The photovoltaic effect

The collection of light-generated carriers does not by itself give rise to power generation. In order to generate power, a voltage must be generated as well as a current. Voltage is generated in a solar cell by a process known as the "photovoltaic effect". The collection of light-generated carriers by the p-n junction causes a movement of electrons to the n-type side and holes to the p-type side of the junction. Under short circuit conditions, there is no build up of charge, as the carriers exit the device as light-generated current.

However, if the light-generated carriers are prevented from leaving the solar cell, then the collection of light-generated carriers causes an increase in the number of electrons on the n-type side of the p-n junction and a similar increase in holes in the p-type material. This separation of charge creates an electric field at the junction which is in opposition to that already existing at the junction, thereby reducing the net electric field. Since the electric field represents a barrier to the flow of the forward bias diffusion current, the reduction of the electric field increases the diffusion current. A new equilibrium is reached in which a voltage exists across the p-n junction. The current from the solar cell is the difference between I_L and the forward bias current. Under open circuit conditions, the forward bias of the junction increases to a point where the light-generated current is exactly balanced by the forward bias diffusion current, and the net current is zero. The voltage required to cause these two currents to balance is called the "open-circuit voltage". The following animation shows the carrier flows at short-circuit and open-circuit conditions.

Simulation of carrier flows in a solar cell under equilibrium, short-circuit current and open-circuit voltage conditions. Note the different magnitudes of currents crossing the junction. In equilibrium (i.e. in the dark) both the diffusion and drift current are small. Under short circuit conditions, the minority carrier concentration on either side of the junction is increased and the drift current, which depends on the number of minority carriers, is increased. Under open circuit conditions, the light-generated carriers forward bias the junction, thus increasing the diffusion current. Since the drift and diffusion current are in opposite direction, there is no net current from the solar cell at open circuit.