**2. Charge transport in polymers**

One of the most important aspects in the conversion of light to electrical energy is the electron transport in the semiconductor. It permits the separation of positive and negative charge and prevents the recombination of the exciton known as "quenching" [9]. Therefore a high conductivity in the organic phase as well intra- as intermolecular is desirable and the contact surface among chains in the same layers as well as between layers should be as large as possible to facilitate electron movement. Here homojunction cells have a clear advantage due to the large transition zone between substrates. Anyhow, as mentioned before, it is very difficult to produce this gradient transition in a reproducible manner resulting in larger experimental errors and poorer comparability of results for different materials. Some of the most important aspects are detailed in the following sections:
