Photovoltaic devices are based on the concept of charge separation at an interface of two materials, which can be n-type and p-type semiconductors, heterojunctions between different semiconductors or semiconductor-metal junctions. Most solar cells presently sold are based on silicon wafers, so-called ‘first generation’ technology. The use of organic bilayers with photo-induced electron transfer is a subject of intensive research in the last decades. In these bilayer donor-acceptor systems the energy conversion is limited by the fact that charge separation only takes very close (10-20 nm) of the interface as a result of the limited exciton diffusion and/or space-charge region. As a result charge carriers generated far from the interface recombine before they reach the interface. Furthermore, since the charge separation is limited to a small region the photovoltaic efficiency is limited by the amount of photons absorbed in this region. An increase of the efficiency has recently been realized by mixing electron-donor type polymers with suitable electron acceptors. Progress has especially been made using conjugated polymer/fullerene solid composites. For these materials systems the efficiency of the charge separation process is nearly 100% as a result of the fast electron transfer from the polymer towards the fullerene (<100 fs). By mixing the two components to a single composite active layer a ‘bulk heterojunction’ is formed between the electron donors and acceptors. Through control of the morphology of phase separation into an interpenetrating network a high interfacial area is achieved within a bulk material. If the donor-acceptor networks are bicontineous also the collection efficiency can be optimized. In present devices, based on various poly-phenylene vinylene derivatives, the limited power conversion efficiency of about 2.5% is mainly due to the spectral mismatch of the absorbing polymer to the solar spectrum.
