Novel π-Conjugated Porphyrin Sensitizers for Dye-Sensitized Solar Cells

Inspired by the efficient energy transfer in naturally occurring photosynthetic reaction centers, numerous porphyrins have been designed and synthesized for dye-sensitized solar cell (DSSC) applications. The intrinsic advantages of porphyrin-based dyes are their rigid molecular structures with large absorption coefficients in the visible region and their many reaction sites, i.e., four meso and eight β positions, available for functionalization. However, porphyrins are known to suffer from dye aggregation due to their planar structural nature. In addition to the molecular design for potential porphyrin sensitizers, we have shown that co-sensitization of a zinc porphyrin with a well designed organic dye is a good strategy to change the aggregation morphology of the porphyrin adsorbed on TiO₂ film to further improve its device performance. In this lecture, I will present our recent results based on a series of novel porphyrin sensitizers together with the approach of co-sensitization of porphyrins with organic dyes and/or porphyrin dimers. Extension of π-conjugation can be achieved by attaching various cyclic aromatic hydrocarbon substituents, such as anthracene, tetracene, pyrene, fluorene, and amino-substituted electron-donating groups opposite to the anchoring group of the target porphyrin. We will demonstrate that devices made of such porphyrins have reached the power conversion efficiency above 10 % using iodine-based electrolyte under one-sun irradiation. For the co-sensitization approach, combination of a porphyrin dye co-sensitized with an organic dye and a porphyrin dimer on a TiO₂ film in a device according to a either a stepwise or a cocktail approach resulted in significant enhancement on both J_SC and V_OC relative to their individual single-dye sensitized devices. For characterizations, two different approaches based on time-resolved and frequency-domain techniques will be introduced. For the time-resolved approach, transient photocurrent and photovoltage decays were obtained based on several CW bias light irradiations of varied intensities at short-circuit and open-circuit conditions, respectively. A small-intensity LED pulse (~50 ms duration) at 630 nm was used as a perturbation probe to monitor the electron-transport kinetics at short-circuit condition or the charge-recombination kinetics at open-circuit condition. For the frequency-domain approach, electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) measurements were also carried out. Both time-resolved and frequency-domain techniques were applied to various highly efficient DSSC systems based on porphyrin and porphyrin + organic dye systems to rationalize their photovoltaic performances.