Altermatt Lecture:   The PV Principle

 
 

1.2:  The photovoltaic (PV) principle

The PV principle

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).

     

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