Photoelectrochemical Behavior of Polychelate Porphyrin Chromophores and Titanium Dioxide Nanotube Arrays for Dye-Sensitized Solar Cells

The influence of TiO2 nanotube array (NTA) morphology on dye-sensitized solar cell performance was studied with five porphyrin chromophores containing up to four carboxylic acid anchor groups. The TiO2 NTAs were anodically grown on Ti foil in NH4F electrolyte containing additives such as poly(ethylene glycol) (PEG 400), ethylene glycol, and glycerol. The NTAs grown in the presence of PEG 400 had the largest nanotube diameter (∼120 nm) and also yielded the best photoresponse with all five porphyrins in iodide/polyiodide electrolyte. Interestingly, nanoporous TiO2 “control” films, anodically grown on Ti foil in the absence of the organic additive, yielded an inferior photoresponse under the same conditions. Two of the porphyrins (P3 and P5), envisioned to have a spider-like configuration on the TiO2 NTA surface, yielded the best photoresponse among the five candidates in the Q(1,0) porphyrin spectral region. This result underlines that the molecular structure of the chromophore affects the directionality of electron transfer and its consequent ability to inject electrons (by the photoexcited porphyrin) into the oxide nanotube host. Finally, some perspectives on the use of TiO2 NTAs and porphyrins in DSSCs are given against the backdrop of the above data.