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 TiO2 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 TiO2 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.