Figure 3. The three generations of solar cells. First-generation cells are based on expensive silicon wafers and make up 85% of the current commercial market. Second-generation cells are based on thin films of materials such as amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium selenide. The materials are less expensive, but research is needed to raise the cells' efficiency to the levels shown if the cost of delivered power is to be reduced. Third-generation cells are the research goal: a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Their design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight concentration, or new materials. The horizontal axis represents the cost of the solar module only; it must be approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost per watt of peak power (Wp). (Adapted from ref. 2, Green.)