Charged defects in soft semiconductors and their influence on organic photovoltaics

Charged defects play an important, but often unrecognized, role in organic semiconductors and in photovoltaic cells made from them. Thermodynamic considerations show that the weak lattice forces characteristic of soft semiconductors inevitably lead to a high concentration of defects, some of which will be charged. The defect density in actual materials is discussed along with the different types of defects and their influence on the electrical properties and chemical stability. Charged defects act as dopants and thus may sometimes be beneficial, although low defect materials with purposely added dopants should be superior.