Photovoltaic effect
The generation of a voltage and electric current in a semiconductor junction when it absorbs photons with energy above the material bandgap.
The photovoltaic effect, first observed by Edmond Becquerel in 1839, is the physical basis of every solar cell. A photon whose energy exceeds the semiconductor bandgap lifts an electron from the valence band to the conduction band, leaving a positively charged hole. The built-in electric field of a p-n junction separates the pair before it recombines, and the separated charges drive current through an external circuit.
Crystalline silicon has a bandgap of 1.12 eV, so it absorbs light up to about 1,100 nm. Two losses set a hard ceiling on efficiency: photons below the bandgap pass through unused, and the excess energy of high-energy photons is lost as heat (thermalisation). The Shockley-Queisser detailed-balance limit for a single junction under the AM1.5 spectrum is about 33.7% at an optimal bandgap near 1.34 eV; for silicon, accounting for Auger recombination, the practical limit is about 29.4% (Richter et al., 2013).
Multi-junction and tandem designs, such as perovskite-on-silicon, stack materials with different bandgaps to reduce thermalisation and push beyond the single-junction limit.
Related terms
All terms →- Solar cell
A semiconductor device, usually a doped silicon wafer with metal contacts, that converts sunlight directly into direct-current electricity via the photovoltaic effect.
- Perovskite solar cell
A solar cell whose absorber is a metal-halide compound with the perovskite crystal structure, offering tunable bandgap, cheap processing and very fast efficiency gains.
- Monocrystalline silicon
Silicon grown as a single continuous crystal, usually by the Czochralski method, giving the highest-efficiency and now dominant wafer type for PV.
- Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).
- Standard Test Conditions (STC)
The reference conditions for rating PV modules: 1,000 W/m² irradiance, 25 °C cell temperature and an AM1.5G solar spectrum.