Joint Photophysical and Electrical Analyses on the Influence of Conjugation Order in D-π-A Photosensitizers of Mesoscopic Titania Solar Cells

Diverse thiophene-containing blocks have been employed as the pi-conjugated spacers of organic D-pi-A dyes. In the case that multiple segments with distinguishable electronic features are applied, their conjugation sequence could potently affect optoelectric behaviors of photosensitizers in mesoscopic titania solar cells. In this work, we address this issue by designing three organic dyes (C225, C226, and C227), wherein the dihexyl-substituted cyclopentadithiophene moiety is stepwise shifted from the electron acceptor side to the donor one, along with the additional use of two 3-hexylthiophene rings as the conjugated spacing unit. With respect to C225 and C226, C227 presents a relatively inefficient photoinduced electron injection as indicated by photoluminescence measurements, which accounts for its lower efficiencies of converting incident monochromatic photons to, collected electrons. Transient absorption measurements suggest that the charge redonibination between oxidized dye molecules and titania electrons gradually decelerates from C225 to C227, while the interception of oxidized dye molecules by iodide ions exhibits an apparent driving force dependent, Marcus normal region behavior.