What are the three processes that must occur within a photovoltaic material in order for an electrical current to be produced in an external circuit?

Answer 1

Absorption, separation, and collection. All three have to succeed in sequence; if any one fails, no current reaches the external circuit.

1. Absorption - a photon creates an electron-hole pair. Light strikes the material and a photon is absorbed, promoting an electron from the valence band into the conduction band and leaving behind a positively charged hole. The condition is energetic: the photon must carry at least the band gap energy, E_photon = hc/λ must be greater than or equal to E_g. Photons with less energy pass straight through and are wasted, which is why a silicon cell with a band gap near 1.1 eV is transparent to most infrared light.

2. Separation - the pair is pulled apart before it recombines. This is the step people forget, and it is the one that actually makes a solar cell a solar cell rather than just a warm piece of silicon. An electron and hole sitting together will simply recombine within microseconds, releasing the energy again as heat or light and accomplishing nothing. Something must physically drive them in opposite directions first. In a conventional cell that something is the built-in electric field at the p-n junction, which sweeps electrons toward the n-side and holes toward the p-side.

3. Collection - the carriers travel to the contacts and out. The separated electrons must reach the metal electrodes and pass into the external circuit. This requires the carriers to survive the trip, so the material's minority carrier diffusion length has to be long compared with the distance to the contact. Impurities and crystal defects create recombination sites that shorten it, which is why photovoltaic silicon has to be so pure.

The unifying idea, if you want one sentence for an exam: a solar cell must absorb light, separate the resulting charges, and collect them before they recombine. Recombination is the enemy at every stage after absorption, and essentially all of solar cell engineering is about the second and third steps rather than the first.

This is also why band gap selection is a compromise rather than an optimisation. A small band gap absorbs more of the solar spectrum but produces a lower voltage per carrier; a large one gives high voltage but ignores most of the incoming light. The balance lands near 1.1 to 1.4 eV, which is why silicon and gallium arsenide dominate.