### 2.2:  Current production

On this page you learn to estimate how much electric current can be maximally produced by absorption of sunlight.

It was mentioned on the previous page that light can pass on solely a fixed quantum of energy to an electron. Such a quantum is very small and is carried by a photon. We can therefore consider light as a photon flux. To calculate the number of electrons excited by sunlight, we assume that each absorbed photon excites a single electron.*

Because the energy of a photon is very small, we use the energy unit "electron volts" (eV), which is volts multiplied by the unit charge q, which is 1.6 x 10–19 ampere seconds (A⋅s). A 40 watt light bulb hence requires the energy of about 1 x 1020 eV per second.

Fortunately, the solar spectrum gives us a huge number of photons per second Nph(E) over its energy range. This will be explained in detail in The Solar Spectrum lecture. A solar cell absorbs many of these photons, so a huge number of electrons is excited from the valence to the conduction band where they move and possibly reach the contacts to enter the external circuit.

Let us assume that all the excited electrons make it to the contact. How much electric current is this? Current is charge per time that flows through a given area. Every electron has unit charge q. To obtain the number of electrons, you can sum up all photons of the solar spectrum with energy larger than Eg. Then, you obtain for the current: This is called the generation current Jgen. In silicon, it is about 46 mA/cm2, i.e., there are about 1017 photons per cm2 per second absorbed in a Si solar cell. Figure 1: The photon flux of the solar spectrum is very large. See The Solar Spectrum lecture for more details. Silicon solar cells can absorb photons with an energy larger than about 1.2 eV (having a wavelength smaller than about 1200 nm).

* This is not strictly true, as you will learn in the lesson on photogenration.