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1.2 physical factors relevant for photoconversion at hybrid interfaces 9donor and the same for the HOMOs, in the type-II (staggered)configuration described above. However the LUMOlevel of the acceptor should not be too low because, forexample, the open circuit voltage of a photovoltaic cell isproportional to the energy difference between the LUMOlevel of the acceptor and the HOMO level of the donor [41].The tuning of these level is a key issue and the determinationof the HOMO and LUMO position is very important.Typically, a compromise between V OC and charge injectionmust be reached, since η ∼ V OC I SC .The possibility of a large interface area and an effectivecontact, critically depends on the adhesion between the organicand inorganic components at the interface. Adhesionis the result of several interatomic force actions includingcovalent, electrostatic, and dispersive ones, the relevanceof each contribution depending both on the chemistry andon the atomic-scale structural properties [42]. In the case ofhybrid polymer/metal oxide systems, strong electrostaticinteractions occur between the ions of the surface and thepartially charged atoms in the polymers due to the ionicityof the metal oxide. However, in general, the polymer doesnot form covalent bonds with the inorganic material. In addition,when the surface is nanostructured, the adhesion ofthe polymer is affected furthermore by the local morphologyand a dependence on the surface curvature is possible[42].As for the optical absorption of the hybrid systems, it istotally due to the optically active polymer, being the metaloxide wide band gap materials (3.4 eV in the case of ZnO[43]) optically transparent.There is a strong dependence of the polymer absorptionon the substrate where the polymer is deposited. For example,Lloyd et al. [2] found a different behavior for the P3HTon glass, on ZnO or on hexadecanethiol (C 16 SH) modifiedZnO. When deposited on glass, P3HT displays two intrachainππ ∗ absorption peaks and a low energy shoulderassociated with interchain interactions that are typical ofhighly crystalline polymers. Conversely, P3HT depositedon ZnO loses its crystalline organization showing a blueshift in the peak of the UV-Vis absorption spectrum andno long wavelength absorption shoulder. The blue shift ofP3HT can be reduced and the low-energy shoulder can be