Sensitization of Nanocrystalline TiO2 with Multibranched Organic Dyes and Co(III)/(II) Mediators: Strategies to Improve Charge Collection Efficiency

The photoelectrochemical properties of thiophene-triphenylamine (TTPA) sensitizers ( TTPA1–3 ) used in association with different cobalt based mediators, including kinetically fast shuttles such as [Co(bpy) 3 ] 3+/2+ and [Co(phen) 3 ] 3+/2+ and sterically hindered couples like [Co(dtb) 3 ] 3+/2+ were studied by combining molecular modeling, spectroscopic, and photoelectrochemical techniques. The results indicate that the presence of multiple anchoring points on the dye structure improves its adsorption stability onto TiO 2 but induces a pseudoplanar adsorption geometry which is not suited to insulate properly the surface of the semiconductor and of the back contact, against electron recombination; particularly when kinetically fast shuttles are employed. This drawback could be successfully countered by exploiting both a compact blocking underlayer and the chemisorption of octyl triethoxysilane (Si–C8) acting as a molecular barrier onto the TiO 2 surface, thus limiting the access of the oxidized mediator to the exposed titania sites. These treatments improve the electron collection efficiency up to 80–90% with the I – /I 3 – couple and up to 70% when the [Co(dtb) 3 ] 3+/2+ mediator was used.