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10 introductionrecovered by the introduction of a C 16 SH layer at the interfacebetween the polymer and the ZnO [2].Figure 1.5.: Normalized UV-Vis absorption spectra for thin films (6nm) of P3HT on glass (squares), ZnO (circles), andC 16 SH modified ZnO (triangles) (figure from ref.[2]).In the direction of better controlling the polymer at theinterface, the use of interlayers between the polymer andthe metal oxide has been recently investigated [44, 45, 46,47]. The motivations for using interlayers are the increaseof the polymer/substrate compatibility, the better chargetransport, the reduced charge recombination, the tunabilityof the work function of the substrate obtained by theintroduction of molecular dipoles and the enlargment ofthe light absorbed spectrum.For example, surface modifications of TiO 2 nanorods bypyridine derivatives before mixing with the polymer, canbe used to improve the device performance by enhancingcharge separation, improving compatibility, and stronglysuppressing back recombination [44].It has been observed that the external quantum efficiencyof a P3HT/ZnO solar cell can be tripled by inserting amonolayer of PCBA between the two components [45]. Infact, the presence of PCBA induces an interfacial dipoleand shifts up the LUMO level of P3HT relative to the conductionband edge of ZnO [45] .Molecular dipoles have been found to modify also thetitania surface in TiO 2 /P3HT interfaces [46]. In fact, a seriesof para-substituted benzoic acids with varying dipoles anda series of multiply substituted benzene carboxylic acids