Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO 2 films have attracted considerable attention because of their great potential in terms of low fabrication costs and high solar-light-to-electricity conversion efficiency. [1] Extensive efforts have been focused on the development of new, highly efficient sensitizers, as they play a critical role in cell performance. Sensitizers exhibiting absorption over a wide range of the solar spectrum and a high molecular extinction coefficient have been investigated for improving the conversion efficiency of DSSCs. Among the most successful of the various sensitizers are complex N3, [Ru(dcbpy) 2 (NCS) 2 ] (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), [2] and complexes of the type [Ru(dcbpy)(L1)(NCS) 2 ], where L1 is a 2,2'-bipyridine with a highly conjugated ancillary group. [3] However, these sensitizers show insufficient light-harvesting efficiencies in the near-IR region. As the solar spectrum has a large photon flux in the near-IR region above 800 nm, the synthesis of efficient near-IR sensitizers is currently one of the most important issues in the development of solar cells.The absorption properties of Ru II complexes can be tuned by careful consideration of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. [4] The absorption band can be extended into a longer wavelength region by either destabilizing the metal t 2g orbital using a strong s-donating ligand or by introducing a ligand with a low-lying p*-level molecular orbital. Complex N749, (TBA) 3 [Ru(tctpy)(NCS) 3 ] (tctpy = 4,4',4''-tricarboxy-2,2':6',2''-terpyridine; TBA = tetra-n-butylammonium), [5] and complexes of type PRT-11-14, (TBA)[Ru(L2)(NCS) 3 ], where L2 is a 4,4'-dicarboxy-2,2':6',2''-terpyridine derivative with a highly conjugated ancillary group, [6] have been reported to exhibit panchromatic sensitization up to 900 nm. Although the introduction of the tctpy ligand improves near-IR sensitization, the main drawbacks of N749 are the inferior incident-photon-to-current conversion efficiency (IPCE) in the shorter wavelength region, and the presence of three NCS ligands. The former problem arises predominantly from the lack of an effective chromophore, whereas the latter is caused by two factors: 1) The linkage isomers of the NCS ligand cause a decrease in the synthetic yield. [5,7] 2) The stability of the complex decreases owing to dye decomposition by weak Ru-NCS bonding. Although NCS-free Ru II complexes with a conversion efficiency of up to 10 % have been reported, [8] these dyes also show relatively low light-harvesting properties over 800 nm.We [9] and others [10] have examined terpyridyl Ru II complexes of the type [Ru(tctpy)(L3)(NCS)] z , where L3 is a bidentate ligand and z = 0 or + 1, in an attempt to optimize near-IR sensitizers. The role of the NCS ligand is to regenerate the sensitizers from the iodine redox. [10a, 11] Among these complexes, cyclometalated Ru II complexes show superior light-harvesting properties in the...

The atomic and electronic structures of two phases of titanium dioxide, anatase and rutile, have been investigated by a first-principles pseudopotential method based on local density approximation in density functional theory. The calculated band structure, equilibrium lattice constants, and bulk modulus of rutile are consistent with experimental data and with other calculations. The calculated structure of anatase is also close to experimental data. The calculated bulk modulus of anatase is found to be smaller than that of rutile, presumably due to the sparsity of anatase. The band structure of anatase is given in comparison with that found in previous works. The energetics between the two phases is also discussed.

Five chlorophyll-a derivatives, chlorins-1-5 possessing C3(2)-carboxy and O17(4)-esterified hydrocarbon groups including methyl, hexyl, dodecyl, 2-butyloctyl, and cholesteryl were synthesized. Their performance as sensitizers in dye-sensitized solar cells (DSSCs) was compared. These sensitizers have similar surface coverage on the unit surface of TiO(2) film and their absorption spectra on transparent TiO(2) films were identical. On the basis of DFT and TD-DFT calculations of these sensitizers in ethanol, a major difference between them was the geometry of the hydrocarbon ester group, to affect their electron injection and charge recombination with the TiO(2) electrode rather than the energy level of their molecular orbitals. DSSC based on chlorin-3 with a dodecyl ester group gave a solar energy-to-electricity conversion efficiency of 8%, which was the highest among all the chlorophyllous sensitizers. The large photocurrent in the chlorin-3 sensitized solar cell can be explained by the least impedance in the electrolyte-dye-TiO(2) interface in electrical impedance spectroscopy measurements. Subpicosecond time-resolved absorption spectroscopic studies have also been carried out to evaluate the electron injection and charge recombination dynamics in the dye-TiO(2) interface. For the electron injection and charge recombination processes, a charge separated state of the dye-TiO(2) complex has been found to be free from the type and concentration of dye sensitizer, reflecting the same type of electron transfer process for all the five chlorin sensitizers. A new quenching pathway of the dye excitation, which is probably from the exciton annihilation, in addition of the charge recombination has been observed for chlorin-1 and chlorin-5, but not for chlorin-3. The higher open-circuit photocurrent observed in the present dyes with larger ester groups can be attributed to the reduced leaking of charges in the TiO(2)-electrolyte interface, which was supported by the longer electron lifetimes.

Two dye sensitizers, methyl trans-3 2 -carboxy-8-deethyl-7-ethyl-8-oxo-pyropheophorbide-a (BChlorin-1) and methyl trans-3 2 -carboxy-7-demethyl-8-methyl-7-oxo-pyropheophorbide-a (BChlorin-2), with stable bacteriochlorin skeletons were synthesized and applied to dye-sensitized solar cells. Both sensitizers absorb the light all over the visible region owing to partial saturation of the two pyrrole rings on the Q x transition dipole. When they were deposited on a TiO 2 film, the J-aggregates of the sensitizers are partially formed to give broad and red-shifted Q y bands. The surface coverage of TiO 2 film by BChlorin-2 is much larger than by BChlorin-1, suggesting the former sensitizer forms more serious aggregation on the surface of TiO 2 , and this could cause more exciton annihilation to reduce the photocurrent of solar cell. The frontier molecular orbitals of both sensitizers obtained from the DFT calculations show no distinguishable difference. Extended calculations on the dye-TiO 2 Na model system suggest that additional LUMO + 2 orbital in BChlorin-1 may also contribute to the difference in photocurrent. The larger photovoltage in BChlorin-1 sensitized solar cell was attributed to a less efficient charge recombination in the dye-TiO 2 interface to give a longer electron lifetime (τ). Additional 4-tert-butylpyridine in the electrolyte significantly reduced the photocurrent and the solar energyto-electricity conversion efficiency (η) of the solar cells, especially when BChlorin-2 was employed as a sensitizer. This dramatic decrease was attributed to the shift of conduction band edge (CBE) of TiO 2 to a negative potential above the molecular Fermi level (MFL) of the sensitizers and suppressed the electron injection from the MFL of sensitizer to CBE of TiO 2 . Coadsorption of BChlorin-1 with chenodeoxycholic acid (CDCA) could break the dye aggregate and improve the incident photon-to-current conversion efficiency at the absorption bands maxima. BChlorin-1 sensitized solar cells coadsorbed with CDCA gave a longer electron lifetime and a larger diffusion coefficient than the cell without CDCA. By coadsorbing with 5 mM CDCA in solution, the BChlorin-1 sensitized solar cell gave a highest performance with short-circuit photocurrent ) 18.4 mA cm -2 , open-circuit photovoltage ) 0.54 V, fill factor ) 0.66, and η ) 6.6% under the air mass AM 1.5 (100 mW cm -2 ) illumination.