Sunlight consists of many tiny energy particles called photons. Photovoltaic systems use semiconductor materials, such as silicon, to absorb some photons and convert them into electrons. This process, called the photoelectric effect, is also the physical process underlying the conversion of sunlight into electricity in photovoltaic cells. Photovoltaic cells have special built-in electric fields that provide the voltage needed for the current to pass through an external load, such as a light bulb.