Dye-sensitized solar cells (DSSCs) have gained great attention as lower-cost alternatives to conventional photovoltaic devices. One way to improve the excellent efficiencies (ca. 11%) exhibited by DSSCs based on ruthenium polypyridyl dyes would consist in using sensitizers with enhanced light-harvesting properties in the red region of the spectrum. Phthalocyanines (Pcs) are very robust molecules which present extraordinary high extinction coefficients in the 600- to 700-nm spectral region. Intensive research has been focused on reducing the undesired aggregation phenomena of Pcs on the metal oxide surface, while keeping a good electronic coupling between the LUMO of the Pcs and the TiO2 conduction band, and a good solubility of the dye in organic solvents. Recently, unsymmetrically substituted “push–pull” Pcs have emerged as efficient red-absorbing dyes, reaching power conversion efficiencies of up to 4.7%, when used as single sensitizers.
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