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2 introduction1-3 eV) and the transition between these two levels can beexcited by light in the visible spectrum [13]. These propertiesmake conjugated organics very interesting for photovoltaicapplications.Furthermore organic semiconductors, having very highabsorption coefficients and quite good charge carrier mobility(0.1 cm 2 V −1 s −1 for the P3HT polymer [14]) allowthe use of very thin films but still absorbing a sufficientportion of the solar spectrum [9]. The reduction in materialused, the low cost manufacturing techniques and thepossibility to produce devices using solution phase methods,such as ink jet printing or various roll to roll techniques[15, 16], make organic materials very attractive tothe photovoltaic market [9] . Moreover their properties anddesigns can be finely tuned and optimized based on materialsversatility, solution-based processing, and mechanicalflexibility [17].Bilayer solar cells are composed by two layers of materials;the one with higher electron affinity and ionization potentialhas the role of electron acceptor, while the other materialis the electron donor and acts also as light absorber(see Figure 1.1). An important example of electron acceptormaterial is the buckminsterfullerene (C 60 ) [18], while thesemiconductor polymer most used as donor is the poly(3-hexylthiophene) (P3HT). The device is completed by twoelectrodes, a semi-transparent anode (e.g. the indium-tinoxide,ITO) and a metallic cathod having a low work functionvalue (e.g. aluminum, lithium) [19]. Special contact layershave been developed to obtain better performance, inparticular the PEDOT:PSS polymer [20] has shown good resultsused as anode due to its high transparency in the visiblerange, high mechanical flexibility, and excellent thermalstability [21].Unlike the silicon case, where the light absorption resultsin the formation of free electrons and holes, in organicsystems electrons are promoted from the HOMOto the LUMO, resulting in the formation of excitons composedby a hole and an electron strongly bound together(usually with a binding energy between 0.5 and 1 eV [13]);a large potential gradient is then necessary to drive thecharge carriers away from the dissociating interface [22],resulting in a lower efficiency of the system. For an ef-