What Are Photocells Made of?

Selenium

• Experiments conducted in the mid-1800s revealed the element selenium produced electricity when exposed to light. Since then, camera light meters and other light-measuring devices have used this material. Until the 1980s, most photocopiers had a metal printing drum coated with light-sensitive selenium. In the periodic table, selenium is a near neighbor with silicon, another semiconductor material. Semiconductors, being partway between insulators and metals, exhibit useful electronic properties, including sensitivity to light.

Cadmium Sulfide

• A photocell made of cadmium sulfide is a photoresistor, meaning its electrical resistance decreases when light falls on it, and increases in the dark. It changes resistance relatively slowly, making it useful for devices such as street light sensors, which turn the street lights on at night and do not turn the lights off again until it receives prolonged sunlight.

Crystalline Silicon

• Silicon is the primary material used for making photovoltaic cells, or solar cells that produce electricity. In its crystalline form, silicon has relatively good efficiency for converting light energy into electricity, about 20 percent. The best cells, called monocrystalline, come from an ingot grown as a single, pure crystal, though these are also the most expensive. Polycrystalline solar cells, those made from multiple crystals, have lower efficiency and lower cost.

Amorphous Silicon

• The least efficient and expensive form of silicon for solar cells, amorphous, does not have a regular crystal structure but is instead glassy. The efficiency for amorphous solar cells runs between 6 and 8 percent. To make them, a vapor deposits a layer of silicon one thousandth of a millimeter thick onto a backing material or substrate. The substrate may be glass, plastic film or stainless steel. The variety of materials opens up new applications for solar power, including automobiles, commercial and residential roofs and tents.