Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

The semiconductor and the surface-adsorbed antenna molecule (dyes or other color species) can constitute efficient photochemical systems for environmental remediation. The major advantage of these systems is that they are able to achieve the degradation of organic pollutants by using visible light from the sun as energy and O2 in the air as the oxidant under ambient conditions. In this tutorial review, the unique mechanistic characteristics, the constitution of photodegradation systems and their performance are described. The involved radical reactions during the degradation are also discussed.

Semiconductor-mediated photodegradation of pollutants under visible-light irradiation

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

Dye-sensitized solar cells (DSCs) with cobalt-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a cobalt polypyridine complex and an organic sensitizer. The effect of the steric properties of two triphenylamine-based organic sensitizers and a series of cobalt polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, have been found to limit the performance of these mediators were avoided by matching the properties of the dye and the cobalt redox mediator. Organic dyes with higher extinction coefficients than the standard ruthenium sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO2 films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than...

Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells

This paper reviews recent studies on the semiconductor photocatalysis based on surface-modified TiO2 of which application is mainly focused on environmental remediation. TiO2 photocatalysis that is based on the photoinduced interfacial charge transfer has been extensively studied over the past four decades. A great number of modification methods of semiconductor photocatalysts have been developed and investigated to accelerate the photoconversion, to enable the absorption of visible light, or to alter the reaction mechanism to control the products and intermediates. In this regard, various modification methods of TiO2 are classified according to the kind of surface modifiers (metal-loading, impurity doping, inorganic adsorbates, polymer coating, dye-sensitization, charge transfer complexation) and their effects on photocatalytic reaction mechanism and kinetics are discussed in detail. Modifying TiO2 in various ways not only changes the mechanism and kinetics under UV irradiation but also introduces visible light activity that is absent with pure TiO2. Each modification method influences the photocatalytic activity and mechanism in a way different from others and the observed modification effects are often different depending on the test substrates and conditions even for the same modification method. Better understanding of the modification effects on TiO2 photocatalysis is necessary to obtain reliable results, to assess the photoconversion efficiency more quantitatively, and to further improve the modification methods.

Surface modification of TiO2 photocatalyst for environmental applications

Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles ( approximately 15 nm in diameter) interconnected in a mesoporous, 10 mum thick film immersed in Li(+)-containing CH(3)CN electrolytes with iodide or phenothiazine donors. Pulsed-laser excitation resulted in rapid excited-state injection and donor oxidation to yield TiO(2)(e(-))s and oxidized donors, while the metal-to-ligand charge-transfer (MLCT) absorption spectrum of the Ru(II) coordination compounds differed from that which was initially excited. The spectral data were consistent with an underlying Stark effect and indicated that the surface electric field was not completely screened from the molecular sensitizer. The magnitude of the electric field was estimated to be approximately 270 MV/m from Li(+) titration experiments, corresponding to a approximately 40 mV potential drop. With iodide donors, the amplitude of the Stark effect decreased over time periods where charge recombination was absent, behavior attributed to "screening" of the electric field by interfacial ionic reorganization. The screening kinetics were nonexponential but were well described by the Kohlrausch-Williams-Watts model, from which a characteristic rate constant, tau(o)(-1), of approximately 1.5 x 10(5) s(-1) was abstracted. At least seven other sensitizers and five different cations, as well as on SnO(2) nanoparticle films, exhibited similar transient absorption behavior with iodide donor molecules indicating that the effect was quite general. In the presence of phenothiazine donors (or in the absence of an external donor), there was no clear evidence for screening, and the Stark effect disappeared concurrent with interfacial charge recombination. Complementary spectroelectrochemical studies of these same sensitized films displayed similar absorption spectra when the TiO(2) thin film was partially reduced with a forward bias. Spectral modeling in the absence of donor molecules as well as studies of TiO(2) thin films sensitized with two different Ru(II) compounds demonstrated that the electric field created by excited-state injection from one sensitizer influenced the absorption spectra of other sensitizers that had not undergone photoinduced electron injection.

Stark Effects after Excited-State Interfacial Electron Transfer at Sensitized TiO2 Nanocrystallites

A new dye sensitization system incorporating Pt(dcbpy)Cl2 on Degussa P-25 TiO2 for the photomineralization of aqueous organic pollutants under visible light irradiation is described. The representative wastewater pollutant, 4-chlorophenol (4-CP), is readily oxidized (ultimately to CO2) when the PtII dye sensitized TiO2 is exposed to visible light in the presence of dissolved O2, and the reaction is accelerated when the solution is purged with O2 gas at 1 atm. The sensitizer is regenerated during the photocatalysis; therefore, 4-CP effectively reduces the oxidized form of the surface bound dye. The experimental data are consistent with parallel oxidative decomposition pathways for 4-CP, one which operates using conduction band electrons to produce hydroxyl radicals and another where the oxidized sensitizer irreversibly oxidizes 4-CP.

Visible-light induced water detoxification catalyzed by PtII dye sensitized titania.

Photophysical studies were performed with [Ru(dtb)2(dcb)](PF6)2 and cis-Ru(dcb)(dnb)(NCS)2, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine, dcb is 4,4′-(COOH)2-2,2′-bipyridine, and dnb is 4,4′-(CH3(CH2)8)2-2,2′-bipyridine), anchored to anatase TiO2 particles (∼15 nm in diameter) interconnected in a mesoporous, thin film (∼10 μm thick) immersed in Li+-containing acetonitrile electrolytes. Pulsed-laser excitation resulted in rapid, nonquantitative excited-state injection into TiO2 with a rate constant that could not be time-resolved, kinj > 108 s−1, to yield an interfacial charge-separated state. Return of this state to ground-state products displayed observation-wavelength-dependent kinetics due to charge recombination and a second process. The second process occurred in parallel and was assigned to a transient Stark effect created by the electric field originating from the electrons in TiO2 on ruthenium sensitizers that had not undergone excited-state injection. The kinetics for this processes were well mod...

Excited-state electron transfer from ruthenium-polypyridyl compounds to anatase TiO2 nanocrystallites: evidence for a Stark effect.

After rapid photoinduced electron injection into TiO2 and regeneration by a donor, D, such as iodide or phenothiazine, sensitizers are present in an environment distinctly different from that prior to light absorption. Significantly, the absorption spectrum of the Ru(II) sensitizer in this new environment is one that is known to be less favorable for excited-state electron injection. The transient absorption features were found to report on photoinduced variations in the local electronic environment of the Ru(II) sensitizer−TiO2 interface that were induced by ion transfer. The data demonstrate that slow (μs to ms) cation transfer follows regeneration to yield the sensitizer that was initially photoexcited.

Slow cation transfer follows sensitizer regeneration at anatase TiO2 interfaces.

A new synthetically facile heteroleptic ruthenium(II) sensitizer (NBu4)[Ru(4,7-dpp)(dcbpyH)(NCS)2], coded as YS5, where NBu4 is tetrabutylammonium, 4,7-dpp is 4,7-diphenyl-1,10-phenanthroline, and dcbpyH is the singly deprotonated surface anchoring derivative of 4,4′-dicarboxy-2,2′-bipyridine (dcbpyH2), was designed, synthesized, and incorporated into regenerative mesoscopic titania-based dye-sensitized solar cells. The sensitizer has characteristic broad, high extinction coefficient MLCT bands spanning the visible spectrum. The compound was fully characterized by 1D and 2D 1H NMR, MALDI-TOF-MS, UV−vis, photoluminescence, Raman, IR, and electrochemistry. YS5 exhibits strong visible absorption properties with a molar extinction coefficient of 1.71 × 104 M−1 cm−1 at its 522 nm maximum. In operational liquid junction-based DSSCs under simulated AM 1.5G one-sun excitation (100 mW/cm2), the photovoltaic performance of YS5 compares almost equally against the current benchmark sensitizer N719 in side-by-side com...

Yali Sun

Papers

Stark Effects after Excited-State Interfacial Electron Transfer at Sensitized TiO2 Nanocrystallites

Visible-light induced water detoxification catalyzed by PtII dye sensitized titania.

Excited-state electron transfer from ruthenium-polypyridyl compounds to anatase TiO2 nanocrystallites: evidence for a Stark effect.

Slow cation transfer follows sensitizer regeneration at anatase TiO2 interfaces.

Viable Alternative to N719 for Dye-Sensitized Solar Cells