Enhancement of photovoltaic performance in dye-sensitized solar cells fabricated with dendritic photosensitizer containing site-isolated chromophores

We designed and synthesized a series of phenothiazine-based donor-acceptor type molecules which consist of different numbers of chromophores in a molecule for use as photosensitizers for dye-sensitized solar cells (DSSCs). Intriguingly, a dendritic molecule containing three chromophores not only gave a new possibility to modify the three dimensional structure, but also reduced aggregation between chromophores inducing dipole-dipole interaction. The DSSCs made of a dendritic photosensitizer system exhibited much higher cell efficiencies than those with the single- or double-chromophoric photosensitizers due to efficient electron extraction pathways in the dendritic molecule which lead to a significantly reduce recombination rate of electrons from the TiO ₂ to the electrolyte when the same numbers of chromophores were loaded on the TiO₂ surface. In particular, the DSSC based on the dendritic molecule exhibited improved open-circuit voltage than that of the single- or double-chromophoric photosensitizers. This can be attributed to strong adsorption properties of the TiO₂ electrode and a screening effect to the electrolyte ions provided by the network structure of the dendritic photosensitizer. The different behavior of these DSSCs was explained by comparing the results of electrochemical impedance spectroscopy and measurement of open-circuit voltage decay.