1.6 Solar Cells

The solar cell in principle consists of a photoactive layer sandwiched between two electrodes [31]. The bandgap of the photoactive layer should be chosen to match the solar irradiance spectrum. In order to create carriers rather than excitons that would be formed in a pure material, polymer solar cells commonly use a combination of two semiconductors with complementary electronic levels. The initial step after photoexcitation is the dissociation of an exciton via charge transfer at the interface of these two materials. After the charge transfer the electrons and holes have to escape their Coulomb potential and migrate towards either electrode due to the internal field. This migration leads to an electric field opposing the external field that will be reduced up to a certain distance from the electrodes. As both carriers will recombine at the opposite electrodes a continuous current flow is created. The resulting electric field in the organic layer depends thus on the free electron and hole concentration, their drift mobility and diffusion coefficient, the generation, and the recombination rate in the bulk and at the electrodes.