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1.1 hybrid interfaces for photovoltaics 3ficient electron-hole separation, the junction between thetwo materials must be of the type-II (staggered), for whichthe HOMO and LUMO positions decrease in energy whenmoving from the donor to the acceptor (see Figure 1.1).Excitons can recombine efficiently unless they diffuse andseparate at the interface within their lifetime. In order toachieve high performance bilayers, trasport must be efficientin comparison to recombination mechanisms, such asluminescence or non-radiative recombination. For the majorityof molecules, the exciton lifetime is in the order ofnanoseconds while the distance that an exciton can crossis limited to about 10 nm [11]. This means that only theexcitons formed within this distance from the interface cancontribute to charge separation.Figure 1.1.: Working principle of an organic bilayer solar cell.The simple bilayer can be replaced by a more complexbulk heterojunction architecture. In this kind of solar cellthe donor and acceptor components interpenetrate one another,giving an interface not planar but spatially distributed[13]. This feature makes possible to partially overcome thelimitation due to the diffusion length of excitons since thelarge surface-to-volume ratio makes possible to collect atthe interface a larger fraction of excitons.Unfortunately, the disadvantages are represented by thedifficult separation of the charges due to the increased dis-