Efficient Dye-Sensitized Solar Cell Based onoxo-Bacteriochlorin Sensitizers with Broadband Absorption Capability

Abstract

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 energy-to-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.