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 <i>meso</i>- position of porphyrin framework (<b>LG11</b>) was designed and applied as sensitizer in DSSC. To reduce the undesirable loss of open-circuit voltage (<i>V</i>_OC) caused by dye aggregation and charge recombination effect, phenyl (<b>LG12</b>) or hexyl phenyl chains (<b>LG13</b> and <b>LG14</b>) were attached to the phenanthroimidazole moiety. The introduction of a simple thiophene unit between the porphyrin and carboxylic acid anchoring group (<b>LG14</b>) has enabled further extension of the absorption to a longer wavelength. Compared to <b>LG11</b> and <b>LG12</b>, hexyl phenyl-substituted <b>LG13</b> and <b>LG14</b> effectively reduced the dye aggregation and allowed enhancement of <i>V</i>_OC to 460 and 650 mV. Both <b>LG13</b> and <b>LG14</b> sensitizers exhibit broader and redshift incident photon conversion efficiency spectra and resulted in highest <i>J</i>_SC of 14.2 and 15.5 mA cm^–2, respectively. Because of all aforesaid properties, <b>LG13</b> and <b>LG14</b> dyes show photovoltaic conversion efficiencies of 6.25% and 7.45%, respectively. Further, a cosensitization of <b>LG14</b> with an organic molecule (<b>HC5</b>) 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 <b>LG11</b>-<b>LG14</b> sensitizers