How do photovoltaic cells produce electricity from sunlight?

• A. Heat energy stored in a battery upon exposure to light.
• B. Mechanical rotation of a turbine driven by sunlight.
• C. Chemical reactions in a liquid generate electrons when illuminated.
• D. The photovoltaic effect in semiconductors creates electron flow when photons excite electrons at a p-n junction.

Answer: (D)

Photovoltaic cells convert light directly into electricity through the photovoltaic effect in semiconductors. When photons with enough energy are absorbed, electrons are excited from the valence band to the conduction band, creating electron–hole pairs. At a p-n junction, the built-in electric field in the depletion region pushes electrons toward the n-type side and holes toward the p-type side, separating the charges. When the cell is connected to a circuit, this separation drives electrons through the external path, producing an electric current and power. Any excess photon energy becomes heat in the material. This is different from heat-driven, mechanical, or purely chemical light–to–electric processes.