Use In Dye-sensitized Solar Cells Research Articles

Preparation of highly porous TiO2 nanofibers for dye-sensitized solar cells (DSSCs) by electro-spinning

TiO2 nanofibers for use in dye-sensitized solar cells (DSSCs) were prepared from a solution of polymerized titanium tetraisopropoxide using a modified electro-spinning process to create fibers with high specific surface areas. The sol–gel technique was utilized to prepare the spinning solution, and glycerin was added to investigate its effects on the surface area and porosity of the TiO2 nanofibers. The spinning rate, tip-to-collector distance, voltage, and amount of glycerin were varied simultaneously and independently to determine the optimal conditions for the preparation of highly porous TiO2 nanofibers. The optimal conditions for producing such electrospun TiO2 nanofibers were 0.4g of glycerin per 29ml of spinning solution, an applied voltage of 22kV, a flow rate of 0.1ml/h, and a tip-to-collector distance (TCD) of 20cm. The thermal decomposition of glycerin during the calcination process increased the surface area of the finished TiO2 nanofibers. SEM and XRD analyses confirmed that the TiO2 nanofibers had an anatase crystallite structure and possessed thicknesses of 80–150nm, a maximum specific surface area of 103.3m2/g, maximum porosity of 80.5% and maximum efficiency of 4.6%, which was significantly higher than that of typical TiO2 nanofibers. Thermogravimetric analysis revealed that the solvent, binder, and impurities were removed at 100°C, 250°C and 450°C, respectively.

Ultrafast Growth of Highly Ordered Anodic TiO2 Nanotubes in Lactic Acid Electrolytes

In the present work, we show that fully functional self-organized TiO(2) nanotube layers can be electrochemically grown with an unprecedented growth rate if lactic acid (LA) is used as an additive during anodization. The main effect of LA addition is that it allows performing nanotube growth at significantly higher anodization voltage than in the LA free case, and this without dielectric oxide breakdown ("burning"). As a result, for example, 15 μm tube thick nanotube layers, suitable for a use in dye-sensitized solar cells (DSSCs) can be grown in 45 s and 7 μm tubes suitable for water splitting can be grown in 25 s.

Photoelectrochemical effects of hyperbranched polyglycerol in gel electrolytes on the performance of dye-sensitized solar cells

A gel electrolyte containing hyperbranched polyglycerol (HPG) was prepared for use in dye-sensitized solar cells (DSSCs) applications, and the effects of HPG on DSSCs performance were studied. The open circuit voltage increased proportionally to HPG composition and the short circuit current increased at optimized HPG ratio due to the expansion of carrier transfer channel and the rapid salt dissociation effects of HPG. Electrochemical impedance spectroscopy of the DSSCs indicated that incorporation of HPG improved the charge transfer properties of the gel electrolytes. Overall, the optimized DSSCs yielded an energy conversion efficiency of 6.78% at 1 sun, with good long-term stability.

Fast Transporting ZnO–TiO2 Coaxial Photoanodes for Dye-Sensitized Solar Cells Based on ALD-Modified SiO2 Aerogel Frameworks

A doubly coaxial photoanode architecture based on templated SiO(2) aerogels was fabricated on transparent conducting oxides for use in dye-sensitized solar cells (DSSCs). These templates were coated with ZnO via atomic layer deposition (ALD) to yield an electronically interconnected, low-density, high-surface-area, semiconductor framework. Addition of a thin conformal layer of a second metal oxide (alumina, zirconia, or titania) via ALD was found to suppress the dissolution of ZnO that otherwise occurs when it is soaked in alcohol solutions containing acidic dyes used for sensitization or in acetonitrile solutions containing a pyridine derivative and the iodide/tri-iodide (I(-)/I(-)(3)) redox shuttle. Electron transport in SiO(2)-ZnO-TiO(2) photoelectrodes was found to be nearly 2 orders of magnitude faster than in SiO(2)-TiO(2) structures, implying that the interior ZnO sheath serves as the primary electron conduit. In contrast, rates of electron interception by the oxidized form of the redox shuttle were observed to decrease when a TiO(2) shell was added to SiO(2)-ZnO, with the decreases becoming more significant as the thickness of the titania shell increases. These effects lead to improvements in efficiency for DSSCs that utilize I(-)/I(-)(3), but much larger improvements for DSSCs utilizing ferrocene/ferrocenium, a notoriously fast redox shuttle. For the former, overall energy conversion efficiencies maximize at 4.0%. From a variety of experiments, the primary factor limiting aerogel-based DSSC performance is light loss due to scattering. Nevertheless, variants of the doubly coaxial structure may prove useful in devising DSSCs that can achieve excellent energy conversion efficiencies even with fast redox shuttles.

Fabrication of hierarchical ZnO nanostructures for dye-sensitized solar cells

Long ZnO nanowires and hierarchical ZnO nanostructures were fabricated by simple anodization and subsequent electrochemical deposition for use in dye-sensitized solar cells (DSSCs). The length of the long ZnO nanowires increased with increasing anodization time at a constant voltage, with a growth rate of 50–80μmh−1. The optimal electrochemical deposition time for preparing hierarchical ZnO nanostructures on ZnO nanowires was approximately 1.5 times the anodization time. Compared to the ZnO nanowires, the hierarchical ZnO nanostructures exhibited higher power conversion efficiency due to an enlargement of the internal surface area. In addition, the hierarchical ZnO nanostructures showed good corrosion resistance in an acidic dye solution.

Dye‐Sensitized TiO2 Nanotube Solar Cells: Rational Structural and Surface Engineering on TiO2 Nanotubes

Owing to well-defined structural parameters and enhanced electronic properties, highly ordered TiO(2) nanotube arrays have been employed to substitute TiO(2) nanoparticles for use in dye-sensitized solar cells. To further improve the performance of dye-sensitized TiO(2) nanotube solar cells, efforts have been directed toward the optimization of TiO(2) photoanodes, dyes, electrolytes, and counter electrodes. Herein, we highlight recent progress in rational structural and surface engineering on anodic TiO(2) nanotube arrays and their effects on improving the power conversion efficiency of dye-sensitized TiO(2) nanotube solar cells.

Study of a sandwich structure of transparent conducting oxide films prepared by electron beam evaporation at room temperature

Transparent conducting ZnO/Ag/ZnO multilayer electrodes having electrical resistance much lower than that of widely used transparent electrodes were prepared by ion-beam-assisted deposition (IAD) under oxygen atmosphere. The optical parameters were optimized by admittance loci analysis to show that the transparent conducting oxide (TCO) film can achieve an average transmittance of 93%. The optimum thickness for high optical transmittance and good electrical conductivity was found to be 11 nm for Ag thin films and 40 nm for ZnO films, based on the admittance diagram. By designing the optical thickness of each ZnO layer and controlling process parameters such as IAD power when fabricating dielectric-metal-dielectric films at room temperature, we can obtain an average transmittance of 90% in the visible region and a bulk resistivity of 5 × 10−5 Ω-cm. These values suggest that the transparent ZnO/Ag/ZnO electrodes are suitable for use in dye-sensitized solar cells.

Controllable hydrothermal synthesis of nanocrystal TiO2 particles and their use in dye-sensitized solar cells

A simple method for the controllable hydrothermal synthesis of nanocrystalline anatase TiO2 (nc-TiO2) particles involving the selection of suitable organic alkali peptizing agents is reported. A dye-sensitized solar cell (DSSC) with square-like nc-TiO2 particles with side lengths about 8–13 nm—prepared using tetraethylammonium hydroxide (TEAOH)—in the photoelectrode showed higher photovoltaic performance than two other DSSCs with square-like nc-TiO2 particles with side lengths about 7–10 nm—prepared using tetrabutylammonium hydroxide—or elongated nc-TiO2 particles with lengths about 18–35 nm and width about 10–18 nm—prepared using tetramethylammonium hydroxide (TMAOH)—in the photoelectrodes. When a scattering layer prepared from sub-micron size spheres or cone-like nc-TiO2 particles—synthesized using a higher concentration of TMAOH—was added on top of the photoelectrode fabricated from nc-TiO2 synthesized with TEAOH, the energy conversion efficiency of the DSSC was markedly increased from 6.77% to 8.18%.

A low-cost counter electrode of ITO glass coated with a graphene/Nafion® composite film for use in dye-sensitized solar cells

Carbon black (CB), multi-walled carbon nanotube (MWCNT), and graphene (GN) were examined as catalytic films on the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). GN exhibits the best performance among these materials for the corresponding cell. A composite film of GN/Nafion® was next used as the catalytic film on the CE of a DSSC. Nafion® is demonstrated to be an excellent dispersant for inhibiting the aggregation of GN. A solar-to-electricity conversion efficiency (η) of 8.19% was achieved for the DSSC sensitized by TBA(Ru[(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-bis(5-(hexylthio)-2,2′-bithien-5′-yl)-2,2′-bipyridine)(NCS)2]), i.e., CYC-B11 dye, which was synthesized by our group, after the optimization of the composition of GN/Nafion®, and an η of 8.89% was exhibited for the cell with a sputtered-Pt (s-Pt) film on its CE under the same conditions. The significance of the cell with the composite film of GN/Nafion® lies in the fact that the expensive Pt is avoided in this case, thereby the cost of the pertinent DSSC was greatly reduced, even though its η (8.19%) is slightly smaller than that of the cell with the s-Pt film (8.89%).

Functionalized organic dyes containing a phenanthroimidazole donor for dye-sensitized solar cell applications

Five functionalized organic dyes (H6-10) containing a phenanthroimidazole unit as an electron donor were synthesized and characterized for use in dye-sensitized solar cell (DSSC) applications. Under standard global AM 1.5 solar conditions, the DSSCs based on dye H6 displayed the best performance, with an incident photon-to-current conversion efficiency (IPCE) exceeding 70% at wavelengths of 400–530 nm, a short-circuit photocurrent density of 10.98 mA cm−2, an open-circuit voltage of 0.68 V, a fill factor of 0.69, and an overall conversion efficiency of 5.12%. This efficiency is ∼94% of that for JK2 cells (5.46%) and ∼72% of that for N719 cells (7.07%) under the same conditions.

Dye Molecular Structure Device Open-Circuit Voltage Correlation in Ru(II) Sensitizers with Heteroleptic Tridentate Chelates for Dye-Sensitized Solar Cells

Dicarboxyterpyridine chelates with π-conjugated pendant groups attached at the 5- or 6-position of the terminal pyridyl unit were synthesized. Together with 2,6-bis(5-pyrazolyl)pyridine, these were used successfully to prepare a series of novel heteroleptic, bis-tridentate Ru(II) sensitizers, denoted as TF-11-14. These dyes show excellent performance in dye-sensitized solar cells (DSCs) under AM1.5G simulated sunlight at a light intensity of 100 mW cm(-2) in comparison with a reference device containing [Ru(Htctpy)(NCS)(3)][TBA](3) (N749), where H(3)tctpy and TBA are 4,4',4"-tricarboxy-2,2':6',2"-terpyridine and tetra-n-butylammonium cation, respectively. In particular, the sensitizer TF-12 gave a short-circuit photocurrent of 19.0 mA cm(-2), an open-circuit voltage (V(OC)) of 0.71 V, and a fill factor of 0.68, affording an overall conversion efficiency of 9.21%. The increased conjugation conferred to the TF dyes by the addition of the π-conjugated pendant groups increases both their light-harvesting and photovoltaic energy conversion capability in comparison with N749. Detailed recombination processes in these devices were probed by various spectroscopic and dynamics measurements, and a clear correlation between the device V(OC) and the cell electron lifetime was established. In agreement with several other recent studies, the results demonstrate that high efficiencies can also be achieved with Ru(II) sensitizers that do not contain thiocyanate ancillaries. This bis-tridentate, dual-carboxy anchor configuration thus serves as a prototype for future omnibearing design of highly efficient Ru(II) sensitizers suited for use in DSCs.

Design of NIR-Absorbing Simple Asymmetric Squaraine Dyes Carrying Indoline Moieties for Use in Dye-Sensitized Solar Cells with Pt-Free Electrodes

Novel near-infrared (NIR)-sensitizing (up to 800 nm) simple asymmetric squaraine dyes (Sq 31 and Sq 33) carrying indoline moieties that did not require the introduction of any linker groups were developed. DSSCs fabricated with Sq 33 exhibited remarkable characteristics in the long-wavelength visible and NIR region (up to 800 nm), such as a conversion efficiency of 3.75% (AM 1.5G) with an incident photon-to-current conversion efficiency of 63% (650 nm), a short-circuit photocurrent density of 13.64 mA, an open-circuit photovoltage of 0.48, and a fill factor of 0.57.

Surface States and Electronic Structures of Various Sintered TiO2 Photoanodes for Use in Dye-Sensitized Solar Cells

Photoanodes used in dye-sensitized solar cells were prepared by sintering hydrothermally synthesized particles under different gases. The obtained photoanodes were then characterized for the crystalline and electronic structures, the surface chemistry, and dye loading capability. The performance of solar cells having the obtained photoelectrodes was evaluated by both the overall conversion efficiency and incident photon-to-electron conversion efficiency. The facile approach of sintering TiO2 under different gases was found to vary the electronic structure and, particularly, the surface state of the TiO2. As compared to the air-sintered film, the nitrogen- and oxygen-sintered TiO2 films reduce the amount of oxygen vacancies and thus increase the Ti4+ (TiO2) concentration, as evident by Ti L-edge and O K-edge X-ray absorption spectra. X-ray photoelectron spectroscopy analysis also shows the same result. It was found that the oxygen-sintered photoanode exhibit the highest oxidation state and has the highest amount of surface C-OH functional group, leading to a 20% increase in the cell efficiency as compare to the cell having the air-sintered TiO2 photoanode. The effect of the resulting TiO2 photoanode characteristics on the cell performance is also discussed.

Anion-controlled morphologies and spectral features of cyanine-based nanoGUMBOS – an improved photosensitizer

The ability to control the morphologies and spectral properties of organic low-dimensional nanomaterials is of paramount importance. The research reported herein demonstrates a template-free approach to tailored morphological and optical properties for a novel class of pseudoisocyanine (PIC)-based fluorescent organic nanoparticles derived from a group of uniform materials based on organic salts (GUMBOS). The synthesized nanoscale PIC-based particles (termed nanoGUMBOS), [PIC][NTf(2)] and [PIC][BETI], exhibit interesting adaptability as a function of the associated anion. The diamond-shaped nanostructures of [PIC][NTf(2)] and [PIC][BETI] nanorods exhibit enhanced fluorescence quantum yields relative to the parent compound, [PIC][I]. As supported by fluorescence lifetime measurements, these enhanced spectral properties can be attributed to differences in molecular self-assembly ordering (e.g., H- vs. J-aggregation) and restricted molecular rotation leading to reduced twisted intramolecular charge transfer in the nanoGUMBOS. The electrochemical properties of the PIC-based GUMBOS suggest their potential use in dye-sensitized solar cells.

Low-cost, quasi-solid-state and TCO-free highly bendable dye-sensitized cells on paper substrate

A highly conductive Ni-coated commercial paper substrate (square resistance <1 Ω) was prepared by chemical deposition, and used as the substrate for a flexible TiO2 film electrode derived from a binder-free TiO2 paste, and heat treated at 250 °C. A low-cost, quasi-solid-state and TCO-free highly bendable paper-based dye-sensitized solar cell with conversion efficiency up to 2.90% is fabricated successfully by applying an iodine-free electrolyte. This encouraging result indicates that commercial paper is a promising material for use in dye-sensitized solar cells because of its flexibility, low cost and relatively high resistance to temperature. Compared with the rigid transparent conducting oxide (TCO) coated glass and flexible conductive plastic substrates, the potential of using mature paper making and coating technologies will greatly reduce the cost of the current photovoltaic devices.

Modulating dye E(S+/S*) with efficient heterocyclic nitrogen containing acceptors for DSCs

Acceptor motifs based on nitrogen containing heterocycles have been synthesized for use in dye-sensitized solar cells (DSCs). Through the selective addition of nitrogen atoms and increased conjugation of the nitrogen containing heterocycles the excited-state oxidation potential, E((S+/S*)), may be conveniently tuned with minimal effect on the ground-state oxidation potential, E((S+/S)), of the dye.

How TiO2 crystallographic surfaces influence charge injection rates from a chemisorbed dye sensitiser

High-energy metal oxide surfaces are considered to be promising for applications involving surface-adsorbate electron transfer, such as photocatalysis and dye-sensitised solar cells. Here, we compare the efficiency of electron injection into different TiO(2) anatase surfaces. We model the adsorption of a carboxylic acid (formic acid) on anatase (101), (001), (100), (110) and (103) surfaces using density functional theory calculations, and calculate electron injection times from a model dye into these surfaces. We find that the different positions of the conduction band edge of these surfaces determine the rate of electron injection (which is faster for the surfaces with lower-lying conduction band, among them the most stable (101) surface). However, if the dye's injection energy is enforced to be at a fixed energy deep inside each surface's conduction band, then several anatase surfaces, such as the synthetically achievable (001) surface, show rates of injection comparable or faster than the (101) surface. Moreover, because of their higher-lying conduction bands, these minority surfaces are likely to offer higher open-circuit voltages in dye-sensitised solar cells. Therefore, synthetically accessible high-energy anatase surfaces, such as (001)-oriented nanostructures, may be promising candidates for use in dye-sensitised solar cells.

Anatase TiO 2 hollow spheres with small dimension fabricated via a simple preparation method for dye-sensitized solar cells with an ionic liquid electrolyte

The anatase TiO 2 hollow spheres with a diameter of approximately 250 nm were synthesized by a chemical template method. Compared to conventional P25 nanocrystalline particles, the TiO 2 hollow structured materials not only have a low density, high specific surface areas, and hierarchically porous structures but also exhibit high light-scattering efficiency and fast motion of the charge carriers. A photoelectrode consisting of TiO 2 hollow spheres for use in dye-sensitized solar cells (DSSCs) is prepared, and the nanostructure and the performance of DSSCs with an ionic liquid electrolyte are investigated. Improved performance of the DSSCs (Jsc = 8.0 mA cm −2, Voc = 0.732 V, FF = 65%, and η = 3.79%) is obtained compared with the P25 TiO 2 nanocrystalline DSSCs (Jsc = 7.25 mA cm −2, Voc = 0.702 V, FF = 59%, η = 2.96%, and measured at 100 mW cm −2, AM 1.5 G). The improved performance is primarily due to the enhanced light scattering by the TiO 2 hollow spheres and the highly porous structure of the hollow sphere electrode, allowing the highly viscous non-volatile electrolytes to thoroughly penetrate the electrode. The results show that the structure of the TiO 2 hollow sphere photoelectrode can greatly improve the performance of the DSSCs with non-volatile electrolytes.

Fabrication of Multiwalled Carbon Nanotube-Titania Nanocomposite for Dye-Sensitized Solar Cell

This work highlights the fabrication of multiwalled carbon nanotube (MWCNT)-TiO2 nanocomposite for use in dye-sensitized solar cells (DSSCs) to improve the photovoltaic conversion efficiency. Morphology of the MWCNT-TiO2 film was investigated by field emission scanning electron microscopy (FESEM). FESEM images demonstrated that MWCNTs were dispersed homogeneously in TiO2 matrix. MWCNT acted as an efficient conduit for electron transfer that can enhance the photovoltaic conversion efficiency of the DSSC. The proposed MWCNT-TiO2 nanocomposite photoelectrode might be beneficial to develop high photovoltaic conversion efficiency of DSSCs.

Unsymmetrical squarylium dyes with π-extended heterocyclic components and their application to organic dye-sensitized solar cells

Novel π-extended squarylium dyes have been synthesized as sensitizers for use in dye-sensitized solar cells (DSSCs). Four quaternary salts with extended π-systems, quinolinium, benz[c,d]indolium, benzopyrylium, and benzo-1-thiopyrylium, were selected as electron-rich heterocyclic components and condensed with indole-based semisquarylium bearing a carboxyl group as an anchor for their immobilization on TiO 2 to obtain four unsymmetrical squarylium dyes ( SQ1–4). They exhibited intense absorption in the near-infrared region in solution and on the TiO 2 surface (absorption edge: approximately 900 nm). Investigation of their electrochemical properties revealed that electron injection from the excited dyes to the TiO 2 conduction band was thermodynamically permitted. A molecular-orbital calculation indicated that the electron distribution moved from the cyclobutene core to the heterocyclic components bearing the carboxyl group by photoexcitation of SQ1–4. The photovoltaic performances of DSSCs based on the squarylium dyes significantly depended on the structure of heterocyclic components. Among the series of squarylium dyes, SQ2–4 achieved comparatively high performance in metal-free NIR sensitizers ( η = 1.1–1.6%), in accordance with the calculation results. These results suggest that the structure of heterocyclic components strongly affects not only the absorption properties of the dyes but also the photovoltaic performances of DSSCs based on these dyes.