Compound photovoltaic cells combined with solar concentrating systems have achieved 35 percent efficiency. Researchers have also made solar cells from a number of other semiconductor materials. Today, solar cells provide power for spacecraft and artificial satellites, handheld calculators, and wristwatches. Solar cells are also used for electric power generation in remote areas, where extending power lines would be difficult or costly. Most photovoltaic systems require a storage facility, which normally consists of batteries.
Excess energy is stored in the batteries during the day and extracted as needed during the night.
Other solar cells. Researchers are also studying solar cells called photoelectrochemical cells. The simplest such cell is similar to the device made by Becquerel in 1839. However, today's cells are much more stable and efficient. In addition to producing electric current, photoelectrochemical cells can be used to chemically split water directly into oxygen and hydrogen gas.