Photophysical properties of new cyclometalated ruthenium complexes and their use in dye sensitized solar cells

A new class of cyclometalated ruthenium complexes, Ru(C^N^N')(N^N'^N')·Cl where N^N'^N' = 4,4',4'-tricarboxy-2,2':6',2'-terpyridine and C^N^N' = substituted 6-phenyl-2,2'-bipyridine, for Dye Sensitized Solar Cells (DSSCs) is proposed. We have investigated the effect of different substituents (R = COOH, thiophen-2-yl, F and OCH(3)) on the ancillary C^N^N' ligand on the photophysical properties and performance of the six different cyclometalated ruthenium complexes in DSSCs. Using an ionic liquid based electrolyte, efficiencies up to η = 3.06% have been attained under 1 sun irradiation. Moreover, the T66 based DSSC exhibited a good stability under 1000 W m(2) light soaking at 60 °C for 24 days, retaining 92.8% of its initial efficiency.     

Synthesis and photophysical properties of phosphole-cored dendrimers

Phosphole-cored dendrimers having poly(benzyl ether) units through the third generation have been synthesized. The dendrimers display intense blue photoluminescence, the quantum yield increasing with the increasing generation of the dendron units. The optical properties are easily tuned by oxidation of the phosphorus atom of the phosphole ring.     

Synthesis and photophysical properties of phenothiazine-labeled conjugated dendrimers

A novel class of conjugated dendrimers bearing phenothiazines as peripheral groups and phenylenevinylene-group as a core has been synthesized through the Wittig-Horner reaction in moderate to good yield.     

Synthesis and photophysical properties of carbazole-based blue light-emitting dendrimers

A new class of highly fluorescent and stable carbazole-based dendrimers (1-5) that contain the ethynylbenzene and diethynylbenzene cores has been synthesized and characterized. They show very high extinction coefficients of absorption (A(max) approximately 328-353 nm) and high quantum yields of fluorescence (lambda(max) approximately 386-437 nm; Phi(F) approximately 0.72-0.89; tau(F) approximately 2.09-3.91 ns) in dichloromethane. The quantum yields of fluorescence of 1-5 in the solid state are equally high (lambda(max) approximately 385-422 nm; Phi(F) approximately 0.40-0.85). These data indicate their potential use as blue-emitting materials in organic light-emitting diodes (OLEDs).     

A swift dye uptake procedure for dye sensitized solar cells

Solar cells based on swift self-assembled sensitizer bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N'-bis(4-carboxylato-4'-carboxylic acid-2,2'-bipyridine)ruthenium(II) (N719) on double layers of 12 + 4 microm thick nanocrystalline TiO2 films exhibit the incident monochromatic photon-to-current conversion efficiency (IPCE) 90% and show a short circuit current density of 17 mA cm(-2), 750 mV open circuit potential and 0.72 fill factor yielding power conversion efficiencies over 9.18% under AM 1.5 sun. For the first time highest power conversion efficiencies are obtained for dye sensitized solar cells using a swift self-assembled procedure.     

Ultrafast fluorescence studies of dye sensitized solar cells

Time-resolved fluorescence spectra from the RuN719 dye exhibit very short lifetimes (<30 fs) in solutions, on non-injecting substrates and on injecting ones. This reveals <10 fs intramolecular energy redistribution competing with the injection. We conclude that injection proceeds on a sub-10 fs time scale from non-thermalized levels of the dye.     

Synthesis and photophysical properties of tetraphenylethylene-based conjugated dendrimers with triphenylamine core

Two new conjugated dendrimers bearing a tetraphenylethylene moiety as dendrons and triphenylamine as a core have been synthesized through a convergent synthetic strategy using threefold Heck/threefold Sonogashira coupling reaction. These dendrimers showed excellent solubility in common organic solvents and emit light in the blue and violet regions.     

Synthesis and photophysical properties of triphenylamine-based dendrimers with 1,3,5-triphenylbenzene cores

Two new conjugated dendrimers bearing a triphenylamine moiety as dendrons and 1,3,5-triphenylbenzene as a core have been synthesized through a convergent synthetic strategy. These conjugated dendrimers have high fluorescence quantum yields and exhibit similar absorption and emission behaviors in solutions and in solid films, which demonstrate that these dendrimers form good amorphous states.     

Photochemical performance of ZnO nanostructures in dye sensitized solar cells

In this work, the photoconversion efficiencies of ZnO having diverse microstructures and structural defects have been investigated. A conversion efficiency of 1.38% was achieved for the DSSCs fabricated with as prepared ZnO nanorods having minimum vacancy defects and a favourable one dimensional directional pathway for electron conduction. The DSSCs fabricated with ZnO nanoparticles exhibited relatively low conversion efficiency of 1.004% probably due to multiple trapping/detrapping phenomena within the grain boundaries and ZnO flowers though exhibited a high dye adsorption capability exhibited the lowest conversion efficiency of 0.59% due to a high concentration of structural defects. Based on the experimental evidences, we believe that the type of defects and their concentrations are more important than shape in controlling the overall performance of ZnO based DSSCs.     

Synthesis of ZnO nanorods and their application in quantum dot sensitized solar cells

ZnO nanorod thin films of different thicknesses and CdS quantum dots have been prepared by chemical method. X-ray diffraction pattern reveals that the CdS quantum dot and ZnO nanorods are of hexagonal structure. Field emission scanning electron microscope images show that the diameter of hexagonal shaped ZnO nanorods ranges from 110 to 200 nm and the length of the nanorod vary from 1.3 to 4.7 μm. CdS quantum dots with average size of 4 nm have been deposited onto ZnO nanorod surface using successive ionic layer adsorption and reaction method and the assembly of CdS quantum dot with ZnO nanorod has been used as photo-electrode in quantum dot sensitized solar cells. The efficiency of the fabricated CdS quantum dot-sensitized ZnO nanorod-based solar cell is 1.10 % and is the best efficiency reported so far for this type of solar cells.     

Sensitizer molecular structure-device efficiency relationship in dye sensitized solar cells

In the Dye Sensitized Solar Cell (DSSC) the dye sensitizer carries out the light harvesting function and is therefore crucial in determining overall cell efficiency. In addition, the dye sensitizer can influence many of the key electron transfer processes occurring at the TiO(2)/dye/electrolyte interface which also determine efficiency. Dye structure can influence and drive forward electron injection into the conduction band of the TiO(2). Conversely, dye structure can help retard loss electron transfer processes such as charge recombination of injected electrons in the TiO(2) with dye cations and also recombination of these electrons with the electrolyte. Therefore tuning dye sensitizer light absorbing properties and control of the aforementioned electron transfer processes through structural design of the dye sensitizer is an important avenue through which optimization of DSSC efficiency should be pursued. In this critical review the latest work focusing on the design of dyes for efficient DSSCs is revised (111 references).     

Energy transfer in new D-pi-A conjugated dendrimers: their synthesis and photophysical properties

A series of new D-pi-A conjugated dendrimers based on benzothiadiazole and triphenylamine were developed via facile synthetic approaches. By changing the types of bridges between the different functional moieties of these dendrimers, their photophysical properties, especially the intramolecular energy transfer process, were effectively modulated.     

Latent gel electrolyte precursors for quasi-solid dye sensitized solar cells

New latent chemically-cross-linked gel electrolyte precursors for quasi-solid dye sensitized solar cells (QDSC) are reported. The gel electrolyte precursors consist of nano-particles and dicarboxylic acids as the latent gelators. The viscosity of the precursor is low at first and does not increase during storage at room temperature. However, when the precursor is baked at 80 degrees C, it solidifies immediately. Photo-voltaic performance is maintained after solidification.     

Preparation of chemically sintered ZnO films and their application in dye sensitized solar cells formed on plastic substrates

Meso-porous zinc oxide films were prepared on tin-doped indium oxide-coated, polyethelene naphthalate substrates from binder-free ZnO slurry. The reaction with ammonium hydroxide was found to increase connection between ZnO grains by forming a nano-rod like structure followed by heating at 150 °C. The enhancement of adhesion among ZnO grains was evaluated using a nano-scratch technique. Two different xanthene dyes were used to sensitize ZnO electrodes, with a photo-voltage of 657 mV, fill-factor of 73% and photo-current of 4.1 mA cm⁻² with a maximum light to-electrical energy conversion efficiency of 2.0% being obtained for the plastic based ZnO|mercurochrome|electrolyte solar cell under 1 sun.     

Detailed investigations of ZnO photoelectrodes preparation for dye sensitized solar cells

Wurtzite ZnO hexagonal nanopyramids were successfully synthesized in the liquid phase from homogeneous methanolic solutions of zinc acetate and tetramethylammonium hydroxide at an excess of zinc ions. The formation and properties of the nanocrystals were examined as a function of synthesis conditions. No significant influence of the [Zn(2+)]/[OH(-)] ratio was noticed on the final particle size, in spite of increased amounts of OH(-) ions, which tend to accelerate the particle nucleation and growth. Nevertheless, the reactant concentration ratio influences the surface properties of the ZnO nanocrystals. Mesoporous ZnO films were prepared by doctor blading ethanolic pastes containing ZnO nanoparticles and ethyl cellulose onto FTO conductive glass substrate followed by calcination. Additionally, the influence of a plasticizer (triacetin)-used during the paste preparation-on the film quality was investigated. A higher content of ZnO nanoparticles and plasticizer in the pastes improved the film quality. Four different temperatures (i.e., 400, 425, 450, and 475 °C) were used for the film calcination and their influence on the structural properties of the films was characterized. In principle, increasing the calcination temperature goes hand in hand with an increase of particle size, as well as the pore diameter and reduction of the surface area. Suitable mesoporous films were employed as photoanodes in dye sensitized solar cells (DSSCs). In order to assess the effect of the varied parameters on complete DSSC devices-using cis-diisothiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II)bis(tetrabutylammonium (N719) as a sensitizer-incident photon to current efficiency (IPCE) and current voltage measurements were carried out. The IPCE measurements confirmed photoinduced electron injection from the dye, reaching IPCE values up to 76%. Furthermore, current-voltage characteristics of complete cells emphasized the importance of the proper preparation methods and temperatures. These features are important assets for the preparation of nanocrystalline ZnO based photoelectrodes and for improving the DSSC performance.     

Synthesis and photophysical properties of chiral dendrimers with quinoline surface group via click chemistry

Synthesis of some novel chiral dendrimers containing quinoline as surface group and 1,2,3-triazole as branching unit is described. The chiroptical property exhibits the widening of the torsional angle in the BINOL core as the generation increases. The photophysical properties indicate an increase in the molar extinction coefficient and fluorescence intensity and a decrease in quantum yield and lifetime as the generation of the dendrimer increases.     

Synthesis,photophysical, and electrochemical properties of triazolyl dendrimers with thiazolylchalcone surface unit

Synthesis of novel triazolyl dendrimers with thiazolylchalcone surface group and triazole as bridging unit has been achieved in good yields by click chemistry through convergent approach. All the triazolyl dendrimers showed an absorption band between 293 and 336?nm and an emission band between 430 and 435?nm. The intensity of absorption and emission bands increases on increasing the number of dendritic wedges and triazole units. Triazolyl dendrimers exhibited quasi-reversible behavior in cyclic voltammetry. A shift in the reversible potential was observed in cyclic voltammetry on increasing the dendritic wedges in the triazolyl dendrimers.     

Coumarin derivatives for dye sensitized solar cells: a TD-DFT study

Time dependent density functional theory (TD-DFT) calculations have been carried out to study the electronic structure and the optical properties of five coumarin based dyes: C343, NKX-2311, NKX-2586, NKX-2753 and NKX-2593. We have found out that the position and width of the first band in the electronic absorption spectra, the absorption threshold and the LUMO energy with respect to the conduction band edge are key parameters in order to establish some criteria that allow evaluating the efficiency of coumarin derivatives as sensitizers in Dye Sensitized Solar Cells (DSSC). Those criteria predict the efficiency ordering for the coumarin series in good agreement with the experimental evidence. Presumably, they might be used in the design of new efficient organic based DSSC.     

Reducing charge recombination losses in solid state dye sensitized solar cells: the use of donor-acceptor sensitizer dyes

Herein we report the application of supramolecular dyes to control charge recombination between photo-injected electrons and oxidized hole-transporting material, resulting in an enhancement in the performance of dye sensitized solar cell devices based upon such dyes.     

New cyclen-cored dendrimers functionalized with pyrene: Synthesis characterization,optical and photophysical properties

New cyclen (1,4,7,10-teraazacyclododecane) cored dendrimers up to the second generation, functionalized with 4, 8 and 16 pyrene units, respectively, were synthesized following a convergent procedure. All new compounds were characterized by NMR spectroscopies and ESI or MALDI TOF mass spectrometry. The optical and photophysical properties of the new dendrimers were studied in THF solution. Absorption spectra showed the typical absorption bands of pyrene moieties. In the fluorescence spectra, monomer as well as excimer emission were observed for all compounds. An increased proportion of excimer emission was observed in the dendrimer of the highest generation.