solar cell
/ SOH-ler SEL /
A light-emitting diode turns electricity into light. A solar cell runs that idea in reverse: it soaks up light and turns it into electricity. Sunlight strikes the material, knocks electrons loose, and a built-in field sweeps them into a current — sunshine in, electric power out, with no moving parts and no fuel.
A solar cell is essentially a large, thin p-n junction exposed to light. When a photon of sunlight is absorbed, it lifts an electron across the band gap, creating a free electron-and-hole pair. The junction's built-in electric field then pulls the electron one way and the hole the other before they can recombine, so they pile up on opposite terminals and drive current through whatever you connect. The output depends on how much of the sunlight the material can absorb and how cleanly the field separates the pairs.
Solar cells matter as a direct, quiet way to generate electricity from an abundant source, and their cost has fallen dramatically. The honest caveats are real: a single junction can only use part of the solar spectrum, so ordinary silicon cells convert roughly a fifth to a quarter of the incoming light into electricity, and the rest becomes heat or passes through. Higher efficiencies need stacked junctions or new materials, and the panels still need sunlight to work.
A rooftop silicon panel in full midday sun delivers roughly 150 to 220 watts per square metre of electrical power, around a fifth of the sunlight energy landing on it. Stack the cell from layers tuned to different colors of light and lab efficiencies climb past 40 percent.
A single silicon junction captures only part of the spectrum; stacked junctions capture more.
Light with less energy than the band gap passes straight through unused, while light with much more energy wastes its excess as heat. This built-in trade-off sets a theoretical ceiling — about 33 percent — on a single-junction cell, known as the Shockley-Queisser limit.