Porphyrin dyes for photovoltaic sensitization: Effects of dye design on porphyrin assembly structure, energy transfer, and solar cell performance

Solar light is the world's single most abundant source of energy and its effective utilization is therefore the most promising means for decreasing global reliance on fossil fuels. The most efficient solar cell technologies to date remain economically uncompetitive, however, due to their high cost. Dye-sensitized solar cells (DSSCs) are among an emerging class of photovoltaics that employ organic molecules as light-harvesters. Though these cells currently demonstrate only moderate efficiencies (∼11%), their inexpensive and easily fabricated components make them a promising solar cell technology.;This research focuses on the enhancement of DSSC efficiency through incorporation of strongly absorbing porphyrin dyes. Specifically, the structure versus function relationship of porphyrins in terms of energy transfer and solar cell sensitization is examined. Novel self-assemblies of conjugated porphyrin monomers are characterized by solution X-ray scattering techniques and energy transfer is examined by transient absorption spectroscopy. One such porphyrin tetramer is found to exhibit a significantly faster rate of energy transfer relative to a simplified, less conjugated tetramer. The effect of porphyrin dye acidity on the sensitization of zinc oxide-nanotube electrodes is studied and a maximum dye pKa is established for this photoanode material. Finally, an original series of four porphyrin dyes is created featuring both zinc(II) and palladium(II) central metals, and meso- and beta-functionalized acid moieties. The performance of these compounds as sensitizers on titanium dioxide and tin dioxide nanoparticle electrodes is investigated. The zinc-metallated porphyrin substituted at the meso-position was found to be the most effective dye on titanium dioxide, whereas the meso-substituted palladium-porphyrin/tin dioxide solar cell was the most efficient. The differences in injection efficiency were analyzed by transient absorption spectroscopy, but data collected thus far do not match observed trends in solar cell performance. Possible reasons for this discrepancy are considered herein.