The first half of this chapter briefly describes the conceptual buildup of a polymer solar cell followed by an overview of the organic materials that have been used. The second half is devoted to a practical guide to making solar cells complete with synthesis, device fabrication, and testing. Today, most polymer solar cells are based on the bulk heterojunction concept described in 1995 by Yu et al. In this type of solar cell, the donor material (typically a polymer) is mixed with an acceptor (a soluble fullerene) in an organic solvent and then spin coated or cast on a substrate of indium-tin oxide (ITO) on glass. During evaporation of the solvent and latter treatments, a microphase separation takes place with the formation of an interpenetrating network. A hole-blocking layer of, e.g., lithium fluoride may be added, and in a last step a metal electrode (aluminum) is evaporated on top (Fig. 2.1).
The bulk heterojunction is important because a large interfacial area between the donor and acceptor materials is created where charge separation can take place.