Donor-π-acceptor, triazine-linked porphyrin dyads as sensitizers for dye-sensitized solar cells

Two porphyrin dyads with the donor-pi-acceptor molecular architecture, namely (ZnP)-[triazine-gly]-(H2PCOOH) and (ZnP)-[triazine-Npip]-(H2PCOOH), which consist of a zinc-metalated porphyrin unit and a free-base porphyrin unit covalently linked at their peripheries to a central triazine group, substituted either by a glycine in the former or a N-piperidine group in the latter, have been synthesized via consecutive amination substitution reactions of cyanuric chloride. The UV-vis absorption spectra and cyclic-voltammetry measurements of the two dyads, as well as theoretical calculations based on Density Functional Theory, suggest that they have suitable frontier orbital energy levels for use as sensitizers in dye-sensitized solar cells. Dye-sensitized solar cells based on (ZnP)-[triazine-gly]-(H2PCOOH) and (ZnP)-[triazine-Npip]-(H2PCOOH) have been fabricated, and they were found to exhibit power conversion efficiency values of 5.44 and 4.15%, respectively. Photovoltaic measurements (J-V curves) and incident photon to current conversion efficiency spectra of the two solar cells suggest that the higher power conversion efficiency value of the former solar cell is a result of its enhanced short circuit current, open circuit voltage, and fill factor values, as well as higher dye loading. This is ascribed to the existence of two carboxylic acid anchoring groups in (ZnP)-[triazine-gly]-(H2PCOOH), compared to one carboxylic acid group in (ZnP)-[triazine-Npip]( H2PCOOH), which leads to a more effective binding onto the TiO2 photoanode. Electrochemical impedance spectra show evidence that the (ZnP)-[triazine-gly]-(H2PCOOH) based solar cell exhibits a longer electron lifetime and more effective suppression of charge recombination reactions between the injected electrons and electrolyte.