Publisher Summary This chapter examines a number of well-known polycrystalline based solar cells and considers a variety of solar cells made from amorphous semiconductors. Single crystal materials exhibit an ordered structure for atomic location, while polycrystalline materials are composed of a number of single crystals oriented at random and separated by “grain boundaries.” Amorphous materials are noted for their complete lack of order when atomic locations beyond the next nearest neighbors are considered. Over and above the lower efficiency exhibited by polycrystalline (pc) semiconductor solar cells and solar cells constructed of amorphous (a) materials, the operating characteristics of polycrystalline based solar cells are dictated, in large part, by the characteristics of the grain boundaries. In the case of amorphous semiconductor based solar cells, the performance is not thoroughly understood, depending on the existence of trace elements, deep lying energy levels within the energy gap and the lack of bonding symmetry. The principal advantage of polycrystalline solar cells is one of cost. Being polycrystalline and thin, they can be fabricated on a wide variety of substrates by mass production techniques. The effective optical absorptivity of most polycrystalline materials is considerably higher than that for the single crystal form of the same material. This enables to construct much thinner solar cells and still capture the incoming photons. A variety of amorphous materials have undergone investigation as solar cell candidates, silicon being the most extensively involved.