(A) The Energy Gap—In a semiconductor, electrons (blue dots) fill up the ladder of allowed energy states to the top of the valence band. An energy region with no energy states (the energy gap) separates the highest rung of the valence band from the bottom rung of the empty conduction band. The gap has an energy value denoted by Egap.
(B) Creating Excitons—An electron absorbs a photon with energy Egap and jumps across the energy gap to the conduction band, where it becomes a negative-charge carrier. The electron leaves a vacancy, a hole that looks like a positive-charge carrier. The electron and hole are slightly bound together, and the pair is called an exciton.
(C) Harvesting the Charge Carriers—A crystalline silicon solar cell quickly separates the excitons’ electrons and holes because the cell contains two silicon layers: a p-doped layer containing “acceptor” atoms, which tend to accept extra electrons, and an n-doped layer containing “donor” atoms, which tend to give away their electrons. When these layers are brought into contact, they form a p-n junction—electrons are exchanged, and ionized donors and acceptors create a strong electric field. It is this electric field that drives the electrons and holes apart as soon as sunlight creates them, thereby preventing recombination (in which the electron falls back into the hole, and solar energy is re-emitted as a photon). The charge carriers collect at opposite conducting terminals and flow around a circuit to power, for example, an electric motor.
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