Compact TiO2 Research Articles

Suppressed Recombination in Quantum Dot-Sensitized Solar Cells with Blocking Layers on FTO Substrates

The compact and thin TiO2 blocking layers (c-TiO2) were formed on F-doped SnO2 (FTO) substrate in quantum dots-sensitized solar cells (QSSCs) by chemical deposition. The c-TiO2 layers induced indirect contact between electrolyte and FTO electrode, which reduced leakage in QSSCs. The QSSCs showed power conversion efficiency (Eff) of 3.85% in the presence of c-TiO2 layers which leads to 21% improved compared to that without c-TiO2 layers (Eff = 3.18%). The presence of the c-TiO2 layers in QSSCs also improved the stability under illumination.

Enhanced efficiency of tri-layered dye solar cells with hydrothermally synthesized titania nanotubes as light scattering outer layer

In the present study dye sensitized solar cells (DSSCs) have been fabricated with a tri-layer photo anode consisting of hydrothermally prepared titania nano tubes (TNT) having a diameter of 9–10nm and length of several micrometers as outer layer, P25 TiO2 powder as transparent light absorbing middle layer and a compact TiO2 inner layer to improve the adhesion of different layers on a transparent conducting oxide coated substrate. In comparison to cells fabricated using TNTs or P25 alone, the tri-layer DSSCs exhibit an enhanced efficiency of 7.15% with a current density of 17.12mAcm−2 under AM 1.5 illumination. The enhancement is attributed to the light scattering generated by TNTs aggregates, reduction in electron transport resistance at the TiO2/dye/electrolyte interface and an improvement in electron life-time.

Effect of optical property of surfactant-treated TiO2 nanostructure on the performance of TiO2 photo-electrochemical cell

TiO2 nanostructures have been treated using different kind of surfactant to modify its optical absorption and morphology. TiO2 nanostructures were prepared via simple method on indium tin oxide (ITO) surface, namely, liquid-phase deposition technique (LPD) at 50°C. The TiO2 nanostructures film with a thickness of about 236 nm was annealed at 400°C for 1 h in the air to enhance the interconnectivity of the particles. The dense and compact TiO2 nanoparticle with different shape, particle size and surface morphology was used as a photovoltaic material in a photo-electrochemical cell of ITO/TiO2/electrolyte/platinum. TiO2 nanostructure films were treated with three different kinds of surfactants: cetyltrimethylammonium bromide (CTAB), hexamethylenetetramine (HMT) and polyvinylpyrrolidone (PVP). It was found that the film treated with HMT performed best, with a Jsc of 57.5 μA/cm2. In conclusion, optical absorption, band gap and grain size of TiO2 nanostructure influenced the performance of the cell.

Structure and Electron-Conducting Ability of TiO2 Films from Electrophoretic Deposition and Paste-Coating for Dye-Sensitized Solar Cells

An electrophoretic deposition (EPD) method, consisting of repetitive short-term depositions with intermediate drying, was developed to prepare nanocrystalline TiO2 films for dye-sensitized solar cells (DSSCs). After calcination, the EPD TiO2 films exhibited a more compact TiO2 network than films derived from the conventional paste-coating (PC) method. X-ray absorption fine structure spectroscopic analysis showed that the EPD films had a higher density of defect states than the PC films because of the higher number of interparticle necking regions created in the EPD films. However, the DSSCs assembled with the EPD films outperformed those with the PC films by 20% in photocurrent and 15% in solar energy conversion efficiency. Intensity-modulated photocurrent spectroscopic analysis showed that the EPD films had a shorter electron transit time than the PC films. Under one-sun illumination on the cells at open-circuit, impedance analysis showed that the EPD films had a constant charge collection efficiency of 95% for thicknesses ranging from 4 to 13 μm, whereas the efficiency of the PC films was not greater than 90% and showed a decreasing trend with increasing film thickness. Concerning the porosity dependence of the electron transport dynamics, the electron diffusivity had much weaker dependence than one would expect from the percolation model with hard spheres. This may result from the fact that interparticle necking causes greater lattice distortion for more compact TiO2 films. The present study demonstrates that an optimized EPD process can construct a nanocrystalline TiO2 architecture with a minimized void fraction to shorten the electron traveling distance and to effectively collect photogenerated charges, even for films with large thicknesses.

Transmittance optimized nb-doped TiO 2/Sn-doped In 2O 3 multilayered photoelectrodes for dye-sensitized solar cells

We adopt a compact TiO 2 (c-TiO 2) layer on Nb-doped TiO 2 (NTO)/Sn-doped indium oxide (ITO) multilayered electrodes, which are transparent conducting oxide (TCO) electrodes, to improve their transmittance for high-efficiency dye-sensitized solar cells (DSSCs). An NTO layer was deposited on ITO by pulsed laser deposition (PLD) and then a c-TiO 2 layer was deposited on the NTO/ITO multilayer by spin-coating a sol. The transmittance spectrum of the c-TiO 2/NTO/ITO multilayered photoelectrodes varied with the thickness of TiO 2. The short circuit current and energy conversion efficiency of the photoelectrodes also varied with the thickness of the c-TiO 2 layer, which is similar behavior to the integrated value of the transmittance in the wavelength range from 500 to 600 nm. Finally, the DSSC employing the 160 nm-thick c-TiO 2 layer exhibited the most improved energy conversion efficiency, compared to the DSSC without the c-TiO 2 layer. Our results demonstrate that the unfavorable optical properties of TCO, e.g. NTO, for DSSCs can be enhanced by the formation of an additional thin layer, e.g. c-TiO 2.

Enhanced Performance of I2‐Free Solid‐State Dye‐Sensitized Solar Cells with Conductive Polymer up to 6.8%

AbstractAn iodine‐free solid‐state dye‐sensitized solar cell (ssDSSC) is reported here, with 6.8% energy conversion efficiency—one of the highest yet reported for N719 dye—as a result of enhanced light harvesting from the increased transmittance of an organized mesoporous TiO2 interfacial layer and the good hole conductivity of the solid‐state‐polymerized material. The organized mesoporous TiO2 (OM‐TiO2) interfacial layer is prepared on large‐area substrates by a sol‐gel process, and is confirmed by scanning electron microscopy (SEM) and grazing incidence small‐angle X‐ray scattering (GISAXS). A 550‐nm‐thick OM‐TiO2 film coated on fluorine‐doped tin oxide (FTO) glass is highly transparent, resulting in transmittance increases of 8 and 4% compared to those of the bare FTO and conventional compact TiO2 film on FTO, respectively. The high cell performance is achieved through careful control of the electrode/hole transport material (HTM) and nanocrystalline TiO2/conductive glass interfaces, which affect the interfacial resistance of the cell. Furthermore, the transparent OM‐TiO2 film, with its high porosity and good connectivity, exhibits improved cell performance due to increased transmittance in the visible light region, decreased interfacial resistance (Ω), and enhanced electron lifetime (τ). The cell performance also depends on the conductivity of HTMs, which indicates that both highly conductive HTM and the transparent OM‐TiO2 film interface are crucial for obtaining high‐energy conversion efficiencies in I2‐free ssDSSCs.

The effect of a blocking layer on the photovoltaic performance in CdS quantum-dot-sensitized solar cells

In order to reduce the surface recombination at the interface between the fluorine-doped tin oxide (FTO) substrate and the polysulfide electrolyte in CdS quantum-dot-sensitized solar cells (QDSCs), compact TiO 2 is deposited on the FTO electrode by sputtering. The TiO 2-coated CdS-sensitized solar cell exhibits enhanced power-conversion efficiency (0.52%) compared with a bare CdS-sensitized solar cell (0.23%). Charge-transfer kinetics are analyzed by impedance spectroscopy, open-circuit decay, and cyclic voltammetry. The TiO 2 layer deposited on the FTO substrate acts as a blocking layer, which plays a significant role in reducing the electron back transfer from the FTO to the polysulfide electrolyte. Interestingly, with respect to the incident photon-to-current conversion efficiency (IPCE) data, asymmetric enhancement is observed from the sample with a thicker blocking layer. This is because CdS quantum dots absorb ultraviolet light completely with the TiO 2 layer because of the high extinction coefficient of the CdS quantum dots compared with dye molecules.

Enhanced performance of dye-sensitized solar cell via surface modification of mesoporous TiO 2 photoanode with electrodeposited thin TiO 2 layer

Surface modification of porous TiO 2 photoanode with a thin compact TiO 2 layer was carried out by means of anodic electrodeposition in an aqueous TiCl 3 electrolyte. Results indicated that the electrodeposited thin TiO 2 layer could bridge gaps between TiO 2 nanoparticles and improve the electrical contact at the FTO (fluoride-doped tin oxide)/TiO 2 interface, leading to an efficient electron transport. In addition, the thin compact TiO 2 layer could cover the exposed surface area of FTO to the electrolyte, which contributes to the suppression of the charge recombination. The surface treatment of mesoporous TiO 2 photoanode via aqueous electrochemical route is an effective way to improve the performance of DSSC (dye-sensitized solar cell), which could increase the short-circuit current density, and reduce the dark current density compared to DSSCs with bare and TiCl 4-treated TiO 2 photoanodes. The photoelectron conversion efficiency of DSSC was increased from 7.3 to 8.2% after employing the TiO 2-modified photoanode.

Efficient and stable, structurally inverted poly(3-hexylthiopen): [6,6]-phenyl-C61-butyric acid methyl ester heterojunction solar cells with fibrous like poly(3-hexylthiopen)

We investigated an inverted organic photovoltaic device structure in which a densely packed ~ 100 nm thin TiO 2 layer on fluorine doped conducting glass serves as anode and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/Au layer on top of the active layer serves as cathode. The active layer is comprised of a blend of poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The rectification behavior of such a device is improved significantly and injection losses are minimized compared to devices without any compact TiO 2 layer. Moreover, nanostructured P3HT active layer was achieved in-situ by spin coating concentrated pure P3HT and P3HT:PCBM blend and solar cell performances on thickness of the active layer were also investigated. For the inverted solar cells constructed with different concentrations of P3HT and PCBM keeping the P3HT:PCBM ratio 1:0.8 (wt.%), the highest short circuit current and efficiency was observed when the P3HT and PCBM concentration was equal to 1.5 (wt.%) and 1.2 (wt.%) respectively. This leads to highly stable and reproducible power conversion efficiency above 3.7% at 100 mW/cm 2 light intensity under AM 1.5 conditions.

Photoluminescence of Dense Nanocrystalline Titanium Dioxide Thin Films: Effect of Doping and Thickness and Relation to Gas Sensing

The photoluminescence (PL) of dense nanocrystalline (anatase) TiO(2) thin films is reported as a function of calcination temperature, thickness, and tungsten and nickel doping. The dependence of the optical absorption, Raman spectra, and PL spectra on heat treatment and dopants reveals the role of oxygen vacancies, crystallinity, and phase transformation in the performance of TiO(2) films used as gas sensors. The broad visible PL from defect states of compact and undoped TiO(2) films is found to be much brighter and less sensitive to the presence of oxygen than that of mesoporous films. The dense nanocrystalline grains and the nanoparticles comprising the mesoporous film are comparable in size, demonstrating the importance of film morphology and carrier transport in determining the intensity of defect photoluminescence. At higher calcination temperatures, the transformation to rutile results in the appearance of a dominant near-infrared peak. This characteristic change in the shape of the PL spectra demonstrates efficient capture of conduction band electrons by the emerging rutile phase. The W-doped samples show diminished PL with quenching on the red side of the emission spectrum occurring at lower concentration and eventual disappearance of the PL at higher W concentration. The results are discussed within the context of the performance of the TiO(2) thin films as CO gas sensors and the chemical nature of luminescent defects.

Dye-Sensitized Solar Cell with Photoanode Made with Polystyrene-Ball-Embedded TiO2 Pastes

We report the effect of varying the concentration of polystyrene (PS) balls embedded in TiO2 paste during the fabrication of TiO2 photoanodes on the performance of dye-sensitized solar cells (DSSCs). We fabricated porous photoanodes using TiO2 pastes mixed with various concentrations of PS balls in aqueous solution. During the TiO2 sintering processes, the PS evaporated, leaving behind large cavities (>1 µm) in the photoanodes. These cavities enhance the scattering of light, leading to improved absorption of light by N3 dyes. DSSC efficiency increases with the increase in PS ball concentration during TiO2 fabrication. As with classical devices, TiCl4 treatment of TiO2 and the use of a compact TiO2 layer both improve the cell efficiency of DSSC devices with our large-cavity TiO2 photoanodes.

P-doped TiO2 nanoparticles film coated on ground glass substrate and the repeated photodegradation of dye under solar light irradiation

The convenient reuse of photocatalysts is essential to the practical application in the degradation of organic pollutant. In this study, compact P-doped TiO2 film coated on ground glass substrate was easily achieved by layer-by-layer assembly technique. Thus such an elaborate complex system exhibited very convenient in recycling photocatalyst in the degradation of dye with high catalytic activity. The excellent performance of P-TiO2 film coated on ground glass substrates endows the assembled route potential in purifying waste water.

The influence of dye structure on charge recombination in dye-sensitized solar cells

Replacing the nonyl groups on the solar cell dye Ru(4,4'-carboxylic acid-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(NCS)(2) (Z-907) with amino groups results in a marked decrease in solar cell performance. This is despite the fact that the amino derivative (Z-960) has more favourable light absorption characteristics than Z-907 when used with thick nanocrystalline TiO(2) layers. Electron transfer to the electrolyte from the exposed fluorine-doped tin oxide (FTO) substrate is particularly fast in cells employing the Z-960 dye if a compact TiO(2) blocking layer is not used. The kinetics of electron transfer from the nanocrystalline TiO(2) layer in DSCs employing Z-960 are comparable to those of bare TiO(2) and ca. 2 to 5 times faster than for cells employing Z-907. The faster charge recombination in cells employing Z-960 lowers open-circuit photovoltage and results in very significant charge collection losses that lower short-circuit photocurrent. Voltammetric measurements show that surface modification of FTO electrodes with Z-960 results in slightly more facile charge transfer to acceptor species in triiodide/iodide electrolytes in the dark. A simpler molecule, p-aminobenzoic acid, more dramatically catalyses this charge transfer reaction. Conversely, chemical modification of FTO electrodes with Z-907 or p-toluic acid retards charge transfer kinetics. Similar results are obtained for nanocrystalline TiO(2) electrodes modified with these benzoic acid derivatives. These results strongly imply that surface adsorbed molecules bearing amino groups, including dye molecules, can catalyse charge recombination in dye-sensitized solar cells.

Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes

At present, the photovoltaic performance of quantum dot-sensitized solar cells (QDSCs) is still much lower than conventional DSCs. Appropriate porous TiO(2) photoanodes for QDSCs need to be further investigated, and optimization of the nanoparticle-based photoanodes is highly desirable as well. In this article, the influence of the structural properties of various TiO(2) photoanodes on CdS/CdSe-sensitized solar cells have been systematically studied. Quantitative analyses of light-harvesting efficiency (LHE) and electron-transfer yield (Φ(ET)) for the QDSCs are investigated for the first time. It is revealed that the LHE increases in the long wavelength region with the addition of large size TiO(2) particles to the transparent film. In the meantime, the balance between the light scattering and surface area also needs to be controlled, which can significantly restrain the dark current of the device. A double-layer photoanodic structure can give 4.92% of light-to-electricity conversion efficiency with a photoactive area of 0.15 cm(2).

Controlled Growth of TiO2 Nanotubes on Conducting Glass

We report on nanotubular TiO2 on top of a compact TiO2 layer on conducting glass substrates. Highly regular structures were obtained from anodization of DC sputtered Ti films of thicknesses between 0.2 and 2 μm in ammonium fluoride containing ethylene glycol solutions. The influence of different anodization parameters was systematically analyzed revealing that full control over tube length, diameter, spacing, and tube wall thickness is possible. Inner tube diameters and spacing between tubes can be controlled via the anodization field. Tube wall thickness can be tuned using different anodization bath temperatures. Tube length can be adjusted by sputtering appropriate layers of Ti. The resulting TiO2 structures can be readily used in poly(3-hexylthiophene)-TiO2 hybrid solar cells and solid state dye sensitized solar cells. In contrast to anodized Ti foils the presented geometry allows the fabrication of photovoltaic devices which can be frontside-illuminated (through the glass substrate).

Perylene imide dyes for solid-state dye-sensitized solar cells: Spectroscopy, energy levels and photovoltaic performance

We report the solid-state dye-sensitized solar cell performances of perylene imide using nanoporous TiO2 electrodes. Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD). The experimental ELUMO levels of perylene imide dyes are found to be 3.75 and 3.77eV, respectively. Therefore, perylene imide dyes can inject electrons to the conduction band of titanium dioxide in organic dye-sensitized solar cells. TiO2 thin films of about 2μm in thicknesses were prepared. Both preparation and thickness of the compact TiO2 layer were optimized using spray pyrolysis. The studies revealed that an optimum film thickness of 130–150nm of compact TiO2 yielded the best rectifying behavior and SDSC performance. In this work, our goal was to investigate the performance of perylene sensitizers in connection with spiro-OMeTAD hole transport material. Short-circuit current densities, open circuit voltages and overall conversion efficiencies of the solar cell with 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-OMeTAD) as a hole conductor and perylene imide as sensitizer on mesoporous TiO2 were investigated.

Solid State Photovoltaic Cells Using Poly-[2-Methoxy-5-(2-Ethylhexyloxy)-Phenylene Vinylene as Hole Transport Material

Solid state photovoltaic cells have been fabricated using a ruthenium dye complex as a sensitizer, and poly[2-methoxy-5-(2-ethyl-hexyloxy)-phenylene vinylene] (MEH-PPV) as a hole transporting layer. A compact TiO2 layers with no pores has been introduced to prevent charge losses related to back reactions. The photovoltaic properties of MEH-PPV/dye coated TiO2 electrodes and MEH-PPV coated TiO2 electrodes with and without using compact TiO2 layers were investigated. The compact TiO2 layer increased the current from 0.09 mA/cm2 to 0.4 mA/cm2. Further improvement in current was achieved by inserting a dye interlayer between nanoporous TiO2 and MEH-PPV. In this case, the current increased up to 2 mA/cm2 yielding an efficiency of 0.3 %.

Micropatterned TiO 2 nanotube surfaces for site-selective nucleation of hydroxyapatite from simulated body fluid

TiO 2 nanotube layers can provide greatly enhanced kinetics for hydroxyapatite formation from simulated body fluid compared with smooth, compact TiO 2 surfaces. In the present work we show how this contrast in reactivity can be used to create highly defined lateral microstructures where bone-like hydroxyapatite can be deposited with very high selectivity. For this we used a photolithographic approach to produce micropatterned TiO 2 nanotube layers surrounded by compact oxide that were then immersed in a simulated body fluid (SBF) solution. Not only the tubular vs. flat geometry but also the finding that compact oxides created in phosphate electrolytes in particular suppress apatite deposition are crucial for a very high reactivity contrast. Overall the results show the feasibility of stimulating hydroxyapatite deposition at surface locations where needed or desired. This provides a valuable tool for biomedical device design.

Role of Polyelectrolyte for Layer-by-Layer Compact TiO2 Films in Efficiency Enhanced Dye-Sensitized Solar Cells

Efficient compact TiO2 films using different polyelectrolytes are prepared by the layer-by-layer technique (LbL) and applied as an effective contact and blocking film in dye-sensitized solar cells (DSCs). The polyanion thermal stability plays a major role on the compact layers, which decreases back electron transfer processes and current losses at the FTO/TiO2 interface. FESEM images show that polyelectrolytes such as sodium sulfonated polystyrene (PSS) and sulfonated lignin (SL), in comparison to poly(acrylic acid) (PAA), ensure an adequate morphology for the LbL TiO2 layer deposited before the mesoporous film, even after the sintering step at 450 °C. The so treated photoanode in DSCs leads to a 30% improvement on the overall conversion efficiency. Electrochemical impedance spectroscopy (EIS) is employed to ascertain the role of the compact films with such polyelectrolytes. The significant increase in Voc of the solar cells with adequate polyelectrolytes in the LbL TiO2 films shows their pivotal role in ...

Efficient Performance of Electrostatic Spray-Deposited TiO2 Blocking Layers in Dye-Sensitized Solar Cells after Swift Heavy Ion Beam Irradiation.

A compact TiO2 layer (~1.1 μm) prepared by electrostatic spray deposition (ESD) and swift heavy ion beam (SHI) irradiation using oxygen ions onto a fluorinated tin oxide (FTO) conducting substrate showed enhancement of photovoltaic performance in dye-sensitized solar cells (DSSCs). The short circuit current density (Jsc = 12.2 mA cm-2) of DSSCs was found to increase significantly when an ESD technique was applied for fabrication of the TiO2 blocking layer, compared to a conventional spin-coated layer (Jsc = 8.9 mA cm-2). When SHI irradiation of oxygen ions of fluence 1 × 1013 ions/cm2 was carried out on the ESD TiO2, it was found that the energy conversion efficiency improved mainly due to the increase in open circuit voltage of DSSCs. This increased energy conversion efficiency seems to be associated with improved electronic energy transfer by increasing the densification of the blocking layer and improving the adhesion between the blocking layer and the FTO substrate. The adhesion results from instantaneous local melting of the TiO2 particles. An increase in the electron transport from the blocking layer may also retard the electron recombination process due to the oxidized species present in the electrolyte. These findings from novel treatments using ESD and SHI irradiation techniques may provide a new tool to improve the photovoltaic performance of DSSCs.