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

Abstract

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 J_sc 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.