There are three basic generations of solar cells, though one of them doesn't quite exist yet, and research is ongoing. They are designated as first, second, and third, and differ according to their cost and efficiency.
The first generation are high-cost, high-efficiency. These solar cells are manufactured in a fashion similar to computers, involving extremely pure silicon, use a single junction for extracting energy from photons, and are very efficient, approaching their theoretical efficiency maximum of 33%. In 2007, first generation products accounted for 89.6% of commercial production, though the market share has declined since. The manufacturing processes that are used to produce them are inherently expensive, meaning that these cells may take years to pay for their purchasing costs. It is not thought that first generation cells will be able to provide energy more cost effective than fossil fuel sources.
The second generation, which has been under intense development during the 1990s and early 2000s, are low-cost, low-efficiency cells. These are most frequently thin film solar cells, designs that use minimal materials and cheap manufacturing processes. The most popular materials used for this type are copper indium gallium selenide, cadmium telluride (CdTe), amorphous silicon, and micromorphous silicon.
A standard example of second generation cells would be those made by Nanosolar, which uses a special machine to print the cells at an extremely fast rate. Though these cells have only 10-15% conversion efficiency, the decreased cost more than makes up for this deficit. Second generation cells have the potential to be more cost effective than fossil fuel.
Third generation solar cells are just a research target and do not really exist yet. The goal of solar energy research is to produce low-cost, high efficiency cells. This is likely to be thin-film cells that use novel approaches to obtain efficiencies in the range of 30-60%. Some analysts predict that third generation cells could start to be commercialized sometime around 2020, but this is just a guess. Technologies associated with third generation products include multijunction photovoltaic cells, tandem cells, nanostructured cells to better pick up incident light, and using excess thermal generation to enhance voltages or carrier collection.