Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO2 electrode

A zinc phthalocyanine with tyrosine substituents (ZnPcTyr), modified for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO2 solar cells, and its ref., glycine-substituted zinc phthalocyanine (ZnPcGly), were synthesized and characterized. The compds. were studied spectroscopically, electrochem., and photoelectrochem. Incorporating tyrosine groups into phthalocyanine makes the dye ethanol-sol. 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. Addn. of 3α,7α-dihydroxy-5β-cholic acid (cheno) and 4-tert-butylpyridine (TBP) to the dye soln. when prepg. 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. Addn. 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 mols. in ∼300 ps. In addn. to surface aggregation, the very fast electron recombination is most likely responsible for the low performance of the solar cell based on ZnPcTyr.