A basic photovoltaic cell consists of a n-type and a p-type semiconductor forming a p-n junction. The upper area is extended and transparent, generally exposed to the sun. These diodes or cells are exceptional that generate a voltage when exposed to light. The cells convert light energy directly into electrical energy.
The following figure shows the symbol of photovoltaic cell.
The construction of a photovoltaic cell is similar to that of a PN junction diode. There is no current flow through the device when no light is applied. In this state, the cell will not be able to generate current.
It is essential to bias the cell properly which requires a fair amount of light. As soon as light is applied, a remarkable state of PN junction diode can be observed. As a result, the electrons acquire sufficient energy and break away from the parent atoms. These newly generated electron-hole pairs in the depletion region crosses the junction.
In this action, the electrons move into the N type material because of its normal positive ion concentration. Likewise holes sweep into the P type material because of its negativeion content. This causes the N type material to instantly take on a negative charge and the P material to take on a positive charge. The P-N junction then delivers a small voltage as a response.
The following figure on the left, shows one of the characteristics, a graph between reverse current (IR) and illumination (E) of a photo diode. IR is measured on the vertical axis and illumination is measured on the horizontal axis. The graph is a straight line passing through the zero position.
i.e, IR = mE
m = graph straight line slope
The parameter m is the sensitivity of the diode.
The figure on the right, shows another characteristic of the photo diode, a graph between reverse current (IR) and reverse voltage of a photo diode. It is clear from the graph that for a given reverse voltage, reverse current increases as the illumination increases on the PN junction.
These cells generally supply electrical power to a load device when light is applied. If a larger voltage is required, array of these cells are used to provide the same. For this reason, photovoltaic cells are used in applications where high levels of light energy are available.