Modified Phthalocyanines for Efficient Near-IR Sensitization of Nanostructured TiO2 Electrode

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Abstract
A zinc phthalocyanine with tyrosine substituents (ZnPcTyr), modified for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO2 solar cells, and its reference, glycine-substituted zinc phthalocyanine (ZnPcGly), were synthesized and characterized. The compounds were studied spectroscopically, electrochemically, and photoelectrochemically. Incorporating tyrosine groups into phthalocyanine makes the dye ethanol-soluble and reduces surface aggregation as a result of steric effects. The performance of a solar cell based on ZnPcTyr is much better than that based on ZnPcGly. Addition of 3α,7α-dihydroxy-5β-cholic acid (cheno) and 4-tert-butylpyridine (TBP) to the dye solution when preparing a dye-sensitized TiO2 electrode diminishes significantly the surface aggregation and, therefore, improves the performance of solar cells based on these phthalocyanines. The highest monochromatic incident photo-to-current conversion efficiency (IPCE) of ∼24% at 690 nm and an overall conversion efficiency (η) of 0.54% were achieved for a cell based on a ZnPcTyr-sensitized TiO2 electrode. Addition of TBP in the electrolyte decreases the IPCE and η considerably, although it increases the open-circuit photovoltage. Time-resolved transient absorption measurements of interfacial electron-transfer kinetics in a ZnPcTyr-sensitized nanostructured TiO2 thin film show that electron injection from the excited state of the dye into the conduction band of TiO2 is completed in ∼500 fs and that more than half of the injected electrons recombines with the oxidized dye molecules in ∼300 ps. In addition to surface aggregation, the very fast electron recombination is most likely responsible for the low performance of the solar cell based on ZnPcTyr.