The working principle of a solar cell

Referencing: Solar Energy - the Physics and Engineering of Photovoltaic Conversion, Technologies and Systems, Chapter 3.

All the basics of a solar cell are described in this post.  Firstly, the photovoltaic (PV) effect is the generation of a potential difference at the junction or interface between 2 different materials due to electromagnetic (EM) radiation.  This effect is closely related to the photoelectric effect, where the light absorbed by a material causes electrons to be emitted.  This is due to the frequency of light, which should be above the material-dependent threshold frequency.

Light, which is part of EM radiation, is understood to consist of energy quanta called photons.  A photon's energy is:

E = hv

where h is Planck's constant, and v is the frequency of the incident light.

Next, the PV effect can be described by 3 processes in sequence (refer to the diagram below):
- Photons are absorbed at the junction of materials, thereby generating charge carriers.
- Due to the junction, the photo-generated charge carriers are separated.
- Charge carriers are collected at the terminals of the junction.

In the first process, we consider ideal semi-conductors as absorbers, where there are 2 energy levels: the lower valence band edge (E_V) and the higher conduction band edge (E_C).  No allowed energy states exist between these bands for electrons.  This energy difference is known as the bandgap (E_G).  A photon is absorbed if it has an energy greater than E_G.  Upon absorption, an electron is excited from E_V to E_C.  Hence:

E_G = E_C - E_V = hv

After excitation, an empty space is created at E_V where the electron once was.  This space is called a hole and is considered a positive charge and behaves like a particle that moves.  Thus, the absorption of a photon creates an electron-hole pair.  The radiative energy of the photon is converted into the chemical energy of the electron-hole pair.

In the second process, the junction of the materials act like semi-permeable membranes, dividing the electron and hole to prevent them from recombining.  This allows the energy stored in the electron-hole pair to be used in an external circuit.  In most solar cells, n and p type materials form the membranes.  They have to be designed to allow the charge carriers to reach the membranes before recombining, which means that the absorber must not be too thick.  The absorber thickness forms part of the distance electrons and holes must travel to the membrane.

In the final process, the charge carriers are extracted from the solar cells using electrical contacts as part of an electric circuit.  The chemical energy of the electron-hole pairs is converted to electricity, where the electrons will go through the external electric circuit, and recombine with the holes at a metal-absorber interface.