Yousuke Ooyama

Photophysical and Electrochemical Properties, and Molecular Structures of Organic Dyes for Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO(2), ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident-solar-light-to-electricity conversion efficiency and low cost of production. To develop high-performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light-harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch-making molecular design of organic dyes for high photovoltaic performance and long-term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.

Dye‐Sensitized Solar Cells Based on Donor‐π‐Acceptor Fluorescent Dyes with a Pyridine Ring as an Electron‐Withdrawing‐Injecting Anchoring Group

A new-type of donor-acceptor π-conjugated (D-π-A) fluorescent dyes NI3-NI8 with a pyridine ring as electron-withdrawing-injecting anchoring group have been developed and their photovoltaic performances in dye-sensitized solar cells (DSSCs) are investigated. The short-circuit photocurrent densities and solar energy-to-electricity conversion yields of DSSCs based on NI3-NI8 are greater than those for the conventional D-π-A dye sensitizers NI1 and NI2 with a carboxyl group as the electron-withdrawing anchoring group. The IR spectra of NI3-NI8 adsorbed on TiO(2) indicate the formation of coordinate bonds between the pyridine ring of dyes NI3-NI8 and the Lewis acid sites (exposed Ti(n+) cations) of the TiO(2) surface. This work demonstrates that the pyridine rings of D-π-A dye sensitizers that form a coordinate bond with the Lewis acid site of a TiO(2) surface are promising candidates as not only electron-withdrawing anchoring group but also electron-injecting group, rather than the carboxyl groups of the conventional D-π-A dye sensitizers that form an ester linkage with the Brønsted acid sites of the TiO(2) surface.

Molecular design of mechanofluorochromic dyes and their solid-state fluorescence properties

Heteropolycyclic donor–acceptor π-conjugated (D–π–A) fluorescent dyes with strong electron-withdrawing substituents show mechanofluorochromism (MFC): grinding of as-recrystallized dyes induces a fluorescent color change accompanying an enhanced fluorescence quantum yield, followed by a reversion to the original fluorescent color by heating or exposure to solvent vapor. Interestingly, both absorption and emission peaks are red-shifted by grinding, and the degrees of the red-shift are dependent on the electron-accepting ability of acceptor, steric sizes of the substituents, and D–π–A system. In order to clarify the mechanism of MFC observed with this new class of D–π–A fluorescent dyes, time-resolved fluorescence spectroscopy, X-ray powder diffractometry, differential scanning calorimetry and density measurements were performed before and after grinding of the solids. On the basis of experimental results and semi-empirical molecular orbital calculations (AM1 and INDO/S), we have revealed that the MFC is attributed to a reversible switching between crystalline and amorphous states with changes of dipole–dipole interaction and intermolecular π–π interaction by changes of the densities of the solids before and after grinding. This study demonstrates that the most important point for developing mechanofluorochromic dyes is to design fluorescent dye molecules of strong D–π–A characters with large dipole moments.

Dye-sensitized solar cells based on D-π-A fluorescent dyes with two pyridyl groups as an electron-withdrawing-injecting anchoring group.

D-π-A fluorescent dye with two pyridyl groups as an electron-withdrawing-injecting anchoring group possessing a high coordinate bonding ability to Lewis acid sites on the TiO2 surface, which can lead to high dye loading on the TiO2 film and efficient electron injection, has been developed as a new type of D-π-A dye sensitizer for dye-sensitized solar cells.

Photovoltaic performance of dye-sensitized solar cells based on D–π–A type BODIPY dye with two pyridyl groups

D–π–A type boron dipyrromethene (BODIPY) dye YH-1, which has two pyridyl groups as electron-withdrawing-anchoring groups at the end of the 3- and 5-positions and a carbazole-diphenylamine moiety as an electron donor at the 8-position on the BODIPY core, was designed and developed as a photosensitizer for dye-sensitized solar cells (DSSCs). It was found that the dye YH-1 possesses a good light-harvesting efficiency (LHE) in the red/near-IR (NIR) region and good adsorption ability on TiO2 film. We demonstrate that the expansion of the π-conjugated system by the introduction of not only the carbazole-diphenylamine moiety and the thiophene unit at the 8-position but also two thienylpyridines at the 3- and 5-positions on the BODIPY core can lead to red-shift and broadening of the absorption band in the red/NIR region. DSSCs based on YH-1 exhibit incident photon-to-current conversion efficiency of ca. 10% over a range of 500 to 700 nm, with an onset at 800 nm.

New molecular design of donor-π-acceptor dyes for dye-sensitized solar cells: control of molecular orientation and arrangement on TiO2 surface

A series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes OH1, OH2, OH4, OH7, and OH17 with carboxyl groups on different positions of a chromophore skeleton are synthesized and applied to dye-sensitized solar cells (DSSCs) as a new class of donor-π-acceptor (D-π-A) photosensitizers. In the dye OH1, a carboxyl group acts as not only the anchoring group for attachment on TiO2 surface but also the electron acceptor. For OH2, OH4, and OH7, on the other hand, a carboxyl group is an anchoring group, but the electron acceptor is not a carboxyl group but a cyano group. The dye OH17 has two carboxyl groups, which are located at the same positions as those of OH1 and OH7. The absorption and fluorescence spectra and cyclic voltammograms of these fluorescent dyes resemble very well, showing a negligible influence of the position of carboxyl group on photophysical and electrochemical properties of these dyes. When these dyes are used in DSSCs, however, their photovoltaic performances differ considerably. In order to elucidate the difference in the DSSC performance among the five dyes, kinetics of the electron injection from the conduction band of TiO2 to dye cation and to I3− ions in solution were studied by employing the transient absorption spectroscopy and the transient photovoltage technique, respectively. It is found that the charge recombination rate for OH2 is similar to that of OH1 and is much slower than those of OH4 and OH17, consistent with the high short-circuit photocurrent densities for OH1 and OH2. On the basis of the MO calculations (AM1 and INDO/S) and the charge recombination kinetics, the differences of the photovoltaic performances among the five dyes are discussed from the viewpoint of configuration of the dyes adsorbed on TiO2 surface, and a new molecular design for D-π-A dye sensitizers based on a control of molecular orientation and arrangement of dye adsorbed on TiO2 surface is proposed.

Photovoltaic performance of dye-sensitized solar cells based on donor–acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes with a carboxyl group at different positions of the chromophore skeleton

Donor-acceptor pi-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b with a carboxyl group at different positions of the chromophore skeleton have been designed and synthesized. The absorption and fluorescence spectra and cyclic voltammograms of the fluorescent dyes agree very well, showing that the position of the carboxyl group has a negligible influence on the photophysical and electrochemical properties of these dyes. When these dyes are used in dye-sensitized solar cells, however, their photovolatic performances are considerably different. The short-circuit photocurrents and energy conversion efficiencies under a simulated solar light increase in the order: 3a (2.12 mA cm(-2), 1.00%) approximately 3b (2.10 mA cm(-2), 1.06%) > 8b (1.50 mA cm(-2), 0.67%) > 8a (0.84 mA cm(-2), 0.34%). Based on semi-empirical molecular orbital calculations (AM1 and INDO/S) together with spectral analyses and their photovolatic performance, the relationships between the observed photovolatic properties and the chemical structures of the benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes are discussed. It is found that strong interaction between a TiO(2) surface and the electron accepting moiety of the dye leads to a high photovoltaic performance.

Development of a D–π–A dye with benzothienopyridine as the electron-withdrawing anchoring group for dye-sensitized solar cells

A D–π–A dye SAT-1 with benzo[4,5]thieno[2,3-c]pyridine as the electron-withdrawing anchoring group capable of forming a hydrogen bond at Bronsted acid sites or a coordinate bond at Lewis acid sites on a TiO2 surface has been developed as a new-type of D–π–A dye sensitizer for dye-sensitized solar cells.

Dye-sensitized solar cells based on novel donor–acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes exhibiting solid-state fluorescence

Novel donor–acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes (2a–2d) with substituents (R = H, butyl, benzyl and 5-nonyl) on the nitrogen of carbazole ring have been designed and synthesized as sensitizers in dye-sensitized solar cells (DSSCs). Absorption and fluorescence properties of 2a–2d are similar in solution. In the solid state, however, the dyes 2c (R = benzyl) and 2d (R = 5-nonyl) with bulky substituents exhibit strong solid-state fluorescence properties, which are attributed to the reduction of the π–π interaction between the dyes. Photovoltaic parameters of DSSCs based on 2a–2d are measured with the amount of dyes adsorbed on TiO2 film as a parameter. It is found that the 2d exhibits a 100% efficiency for absorbed photon-to-current conversion, demonstrating that bulky substituents in 2d can efficiently prevent intermolecular energy transfer between the dyes in molecular aggregation states.

Development of D–π–Cat fluorescent dyes with a catechol group for dye-sensitized solar cells based on dye-to-TiO2 charge transfer

D–π–Cat fluorescent dyes YM-1 and YM-2 with a diphenylamine moiety as the electron-donating group, a catechol (Cat) unit as the anchoring group and fluorene or carbazole as the π-conjugated system were designed and developed as a photosensitizer for type-II dye-sensitized solar cells (DSSCs), which have a direct electron-injection pathway from the dye to the conduction band (CB) of the TiO2 electrode by photoexcitation of the dye-to-TiO2 charge transfer (DTCT) bands. Furthermore, not only to gain insight into the influence of the molecular structure of D–π–Cat dyes on the appearance of a DTCT band and the electron-injection mechanism, but also to investigate the impacts of the DTCT characteristics of D–π–Cat dyes on the photovoltaic performances of DSSCs, a D–π–Cat fluorescent dye YM-3 with carbazole–terthiophene as the π-conjugated system was also synthesized. It was found that the D–π–Cat dyes possess a good light-harvesting efficiency (LHE) in the visible region due to a broad absorption band corresponding to DTCT upon binding to a TiO2 film. The incident photon-to-current conversion efficiency (IPCE) corresponding to the DTCT band for DSSCs based on YM-1 and YM-2 is higher than that for YM-3. This work indicates that the stabilization of the LUMO level and the expansion of the π-conjugated system by the introduction of a long π-bridge such as terthiophene on the Cat moiety can lead to an increase in the intramolecular charge transfer (ICT) excitation based on π → π* transition with a decrease in the DTCT characteristics, resulting in enhancement of an indirect electron-injection pathway from the excited dye to the CB of TiO2 by photoexcitation of the local band of the adsorbed dye on TiO2.