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

Abstract

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.

Improved performance of porphyrin-based dye sensitised solar cells by phosphinic acid surface treatment

“…Thus, in order to have a significant effect, a decrease in J sc may be inevitable. 24,25 For the case of BMPPA, the increase of J sc is partly due to the positive shift of the quasi-Fermi level and, thus, the effect is expected for all dyes having relatively low LUMO levels. The drawback of the positive shift is the decreased V oc arising from a decreased difference between the conduction band edge and I À /I 3 À redox potential.…”

Recent progress in interface modification for dye-sensitized solar cells

“…Tamiaki et al reported that compared to the blank sample, the CDCA coadsorbent enhanced the conversion efficiency from 6.2% to 6.6% at AM1.5, 100mW/cm 2 [73]. The same effect could be found when the PVA was used as a coadsorbent.…”

“…The commonly used efficient coadsorbents are chenodeoxycholic acid (CDCA) [73] and pivalic acid (PVA) [74]. Tamiaki et al reported that compared to the blank sample, the CDCA coadsorbent enhanced the conversion efficiency from 6.2% to 6.6% at AM1.5, 100mW/cm 2 [73].…”

“…The conversion efficiency increased from 5.77% to 6.25%. It was shown that the efficiency enhancement was mainly caused by the increasing of V oc , indicating the recombination was retarded by the coadsorbent [73].…”

Chlorin e6 sensitized photovoltaic cells: effect of co-adsorbents on cell performance, charge transfer resistance, and charge recombination dynamics

“…However, compounds derived from naturally occurring chlorophylls are appealing as they are abundant, inexpensive in nature, and are feasible for molecular structure modification methods. [14][15][16][17] A large number of studies have focused on chlorin e6 due to its ease of synthesis from the precursor molecule, chlorophyll a. [18][19][20][21][22] In the construction of solar cells, co-adsorbents were used to minimize the strong aggregation effects caused by chlorin e6.…”