Pyridinium p-DSSC dyes: An old acceptor learns new tricks

A family of six (five new) thiophenyl bridged triarylamine-donor based dyes with pyridine anchoring groups have been synthesized and studied as sensitizers for the p-type dye-sensitized solar cell (p-DSSC). They comprise bis-dicyano acceptor systems with a single pyridyl binder incorporated directly into the triarylamine (1), or separated by a phenyl group (2); a mono-dicyano with two phenyl pyridine binders (3); and respective homologues 4 to 6 with pyridinium acceptors. In all cases, compared to their dicyano counterparts, the pyridinium based dyes have higher extinction coefficients and smaller HOMO-LUMO gaps that give broader spectrum absorption. Thus, despite lower dye uptake, devices based on pyridiniums 4 and 6 have identical power conversion efficiencies (η) to the equivalent dicyano systems 1 and 3. However, the best performing device (η = 0.06%) is based on the known bis-acceptor dicyano system 2, as the large size and double positive charge of 5 leads to a substantial disadvantage in loading on NiO. Absorbed-photon-to-current efficiencies for 5 are competitive with or higher than those of 2, implying a better per dye performance consistent with the absorption profile, and DFT calculations suggesting better charge separation. Thus, pyridiniums may provide a new, and easily accessible high performance acceptor for p-DSSC dyes, but are likely better paired with anionic binding groups such as carboxylates.