Kohjiro Hara

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO2, Nb2O5, ZnO, SnO2, and In2O3 with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400‐600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochromesensitized ZnO solar cell with an I ‐ /I3 ‐ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99mWcm ‐2 ) reached 2.5% with a short-circuit photocurrent density (Jsc) of 7.44mA cm ‐2 , a open-circuit photovoltage (Voc) of 0.52 V, and a fill factor (ff) of 0.64. The Jsc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the Voc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.

Design of new coumarin dyes having thiophene moieties for highly efficient organic-dye-sensitized solar cells

We have developed new coumarin dyes having thiophene moieties in order to improve the photovoltaic performance of dye-sensitized nanocrystalline TiO2 solar cells based on the organic dyes as photosensitizers. A solar-energy-to-electricity conversion efficiency (η) of 7.7% was attained under AM 1.5 irradiation (100 mW cm−2) with a short-circuit current density (Jsc) of 14.3 mA cm−2, an open-circuit voltage (Voc) of 0.73 V, and a fill factor (ff) of 0.74.

Interfacial Electron-Transfer Kinetics in Metal-Free Organic Dye-Sensitized Solar Cells: Combined Effects of Molecular Structure of Dyes and Electrolytes

Electron diffusion coefficient, lifetime, and density in the TiO(2) electrode of dye-sensitized TiO(2) solar cells (DSCs) employing I(-)/I(3)(-) redox couples were measured with eight different metal-free organic dyes and three Ru complex dyes. At matched electron density, all DSCs using organic dyes (ODSCs) showed shorter electron lifetime with comparable or larger diffusion coefficients in comparison to the DSCs using the Ru dyes (RuDSC). The shorter lifetime was attributed partially to the slower dye cation reduction rate of the organic dyes by I(-), faster electron diffusion coefficient in the TiO(2), and mostly higher I(3)(-) concentration in the vicinity of the TiO(2) surface. Whereas a slight shift of the conduction band edge potential (E(cb)) of the TiO(2) was seen with a few organic dyes, no correlation was found with the dipole moment of the adsorbed dyes. This implies that the adsorbed dyes interact with cations in the electrolyte, so the direction of the dipole is altered or simply screened. The increase of [I(3)(-)] in the vicinity of the TiO(2) surface was interpreted with partial charge distribution of the dyes. Under one-sun conditions, less electron density due to shorter electron lifetime was found to be the main reason for the lower values of V(oc) for all ODSCs in comparison to that of RuDSCs. Among the organic dyes, having larger molecular size and alkyl chains showed longer electron lifetime, and thus higher V(oc). Toward higher open circuit voltage, a design guide of organic dyes controlling the electron lifetime is discussed.

A coumarin-derivative dye sensitized nanocrystalline TiO2 solar cell having a high solar-energy conversion efficiency up to 5.6%

It is found that newly synthesized coumarin derivatives work as highly efficient photosensitizers for dye-sensitized nanocrystalline TiO2 solar cells producing a 5.6% solar-light-to-electricity conversion efficiency, the highest efficiency so far among organic dye-sensitized solar cells, with a short-circuit current density of 13.8 mA cm−2, an open-circuit photovoltage of 0.63 V, and a fill factor of 0.63 under standard AM 1.5 irradiation (100 mW cm−2).

Dynamics of efficient electron–hole separation in TiO2 nanoparticles revealed by femtosecond transient absorption spectroscopy under the weak-excitation condition

The transient absorption of nanocrystalline TiO(2) films in the visible and IR wavelength regions was measured under the weak-excitation condition, where the second-order electron-hole recombination process can be ignored. The intrinsic dynamics of the electron-hole pairs in the femtosecond to picosecond time range was elucidated. Surface-trapped electrons and surface-trapped holes were generated within approximately 200 fs (time resolution). Surface-trapped electrons, which gave an absorption peak at around 800 nm, and bulk electrons, which absorbed in the IR wavelength region, decayed with a 500-ps time constant due to relaxation into deep bulk trapping sites. It is already known that, after this relaxation, electrons and holes survive for microseconds. We interpreted these long lifetimes in terms of the prompt spatial charge separation of electrons in the bulk and holes at the surface.

Visible-Light-Induced Water Splitting Based on Two-Step Photoexcitation between Dye-Sensitized Layered Niobate and Tungsten Oxide Photocatalysts in the Presence of a Triiodide/Iodide Shuttle Redox Mediator

Water splitting into H2 and O2 under visible light was achieved using simple organic dyes such as coumarin and carbazole as photosensitizers on an n-type semiconductor for H2 evolution, a tungsten(VI) oxide (WO3) photocatalyst for O2 evolution, and a triiodide/iodide (I3(-)/I(-)) redox couple as a shuttle electron mediator between them. The results on electrochemical measurements revealed that the oxidized states of the dye molecules having an oligothiophene moiety (two or more thiophene rings) in their structures are relatively stable even in water and possess sufficiently long lifetimes to exhibit reversible oxidation-reduction cycles, while the carbazole system required more thiophene rings than the coumarin one to be substantially stabilized. The long lifetimes of the oxidized states enabled these dye molecules to be regenerated to the original states by accepting an electron from the I(-) electron donor even in an aqueous solution, achieving sustained H2 and I3(-) production from an aqueous KI solution under visible light irradiation when they were combined with an appropriate n-type semiconductor, ion-exchangeable layered niobate H4Nb6O17. The use of H4Nb6O17 loaded with Pt cocatalyst inside the interlayer allowed the water reduction to proceed preferentially with a steady rate even in the presence of a considerable amount of I3(-) in the solution, due to the inhibited access of I3(-) to the reduction site, Pt particles inside, by the electrostatic repulsion between the I3(-) anions and the negatively charged (Nb6O17)(4-) layers. It was also revealed that the WO3 particles coloaded with Pt and IrO2 catalysts exhibited higher rates of O2 evolution than the WO3 particles loaded only with Pt in aqueous solutions containing a considerable amount of I(-), which competitively consumes the holes and lowers the rate of O2 evolution on WO3 photocatalysts. The enhanced O2 evolution is certainly due to the improved selectivity of holes toward water oxidation on IrO2 cocatalyst, instead of undesirable oxidation of I(-). Simultaneous evolution of H2 and O2 under visible light was then achieved by combining the Pt/H4Nb6O17 semiconductor sensitized with the dye molecules having an oligothiophene moiety, which can stably generate H2 and I3(-) from an aqueous KI solution, with the IrO2-Pt-loaded WO3 photocatalyst that can reduce the I3(-) back to I(-) and oxidize water to O2.

Robust dye-sensitized overall water splitting system with two-step photoexcitation of coumarin dyes and metal oxide semiconductors

Photocatalytic splitting of water into H(2) and O(2) under visible light irradiation is achieved using a coumarin-dye-adsorbed lamellar niobium oxide for hydrogen evolution.

Substituted carbazole dyes for efficient molecular photovoltaics: long electron lifetime and high open circuit voltage performance

We designed and synthesized new substituted carbazole dyes, MK-14 and -16, for dye-sensitized solar cells (DSSCs) employing the I−/I3−redox couple. By the addition of a hexyloxyphenyl substituent to previously reported carbazole dyes MK-1 and -2, the electron lifetime and open circuit voltage of the DSSCs employing these dyes were increased, showing comparable values with those using a conventional Ru complex dye. This result was achieved by the retardation of the charge recombination, caused by more effective blocking of the I3− ion in the electrolyte than that in the cases of MK-1 and -2. The result shows the importance of the position of alkyl chains attached to the main framework of dye molecules.

PHOTOSENSITIZATION OF A POROUS TIO2 ELECTRODE WITH MEROCYANINE DYES CONTAINING A CARBOXYL GROUP AND A LONG ALKYL CHAIN

Porous TiO2 electrodes sensitized using merocyanine dyes containing a carboxyl group and a long alkyl chain, in particular 3-carboxymethyl-5-[2-(3-alkyl-2-benzothiazoli- nyldene)ethylidene]-2-thioxo-4-thiazolidinone, showed remarkably high solar–energy efficiency (4.2%, AM-1.5, 100 mW cm−2).

Near-infrared photoconductive and photovoltaic devices using single-wall carbon nanotubes in conductive polymer films

We have fabricated prototypical Al/single-wall carbon nanotube (SWNT)-polymer/indium tin oxide thin-film devices that exhibit promising photoconductive and photovoltaic responses in a broad spectral range, typically from 300to1600nm. This achievement was made possible by finely dispersed SWNT powders in polymer matrices such as poly-phenylene-vinylene and poly-thiophene. These devices utilize (i) the intrinsic near-infrared light harvesting properties of semiconducting SWNTs, (ii) the electronic transport properties of both semiconducting and metallic SWNTs in combination with those of the polymer matrices, and (iii) probably charge/energy transfer processes between SWNTs and the polymers. By selecting different sources of SWNTs and polymers, we have shown that the optoelectronic properties of these devices are potentially tunable. To support our investigation, several techniques including spectrally resolved photoconductivity, optical absorption, and photoluminescence spectroscopy were utilized.