Akihiro Furube

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.

Zn−Zn Porphyrin Dimer-Sensitized Solar Cells: Toward 3-D Light Harvesting

Zn-Zn porphyrin dimers have been incorporated into thin dye-sensitized solar cells (DSSCs) to boost their light harvesting efficiency. The photoexcited dimers show efficient and fast electron injection into TiO(2) indicating that both photoexcited chromophores contribute to current generation. The improved light harvesting ability coupled to enhanced DSSC performance demonstrates the potential of 3-D light harvesting arrays as next generation light harvesters for artificial solar energy conversion systems.

Long-term stability of organic–dye-sensitized solar cells based on an alkyl-functionalized carbazole dye

We investigated the long-term stability of performance for dye-sensitized solar cells (DSSCs) based on an alkyl-functionalized carbazole dye (MK-2) and used in conjunction with ionic liquid-based electrolytes. We observed good long-term stability of the performance of the DSSCs during 60 days under visible-light irradiation at ca. 50 °C. The performance of the DSSC decreased gradually under white-light irradiation including UV light or at 80 °C under dark conditions. However, no decomposition or detachment of the dye molecule from the TiO2electrode was observed after these measurements. These results indicate that the MK-2dye molecule in the cell was stable even under white-light irradiation and at 80 °C under dark conditions.

Enhancing the performance of quantum dots sensitized solar cell by SiO2 surface coating

This letter reports enhanced performance of quantum dots sensitized solar cells by selectively deposition an insulating SiO2 layer over the quantum dots (QDs) sensitized mesoporous TiO2 photoanode. The incident photon to current conversion efficiency of the CdS/CdSe QDs cosensitized solar cell with SiO2 coating can reach 83%. A power conversion efficiency of 2.05% was obtained.

Photoinduced electron injection in black dye sensitized nanocrystalline TiO2 films

Photoinduced electron injection in nanocrystalline TiO2 films sensitized with black dye [trithiocyanato(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)ruthenium(II), Ru(tcterpy)(NCS)3] was studied by means of transient absorption (TA) spectroscopy. We examined the effect of the additive 4-tert-butylpyridine (TBP) in acetonitrile, which is known to markedly improve the performance of solar cell devices. The efficiency of electron injection for the film immersed in acetonitrile was 50% higher than that for the film dried in air. Femtosecond TA measurements indicated that this efficiency enhancement was due to the opening of an additional injection pathway from a lower-lying energy level. Upon the addition of 1 M TBP, which is known to raise the level of the conduction band of TiO2, the enhancement induced by acetonitrile disappeared. We discuss the mechanism of electron injection by comparing our results with results previously reported for the N3/TiO2 system.

Influence of TiCl4 treatment on back contact dye-sensitized solar cells sensitized with black dye

To better understand why titanium tetrachloride (TiCl4) treatment improves short circuit current, we studied its effects on back contact dye-sensitized solar cells sensitized with black dye [tri(thiocyanato)(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)ruthenium(II), Ru(tcterpy)(NCS)3] using transient absorption spectroscopy and electrochemical impedance spectroscopy. We found that the TiCl4 treatment improved short circuit current and achieved an overall energy conversion efficiency of 8.9%. The transient absorption signals did not change as a result of the treatment, suggesting that electron injection efficiency is not affected by the treatment. The impedance related to electron transport between TiO2 particles decreased and the peak frequency of the imaginary part of the electrochemical impedance spectra assigned to electron transfer from TiO2 to the redox couple was shifted to lower frequency by the treatment. This clearly indicates that TiCl4 treatment improved electron transport in the nanocrystalline TiO2 film in back contact dye-sensitized solar cells.

Trapped State Sensitive Kinetics in LaTiO2N Solid Photocatalyst with and without Cocatalyst Loading

In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by existing defects, which can both enhance the photocatalytic activity by promoting electron-hole separation or can deteriorate the activity by serving as recombination centers, is also very crucial to the overall performance of the photocatalyst. In this work, using femtosecond diffuse reflectance spectroscopy we have provided evidence for the existence of energetically distributed trapped states in visible-light responsive solid photocatalyst powder material LaTiO2N (LTON). We observe trapped state sensitive kinetics in bare-LTON. CoOx cocatalyst loading (2 wt % CoOx-LTON) shows effect on the kinetics only when presence of excess energy (for above bandgap excitation) results in the generation of surface carriers. Thus, the kinetics show appreciable excitation wavelength dependence, and the experimental results obtained for different λexc have been rationalized on this basis. In an earlier work by Domen and co-workers, the optimized CoOx/LTON has been reported to exhibit a high quantum efficiency of 27.1 ± 2.6% at 440 nm, the highest reported for this class of photocatalysts (J. Am. Chem. Soc. 2012, 134, 8348-8351). In the present work, the mechanism is addressed in terms of picosecond charge carrier dynamics.

Femtosecond diffuse reflectance transient absorption for dye-sensitized solar cells under operational conditions: effect of electrolyte on electron injection.

By applying diffuse reflectance transient absorption spectroscopy to dye-sensitized solar cells under operational conditions, we were able to directly correlate interfacial electron injection kinetics from a Ru complex sensitizer to a TiO(2) nanocrystalline electrode with the incident photon to current efficiency values. It was revealed that ionic liquid electrolytes reduced the initial electron injection efficiency by suppression of the approximately 100 ps process.

Remarkable Dependence of the Final Charge Separation Efficiency on the Donor–Acceptor Interaction in Photoinduced Electron Transfer

The unprecedented dependence of final charge separation efficiency as a function of donor-acceptor interaction in covalently-linked molecules with a rectilinear rigid oligo-p-xylene bridge has been observed. Optimization of the donor-acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100% efficiency. This finding is extremely useful for the rational design of artificial photosynthesis and organic photovoltaic cells toward efficient solar energy conversion.

Matter of minutes degradation of poly(3-hexylthiophene) under illumination in air

Films of regioregular poly(3-hexylthiophene) [P3HT] were monitored for degradation by means of in situ fluorescence quenching measurements. The films were found to be stable under UV or VIS energy illumination in vacuo and stable to the air when kept in the dark. Under illumination in air however, degradation products were formed immediately as seen by the fluorescence quenching. The quantum yield for defect product formation was calculated to be around 3 × 10−6 per incident photon of above band-gap energy, a relatively rapid reaction that could create over 1016 defects per cm3 in optically thin films of P3HT in less than a second under sunlight, or in only a matter of minutes under room lighting. The degradation pathway is thought to involve attack of ground state molecular oxygen on the P3HT excited state. The reaction itself is self-inhibiting, as the P3HT excited state is thought to be effectively quenched by the increasing formation of defects, leading to slower and slower defect formation until the film enters a ‘quasi-stable’ state where no further degradation by this mechanism will occur. The upper limit to the number of defects to be found in films of P3HT formed by this mechanism is estimated to be around 2 × 1017 cm−3.