**2.1 A brief history**

The first organic solar cells consisted of a single layer of photoactive material sandwiched between two electrodes of different work functions (Chamberlain, 1983; Wohrle & Meissner, 1991). However, due to the high binding energy of the primary photoexcitations, the separation of the photogenerated charge carriers was so inefficient that far below 1% power conversion efficiency could be achieved.

The next breakthrough was achieved in 1986 by introducing the bilayer heterojunction concept, in which two organic layers with specific electron or hole transporting properties were sandwiched between the electrodes (Tang, 1986). In this organic bilayer solar cell were consisting of a light-absorbing copper phthalocyanine layer in conjunction with an electronegative perylene carboxylic derivative. The differing electron affinities between these two materials created an energy offset at their interface, thereby driving exciton dissociation.

The efficiencies of the first organic solar cells reported in the 1980s were about 1% at best at that time. Primarily, this is due to the fact that absorption of light in organic materials almost always results in the production of a mobile excited state, rather than free electronhole pairs as produced in inorganic solar cells. This occurs because in organic materials the weak intermolecular forces localize the exciton on the molecules. Since the exciton diffusion lengths in organic materials are usually around 5-15 nm (Haugeneder et al., 1999), much shorter than the device thicknesses, exciton diffusion limits charge-carrier generation in these devices because most of them are lost through recombination. Photogeneration is therefore a function of the available mechanisms for excitons dissociation.

The discovery of ultrafast photoinduced electron transfer (Sariciftci et al., 1992) from a conjugated polymer to buckminsterfullerene (C60) and the consequent enhancement in charge photogeneration provided a molecular approach to achieving higher performances from solution-processed systems. In 1995 the first organic bulk heterojunction organic solar cell was fabricated based on a mixture of soluble *p*-phenylene-vinylene (PPV) derivative with a fullerene acceptor (Yu et al., 1995). In 2001, Shaheen et al. obtained the first truly promising results for bulk heterojunction organic solar cells when mixing the conjugated polymer poly(2-methoxy-5-(3',7'-dimethyl-octyloxy)-*p*-phenylene vinylene) (MDMO-PPV) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) yielding a power conversion efficiency of 2.5% (Shaheen et al., 2001).

Padinger *et al.* (Padinger et al., 2003) presented a further increase in the power conversion efficiency by using a blend, which is nowadays the best investigated organic solar cell system: a poly(3-hexyl thiophene) donor (P3HT) in conjunction with PCBM. It was shown that annealing at a temperature above the glass transition of the polymer enabled an enhancement of the efficiency from 0.4% to 3.5%.

In the following years, the power conversion efficiency could be increased steadily. This is, to a large fraction, due to the considerable amount of time that has been spent by many laboratories around the world on the optimization of bulk heterojunction solar cells—many of them using P3HT:PCBM—but also by new approaches. Additives have been used in order to allow an increased control of the phase segregation during film formation of a copolymer–fullerene blend (Park et al., 2009; Peet et al., 2007), thus yielding efficiencies of up to 6%. The process additive is a solvent for the fullerene, but not the polymer, thus allowing the PCBM an extended time for self-organization during the drying process. A positive effect by heating the solvent before the film application could also be shown (Bertho et al., 2009). Today, up to 8% power conversion efficiency are reported in this kind of organic solar cells (Park et al., 2009; Green et al., 2010).
