Publisher Summary This chapter introduces a number of concepts and terms that are most useful in considering aspects of the design, construction, and operation of solar (photovoltaic) cells. Theoretically, solar cells are constructed of various semiconductors and their performance is dependent upon the properties of these materials. These properties are best understood within the framework of quantum mechanics and solid state physics. Electrically, all materials can be divided into three classes, relating to the ability of a specific material to carry electrical current: (1) Conductors that contain many free electrons, which are not bound to specific sites within the material, hence, are free to move and generate current; (2) insulators that have no mobile electrons and, therefore, are unable to sustain an electrical current; and (3) semiconductors who have the ability to carry an electrical current lying between these two extremes. Semiconductors exhibit all three forms of crystal arrangement: crystalline, polycrystalline, and amorphous. Many of the properties that are found to be useful in semiconductors and solar cells arise from the basic crystal structure of the semiconductor and are dependent on the direction in the crystal toward , which the observer is facing. There are a number of methods for introducing impurities into any particular semiconductor. We can insert the impurities into the crystal when the crystal is initially formed. This initial construction is normally performed by pulling the semiconductor crystal from a molten bath, using a seed crystal to define the nature and orientation of the lattice.