We have developed novel coumarin dyes for use in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). The absorption spectra of these novel coumarin dyes are red-shifted remarkably in the visible region relative to the spectrum of C343, a conventional coumarin dye. Introduction of a methine unit (−CHCH−) connecting both the cyano (−CN) and carboxyl (−COOH) groups into the coumarin framework expanded the π conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. The monochromatic incident photon-to-current conversion efficiency (IPCE) from 420 to 600 nm for a DSSC based on NKX-2311 was over 70% with the maximum of 80% at 470 nm, which is almost equal to the efficiency obtained with the N3 dye system. The IPCE performance of DSSCs based on coumarin dyes depended remarkably on the LUMO levels of the dyes, which are estimated from the oxidation potential and 0−0 energy of the dye. The slow charge recombination, on the order of micro to milliseconds, between NKX-2311 cations and injected electrons in the conduction band of TiO2 (observed by transient absorption spectroscopy) resulted in efficient charge separation in this system. A HOMO−LUMO calculation indicated that the electron moves from the coumarin framework to the −CHCH− unit by photoexcitation of the dye (a π−π* transition). Our results strongly suggest that molecular design of the sensitizer is essential for the construction of highly efficient DSSCs. The structure of NKX-2311, whose carboxyl group is directly connected to the −CHCH− unit, is advantageous for effective electron injection from the dye into the conduction band of TiO2. In addition, the cyano group, owing to its strong electron-withdrawing ability, might play an important role in electron injection in addition to a red shift in the absorption region.

Reactive species, holes, and electrons in photoexcited nanocrystalline TiO 2 films were studied by transient absorption spectroscopy in the wavelength range from 400 to 2500 nm. The electron spectrum was obtained through a hole-scavenging reaction under steady-state light irradiation. The spectrum can be analyzed by a superposition of the free-electron and trapped-electron spectra. By subtracting the electron spectrum from the transient absorption spectrum, the spectrum of trapped holes was obtained. As a result, three reactive speciess trapped holes and free and trapped electronsswere identified in the transient absorption spectrum. The reactivity of these species was evaluated through transient absorption spectroscopy in the presence of hole-and electronscavenger molecules. The spectra indicate that trapped holes and electrons are localized at the surface of the particles and free electrons are distributed in the bulk.

The efficiency of electron injection from excited N3 dye (cis-bis-(4,4′-dicarboxy-2,2′-bipyridine) dithiocyanato ruthenium(II), Ru(dcbpy) 2 (NCS) 2 ), into various nanocrystalline semiconductor (ZrO 2 , TiO 2 , ZnO, Nb 2 O 5 , SnO 2 , In 2 O 3 ) films was studied by transient absorption spectroscopy. For TiO 2 , ZnO, Nb 2 O 5 , SnO 2 , or In 2 O 3 films, injection efficiencies were found to be very high; for ZrO 2 film, the efficiency was very low. These findings indicate that electron injection occurs efficiently if the LUMO level of N3 dye is located sufficiently far above the bottom of the conduction band of the semiconductor film. On the basis of the results, we discuss the reason TiO 2 exhibits higher solar cell performance than other materials.

We have developed oligothiophene-containing coumarin dyes fully functionalized for dye-sensitized nanocrystalline TiO(2) solar cells (DSSCs). DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400-800 nm. A solar energy-to-electricity conversion efficiency (eta) of 7.4% was obtained with a DSSC based on 2-cyano-3-[5'-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-[2,2']bithiophenyl-5-yl]acrylic acid (NKX-2677) under simulated AM 1.5G irradiation (100 mW cm(-2)) with a mask: short-circuit current density (J(sc)) = 13.5 mA cm(-2); open-circuit voltage (V(oc)) = 0.71 V; fill factor (FF) = 0.77. Transient absorption spectroscopy measurements indicated that electron injection from NKX-2677 to the conduction band of TiO(2) is very rapid (<100 fs), which is much faster than the emission lifetime of the dye (1.0 ns), giving a highly efficient electron injection yield of near unity.

To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystalline TiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron-hole recombination. Accordingly, the rates and yields of photocatalytic oxidation of several alcohols adsorbed on TiO2 were evaluated successfully.

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.

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO2 solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (Jsc) = 12.9 mA cm–2, open‐circuit voltage (Voc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO2 occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO2 surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.

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