Solar Cell

A solar cell is essentially a large-area p-n junction operated without any applied bias, relying on the photovoltaic effect. When sunlight with photon energy greater than the band gap is absorbed near the junction, it creates electron-hole pairs. The built-in electric field of the depletion region then sweeps electrons toward the n-side and holes toward the p-side, separating the charges. This charge separation accumulates as a potential difference across the terminals, the photo-voltage, which can drive current through an external load such as a charging battery. The energy of the output therefore comes entirely from the absorbed photons, with the junction acting as a built-in pump that sorts the carriers; brighter illumination frees more pairs and increases the current the cell can supply. The option invoking an external battery is wrong because a solar cell is a source, not a biased load. The thermocouple option is wrong; the output arises from photon absorption, not a temperature gradient. The reverse-breakdown option is wrong because the cell operates well below breakdown, in the photovoltaic regime. As a consistency check, the device converts light energy directly into electrical energy with no external supply, which is exactly the defining feature of a solar cell and distinguishes it from a photodiode used merely as a detector.