Photovoltaics (PVs) are semiconductor devices that absorb light energy and transform it directly into electrical energy. They function much like batteries, which turn chemical energy into electricity, but a photocell power supply never runs down or needs to be recharged; it must though be illuminated. Moreover, because standard silicon photovoltaics have a lifetime of 20-25 years — something unheard of in batteries — they successfully compete with batteries for small desktop devices. They are also the major power source for space satellites due to their relatively light weight and their ability to capture primordial solar irradiation.
In the eighteenth and nineteenth century, early electricity researchers began exploring the ability of solids to conduct electricity. Using primitive batteries, they recognized that metals, such as copper, gold and silver, readily conduct electricity, while materials they called insulators, such as glass, blocked current flow. Other substances were intermediate, but poor conductors, enabling flow of a billionth as much electricity using identical voltage sources. These latter materials, including silicon, gallium, and selenium, were given the name semiconductors.
In these studies, the nineteen-year-old experimental French physicist, Edmond Becquerel, was apparently the first, in 1841, to discover that electrodes in salt solutions coated with the inorganic semiconductor silver chloride would produce electricity when exposed to light. In these early days, this photoelectric effect remained a curious physical phenomenon for which no one had any explanation. While research into various photoelectric solids continued, these materials were thought to be of little interest except to physicists themselves.
However in 1883, basing himself on William Adams and Richard Day’s studies of the photoelectric selenium, the American inventor Charles Fritts was able to take PV cells out of the laboratory. His design of the first large area solar cells, made from selenium wafers on brass, led to the first practical use of PV cells as light-measuring devices. This development was immediately adopted by photographers who used the new light meters for setting their exposure settings. However, efficiency of these cells was extremely low, lower than 1 percent.
It was only in the 1930s — due to the work of British, Russian and German scientists — that became apparent that the photoelectric and conductive properties of semiconductors are greatly affected by small amounts of impurities. In silicon, the material most widely used in contemporary PV devices, elemental phosphorous — which contains an extra outer electron as compared to silicon — provides easily discharged electrons participate in conductivity and photovoltaic activity. Such doped materials are called, for short, n-type (negative) semiconductors. It was also noted that other atoms, such as boron with one less outer electron than silicon, are also useful for semiconductors. When present in crystalline silicon, boron atoms bind up electrons from the bulk silicon electron complement, leaving positively charged “holes” in the semiconductor, which is also able to increase conductivity via flow of the “holes.” This variety of semiconductors is known as p-type, for its electrically positive character.
N- or p-type semiconductors with identical contact leads do not show a photovoltaic effect. The semiconductor could absorb light energy that loosens electrons, but there would be no force to get them moving in any direction. For photocells, an electrically asymmetric junction must be created, where positive and negative charges build up on either side of the junction, a situation known to physicists as an electric field. Here, when illumination frees electrons, they are repelled by the fixed negative charges in the junction and move through the n-semiconductor contact leads, external circuit, and the second lead to the positive side of the junction.
In the early studies, despite the use of a single, uniform semiconductor material, such as Becquerel’s silver chloride, photovoltaic action could be observed due to the metal electrode/silver chloride junction on one side of his cell and the electrode solution junction on the opposite side. Since the physical properties of the materials comprising the two junctions were different, a small electric field was set up across the junction and a photovoltaic effect was obtained. However, because the metal and solution have such high conductivities, the field is extremely small and the efficiency of the photovoltaic effect is also very small.
For obtaining the high 15 to 23 percent efficiencies of commercial PV cells, producers sandwich together n- and p-doped silicon or other semiconductors. Because of the lower conductivity of the components, a much larger electric field is set up at the junction. When electrons on the n-side of the junction are excited by light, they are efficiently pushed through the negative lead, provide electrical energy to the circuit, and return to the p-side.
Studied cells and Eppley PSP pyranometer mounted on a solar tracker.
Previous section Next section
