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J. Phys. Chem. C 2009, 113, 7954–7961
Efficient Dye-Sensitized Solar Cell Based on oxo-Bacteriochlorin Sensitizers with Broadband Absorption Capability Xiao-Feng Wang,†,| Osamu Kitao,† Haoshen Zhou,*,† Hitoshi Tamiaki,*,‡ and Shin-ichi Sasaki‡,§ Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan, Department of Bioscience and Biotechnology, Ritsumeikan UniVersity, Kusatsu, Shiga 525-8577, Japan, and Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: March 23, 2009
Two dye sensitizers, methyl trans-32-carboxy-8-deethyl-7-ethyl-8-oxo-pyropheophorbide-a (BChlorin-1) and methyl trans-32-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 Qx transition dipole. When they were deposited on a TiO2 film, the J-aggregates of the sensitizers are partially formed to give broad and red-shifted Qy bands. The surface coverage of TiO2 film by BChlorin-2 is much larger than by BChlorin-1, suggesting the former sensitizer forms more serious aggregation on the surface of TiO2, 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-TiO2Na 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-TiO2 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 TiO2 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 TiO2. 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. Introduction Since the first high efficient dye-sensitized solar cell (DSSCs) with the solar energy-to-electricity conversion efficiency (η) of 7.1% published in 1991 by Prof. Gra¨tzel,1 further development of this particular device toward more favorable characters becomes a solid target for all the scientists in this field.2-4 In recent years, a large quantity of the progress in DSSCs research can be attributed to the success in finding new efficient dye sensitizers. For ruthenium polypyridyl-based sensitizers, the highest η value of 11.1% was obtained based on black-dye,5 and for metal-free organic sensitizers, indoline dyes give rise to the η values of more than 9%.6,7 In contrast to traditional silicon solar cells that can efficiently use the photons of sunlight up to 1200 nm for electricity generation, most of the highly efficient dye sensitizers in DSSCs lack the light-harvesting * To whom correspondence should be addressed. E-mail: hs.zhou@ aist.go.jp (H.Z.);
[email protected] (H.T.). † National Institute of Advanced Industrial Science and Technology. ‡ Ritsumeikan University. § Nagahama Institute of Bio-Science and Technology. | Present address: Environmental and Renewable Energy Systems Division Graduate School of Engineering, Gifu University Yanagido 1-1, Gifu 501-1193, Japan.
ability in the near-infrared region. For example, DSSCs based on one of the most efficient organic dye, D-149, fully used the solar energy in 400-700 nm if taking the light reflection by the conductive glass into account.7 The photocurrent of DSSCs based on this kind of narrow wavelength absorption sensitizers has no more potential to be further improved. Extension of the absorption region of the dye sensitizers to efficiently use light energy of more than 700 nm is absolutely necessary for the future development of DSSCs. To expand the absorption region of DSSCs, many efforts were paid to develop new porphyrintype sensitizers, and the highest η value of the best cell based on porphyrin-type sensitizers was approaching 7.1%.8 Light-harvesting complexes (LHC) of the photosynthetic purple bacteria are constructed mainly by bacteriochlorophyll (BChl) molecules having a bacteriochlorin macrocycle (7,8,17, 18-tetrahydroporphyrin).9,10 By partial saturation of the two pyrrole rings along the Qx transition dipole, solar energy in the near-infrared region can be efficiently captured by BChls depending on the size of different forms of their circular aggregates.11 In vivo, the Qy-band of BChls can be red-shifted even up to 1020 nm, which is far greater than Chls possessing a chlorin macrocycle (17,18-dihydroporphyrin) with Qy bands
10.1021/jp900328u CCC: $40.75 2009 American Chemical Society Published on Web 04/10/2009
Efficient Dye-Sensitized Solar Cell at
