Use In Dye-sensitized Solar Cells Research Articles

Comparative Interface Metrics for Metal-Free Monolayer-Based Dye-Sensitized Solar Cells

The first quantitative comparison between self-assembled monolayers of homologous carboxylate- and phosphonate-terminated organic dyes that are of use in dye-sensitized solar cells (DSSCs) is reported. (Cyanovinyl)phosphonate-terminated oligothiophenes and (cyanovinyl)carboxylate-terminated oligothiophenes were synthesized on TiO(2) thin film electrodes. Structurally analogous organics were compared for the effect of the anchoring groups on photochemical properties in solution as measured by UV/vis spectroscopy and for reactivity with the electrode surface. Monolayers were grown on the TiO(2) electrodes either by "tethering by aggregation and growth" (T-BAG) or by solution dipping. Surface roughness and homogeneity, elemental composition, and thickness of the monolayers were evaluated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ellipsometry. Molecular loadings for each monolayer on TiO(2) were quantified by quartz crystal microgravimetry (QCM), and the stability of bonding between each class of dyes and the TiO(2) was evaluated by measuring desorption, also by QCM; the carboxylates underwent significant dissociation in aqueous media but the phosphonates did not. DSSCs were prepared from each congener and from simple oligothiophene phosphonates to determine the effect of the cyanovinyl group on device behavior; all DSSCs were studied under irradiation from a AM 1.5G solar light source; the effect of cyanovinyl group termination was comparable to that of adding a thiophene moiety, and the DSSC using a self-assembled monolayer of (sexithiophene)phosphonate (6TP) had total power conversion efficiency (η) of ca. 5%.

Theoretical study of Ru (II) complexes by using the conjugated-donor antenna moiety for DSSCs

In this study, a novel heteroleptic ruthenium complex of [Ru(dcbpy)(L)(NCS)2] (CQ), where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine and L = a coumarin derivative, was designed and studied theoretically for its photoelectrochemical properties for use in dye-sensitized solar cells(DSSCs). We compared the well-known [Ru(dcbpy)2(NCS)2] (N3) and CQ. To gain insight into the structural, electronic, and optical properties of the heteroleptic ruthenium complex, we performed density functional theory (DFT)/time-dependent DFT (TDDFT) calculations on the complexes of the heteroleptic ruthenium complex. A molecular orbital analysis confirmed that the highest occupied molecular orbitals (HOMOs) of complexes were localized above the ruthenium center, the NCS ligand, and the coumarin moiety. The lowest unoccupied molecular orbital (LUMO) was essentially localized above the dcbpy. The TDDFT results showed that the CQ with a chromophore ligand had broader, stronger absorption spectra in the region above 400 nm compared to the N3 dye. From these results, we were able to determine how substituted antenna moiety influenced the electronic and the optical properties. We expect the ruthenium complex to perform better than the N3 dye in terms of conversion efficiency in DSSCs.

Tuning the Electrical and Optical Properties of Diketopyrrolopyrrole Complexes for Panchromatic Dye‐Sensitized Solar Cells

A series of metal-free organic dyes that were bridged by a diketopyrrolopyrrole moiety and were composed of indoline and triphenylamine as donor groups and furan and benzene as conjugated spacer groups were designed and synthesized for use in dye-sensitized solar cells (DSCs). The photophysical properties, electrochemical properties, and performance of the DSCs were related to the structure of their corresponding dyes. Their absorption spectra broadened upon the introduction of the indoline and heterocyclic furan moieties through fine-tuning of their molecular configuration. The overall conversion efficiencies of DSCs that were based on these dyes ranged from 5.14-6.53%. Among the four dyes that were tested, indoline-based ID01 and ID02 showed higher efficiencies (6.35% and 6.53%) as a result of their improved light-harvesting efficiency and larger electron driving force. The ID01 dye, which contained an indoline moiety as an electron donor and a furan group as a π-conjugated linker, showed an excellent monochromatic incident-photon-to-current-conversion efficiency (IPCE) spectrum (350-650 nm) with a maximum value of 78% in the high plateau region and an onset value close to 800 nm. Intensity-modulated photovoltage spectroscopy (IMVS) and impedance spectroscopy (IS) revealed that dyes that contained benzene conjugation spacers suppressed the charge-recombination rate more efficiently than dyes that contained furan spacers, thereby resulting in improved photovoltage.

D-A-π-A organic sensitizers containing a benzothiazole moiety as an additional acceptor for use in solar cells

Three novel triphenylamine-based D-A-π-A-featured dyes (Z1–Z3) have been designed, synthesized and characterized for use in dye-sensitized solar cells. Benzothiazole was incorporated as an additional acceptor, which greatly enhanced the molar extinction coefficient of the dyes. Various conjugated linkers, such as benzene, furan and thiophene, were also introduced to configure the novel D-A-π-A framework in order to prolong electron flow and active transportation. Among all dyes, Z2 containing a thiophene linker exhibited the maximum overall conversion efficiency (η) of 4.16% (J sc = 9.27 mA cm−2, V oc = 642 mV, FF = 0.70) under standard global AM 1.5 G solar condition.

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.