Photoelectric Effect

What Is the Photoelectric Effect?

The photoelectric effect describes the release of electrons from a material by incident light. In solar cells, a specific form is utilised — the internal photoelectric effect — in which photons lift electrons in the semiconductor from the valence band to the conduction band, thereby generating electrical current.

Physical Background

Albert Einstein explained the photoelectric effect in 1905 and received the Nobel Prize for it in 1921. The key principles:

• Light consists of energy quanta (photons)
• Each photon has the energy E = h x f (h = Planck’s constant, f = frequency)
• Only photons with sufficient energy can release electrons
• The band gap of the semiconductor determines which wavelengths can be used

Significance for Solar Cells

Material	Band Gap	Usable Spectrum
Silicon	1.12 eV	up to approx. 1,100 nm (infrared)
Perovskite	1.5—1.7 eV	up to approx. 800 nm (visible)
CdTe	1.45 eV	up to approx. 850 nm

Photons with less energy than the band gap cannot be used. Photons with more energy lose the excess as heat. This limit (Shockley-Queisser limit) restricts the theoretical efficiency of a single-junction cell to approximately 33%.

Practical Tip

The photoelectric effect also works under diffuse light and overcast skies — just with lower intensity. Therefore, solar systems also generate electricity on cloudy days, although significantly less.