Dye-sensitized solar cells based on triazine-linked porphyrin dyads containing one or two carboxylic acid anchoring groups

Abstract

Two porphyrin dyads (5a and 5b) consisting of two meso aryl-substituted zinc-metallated porphyrin units, which are covalently linked through peripheral aryl-amino groups by a 1,3,5-triazine moiety, have been synthesized via stepwise substitution reactions of cyanuric chloride. Both porphyrin dyads 5a and 5b contain a carboxyphenyl meso substituent, while in the former the triazine ring is functionalized by a glycine group, and in the latter by a piperidine group. Photophysical and electrochemical studies of the two dyads, in combination with DFT theoretical calculations, suggest that there is negligible electronic interaction between the porphyrin units in the dyads’ ground states, but the frontier orbital energy levels are suitable for use as sensitizers in dye-sensitized solar cells (DSSCs). Solar cells sensitized by dyads 5a and 5b have been fabricated, and they were found to exhibit power conversion efficiencies (PCEs) of 5.28 and 3.50%, respectively, under illumination (AM1.5, 100 mW cm⁻²), and TiO₂ films of 10 μm thickness. As revealed by the photovoltaic measurements (J–V curves) and the incident photon to current conversion efficiency (IPCE) spectra of the two solar cells, the higher PCE value of the 5a based solar cell is attributed to its enhanced short circuit current (J_sc), higher open circuit voltage (V_oc), and fill factor (FF) values. Also, the 5a sensitized solar cell exhibits a larger dye loading value. This is attributed to the presence of two carboxylic acid anchoring groups in its molecular structure (compared to one carboxylic acid and one hindered piperidine-type anchoring group of 5b), which result in a more effective binding capacity onto the TiO₂ film. Furthermore, electrochemical impedance spectra (EIS) demonstrated that the 5a based solar cell exhibits longer electron lifetime (τ_e) and more effective suppression of the recombination reactions of the injected electrons and the electrolyte.