Photovoltaic principles and organic solar cells

The fundamentals of photovoltaics are reviewed. The necessary requirements for material properties are discussed. Achieving high efficiency with organic solar cells is known to be difficult for two main reasons. Photon absorption does not directly lead to free charge carriers but to excitons with large binding energies which are difficult to dissociate. These excitons have diffusion lengths much smaller than the penetration depth of the incident light, making them difficult to process before their recombination. Ways are discussed how these shortcomings may be overcome. In principle, organic solar cells can be as efficient as inorganic solar cells if the right combination of materials can be found.