Control of the distance between porphyrin sensitizers and the TiO<inf>2</inf> surface in solar cells by designed anchoring groups

© 2019 Elsevier B.V. Unsymmetrical porphyrins were rationally-designed and synthesized to investigate the relation between their structure, properties and adsorption geometries, and their relative performance as dyes in dye-sensitized solar cells. Photophysics, electrochemical and TiO2 anchoring properties of the new unsymmetrical N-glycolic acid amino phenyl porphyrins were evaluated. Most dyes showed good energy matching between excited state energies and the TiO2 conduction band. Depending on the porphyrins, anchoring to TiO2 occurred with only one carboxyl anchor group or with two N-glycolic acid amino phenyl connected to opposite and adjacent phenyl groups. It was found that cell efficiencies normalized for surface coverage are strongly affected by the adsorption geometry and spacer linker flexibility. The effective distance between the porphyrin core and the TiO2 surface has key importance in cell efficiencies. The data is consistent with a through-space electron transfer and anchoring via N-glycolic acid substituents located in adjacent phenyl groups results in higher surface coverage normalized cell efficiencies.