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1.2 physical factors relevant for photoconversion at hybrid interfaces 11can be used to cause a band edge shift in titania, resultingin a change in the open-circuit voltage [46].Recently also small molecules, such as catechol or isonicotinicacid, have attracted attention as interface modifiersin hybrid systems [48, 49, 50]. In particular, catechol isused as an anchoring group for organic and organometallicdyes due its efficient adsorption onto TiO 2 via formationof a strong adsorbate-substrate complex [48, 49, 50] and forthe type II hybrid junction that forms in combination withTiO 2 [48, 51, 52].Again, an ordered molecular layer composed by the 4-mercaptopyridine (4-MP) molecules between a TiO 2 surfaceand a polymer, has produced an overall efficiency ofthe device which overcomes the 1% limit [47]. The presenceof the oriented molecular layer, triggered by selective interactionswith the TiO 2 surface, drives local ordering smoothingthe otherwise abrupt interface. This result shows theimportance of molecular interactions and local morphologyin hybrid interfaces and their implications on chargeseparation and recombination [47].High-efficient solid-state hybrid polymer/metal oxide solarcells have been obtained by depositing Sb 2 S 3 as sensitizerand P3HT as hole conductor and light absorber ona titania surface [53]. This cells exhibit good conversionefficiency and it is highly stable in air, even without encapsulation[53].The most important modification of the hybrid polymer/metaloxide interface is obtained by inserting optically active interlayersthat can contribute to light absorption and injection.This is possible by using dyes and sensitizers toload the metal oxide surface. Most of the information inthis approach comes from the research on DSSCs. Amongthe most widely used sensitizers there are the porphyrins,partly because the their structure synthetically analogue ofchlorophyll [54]. Porphyrins have extensively conjugatedπ systems, are favourable to fast electron transfer to an acceptorand absorb light well in the blue and moderately inthe green regions of the visible spectrum with high molarabsorption coefficients [54]. In particular, one of themost notable efficiency improvements in hybrid devices(3%) has been obtained by using porphyrins as dyes in aP3HT/TiO 2 system [55].