Solar Power Cells Solar power (photovoltaic) cells convert light energy into electricity at the atomic level.   Illustration Courtesy of U.S. Department of Energy's Photovoltaics Program Although first discovered in 1839, the process of producing electric current in a solid material with the aid of sunlight wasn't truly understood for more than a hundred years. Throughout the second half of the 20th century, the principals underlying the photovoltaic effect have been determined and the manufacturing processes have been more fully refined. As a result, the cost of these devices has put them into the mainstream of modern energy producers. This was caused in part by advances in the technology - in which solar electric conversion efficiencies have improved considerably - and in part by improvements in manufacturing all the other components in a complete system. The conversion efficiency of a solar power cell is defined as the ratio of the sunlight energy that hits the cell divided by the electrical energy that is produced by the cell. This is very important when discussing solar electric devices, because by affordably improving this efficiency, solar power energy becomes competitive with fossil fuel sources. For comparison, the earliest solar energy devices converted about 1%-2% of sunlight energy into electric energy. Today's solar energy devices convert 7%-17% of light energy into electric energy. Moreover, today's mass produced panel systems are substantially less expensive than earlier systems. The "photovoltaic effect" is the basic physical process through which a solar power cell converts sunlight into electricity. Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum.                                              Illustration Courtesy of U.S. Department of Energy's Photovoltaics Program When photons strike a solar power cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the solar electric cell (which is actually a semiconductor). With its newfound energy, the electron is able to escape from its normal position associated with that atom to become part of the current in an electrical circuit. By leaving this position, the electron causes a "hole" to form. Special electrical properties of the solar power cell—a built-in electric field—provide the voltage needed to drive the current through an external load (such as a light bulb).   The most important parts of a solar energy cell are the semiconductor layers, because this is where the electron current is created. There are a number of different materials suitable for making these semiconducting layers, and each has benefits and drawbacks. Unfortunately, there is no one ideal material for all types of solar electric cells and applications. In addition to the semiconducting materials, solar electric cells consist of a top metallic grid or other electrical contact to collect electrons from the semiconductor and transfer them to the external load, and a back contact layer to complete the electrical circuit. Then, on top of the complete cell is typically a glass cover or other type of transparent encapsulant to seal the cell and keep weather out, and an antireflective coating to keep the solar electric cell from reflecting the light back away from the cell.