Synthesis and characterization of a conjugated porphyrin dyad entangled with carboxyl functionalized benzimidazolium: an efficient metal free sensitizer for DSSCs

Abstract

As an archetypal functional molecule, porphyrin is the nature's best photosynthetic pigment in plants, which mimics the principles of solar energy conversion. Considering the prodigious ability of porphyrin to harvest light, we present the stepwise synthesis of a novel conjugated meso-substituted porphyrin dyad entangled with carboxyl functionalized benzimidazolium moieties (CPDCFBM) as a sensitizer for application in a dye sensitised solar cell (DSSC). 1,4-Di(4-amidostyryl)benzene functionalised with a benzimidazole moiety served as a spacer between two substituted porphyrin moieties. The dyad was characterized using analytical techniques such as Fourier-transform infrared spectroscopy, ultraviolet-visible absorption spectroscopy, nuclear magnetic resonance spectroscopy, thermogravimetric analysis, BET surface area analysis, cyclic voltammetry (CV) studies and electrochemical impedance spectroscopy (EIS). The band gap measured using UV-vis spectroscopy was found to be 1.42 eV. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels calculated from CV studies revealed that the frontier orbital energy levels make the system suitable for use as a sensitizer in conjugation with TiO₂ in DSSCs. A DSSC fabricated using dyad sensitized TiO₂ as an iodide tri-iodide redox couple exhibited a power conversion efficiency of 6.9%, under illumination (AM 1.5, 100 mW cm⁻²) with a sensitization time of 4 h without adding any efficiency booster and anti-aggregating agents. The strong tethering of the dyad to the TiO₂ surface was achieved via linking through six carboxylate groups to form an envelope around TiO₂ to ensure excellent electronic communication and good stability against the aggregation of the dyad. An EIS study demonstrated that the dyad shows faster electron injection than recombination.