Koji Uenaka

Effective co-sensitization using D–π–A dyes with a pyridyl group adsorbing at Brønsted acid sites and Lewis acid sites on a TiO2 surface for dye-sensitized solar cells

Effective and convenient co-sensitization to enhance dye coverage on TiO2 electrodes for dye-sensitized solar cells have been achieved successfully by employing two kinds of D–π–A dyes with a pyridyl group capable of adsorbing at the Bronsted acid sites and the Lewis acid sites on a TiO2 surface.

Development of highly-sensitive fluorescence PET (photo-induced electron transfer) sensor for water: anthracene–boronic acid ester

Anthracene–boronic acid ester OF-2 having a cyano group as an electron-withdrawing substituent was designed and developed as a highly-sensitive fluorescence PET sensor for detection of a trace amount of water in various solvents (polar, less polar, protic and aprotic solvents).

Synthesis, optical, electrochemical and photovoltaic properties of a D–π–A fluorescent dye with triazine ring as electron-withdrawing anchoring group for dye-sensitized solar cells

The D–π–A fluorescent dye OUJ-1 with 1,3,5-triazine ring as electron-withdrawing anchoring group and (diphenylamino)carbazole containing a thiophene ring as D–π moiety have been newly developed and their optical and electrochemical properties, adsorption states on TiO2 nanoparticles, and photovoltaic performance in dye-sensitized solar cell (DSSC) were investigated. The absorption maximum (λabsmax) for the intramolecular charge-transfer (ICT) absorption band of the D–π–A dye OUJ-1 occurs at a longer wavelength than those of D–π–A dye NI-6 with a pyridyl group and D–π–A dye OUK-1 with a pyrzayl group. Moreover, the dye OUJ-1 exhibits significant fluorescence solvatochromic properties, that is, a bathochromic shift of the fluorescence band and a decrease in the fluorescence quantum yield (Φ) due to a change from the 1ICT excited state to the twisted intramolecular charge transfer (TICT) excited state with increasing solvent polarity were observed. The photovoltaic performance of a DSSC based on OUJ-1 is lower than those of NI-6 and OUK-1 due to the low dye loading of OUJ-1 on TiO2 electrode. It was found that the dye OUJ-1 is adsorbed onto the TiO2 surface through the formation of trizainium ions at the Bronsted acid sites (surface-bound hydroxyl groups, Ti–OH) on the TiO2 surface, although the dye NI-6 was predominantly adsorbed on the TiO2 through coordinate bonding between the pyridyl group of the dye and the Lewis acid sites (exposed Tin+ cations) on the TiO2 surface, and the dye OUK-1 was adsorbed on the TiO2 surface through both the formations of hydrogen bonding of pyrazyl groups and pyrazinium ions at Bronsted acid sites on the TiO2 surface. This work revealed that the binding mode of D–π–A dye sensitizers with azine rings on the TiO2 surface can be changed by control of the basicity and electron density of the azine rings.

Development of D–π–A dyes with a pyrazine ring as an electron-withdrawing anchoring group for dye-sensitized solar cells

D–π–A and (D–π–)2A fluorescent dyes (OUK-1 and OUK-2) with a pyrazine ring as an electron-withdrawing anchoring group capable of forming a hydrogen bond or a pyrazinium ion at the Bronsted acid sites on TiO2 surface have been developed as a new type of D–π–A dye sensitizer for dye-sensitized solar cells.

Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C60

Cyclic free-base porphyrin dimers (H4-C4-CPDPy(TEO) and H4-Ptz-CPDPy(TEO)) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes (C60⊂H4-C4-CPDPy(TEO) and C60⊂H4-Ptz-CPDPy(TEO)) with fullerene C60 have been applied to dye-sensitized solar cells (DSSCs) as a new class of porphyrin dye sensitizers with pyridyl anchoring groups for attachment on a TiO2 electrode. The FTIR spectra of the porphyrin dimers adsorbed on TiO2 nanoparticles demonstrated that these porphyrin dimers are adsorbed on the TiO2 surface through the formation of hydrogen bonding of pyridyl groups and/or pyridinium ions at Bronsted acid sites on the TiO2 surface. The adsorption amount of the porphyrin dimers adsorbed on the TiO2 electrode is 2.0 × 1017 molecules per cm2, that is, the adsorption amount of the porphyrin unit is 4.0 × 1017 cm−2, which is higher than that of dye sensitizers with pyridyl groups reported so far. The photovoltaic performance of DSSCs based on phenothiazine-bridged cyclic porphyrin dimer H4-Ptz-CPDPy(TEO) is higher than that of DSSCs based on butadiyne-linked cyclic porphyrin dimer H4-C4-CPDPy(TEO). Moreover, the photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers are higher than those of DSSCs based on their C60 inclusion complexes C60⊂H4-C4-CPDPy(TEO) and C60⊂H4-Ptz-CPDPy(TEO). On the basis of the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy, the differences in the photovoltaic performances among these cyclic free-base porphyrin dimers are discussed from kinetic and thermodynamic considerations concerning the electron transfer processes in DSSCs.

Development of a functionally separated D–π-A fluorescent dye with a pyrazyl group as an electron-accepting group for dye-sensitized solar cells

A functionally separated D–π-A dye OUK-3 with a pyrazyl group as an electron-withdrawing anchoring group and a carboxyl group as an additional anchoring group has been newly developed as a photosensitizer for dye-sensitized solar cells. The optical and electrochemical properties, adsorption states on TiO2 nanoparticles, and photovoltaic performances in dye-sensitized solar cells (DSSCs) were investigated. It was found that the maximum adsorption amount of dye adsorbed on the TiO2 electrode for OUK-3 is 3 times as much as that of the D–π-A dye sensitizer OUK-1 with a pyrazyl group as an electron-withdrawing anchoring group. Moreover, this work revealed that the DSSC based on the dye OUK-3 exhibits a higher photovoltage (Voc) value than the DSSC based on the dye OUK-1. On the basis of the FTIR spectra of the dyes adsorbed on TiO2 nanoparticles and the electrochemical impedance spectroscopy (EIS) analysis of DSSCs based on the two dyes OUK-1 and OUK-3, the differences of photovoltaic performances between the two dyes are discussed by taking into account the adsorption states of the dyes adsorbed on the TiO2 surface. This work demonstrates that functionally separated D–π-A dye sensitizers can achieve effective surface coverage of the TiO2 electrode due to their high adsorption ability onto the TiO2 electrode, leading to not only the improvement of light-harvesting efficiency (LHE), but also an increase in the number of injected electrons in the CB of TiO2, which is responsible for the higher Voc value of functionally separated D–π-A dye sensitizers.

Effect of Substituents in Catechol Dye Sensitizers on Photovoltaic Performance of Type II Dye‐Sensitized Solar Cells

In order to provide a direction in molecular design of catechol (Cat) dyes for type II dye-sensitized solar cells (DSSCs), the dye-to-TiO2 charge-transfer (DTCT) characteristics of Cat dyes with various substituents and their photovoltaic performance in DSSCs are investigated. The Cat dyes with electron-donating or moderately electron-withdrawing substituents exhibit a broad absorption band corresponding to DTCT upon binding to TiO2 films, whereas those with strongly electron-withdrawing substituents exhibit weak DTCT. This study indicates that the introduction of a moderately electron-withdrawing substituent on the Cat moiety leads to not only an increase in the DTCT efficiency, but also the retardation of back electron transfer. This results in favorable conditions for the type II electron-injection pathway from the ground state of the Cat dye to the conduction band of the TiO2 electrode by the photoexcitation of DTCT bands.