Bulky Nature Phenanthroimidazole-Based Porphyrin Sensitizers for Dye-Sensitized Solar Cell Applications

Abstract

Porphyrin-based sensitizers have attracted significant attention due to their excellent performance in dye-sensitized solar cells (DSSCs). Herein, a novel electron donor−π-bridge–electron acceptor (D−π–A) based porphyrin sensitizer having strong electron donating methyl phenanthroimidazole ring and ethynylcarboxyphenyl group at meso- position of porphyrin framework (LG11) was designed and applied as sensitizer in DSSC. To reduce the undesirable loss of open-circuit voltage (VOC) caused by dye aggregation and charge recombination effect, phenyl (LG12) or hexyl phenyl chains (LG13 and LG14) were attached to the phenanthroimidazole moiety. The introduction of a simple thiophene unit between the porphyrin and carboxylic acid anchoring group (LG14) has enabled further extension of the absorption to a longer wavelength. Compared to LG11 and LG12, hexyl phenyl-substituted LG13 and LG14 effectively reduced the dye aggregation and allowed enhancement of VOC to 460 and 650 mV. Both LG13 and LG14 sensitizers exhibit broader and redshift incident photon conversion efficiency spectra and resulted in highest JSC of 14.2 and 15.5 mA cm–2, respectively. Because of all aforesaid properties, LG13 and LG14 dyes show photovoltaic conversion efficiencies of 6.25% and 7.45%, respectively. Further, a cosensitization of LG14 with an organic molecule (HC5) to increase the absorption valley in the visible region has seen efficiency enhanced to 8.27%. This work provides a potential approach to molecular design of porphyrin sensitizers and selection of cosensitizers to build better efficient and stable DSSC system. We have adopted intensity-modulated photo voltage spectroscopy and nanosecond laser flash photolysis spectroscopy to explain the efficiency and structure relationship of these LG11-LG14 sensitizers.