Zinc octacarboxylic phthalocyanine/lutein dyads co-adsorbed nanocrystalline TiO₂ electrode: enhancement in photovoltaic performance of dye-sensitized solar cells

A novel zinc phthalocyanine containing eight carboxyl groups was synthesized and utilized as a co-adsorbent with lutein for dye-sensitized solar cells enhance the photoelectric conversion efficiency. The effects of various phthalocyanine/lutein mole ratios on the performances of the fabricated solar cells were investigated. The results showed that zinc octacarboxylic phthalocyanine aggregation decreased with the increasing lutein/phthalocyanine mole ratios and zinc octacarboxylic phthalocyanine fluorescence was strongly quenched due to energy transfer from the phthalocyanine to the excited lutein. The photoelectric conversion efficiency reached its maximum when zinc octacarboxylic phthalocyanine/lutein mole ratio was 4:1. Moreover, the charge-transfer resistances and electron lifetimes at the TiO₂/dye/electrolyte interface also showed great dependency on the phthalocyanine/lutein mole ratios by electrochemical impedance spectroscopy. The density-functional theory calculation of zinc phthalocyanine suggests that the electronic cloud density distribution move from the phthalocyanine ring framework toward the anchoring carboxylic group and further to the conduction band of TiO₂, which results in efficient electron transfer.