Though a brief introduction of organic solar cells (OSCs) has already been given in Chapter 1, here I will discuss more about the materials, processing, limitations, and recent developments in OSCs. As mentioned earlier, OSCs are based on organic light absorbers, which are special types of hydrocarbons. These materials possess energy band gaps ranging from about 1.2 eV to about 3.5 eV and are known as organic semiconductors. These materials possess alternate single and double bonds among the C atoms, because of which they are also known as conjugated semiconductors. In pure form these materials are insulators, but they possess many properties similar to semiconductors like band gaps and temperature-dependent electrical conductivity, and therefore are called semiconductors. Though organic semiconductors have been known since the 1950s, their potential was realized in the early 1970s. Realization of very high conductivity in doped conjugated organic semiconductors generated a great interest in these materials for their applications in various electronic devices such as light-emitting diodes, solar cells, photodiodes, transistors, lasers, and sensors.