Another way to categorize photovoltaic materials is based on whether thep andnregions of the semiconductor are made of the same material (with different dopings, of course) —for example, silicon. These are calledhomojunction photovoltaics. When thep–njunction is formed between two different semiconductors, they are calledheterojunctionPVs. For example, one of the most
promising heterojunction combinations uses cadmium sulfide (CdS) for thentype layer and copper indium diselenide (CuInSe2, also known as “CIS”) for the p-type layer.Other distinctions include multiple junction solar cells (also known as cascade or tandem cells) made up of a stack ofp–njunctions with each junction designed to capture a different portion of the solar spectrum. The shortest-wavelength,highest-energy photons are captured in the top layer while most of the rest pass through to the next layer. Subsequent layers have lower and lower band gaps, so they each pick off the most energetic photons that they see, while passing the rest down to the next layer. Very high efficiencies are possible using this approach.Finally, some cells are specifically designed to work best with concentrated sunlight while others are used in nonconcentrating flat-plate systems. Figure 8.45 provides a rough road map for our subsequent discussion of each technology.