Larger Dye Loading Research Articles

Guest-Host Interactions in Symmetrical Carboxy Heptamethine Cyanine Dyes-Titanium Dioxide Systems: Synthesis, Theoretical Calculations, Aggregation Properties, and Application in Dye-Sensitized Solar Cells

In this work, the role of deoxycholic acid (DCA) as a coadsorbent was investigated in the sensitization of mesoporous TiO2 layers (host) with symmetrical carboxy heptamethine cyanine dyes (guest). Different approaches have been tested, aimed at reducing the H-aggregation and minimizing the competition between cyanine molecules and DCA for active sites of the host, thus improving solar cell efficiency. Heptamethine cyanines containing carboxylic anchoring groups were obtained with good yields. The cyanines present UV-Vis absorption in methanol and dimethylformamide solutions ascribed to fully allowed electronic transitions ( 1 π π ∗ ), as well as fluorescence emission in the NIR region, with any evidence of aggregations in both ground and excited states. TD-DFT calculations were also performed in order to study the geometry and charge distribution of these compounds in their ground and excited electronic states. Solid-state photophysics indicates that the cyanines showed excellent adsorption on TiO2, which can be justified by the presence of the -COOH moieties in the structure. Photophysical measurements have revealed the best concentrations of dye and DCA, which resulted in efficient inhibition of cyanine H-aggregates on the TiO2 surface in addition to allow large dye loading. HOMO and LUMO energy levels of the dyes were identified by cyclic voltammetry, showing oxidation and reduction potentials within acceptable limits for application as a photosensitizer in dye-sensitized solar cells (DSSCs) based on a TiO2 mesoporous photoanode. Assembled DSSCs have shown a large improvement of the electrical parameters and efficiency when a balance between dye aggregation and the competition to the host active sites was reached.

Facile non-pressurized synthesis of nanowire-constructed hierarchical TiO2 nanomaterials with improved dye sensitized solar cell performance

Nanowire-constructed hierarchical TiO2 nanomaterials (HTN) are prepared by a facile non-hydrothermal method at atmospheric pressure. The product is characterized in detail using powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV–Vis spectrophotometer. The synthesized TiO2 hierarchical nanomaterials consist of nanowires of 15 nm in diameter, it possesses a high surface area of 144.65 m2/g, and demonstrate an excellent dye absorption ability (155.55 nmol/cm2), which is 2.7 times more than that of the Degussa P25. Corresponding photovoltaic performance of dye-sensitized solar cells (DSSCs) fabricated from HTN is also investigated. It exhibits an overall light-to-electricity conversion efficiency of 8.9% accompanying a short-circuit current density of 20.43 mA/cm2, an open-circuit voltage of 720 mV and fill factor of 0.61, which is much higher than that of P25 (5.5%). The significant enhancement of short-circuit current density and power conversion efficiency of HTN can be attributed to its larger dye loading amount, better charge transportation ability and excellent light scattering ability.

Aqueous Solution-Processed Multifunctional SnO2 Aggregates for Highly Efficient Dye-Sensitized Solar Cells

A room temperature based environmentally friendly aqueous solution synthesis route was developed to fabricate highly dispersed small SnO2 nanoparticles (3–5 nm) without the application of any pressurized reaction vessel or organic solvents. The subsequent treatments, that is, dialyzing and freezing, allow for the acquision of dual-function nanostructures with sheet-like feature and large aspect ratio except for the ordinary irregular aggregates. Dye-sensitized solar cells (DSCs) constructed with the resultant multifunctional SnO2 showed an outstanding photovoltaic conversion efficiency (PCE) of 6.92% and an unexpected JSC of 19.5 mA cm–2 at an optimized thickness of 14.1 μm. The excellent performance can be ascribed to the effective coordination of the favorable features (i.e., strong light scattering, large dye loading capability, and fast electron transport) via rational film thickness control as indicated by diffused reflectance spectra, UV–vis absorption spectra, and electrochemical impedance spectros...

Enhancing the Photovoltaic Performance of Dye-Sensitized Solar Cells with Rare-Earth Metal Oxide Nanoparticles

Doping titania with rare-earth metals improves the photoconversion efficiency (PCE) of dye sensitized solar cells (DSSCs). In most reported cases, the overall cell quality is not optimal, making it unclear what role the rare earth oxide metal plays in the improvement, if any. Here we incorporate <100 nm diameter nanoparticles of neodymium and erbium oxide into titania films and test their performance in high quality DSSCs. The doping level for optimized cells is 2.0% and 1.0% for neodymium oxide and erbium oxide by weight, respectively. This results in a 5 to 10% increase in PCE for the rare earth oxide doped DSSCs relative to undoped DSSCs, with erbium oxide being the best. The rare-earth metal-doped films exhibit a 20–25% larger dye loading and the loading was maximum at the same doping levels where the PCE was maximum, showing this is a big contributor to the improvement. The surface area is larger with rare-earth doped titania as determined by atomic force microscopy and BET analysis, explaining the higher dye loading. The cell impedance is lower with rare-earth doped samples, showing that the rare-earth metal oxide also facilitates electron transport through the mesoporous titania, improving the collection efficiency of the photoexcited electrons.

Synthesis of TiO2 nanorice and their improved dye sensitized solar cells performance

TiO2 nanostructures with different morphologies were successfully synthesized via a solvothermal reaction of titanium isopropoxide and acetic acid. The synthesized TiO2 nanostructures were characterizated in detail by powder X-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectroscopy and Raman etc. technologies. The effect of growth time on the composition, morphology and crystal phase of synthesized TiO2 nanostructures were studied, and corresponding photovoltaic performance of dye-sensitized solar cell (DSSC) fabricated from TiO2 nanorice was also investigated. The DSSC based on the 24 h anatase TiO2 nanorice photoelectrode shows an overall light-to-electricity conversion efficiency of 7.45% accompanying a short-circuit current density of 18.83 mA cm−2, an open-circuit voltage of 788 mV and fill factor of 0.5, which is much higher than that of 12 h (5.25%), 16 h (5.73%) and 20 h (6.20%) samples. The significant enhancement of short-circuit current density and power conversion efficiency for the 24 h nanorice-based DSSC is mainly attributed to its larger dye loading amount.

Growth of TiO2 nanoflowers photoanode for dye-sensitized solar cells

Directly growth of TiO2 nanoflowers on fluorine drop tin oxide (FTO) glass substrates were achieved by a one-step hydrothermal process. These TiO2 nanoflowers films are preeminent with strong adhesion, low internal resistance and large dye loading. They are beneficial for improving the photoelectric properties of DSSCs (dye sensitized solar cells). In the process of preparing TiO2 nanoflowers films, gradually increasing the dosage of tetrabutyl titanate, compact and uniform Rutile TiO2 nanoflowers layer can be achieved, the coverage of TiO2 nanoflowers increased along with it. Employ these TiO2 nanoflowers as the photoanode in DSSCs, the optimal DSSCs performed with a short-circuit current density of 5.77 mA cm−2, an open-circuit voltage of 0.65 V, fill factor of 48.66 and an energy conversion efficiency of 1.83% because of large dye loading and high light harvesting ability of the DSSCs with appropriate tetrabutyl titanate and reaction temperature. Excessive or insufficient tetrabutyl titanate, or lower reaction temperature will endanger the performance of DSSCs.

Positional isomers of pyridine linked triphenylamine-based donor-acceptor organic dyes for efficient dye-sensitized solar cells

Two donor-acceptor (D-A) metal free dyes 3-{4-[Bis-(4-pyridin-2-ylethynyl-phenyl)-amino]-phenyl}-2-cyano-acrylic acid (D1) and 3-{4-[Bis-(4-pyridin-3-ylethynyl-phenyl)-amino]-phenyl}-2-cyano-acrylic acid (D2) have been designed, synthesized and used as sensitizers for the dye sensitized solar cells (DSSCs). The DSSCs based on D1 and D2 exhibit the power conversion efficiency (PCE) of 5.16% and 4.27%, respectively. The higher PCE value of D1 based DSSC is attributed to its enhanced short circuit current (Jsc) and open-circuit voltage (Voc) and fill factor (FF) values. The electrochemical impedance spectra demonstrated shorter electron transport time, longer electron lifetime and high charge recombination resistance for DSSC based on D1, as well as larger dye loading onto TiO2 are attributed to the higher PCE.

Bi-layer of nanorods and three-dimensional hierarchical structure of TiO2 for high efficiency dye-sensitized solar cells

A novel bi-layer structure assembled by nanorods and three-dimensional hierarchical TiO2 is synthesized by a facile two step hydrothermal method. By adjusting the acid concentration, the morphology of three-dimensional hierarchical TiO2 can be well controlled. This bi-layer structure combines the merits of one-dimensional nanorods which can serve as direct electrons transport pathways and three-dimensional hierarchical structure supplying light scattering ability and large specific surface area for dye loading. Hence, the photovoltaic performance of the dye-sensitized solar cells based on the bi-layer TiO2 is greatly enhanced compared to that of single nanorods film. The maximum short-circuit current and power conversion efficiency of the DSSCs based on bi-layer TiO2 structure reach 12.55 mA/cm2 and 5.61% respectively, which are remarkably larger than those of 5.00 mA/cm2 and 2.38% for the DSSC based on a single layer TiO2 nanorods film. The superior performance of bi-layer TiO2 structure is attributed to the large dye loading amount and light scattering properties due to the unique hierarchical structure.

A Propeller‐Shaped, Triazine‐Linked Porphyrin Triad as Efficient Sensitizer for Dye‐Sensitized Solar Cells

AbstractA propeller‐shaped, triazine‐linked, unsymmetrical porphyrin triad – consisting of two zinc‐metallated porphyrin units and one free‐base porphyrin unit that is functionalized by a carboxylic acid – has been synthesized by stepwise amination reactions of cyanuric chloride. Photophysical, electrochemical, and DFT studies of the triad revealed no significant electronic interactions between the porphyrin units in the triad ground state but frontier orbital energy levels suitable for use as sensitizer in dye‐sensitized solar cells (DSSCs). Furthermore, the triad can be described as a 2D–π–A system (D: donor, A: acceptor) that has the potential to promote electron transfer and injection into the TiO2 electrode. Solar cells sensitized by the triad were fabricated with two different TiO2 photoanodes, one processed by the paste‐coating (PC) method and the other by the electrophoretic deposition (EPD) method. They were found to exhibit power conversion efficiencies (PCEs) of 3.80 and 4.91 %, respectively. The higher PCE value of the latter is attributed to its larger dye loading, as well as to its enhanced short‐circuit current (Jsc) and higher open‐circuit voltage (Voc) and fill factor (FF) values. In addition, differences in the surface morphology between the two different TiO2 photoanodes result in different electron‐transport kinetics. Electrochemical impedance spectra (EIS) demonstrated that the solar cell with the EPD‐processed photoanode exhibits a shorter electron‐transport time (τd), a longer electron lifetime, and higher charge‐recombination resistance (Rct). The PCE of the latter solar cell was further improved up to 5.56 % by incorporating chenodeoxycholic acid in the dye solution as co‐sensitizer.

Double layered nanoarchitecture based on anodic TiO2 nanotubes for dye-sensitized solar cells

Based on the TiO2 nanotube (TONT) with three different thicknesses (10, 18, 28 μm) prepared by electrochemical anodization, a double layered electrode consisting of TONT and TiO2 nanoparticles (TONPs) with a size of approximately 20 nm was developed. The anodic TONT electrode has been actively investigated due to its beneficial properties such as a favorable electron transport rate and light scattering ability. However, the small specific surface area caused by the large pores and vacant space between 1-D NTs impedes the fabrication of high efficient DSSCs. Hence, the TONP layer with a thickness of 10 μm is additionally deposited on the top surface of the TONT film to offer a sufficient surface area for large dye loading. Compared to a single TONT electrode, the double layered TONT/NP electrodes provide a high short-circuit current density (Jsc) and fill factor (FF) while maintaining a constant open-circuit voltage (Voc), finally disclosing a higher conversion efficiency (η), attributed to the high loading of dye molecules, light scattering ability, low resistance in the photoanode/electrolyte interface, and longer electron lifetimes. In particular, the optimal η was attained with the TONT/NP (18 μm) electrode having a Voc of 0.80 V, a Jsc of 7.80 mA/cm2, a FF of 62.7%, and a η of 3.93%. Incidentally, when using a longer TONT film (28 μm), η decreases steadily due to the fast charge recombination process. A more detailed discussion is included below.

Efficient Sensitization of Dye-Sensitized Solar Cells by Novel Triazine-Bridged Porphyrin–Porphyrin Dyads

Two novel porphyrin-porphyrin dyads, the symmetrical Zn[Porph]-Zn[Porph] (2) and unsymmetrical Zn[Porph]-H2[Porph] (4), where Zn[Porph] and H2[Porph] are the metalated and free-base forms of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin, respectively, in which two porphyrin units are covalently bridged by 1,3,5-triazine, have been synthesized via the stepwise amination of cyanuric chloride. The dyads are also functionalized by a terminal carboxylic acid group of a glycine moiety attached to the triazine group. Photophysical measurements of 2 and 4 showed broaden and strengthened absorptions in their visible spectra, while electrochemistry experiments and density functional theory calculations revealed negligible interaction between the two porphyrin units in their ground states but appropriate frontier orbital energy levels for use in dye-sensitized solar cells (DSSCs). The 2- and 4-based solar cells have been fabricated and found to exhibit power conversion efficiencies (PCEs) of 3.61% and 4.46%, respectively (under an illumination intensity of 100 mW/cm(2) with TiO2 films of 10 μm thickness). The higher PCE value of the 4-based DSSC, as revealed by photovoltaic measurements (J-V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two cells, is attributed to its enhanced short-circuit current (J(sc)) under illumination, high open-circuit voltage (V(oc)), and fill factor (FF) values. Electrochemical impedance spectra demonstrated shorter electron-transport time (τd), longer electron lifetime (τe), and high charge recombination resistance for the 4-based cell, as well as larger dye loading onto TiO2.

Mixed photoelectrode based on spherical TiO2 nanorod aggregates for dye-sensitized solar cells with high short-circuit photocurrent density

Mixed photoelectrode was successfully prepared by dispersing spherical TiO2 nanorod aggregates with high surface area into nanocrystallites for dye-sensitized solar cells. This type of mixed photoelectrode demonstrated a considerably higher short-circuit photocurrent density value due to the multifunctions of the spherical TiO2 nanorod aggregates, including high surface area for large dye loading, sub-micrometre size for superior light-scattering ability and one-dimensional building units for fast electron transport. After maximizing the combined dye loading, light-scattering efficiency, and electron transport property in the mixed photoelectrode by adjusting the concentrations of the spherical TiO2 nanorod aggregates, a highest Jsc of 20.36 mA cm−2 and a highest energy conversion efficiency of 6.96% were obtained when the concentration of the spherical TiO2 nanorod aggregates was 40 wt%.

A Two-step anodization to grow high-aspect-ratio TiO 2 nanotubes

High-aspect-ratio TiO 2 nanotubes with small diameter are favored in dye-sensitized solar cells for large dye loading provided by high surface areas. However, long TiO 2 nanotubes with small diameter are difficult to grow under usual anodizing conditions due to unavoidable chemical dissolution of the top portion of the as-fashioned tubes. In the present work, two kinds of double-layered TiO 2 nanotube arrays were prepared by changing voltage from high to low (i.e., from 30 V to 15 V) or from low to high (i.e., from 15 V to 30 V). It is found that the top layer can serve as a sacrificial layer to protect the continuous growth of the bottom layer from chemical dissolution. Accordingly, the two-step anodization from high voltage to low voltage is proposed to produce high-aspect-ratio TiO 2 nanotubes with small diameter underneath a sacrificial top layer.