Sequential "click" functionalization of mesoporous titania for energy-relay dye enhanced dye-sensitized solar cells.

Energy relay dyes (ERDs) have been investigated previously as a mean to achieve panchromatic spectral response in dye-sensitized solar cells via energy transfer. To reduced the distance between the ERDs and energy-accepting injection dyes (IDs) on the surface of a mesoporous titanium dioxide electrode, the ERDs were immobilized adjacent to the IDs via a sequential functionalization approach. In the first step, azidobenzoic acid molecules were co-adsorbed on the mesoporous titanium dioxide surface with the ID. In the second step, the highly selective copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition "click" reaction was employed to couple an alkyne-functionalized ERD to the azidobenzoic acid monolayer. The cycloaddition step in the mesoporous electrode was slowed dramatically due to reactants and catalysts forming agglomerates. In solar cell devices, the close proximity between the surface-immobilized ERD and energy-accepting squaraine sensitizer dyes results in energy transfer efficiencies of up to 91%. The relative improvement in device performance due to the additional ERD spectral response was 124%, which is among the highest reported. The sequential functionalization approach described herein is transferrable to other applications requiring the functionalization of electrodes with complex molecules.