“Scorpion”-shaped mono(carboxy)porphyrin-(BODIPY)2, a novel triazine bridged triad: synthesis, characterization and dye sensitized solar cell (DSSC) applications

A novel BODIPY-porphyrin triad PorCOOH-(BDP)2 (scorpion shaped) was prepared, in which two BODIPY molecules are covalently attached via a 1,3,5-triazine molecule to a free-base carboxyphenyl meso-substituted porphyrin. The chromophore was synthesized via stepwise substitution reactions of cyanuric chloride. Photophysical and electrochemical studies of this triad, in combination with DFT theoretical calculations, suggest that there is negligible electronic interaction between the porphyrin and BODIPY moieties in the triad's ground states, but the frontier orbital energy levels are suitable for use as sensitizers in dye-sensitized solar cells (DSSCs). Solar cells sensitized by triad PorCOOH-(BDP)2 were fabricated, and were found to exhibit a power conversion efficiency (PCE) value of 5.17% under illumination (AM1.5, 100 mW cm2) and with a TiO2 film of 12 μm thickness, with a sensitization time of 3 h without any coadsorbent. The PCE value of DSSC was enhanced to 6.20% when a thin layer of reduced graphene oxide (rGO) was incorporated between TiO2 and PorCOOH-(BDP)2 dye, which is attributed to the formation of an rGO–TiO2 Schottky barrier in the device, leading to efficient charge transport of injected electrons towards the external circuit, resulting in higher Jsc and FF. Electrochemical impedance spectra (EIS) demonstrate that a photoelectrode with rGO–TiO2 exhibits shorter transport time of electrons, longer electron lifetime and lower charge transfer resistance.