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

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 (V OC) 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 V OC to 460 and 650 mV. Both LG13 and LG14 sensitizers exhibit broader and redshift incident photon conversion efficiency spectra and resulted in highest J SC 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.