Fabrication of Three-Dimensionally Ordered Macroporous TiO2 Films with Enhanced Photovoltaic Conversion Efficiency

Negative-charged polystyrene (PS) microspheres were prepared through a soap-free emulsion polymerization method using potassium persulfate as initiator. Three-dimensionally ordered macroporous TiO₂ films were fabricated using the high-quality PS colloidal crystals templates obtained via a horizontal deposition method. The as-prepared macroporous TiO₂ films were applied as the photoanodes in dye-sensitized solar cell (DSSC). The microstructure of the products were characterized by X-ray diffractometer, fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption–desorption analyzer. The results showed that the macroporous TiO₂ films replicated well the 3D ordered structure derived from PS colloidal crystal templates and revealed a relatively large specific surface area (69.99 m²/g), which could increase the capacity of TiO₂ film anode for absorbing dyes and scattering light. The photocurrent–voltage (I–V) characteristics of DSSC were measured by a digital source meter under simulated solar light. The results indicated that the introduction of an ordered macroporous TiO₂ interfacial layer increased the photovoltaic conversion efficiency, which was improved by 68 % from 3.61 to 6.08 %, as compared to a device using a bare P25 TiO₂ photoanode. Our results showed that the hierarchically ordered macroporous TiO₂ bilayer films photoanode for DSSC could be helpful to improve the photovoltaic conversion efficiency.     

Flexible photoanodes of TiO2 particles and metallic single-walled carbon nanotubes for flexible dye-sensitized solar cells

Recent advance in flexible electronics demands development of flexible energy sources. Of particular interests are flexible dye-sensitized solar cells (DSCs). However, a brittle nature of TiO₂ materials is one of hurdles to realize flexible DSCs. Here we synthesized flexible photoanodes of TiO₂ particles and single-walled carbon nanotubes (SWNTs). Metallic SWNTs provided a greater photovoltaic conversion efficiency than semiconducting SWNTs due to the more efficient electron transport. The metallic SWNTs also constructed effective mechanical network among TiO₂ particles providing flexibility and durability. The photoanode was transferred on an indium tin oxide (ITO)-coated polyethylene terephthalate film and characterized for front-illuminated DSCs under the AM 1.5 simulated sunlight. There was only a small decrease in photovoltaic conversion efficiency with bending which was primarily caused by cracking of the ITO layer. Due to this limitation, the TiO₂–metallic SWNT photoanode was transferred on a Ti foil and went through up to 1000 bending cycles. The cycled photoanode was assembled for back-illuminated DSCs due to the non-transparent Ti foil. There was no decrease in photovoltaic conversion efficiency even after 1000 bending cycles demonstrating excellent flexibility and durability.     

Au Nanoparticles as Interfacial Layer for CdS Quantum Dot-sensitized Solar Cells

Quantum dot-sensitized solar cells based on fluorine-doped tin oxide (FTO)/Au/TiO₂/CdS photoanode and polysulfide electrolyte are fabricated. Au nanoparticles (NPs) as interfacial layer between FTO and TiO₂ layer are dip-coated on FTO surface. The structure, morphology and impedance of the photoanodes and the photovoltaic performance of the cells are investigated. A power conversion efficiency of 1.62% has been obtained for FTO/Au/TiO₂/CdS cell, which is about 88% higher than that for FTO/TiO₂/CdS cell (0.86%). The easier transport of excited electron and the suppression of charge recombination in the photoanode due to the introduction of Au NP layer should be responsible for the performance enhancement of the cell.     

Enhancement of the Mie scattering effect using floatstone-like TiO2 spherical micropigment

Floatstone-like TiO₂ microparticles with an efficiently enhanced Mie scattering effect were developed by thermofusing of rutile-type TiO₂ submicron particles on the template core of poly(acrylate) microspheres. The light reflectance of the floatstone-like TiO₂ microparticles was approximately 10% higher than that of the usual rutile-type TiO₂ submicron particles in a wavelength range from 400 to 850 nm. The diffuse reflectance of the microparticles is enhanced by the formation of a floatstone-like structure, whereas there is no difference in the specular reflectance of the particles. As an application of the floatstone-like TiO₂ microparticles, the contribution of the particles to the light-trapping effect of a solar cell was examined. The power conversion efficiency of the solar cell was enhanced by 22% with the introduction of a light-scattering layer composed of the floatstone-like TiO₂ microparticles.     

Titanium flexible photoanode consisting of an array of TiO2 nanotubes filled with a nanocomposite of TiO2 and graphite for dye-sensitized solar cells

A flexible dye-sensitized solar cell (DSSC) was fabricated using a photoanode consisting of an array of TiO₂ nanotubes (TNT) filled with a nanocomposite of TiO₂ (P90) and nanographite. The array of TNT was obtained by anodic oxidation of Ti foil, and this Ti foil with TNT was used as the photoanode of the DSSC. Each tube in the array has an average diameter of 100 nm. The morphologies of the array of TNT were obtained both after and before filling them with the TiO₂/graphite nanocomposite, using a field-emission scanning electron microscopy (FE-SEM). DSSC with photoanode consisting of the nanocomposite (photoanode designated as Graphite/P90-TNT) rendered a light-to-electricity conversion efficiency (η) of 5.75%. In contrast, the cells with photoanodes consisting of only TNT (photoanode designated as TNT) and TNT filled with P90-TiO₂ (photoanode designated as P90-TNT) exhibited efficiencies (η) of 4.44% and 5.14%, respectively. The enhancements in the η’s in favor of the cells with P90-TNT and Graphite/P90-TNT were attributed to the filled P90 and nanocomposite, respectively. The filled particles were assumed to provide more conductive pathways for electron transfer and prolonged lifetime for electrons in the film of TNT. The results were substantiated by light-absorption values, incident-photo-to-current efficiency (IPCE) curves, Nyquist and Bode plots of electrochemical impedance spectroscopy (EIS), and photopotential transient curves.     

Polyvinyl pyrrolidone aided preparation of TiO2 films used in flexible dye-sensitized solar cells

Polyvinyl pyrrolidone (PVP) is introduced to low temperature preparation of a good quality TiO₂ film used in flexible dye-sensitized solar cells (DSSCs). The samples are characterized by scanning electron microscopy and UV–vis absorption spectra, the photovoltaic performance of the DSSC is measured. It is found that PVP can improve the dispersion of TiO₂ particles and the adherence of TiO₂ particles to flexible substrate, as well as the adsorption of sensitized dye to TiO₂ film. Additionally, ultraviolet light irradiation can eliminate organics remained on the surface the TiO₂ film and improve the surface state of TiO₂ film. Under an optimal condition, a flexible DSSC using TiO₂ film doped PVP and UV irradiation treated achieves a light-to-electric energy conversion efficiency of 3.02% under irradiation with a simulate solar light intensity of 100 mW cm⁻².     

Partially dyed-TiO2 dispersions for adaptation to the continuous fabrication of photoanodes

A new fabrication method of photoanodes for dye-sensitized solar cells (DSCs) is proposed using partially N3-dyed-TiO₂ nanoparticle dispersions. The photoanode fabrication method presented here is contrasted with a conventional low-temperature compression process, in that a time-consuming dye-adsorption step can be omitted, making DSCs more adaptable to a continuous printing process. The effects of the amount of dye-loadings, the degree of compression, and the film thickness on photovoltaic performances were examined. All processes were conducted at room temperature. The optimized film composed of partially dyed-TiO₂ led to sufficient interparticle connections to reach a power conversion efficiency of 5.0% under 100 mW cm⁻² AM1.5 illumination.     

Large-diameter titanium dioxide nanotube arrays as a scattering layer for high-efficiency dye-sensitized solar cell

Large-sized titanium dioxide (TiO₂) nanotube arrays with an outer diameter of approximately 500 nm have been successfully synthesized by potentiostatic anodization at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. It is found that the synthesized large-diameter TiO₂ nanotube array shows a superior light scattering ability, which can be used as a light scattering layer to significantly enhance the efficiency of TiO₂ nanoparticle-based dye-sensitized solar cells from 5.18% to 6.15%. The remarkable light scattering ability makes the large-diameter TiO₂ nanotube array a promising candidate for light management in dye-sensitized solar cells (DSSCs).     

KrF excimer laser irradiated nanoporous TiO2 layers for dye-sensitized solar cells: Influence of laser power density

This paper reports the performance of dye-sensitized solar cells (DSSCs) with nanoporous TiO₂ photoanodes irradiated by KrF excimer laser beams of various power densities. The laser induces surface remelting and solidification to create textures on the TiO₂ layers. After laser irradiation, TiO₂ also undergoes a phase transition from anatase to rutile; the amount of the transformed phase increases with the laser power density. For dye-anchored TiO₂ layers, light absorption increases first and then decreases as the laser power density increases. The assembled cell efficiency, strongly correlated with the photocurrent density, also increases first and then decreases as the laser power density rises. This indicates that laser treatment creates surface textures on TiO₂ that improve light trapping in the dye-anchored TiO₂ photoanodes, thereby increasing the photocurrent level and cell efficiency. For high laser irradiation power density, the ablation of the TiO₂ layer becomes significant, leading to a decrease in cell efficiency. The surface remelting and solidification process reduces the density of the surface recombination centers on TiO₂, resulting in an increase of open-circuit voltage.     

Quasi solid state dye-sensitized solar cells with modified TiO2 photoelectrodes and triphenylamine-based dye

Quasi solid state dye-sensitized solar cells (DSSCs) have been fabricated with organic sol or TiCl₄ modified TiO₂ and porous TiO₂ photoanode and a triphenylamine-based dye (TPAR3) used as photosensitizer. Dark current measurements suggested that both modified TiO₂ photoelectrodes had significantly reduced the recombination rate of photoelectrons due to the reduced bare FTO surface in comparison to porous photoelectrode. The DSSC based on modified TiO₂ photoelectrodes showed improved photovoltaic parameters compared to the porous TiO₂ photoelectrode. The overall power conversion efficiency (PCE) is 3.27%, 4.73% and 6.8% for porous, TiCl₄ modified and sol modified TiO₂ photoelectrodes, respectively. The improved PCE with modified TiO₂ electrodes was attributed to the formation of a compact layer. This effectively improves adherence of TiO₂ to FTO surface, providing a larger TiO₂/FTO contact area and reducing the electron recombination by blocking the direct contact between redox electrolyte and the conductive FTO surface and enhances the electron collection efficiency.     

Efficient PbS/CdS co-sensitized solar cells based on TiO2 nanorod arrays

Narrow bandgap PbS nanoparticles, which may expand the light absorption range to the near-infrared region, were deposited on TiO₂ nanorod arrays by successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The thicknesses of PbS nanoparticles were optimized to enhance the photovoltaic performance of PbS QDSCs. A uniform CdS layer was directly coated on previously grown PbS-TiO₂ photoanode to protect the PbS from the chemical attack of polysulfide electrolytes. A remarkable short-circuit photocurrent density (approximately 10.4 mA/cm²) for PbS/CdS co-sensitized solar cell was recorded while the photocurrent density of only PbS-sensitized solar cells was lower than 3 mA/cm². The power conversion efficiency of the PbS/CdS co-sensitized solar cell reached 1.3%, which was beyond the arithmetic addition of the efficiencies of single constituents (PbS and CdS). These results indicate that the synergistic combination of PbS with CdS may provide a stable and effective sensitizer for practical solar cell applications.     

The influence of anatase-rutile mixed phase and ZnO blocking layer on dye-sensitized solar cells based on TiO2nanofiberphotoanodes

High performance is expected in dye-sensitized solar cells (DSSCs) that utilize one-dimensional (1-D) TiO₂ nanostructures owing to the effective electron transport. However, due to the low dye adsorption, mainly because of their smooth surfaces, 1-D TiO₂ DSSCs show relatively lower efficiencies than nanoparticle-based ones. Herein, we demonstrate a very simple approach using thick TiO₂ electrospun nanofiber films as photoanodes to obtain high conversion efficiency. To improve the performance of the DSCCs, anatase-rutile mixed-phase TiO₂ nanofibers are achieved by increasing sintering temperature above 500°C, and very thin ZnO films are deposited by atomic layer deposition (ALD) method as blocking layers. With approximately 40-μm-thick mixed-phase (approximately 15.6?wt.% rutile) TiO₂ nanofiber as photoanode and 15-nm-thick compact ZnO film as a blocking layer in DSSC, the photoelectric conversion efficiency and short-circuit current are measured as 8.01% and 17.3?mA?cm^?2, respectively. Intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy measurements reveal that extremely large electron diffusion length is the key point to support the usage of thick TiO₂ nanofibers as photoanodes with very thin ZnO blocking layers to obtain high photocurrents and high conversion efficiencies.     

Micron-sized silica mesh for enhancing the performance of quantum dot sensitized solar cells

Three dimensional silica mesh structures are prepared through a new and simple method for enhancing the quantum dot sensitized solar cells performance and stability.Silica patterns are made on the top of the TiO₂ photoanodes and a marked improvement in light scattering properties of meshed structures is confirmed by diffuse reflectance spectroscopy measurements. This improvement enhances the current density and consequently the cells ‘efficiency. Parameters of electron transport in cells are explored by electrochemical impedance spectroscopy (EIS). According to the EIS results, silica mesh declines the recombination rate in cells in a clear way. Here more than 50% efficiency improvement is obtained in meshed structures in comparison to cells with normal TiO₂ photoanode structures. The insulated silica mesh, reduces the electrolyte's deleterious effect on the semiconductor layers and the cells’ stability is improved.     

Influence of 1-methylbenzimidazole interactions with Li and TiO2 on the performance of dye-sensitized solar cells

In the present work, the effect and the mechanism of the interaction between 1-methylbenzimidazole (MBI) with Li⁺ and TiO₂ on the performance of dye-sensitized solar cells were explored. A negative shift of the TiO₂ Fermi level of the nanostructured TiO₂ electrode could attribute to the interaction of MBI with Li⁺ and TiO₂, which shown that MBI affects the surface state of the TiO₂ film by decreasing the amount of the adsorbed Li⁺ ions. When MBI was added in the electrolyte, the interfacial exchange current density (j₀) reduced and the cathodic Tafel slope (β_c) increased in dye-sensitized solar cells due to the reduction of the surface adsorption of Li⁺ and ions from the electrolyte. The photovoltaic performance of the dye-sensitized solar cells under UV illumination suggests that the addition of MBI in the electrolyte could improve the efficient and stability of dye-sensitized solar cells.     

TiO2/CdS/CdSe quantum dots co-sensitized solar cell with the staggered-gap (type-II) heterojunctions for the enhanced photovoltaic performance

The effect of co-sensitization and ZnS passivation on the photovoltaic performance of CdS quantum dot sensitized solar cells (QDSSCs) were investigated. The deposition of CdS, CdSe quantum dots (QD) and ZnS passivation on TiO₂ photoanode was carried out by successive ionic layer adsorption and reaction (SILAR) method. CdS/CdSe co-sensitization developed two staggered type-II heterojunctions at TiO₂/CdS and CdS/CdSe interfaces and resulted a cascade energy band structure. This suitable band alignment facilitated the double charge transfer mechanism at each heterojunction and transported the electrons easily into the photoanode. The narrow bandgap sensitizers CdS and CdSe significantly improved the potential utilization of solar spectrum with more charge carrier generation. ZnS passivation on QD surface suppressed electrode/electrolyte interfacial charge recombination and facilitated more electron injection from QDs into TiO₂ photoanode. The EDAX elemental mapping results inferred that CdS, CdSe and ZnS have efficiently covered the TiO₂ surface. TiO₂/CdS and CdS/CdSe interfaces and the amorphous nature of ZnS could be verified with HRTEM images. Hence, the co-sensitization and surface passivation played a significant role to enhance the PCE of CdS QDSSC from 1.9% to 4.05%.     

Mesoporous TiO<Subscript>2</Subscript> microspheres synthesized via a facile hydrothermal method for dye sensitized solar cell applications

Mesoporous TiO₂ microspheres were successfully synthesized by a facile hydrothermal process and the obtained product was sintered at 450 °C. The sintered TiO₂ powder was characterised by powder X-ray diffraction pattern and the result shows pure anatase phase with good crystalline nature. The morphological image of field emission scanning electron microscopy and high resolution transmission electron microscopy shows spherical shape and size of the particles is around 100 to 300 nm. The Brunauer–Emmett–Teller surface area of synthesized TiO₂ material was 56.32 m² g^?1 and average pore width of synthesized materials was 7.1 and 9.3 nm. Bimodal pore structure of TiO₂ microspheres has been very effective for electrolyte diffusion into photoanode in dye sensitized solar cells. The synthesized anatase TiO₂ microsphere based dye sensitized solar cells have high surface area with light scattering effect to enhance the photocurrent and conversion efficiency than the commercial P25 photoanode material. The power conversion efficiency of synthesized mesoporous TiO₂ microspheres and commercial P25 material is 4.2 and 2.7 % respectively. Therefore bimodal mesoporous anatase TiO₂ microsphere appears to be a promising and potential candidate for dye sensitized solar cells (DSSC) application.     

A natural tomato slurry as a photosensitizer for dye-sensitized solar cells with TiO2/CuO composite thin films

We have studied the performance of dye-sensitized solar cells by employing natural dye “anthocyanins” extracted from the tomato slurry as a sensitizer for the TiO₂/CuO photoanode. The extracts were anchored on TiO₂/CuO films deposited on an ITO substrate which was used as a photoanode. The dye adsorbed TiO₂/CuO films electrode, the copper plate as a counter electrode, and iodolyte as an electrolyte were assembled into DSSCs. The conversion efficiency of the DSSCs was found to be 2.96% with a V_OC of 0.615 V, J_SC of 6.6 mA/cm², and an FF of 0.73. This work highlights the use of contribution of the tomato slurry as a natural sensitizer to enhance the efficiency of DSSCs.     

Mixed 3D/2D dimensional TiO2 nanoflowers/MoSe2 nanosheets for enhanced photoelectrochemical hydrogen generation

Recently, v arious kinds of methods have been implemented to broaden the visible light response and fasten the carrier's separation of TiO₂-based photoanodes. As a promising hydrogen evolution reaction catalyst, MoSe₂ is rarely investigated especially combined with TiO₂ photoanode. In this study, we report a composite photoanode of MoSe₂ nanosheets (with 1T and 2H phase)-modified 3D TiO₂ nanoflowers (NFs).The hybrid of 3D TiO₂ NFs/2D MoSe₂ holds great promise in boosting the PEC water splitting performance. TiO₂ NFs/MoSe₂-15 showed the largest photocurrent density of 1.40 mA/cm², which was five times higher than that of pure TiO₂ NFs under AM1.5G illumination. Moreover 10 times improvement in current density was observed for the TiO₂ NFs/MoSe₂-15 under visible light. This increase could be ascribed to synergistic effects of light absorption enhancement and more efficient carrier separation after MoSe₂ modification. This study not only provides a reference to boost the photoelectrochemical performance of photoelectrodes but also renders a perspective on the potential applications of MoSe₂ nanosheets.     

A cylindrical core-shell-like TiO<Subscript>2</Subscript> nanotube array anode for flexible fiber-type dye-sensitized solar cells

A versatile anodization method was reported to anodize Ti wires into cylindrical core-shell-like and thermally crystallized TiO₂ nanotube (TNT) arrays that can be directly used as the photoanodes for semi- and all-solid fiber-type dye-sensitized solar cells (F-DSSC). Both F-DSSCs showed higher power conversion efficiencies than or competitive to those of previously reported counterparts fabricated by depositing TiO₂ particles onto flexible substrates. The substantial enhancement is presumably attributed to the reduction of grain boundaries and defects in the prepared TNT anodes, which may suppress the recombination of the generated electrons and holes, and accordingly lead to more efficient carrier-transfer channels.     

Enhanced power conversion efficiency in dye-sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes

Photoelectrodes of mixed microsized TiO₂ aggregates and individually dispersed TiO₂ nanocrystallites with different ratios were fabricated and studied for improved power conversion efficiency in dye-sensitized solar cells (DSCs). TiO₂ aggregates/nanocrystallites composites possess several advantages for high performance of DSCs, including the light scattering by the microsized TiO₂ aggregates and the high surface area of nanocrystallites both in aggregates and individually dispersed. A high power conversion efficiency of 7.59% was achieved with mixed TiO₂ aggregates/nanocrystallites photoelectrode using conventional dye N3, without applying anti-reflection coating, back-scattering layer, or chemical treatment. The electron transport properties of DSCs with mixed photoelectrodes were investigated by electrochemical impedance spectra, and the results showed that such a photoelectrode with mixed aggregates and nanocrystallites possess better connectivity for efficient electron transport.