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How solar cells work Like all semiconductor devices, solar cells work with a semiconductor that has been doped to produce two different regions separated by a p-n junction. Across this junction, the two types of charge carrier â€" electrons and holes â€" are able to cross. In doing so, they deplete the region from which they came and transfer their charge to the new region. This migration of charge results in a potential gradient or electrical slope, down which charge carriers tend to slide as they approach the junction. Simplified operation of a solar cell. Image source: US Dept of Energy When sunlight strikes a solar cell, atoms are bombarded with particles of light called photons, and give up electrons. When an electron is kicked out of an atom, it leaves behind a hole, which has an equal and opposite (positive) charge. If either carrier wanders across the junction, the field and the nature of the semiconductor material discourage it from recrossing. A proportion of carriers that cross the junction can be harvested by completing a circuit from a grid on the cell's surface to a collector on the backplane. In the cell, the light "pumps" electrons out one side of the cell, through the circuit, and back to the other side, energizing any electrical device that is connected along the way. The current generated in the semiconductor is extracted by contacts at the top and bottom of the cell. The top contact structure, which must allow light to pass through, is made of thin, widely-spaced metal strips (usually called fingers) that supply current to a larger bus bar. The cell is covered with a thin layer of dielectric material â€" the anti-reflection coating â€" to minimize light reflection from the top surface.

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