6: Photovoltaics

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This chapter discusses solar cells and optical detectors, both of which are devices that convert optical electromagnetic energy to electricity. The next chapter discusses lamps, LEDs, and lasers which convert energy in the opposite direction. The photovoltaic effect is the idea that if a light shines on a pure piece of semiconductor, electron-hole pairs form. In the presence of an external electric field, these charges are swept apart, and a voltage develops across the terminals of the semiconductor. It was first demonstrated in 1839 by Edmond Becquerel. In a photovoltaic device, also called a solar cell, this effect typically occurs at a semiconductor pn junction. This same effect occurs on a smaller scale in photodiodes used to detect light and in optical sensors in digital cameras. To understand the physics behind these devices, we need to further study crystallography in semiconductors. Energy level diagrams, which illustrate the energy needed to remove an electron from a material, are another topic studied in this chapter.

Unlike fossil fuel based power plants, photovoltaic cells produce energy without contributing to pollution. The solar power industry is growing at a fast pace. Worldwide as of April 2017, photovoltaic cells were capable of generating over 303 GW of power, and 75 GW of this total was installed within the past year [67]. This generating capacity was sufficient to satisfy 1.8% of the worldwide demand for electricity [67]. In the United States as of April 2017, photovoltaic cells installed were capable of generating 14.7 GW [67].

Thumbnail: Photovoltaic systems use semiconductor cells that convert sunlight directly into electricity. Direct current from the PV cells, which are arrayed in flat panels, flows to inverters that change it to alternating current. (Public Domain; Tennessee Valley Authority via Wikipedia)