized solar cell (DSC), TiO₂/SiO₂ nanocomposite materials were electrodeposited on transparent fluorine doped tin oxide-coated glass by cathodic electrodeposition at room temperature. The electrode and DSC fabricated with TiO₂/SiO₂ nanocomposite were characterized with photocurrent density, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and a photovoltaic performance test. On the electrodeposition, the addition of an appropriate amount of SiO₂ in the bath containing TiO₂ slurry was essential to achieve the superior crystallinity, photocurrent density and photovoltaic performance of the resulting TiO₂/SiO₂ electrode, which was significantly superior to a bare TiO₂ electrode. This enhanced performance of optimized TiO₂/SiO₂ electrode was ascribed to the role of SiO₂ as an energy barrier, increasing the physical separation of injected electrons and oxidized dyes/redox couple, and thereby retarding the recombination reactions in the resulting DSC.     

Characterization of N, N′-bis-2-(1-hydoxy-4-methylpentyl)-3, 4, 9, 10-perylene bis (dicarboximide) sensitized nanocrystalline TiO2 solar cells with polythiophene hole conductors

We have fabricated solid-state, dye-sensitized nanocrystalline TiO₂ solar cells (DSSC) based on perylene derivative dye, N,N′-bis-2-(1-hydoxy-4-methylpentyl)-3,4,9,10-perylene bis (dicarboximide) (HMPER) with two different polythiophenes as hole conductors; i.e. poly (3-octyl thiophene) (P3OT) and poly (3-hexyl thiophene) (P3HT), respectively. HMPER adsorbs strongly to the surface of nanocrystalline TiO₂ and inject electrons into TiO₂ conduction band upon absorption of light. Polythiophene derivatives are well-known materials as hole conductors in solid-state dye-sensitized solar cells. We obtained quite similar results with P3OT and P3HT yielding a short-circuit current density of around 80 μA/cm² and open-circuit voltage of around 0.7 V at 80 mW/cm² AM 1.5 light intensity. The results are compared with Ru-535 TBA-sensitized nc-TiO₂ cells prepared by using the same polythiophene derivatives.     

High quantum efficiency Pt/TiO2 catalyst for sacrificial water reduction

Platinum photodeposition on TiO₂ from methanolic solution of chloroplatinic acid (CPA) is investigated to determine the conditions that give optimum photocatalytic activity towards dehydrogenation of methanol. Conditions favoring nucleation of Pt islets rather than their autocatalytic growth enhance the catalytic activity. Photoplatinization from idoplatinic acid, adsorbing more strongly on TiO₂ than CPA produced more active Pt/TiO₂ catalysts. The best catalyst prepared from CPA yielded H₂ from 12.5% methanol solution at a quantum efficiency of 23.9% whereas for idoplatinic acid based catalysts, the quantum efficiency increased to 42.5%.     

Photoelectrochemical studies of nanocrystalline TiO2 co-sensitized by novel cyanine dyes

Pentamethylcyanine derivative (A), trimethylcyanine derivative (B) and their mixtures (AB) were used as sensitizers in nanocrystalline TiO₂ solar cells to improve photoelectric conversion efficiency. It was found that the aggregates of the cyanine dyes were efficient in light harvesting and that the mixture of A and B could be employed to sensitize the solar cell over the entire visible spectrum. The sensitizing properties of three mixed dyes with different A-to-B ratio were systematically studied, and it was found that A1B3 (A:B=1:3 V/V)-sensitized solar cell generated the highest photoelectric conversion yield of 3.4%. The effect of co-adsorption of A and B on their aggregation behavior and photosensitization was also investigated. Co-sensitization was found to suppress the aggregation and affect the sensitization performance profoundly.     

A dye sensitized TiO2 photovoltaic cell constructed with an elastomeric electrolyte

A photovoltaic solar cell employing an elastomeric electrolyte and using a dye-sensitized nanoporous TiO₂ electrode has been assembled. The polymeric electrolyte is poly(epichlorohydrin-co-ethylene oxide) filled with NaI/I₂. This cell exhibits an open-circuit voltage of 0.71 V and a short-circuit current of 0.46 mA cm⁻² under 120 mW cm⁻² of white-light illumination. The overall conversion efficiency of the cell is 0.22%. The polymeric electrolyte behavior under different conditions of external resistance and intensity of light as well as the performance of this photoelectrochemical cell are discussed.     

Green synthesis of fibrous hierarchical meso-macroporous N doped TiO2 nanophotocatalyst with enhanced photocatalytic H2 production

A new approach to prepare hierarchical and fibrous meso-macroporous N-doped TiO₂ is attempted at room temperature without using templates by the addition of titanium isopropoxide droplets to the ammonia solution. The catalysts are thoroughly characterized by physico-chemical and spectroscopic method to explore the structural, electronic and optical properties. The photocatalytic activities of the catalyst were evaluated with hydrogen generation. NTP catalyst calcined at 400 °C (NTP-400) exhibited 602.7 μmol/3 h H₂ generation from 10 vol.% methanol under visible light. The excellent photocatalytic activity for NTP-400 is attributed to the porous networks existing in our system with uniform N dispersion throughout the catalyst. The hierarchical and fibrous structures allow easy channelization of electron as in the case of nanotubes for effective surface charge transfer. Along with macroporosity, nitrogen incorporation and mesoporosity play some important roles for enhanced photoactivities.     

Photoelectric behavior of nanocrystalline TiO2 electrode with a novel terpyridyl ruthenium complex

The photoelectric behavior of a black dye, tris (isothiocyanato)-[N-(2,2′:6′,2″-terpyridine-4′-(4-carboxylic acid) phenyl)] ruthenium (II) complex, was examined under different conditions. The dye was adsorbed on nanocrystalline TiO₂ surface strongly and generated incident monochromatic photon-to-current conversion efficiency (IPCE) of about 90% at maximum absorption wavelength and greater than 20% in the near-IR region. A sandwich-type solar cell fabricated by this dye-sensitized nanocrystalline TiO₂ film generated 6.1 mAcm⁻² of short-circuit photocurrent, 0.58 V of open-circuit photovoltage and 2.9% of overall yield under irradiation of white light (78.0 mWcm⁻²) from a Xe lamp. Since the title dye shows better photoresponse than the N3 dye in the near-IR region, it would be a promising panchromatic sensitizer after optimization.     

Sol-gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol-gel dip-coating technique can be used to fabricate a SiO₂/TiO₂ bilayer film with self-cleaning and antireflection properties. The bottom SiO₂ layer acts as an antireflection coating due to its lower refractive index; the top TiO₂ layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO₂/TiO₂ bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO₂ nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO₂ nanoparticles. After illuminated by ultraviolet light, the SiO₂/TiO₂ bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.     

Oxidative photoelectrochemical technology with Ti/TiO2 anodes

Rutile and anatase TiO₂ films have been grown on Ti plates by thermal (500–800°C) and anodic oxidation followed by thermal annealing (400–500°C), respectively. The photoelectrochemical efficiency of these photoanodes, evaluated by current density measurements in the photooxidation of 4-methoxybenzyl alcohol in deaerated CH₃CN, has been determined. The photocurrent efficiency increases with the thickness of the TiO₂ rutile film up to 1 μm (the most efficient thickness). At the wavelengths furnished by the irradiation apparatus similar thicknesses of anatase and rutile films show nearly the same efficiencies. Anodic bias produces similar relative increases of current intensity in both crystalline forms.     

Electron microscopy studies of TiO2 micelles

The microstructural properties of nanosized TiO₂ micelles, generated in situ in a water-in-oil (w/o) microemulsion composed of water, dioctyl sulfosuccinate sodiun salt (AOT) and cyclohexane, by controlled hydrolysis of TiCl₄, were investigated. The samples were characterized by analytical electron microscopy combining electron diffraction. TiO₂ film consisted of isotropic grains of anatase. The grain sizes were 5 nm, which is in agreement with the average grain size R, previously obtained by grazing-incidence wide-angle X-ray diffraction (GIWAXD), of 5.0±1.3 nm. Grains were gathered in a number of clusters, differently oriented with respect to the electron beam.     

Electrochromic devices based on surface-modified nanocrystalline TiO2 thin-film electrodes

Nanocrystalline TiO₂ thin film electrodes on conductive glass were modified with monolayers of different electrochromic compounds (mono-, di- and trimeric N,N′-dialkyl- or-diphenyl-4,4′-bipyridinium salts) equipped with TiO₂ anchoring groups (An=benzoate, salicylate, phosphonate). The synthesis of these compounds is reported. Different approaches have been studied to increase the surface concentration Γ_CS of electrochemically active coloring centers (CS) on TiO₂. The electrodes were checked coulometrically and spectroelectrochemically under potentiostatic conditions in MeCN/TEAP. Γ_CS of mono- and oligomeric viologens was shown to depend on the ratio (CS/An) of CS to anchoring groups (An). A cone-shaped trimeric arborol-type viologen was prepared with the intention to fill out the space above the convex surface of the nanoparticles particularly well. Preliminary results of a new type of TiO₂ solid-phase supported synthesis of the viologens is reported. Electrochromic devices including filters and displays have been prepared. The filter devices (12–100 cm²) consist generally of OTE/TiO₂-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE and exhibit optical density changes up to 2 (transparent to blue or yellowish to green and red-brown (at higher potential)) at switching times in the range of 1–3 s. Even higher optical density changes (at slower switching times) were achieved with systems such as OTE/TiO₂-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue-TiO₂/OTE. The display devices prepared include reflective displays with two to four separately addressable segments ((OTE/TiO₂ (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/molten salt+spacer/Zn) or (OTE/TiO₂ (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE), as well as transparent systems with up to four addressable segments such as: OTE/TiO₂ (both structured)-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE.     

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.     

Coloured electrochromic windows based on nanostructured TiO2 films modified by adsorbed redox chromophores

A series of viologens has been synthesised, characterised and tested for their suitability as redox chromophores in electrochromic devices. These viologens contain a phosphonic acid moiety and are irreversibly adsorbed at a transparent nanoporous-nanocrystalline TiO₂ electrode. An electrochromic device consisting of a sandwich of a viologen-modified TiO₂ electrode/electrolyte (γ-butyrolactone, 0.05M LiClO₄, 0.05M ferrocene)/conducting glass shows excellent electrochromic properties: fast switching times (1–2 s), large changes in absorbance, high colouration efficiencies (up to 200 cm²/C) and good long-term stability (>10 000 cycles). Further, the colour changes from transparent or a faint yellow to either a deep blue or a deep green, depending on the nature of the viologen.