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Three novel metal-free organic dyes (DN458, DN475 and DN484) were designed for use in plastic-substrate dye-sensitized solar cells (PDSCs). The photoelectric conversion region of DN475 was successfully expanded into the near-infrared region. As a result, an energy conversion efficiency of 5.76% was achieved. [ABSTRACT FROM AUTHOR]

A novel ruthenium sensitizer with a terpyridine ligand having a hexylthiophene unit at the 4-position (TUS-38) has been synthesized to investigate the effects of the substituent position on the photo- and electrochemical properties and on solar cell performance. The dye-sensitized solar cell (DSC) with TUS-38 showed a 10.6 % conversion efficiency under AM 1.5 (100 mW/cm2) irradiation, which is much higher than that of the DSC with a previously reported ruthenium sensitizer with a terpyridine ligand having two hexylthiophene units at the 5- and 5″-positions. [ABSTRACT FROM AUTHOR]

Co-sensitized dye-sensitized solar cells using black dye and a pyridine-anchor dye (NI5 or YNI-2) showing site-selective adsorption behaviour at the TiO2 surface have been prepared for the first time to reduce the competitive adsorption between the two dyes. [ABSTRACT FROM AUTHOR]

Abstract: The monomer and charge-transfer complexes of 14 different imidazole derivatives with diiodine were studied by a density functional theory (DFT) method. DFT calculations revealed that the σ* orbital of iodine interacts with the nitrogen lone pair of imidazoles at position 3. The influence of these imidazoles addition on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized nanocrystalline TiO2 solar cell with an I−/I3 − redox electrolyte in acetonitrile was also studied. All of the imidazole derivatives enhanced the open-circuit photovoltage (V oc). The resulting V oc values of solar cell were compared to computational calculations on the interaction between imidazoles and I2 by a DFT method. Optimized geometries, frequency analyses, and interaction energies suggest that the V oc value is higher, the more the imidazole complexes with I2. [Copyright &y& Elsevier]

The influence of pyrazole additives in an I-/I3- redox electrolyte solution on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO2 solar cell was studied. The current–voltage characteristics of the cell were measured using 18 different pyrazole derivatives. All of the pyrazole additives enhanced the open-circuit photovoltage (Voc) and the solar energy conversion efficiency (η), but reduced the short-circuit photocurrent density (Jsc). Most of the pyrazoles improved fill factor (ff). The physical and chemical properties of the pyrazoles were computationally calculated in order to elucidate the reasons for the additive effects on cell performance. The greater the partial charge of the nitrogen atom at position 2 in the pyrazole group, the larger the Voc, but the smaller the Jsc values. As the dipole moment of the pyrazole derivatives increased, the Voc value increased, but the Jsc value decreased. The Voc of the cell also increased as the ionization energy of the pyrazoles decreased. These results suggest that the electron donicity of the pyrazole additives affected the interaction with the nanocrystalline TiO2 photoelectrode, the I-/I3- electrolyte, and the acetonitrile solvent, which changed the Ru(II)-dye-sensitized solar cell performance. [Copyright &y& Elsevier]

The influence of benzimidazole additives on the performance of a bis(tetrabutylammonium)-cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO2 solar cell with an I−/I3− redox electrolyte in acetonitrile was investigated by measuring the current–voltage characteristics of more than 20 different benzimidazole derivatives under AM 1.5 (100 mW/cm2). The benzimidazole additives tested had varying effects on the cell performance. Adding benzimidazole drastically enhanced the open-circuit photovoltage (Voc) and the fill factor (ff), but reduced the short-circuit photocurrent density (Jsc) of the solar cell. In order to determine the reasons for the additive effects on cell performance the physical and chemical properties of the benzimidazoles were computationally calculated. Consequently, the greater the calculated partial charge of the nitrogen atoms in position 3 of the benzimidazole groups, the larger the Voc, but the smaller the Jsc values. The Voc values also increased as the molecular size of the benzimidazole derivatives decreased. Moreover, the greater the absolute difference between the calculated dipole moment of the benzimidazole and acetonitrile, the larger the Jsc value. These results suggest that these properties of the benzimidazoles influenced the extent of interaction between the TiO2 electrode and electrolyte solvent, which changed the dye-sensitized solar cell performance. [Copyright &y& Elsevier]

The influence of alkylaminopyridine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO2 solar cell with an I−/I3− redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 20 different alkylaminopyridines under AM 1.5 (100 mW/cm2). The alkylaminopyridine additives tested had varying effects on the performance of the cell. All the additives decreased the short circuit photocurrent density (Jsc), but increased the open-circuit photovoltage (Voc) of the solar cell. Molecular orbital calculations imply that the dipole moment of the alkylaminopyridine molecules influences the Jsc of the cell and that the size, solvent accessible surface area, and ionization energy all affect the Voc of the cell. The highest Voc of 0.88 V was observed in an electrolyte containing 4-pyrrolidinopyridine, which is comparable to the maximum Voc of 0.9 V for a cell consisting of TiO2 electrode and I−/I3− redox system. [Copyright &y& Elsevier]

The influence of pyrimidine additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO2 solar cell with an I−/I3− redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured for more than 10 different pyrimidine derivatives under AM 1.5 (100 mW/cm2). The pyrimidine additives tested had varying effects on the performance of the cell. The additives drastically enhanced the open-circuit photovoltage (Voc) and the solar energy conversion efficiency (η), but usually reduced the short circuit photocurrent density (Jsc) of the solar cell. Physical and chemical properties of the pyrimidines were computationally calculated in order to determine the reasons for the additive effects on cell performance. Consequently, the greater the calculated partial charge of the nitrogen atoms in the pyrimidine groups, the larger the Voc but the smaller the Jsc values. The Voc of the cell also increased as the ionization energy of the pyrimidine molecules decreased. Moreover, as the calculated dipole moment of the pyrimidine derivatives increased, the Jsc value was reduced, but the Voc value was enhanced. These results suggest that the electron donicity of pyrimidine additives influenced the interaction with TiO2 electrode and I−/I3− electrolyte, which lead to the changes in dye-sensitized solar cell performance. [Copyright &y& Elsevier]

The ethylenediamine Ru(II) tricarboxyterpyridine complex [Ru(tctpy)(en)(NCS)]Cl (1) was newly synthesized, and its photophysical and photovoltaic properties were characterized. Solar cells based on nanocrystalline TiO2 films sensitized with 1 exhibited efficient sensitization over the visible and near-infrared region. These solar cells generated a large short-circuit photocurrent of 17.7 mA cm-2 and an open-circuit potential of 0.68 V under simulated AM 1.5 solar irradiation (100 mW cm-2) with a solar energy conversion efficiency of 8.5%. [ABSTRACT FROM AUTHOR]

We have developed a novel and efficient organic liquid electrolyte composed of 1-ethyl-3-methylimidazolium iodide, iodine (I[sub2]), and acetonotrile solvent for a nanocrystalline TiO[sub2] solar cell sensitized with a Ru(II) terpyridyl complex. A solar energy-to-electricity conversion efficiency of 8.0% was attained under AM 1.5 irradiation (100 mW cm[sup-2]). [ABSTRACT FROM AUTHOR]

Abstract: The influence of nitrogen-containing heterocyclic additives on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized nanocrystalline TiO2 solar cell with an I-/I3- redox electrolyte in acetonitrile was studied. The current–voltage characteristics were measured under AM 1.5 (100mW/cm2) for 10 different five- and six-membered nitrogen-containing heterocyclic compounds. The heterocyclic additives tested had varying effects on the cell performance. Most of the additives enhanced the open-circuit photovoltage (Voc), fill factor (ff), and the solar energy conversion efficiency (η), but reduced the short circuit photocurrent density (Jsc). The physical and chemical properties of the nitrogen-containing heterocycles such as the partial charge, dipole moment, and ionization energy were computationally calculated in order to elucidate the reasons for the additive effects on the cell performance. The greater the lowest partial charge of the nitrogen atoms in the heterocyclic ring, the larger the Voc, but the smaller the Jsc values. The dark current at the bare TiO2 electrode without a dye coating decreased as the lowest partial charge of the nitrogen atoms increased. The Voc of the cell also increased as the ionization energy of the nitrogen-containing heterocyclic molecules decreased. These results suggest that the electron donicity of the nitrogen-containing heterocyclic additives influenced the interaction with the nanocrystalline TiO2 photoelectrode and the I-/I3- electrolyte, which altered the Ru-dye-sensitized solar cell performance. [Copyright &y& Elsevier]

Transition metal complexes using polypyridine ligands are finding increasing use in the dye sensitized solar cells. To improve further the efficiency of this cell, an enhanced spectral response of the sensitizer in the lower energies is required. In this paper we review our effort in the molecular design of the ruthenium polypyridyl complexes for nanocrystalline TiO2-based solar cells. The poor cell efficiency in Ru(4,4′-dicarboxy-2,2′-biquinoline)2(NCS)2/TiO2 system may be ascribed to the low excited-state oxidation potential, which plays a crucial role in the electron-transfer process. Ru(2-(2-(4-carboxypyridyl))-4-carboxyquinoline)2(NCS)2, when anchored to nanocrystalline TiO2 films, achieves efficient sensitization over the whole visible range extending up to 900 nm, yielding incident photon-to-current conversion efficiency (IPCE) of 55%. The low cell efficiency of Ru(4-4′-dicarboxy-2,2′-bypyridine)2(ethyl-2-cyano-3,3-dimercaptoacrylate) may be due to slow regeneration of the dye by electron donation from iodide following charge injection into the TiO2. Tuning of HOMO and LUMO energy level show that an efficient sensitizer should possess ground-state and excited-state redox potentials of 0.5 and −0.8 V vs. SCE, respectively. Transient absorption studies of Ru phenanthroline complexes show that the efficiency of electron injection is strongly affected by the number of carboxyl groups of the sensitizing dye. The β-diketonate complex [Ru(4-4′-dicarboxy-2,2′-bypyridine)2(acetylacetonato)]Cl, when anchored to nanocrystalline TiO2 films, achieves very efficient sensitization across the entire visible region, yielding 60% IPCE. A new series of panchromatic sensitizers of Ru(tricarboxyterpyridine)(β-diketonato)(NCS) type have been developed. Ru(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)(1,1,1-trifluoropentane-2,4-dionato)(NCS) achieved an efficient sensitization of nanocrystalline TiO2 solar cells over the whole visible range extending into near IR region and displaying a maximum around 600 nm, where IPCE approaches a high value of 70%. [Copyright &y& Elsevier]

We have investigated the influence of electrolyte composition on the photovoltaic performance of a dye-sensitized nanocrystalline TiO2 solar cell (DSSC) based on a Ru(II) terpyridyl complex photosensitizer (the black dye). We have also spectroscopically investigated the interaction between the electrolyte components and the adsorbed dye. The absorption peaks attributed to the metal-to-ligand charge transfer transitions of the black dye in solution and adsorbed on a TiO2 film, were red-shifted in the presence of Li cations, which led to an expansion of the spectral response of the solar cell toward the near-IR region. The photovoltaic performance of the DSSC based on the black dye depended remarkably on the electrolyte composition. We developed a novel efficient organic liquid electrolyte containing an imidazolium iodide such as 1,2-dimethyl-3-n-propylimidazolium iodide or 1-ethyl-3-methylimidazolium iodide (EMImI) for a DSSC based on the black dye. A high solar energy-to-electricity conversion efficiency of 9.2% (Jsc=19.0 mA cm-2, Voc=0.67 V, and FF=0.72) was attained under AM 1.5 irradiation (100 mW cm-2) using a novel electrolyte consisting of 1.5 M EMImI, 0.05 M iodine, and acetonitrile as a solvent with an antireflection film. [Copyright &y& Elsevier]

Diimine ruthenium(II) complexes with β-diketonate ligands were synthesized. Photophysical and electrochemical properties of these complexes were investigated. The electron-donating strength of β-diketonate can be tuned by choosing alkyl substitutents of the diketones. The complexes examined showed excellent photosensitization ability toward TiO2. The photoelectrochemical cell sensitized with [Ru(dcbpy)2(dpmo)]Cl (dpmo=dipivaloylmethanato) showed a solar light-to-electricity conversion efficiency of 6.9%. Electronic absorption spectra of the complexes were assigned by a computational calculation of molecular orbital. Transition from HOMO-2 to the LUMO and LUMO+2, which come from diketonate ligand, play an important role for absorption spectra in the longer wavelength response. [Copyright &y& Elsevier]

We constructed a transient absorption microscope as a new tool for studying heterogeneous photochemical systems. We have used the instrument to study the process of electron injection from an excited dye into a nanocrystalline ZnO film. From microscopic images of ground-state absorption and fluorescence, we found that the distribution of dye molecules on the surface was not homogeneous and that aggregates between dye molecules and Zn2+ ions were formed on the surface. However, the distribution of oxidized dye molecules generated by electron injection was homogeneous, according to the microscopic image of transient absorption. This indicates that dye molecules that are active for electron injection are attached directly to the ZnO surface and that aggregates that are inactive for electron injection are formed on this layer. [Copyright &y& Elsevier]

Since Grätzel’s group reported an overall efficiency of 10% for dye-sensitized solar cell using cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II), many other institutions have tried to reproduce it. However, no other institutions have, so far, reported such a high efficiency. In order to develop high efficiency dye-sensitized solar cells, the tuning of TiO2 photoelectrode morphology towards optimization of solar energy conversion efficiency has been investigated. TiO2 photoelectrodes with six different structures, with layers of nanoparticles, light-scattering particles, and mixture of nanoparticles and light-scattering particles on the conducting glass at a desirable sequence and thickness, were designed and investigated. The profiles of photocurrent action spectra were compared in terms of light scattering and the suppression of light loss due to the back-scattering of large particles near the conducting glass. The data show that the multilayer structure is superior to the mono- and double-layer structure, with ∼85% of incident monochromatic photon-to-electron conversion efficiency (IPCE) below 620 nm and ∼45% of IPCE at 700 nm. The solar-to-electric energy conversion efficiency of N719 dye-sensitized solar cell has been improved significantly from 7.6 to 9.8% by tuning the film structure from monolayer to multilayer. The best efficiency of 10.2% under illumination of simulated AM1.5 solar light (100 mW cm-2) was attained with a multilayer structure using an anti-reflection film on the cell surface. [Copyright &y& Elsevier]

The influence of alkylpyridines additive to an I−/I3− redox electrolyte in acetonitrile on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO2 solar cell was studied. I–V measurements were performed using more than 30 different alkylpyridines. The alkylpyridine additives showed a significant influence on the performance of the cell. All the additives decreased the short-circuit photocurrent (Jsc), but most of the alkylpyridines increased the open-circuit photovoltage (Voc) and fill factor (ff) of the solar cell. The results of the molecular orbital calculations suggest that the dipole moment of the alkylpyridine molecules correlate with the Jsc of the cell. These results also suggest that both the size and ionization energy of pyridines correlate with the Voc of the cell. Under AM 1.5 (100 mW/cm2), the highest solar energy conversion efficiency (η) of 7.6% was achieved by using 2-propylpyridine as an additive, which was more effective than the previously reported additive, 4-t-butylpyridine. [Copyright &y& Elsevier]

cis-[Ru(Hmcpq)2(NCS)2] (1; Hmcpq=4-carboxy-2-(2′-pyridyl)quinoline) was newly synthesized, and its spectral (absorption, luminescence) and electrochemical properties were compared with those of cis-[Ru(H2dcpq)2(NCS)2] (2; H2dcpq=4-carboxy-2-[2′-(4′-carboxypyridyl)]quinoline). Solar cells based on nanocrystalline TiO2 film sensitized with 1 showed efficient photosensitization over a large portion of the visible and near-IR spectral region. These solar cells generated a large short-circuit photocurrent (12 mA cm−2), produced an open-circuit voltage of 0.53 V, and exhibited a solar energy conversion efficiency of 4.6% under simulated AM 1.5 solar irradiation (100 mW cm−2). The effects of the number of carboxyl groups in 1 and 2 on the binding to nanocrystalline TiO2 and on the photovoltaic performance of the solar cells were investigated. [Copyright &y& Elsevier]

Anodic photocurrent loss processes have been investigated for dye sensitized nanocrystalline SnO2 and TiO2 solar cells by means of electrical bias dependent photocurrent spectroscopy. Electron injection efficiency is not dependent on visible excitation wavelengths under electrical bias application for both films. Under an identical cell configuration, the unsensitised SnO2 film exhibited cathodic photocurrent generation while no cathodic photocurrent was detected for the TiO2 film. The origins of these observations are discussed. The experimental results suggest that the electron transfer reaction from the SnO2 to the electrolyte must be accelerated at the SnO2 surface by the bias application. [Copyright &y& Elsevier]