Photovoltaic Cell (Solar Cell)

A photovoltaic cell, commonly called a solar cell, turns light directly into electrical current via the photovoltaic effect, first observed by Edmond Becquerel in 1839 and made practical at Bell Labs in 1954. Unlike a solar thermal collector, it produces electricity without any moving parts or intermediate heat stage. The active component is a semiconductor p-n junction, usually crystalline silicon doped with boron on one side (p-type) and phosphorus on the other (n-type). Doping introduces mobile charge carriers and creates a built-in electric field across the junction. When a photon with energy above the material's bandgap (about 1.1 eV for silicon) is absorbed, it promotes an electron from the valence band to the conduction band, generating an electron-hole pair. The junction's field separates the two: electrons drift toward the n-side and holes toward the p-side. If the cell is wired into an external circuit, the accumulated electrons flow through it as direct current before recombining, doing useful work along the way. A single-junction cell faces a hard ceiling. Photons below the bandgap pass through uncaptured, while photons well above it waste their surplus energy as heat. These losses set the Shockley-Queisser limit at roughly 33% for an ideal single-junction silicon cell. Real commercial silicon modules reach about 20-25%; stacked multijunction cells under concentrated sunlight have exceeded 47% by tuning each layer to a different slice of the spectrum. Silicon dominates around 95% of the market, split between higher-efficiency, costlier monocrystalline wafers and cheaper polycrystalline ones. Thin-film technologies such as cadmium telluride and CIGS use far less material and bend to flexible or building-integrated forms, trading some efficiency for cost and weight. Plummeting prices, roughly $96/watt in the 1970s to well under $1/watt today, have pushed cells from powering satellites into grid-scale farms, rooftops, and lightweight craft. The same energy-budget physics constrains Solar-Powered Aircraft and is central to Why Perpetual Solar Flight Is Hard: The Aerodynamics and Energy Budget, while rooftop arrays underpin Net-Zero Homes: Buildings That Produce as Much Energy as They Consume.