The photovoltaic effect is the physical process of converting light into electricity.
The photovoltaic principle has the following minimal requirements:
A two-level system, where there is at least one type of carrier that can make a transition from the lower (relaxed) to the upper (excited) state.
Carrier-photon interaction, so the carrier can be excited by light from the lower to the upper energy level.
Transport of this carrier to a contact.
A contact that enables the photo-excited carrier to go to the external circuit, or at least that enables the photo-excited carrier to transfer its energy to the electrons in the external circuit.
In the external circuit, the electrons can deliver work and hereby loose energy.
A second contact for the relaxed electrons to enter the two-level system, or at least to communicate with the relaxed carrier in the two-level system.
Transport of the relaxed carrier through the second contact back to where it started, so the cycle can start again.
Please note the following two aspects:
There must be two different kinds of contacts: one for the excited carrier, one for the relaxed one—otherwise the net carrier
charge-flow to the external circuit would be zero. We call them selective contacts, and this is a central requirement for the PV effect to happen,
as you will appreciate in the coming pages.
As many charge-carriers must leave as must return to the two-level system, otherwise there is an accumulation or depletion of carriers
within the system. This may seem obvious at first. However, in systems with a weak PV effect, you need to take care that you don't measure merely an accumulation or
depletion of carriers. Hence, you better don't trust reported efficiencies that are below 1% and are measured over short term only.
By the way, there are many different ways of transport to and through the contacts. For example, in very small systems where the carriers are confined,
tunneling is a sufficient requirement (as opposed to larger, unconfined systems, where the carriers need some degree of mobility within the two-level
system to reach the contacts).