|To understand the operation of a PV cell, we need to consider
both the nature of the material and the nature of sunlight. Solar cells consist
of two types of material, often p-type silicon and n-type silicon. Light of
certain wavelengths is able to ionize the atoms in the silicon and the internal
field produced by the junction separates some of the positive charges
("holes") from the negative charges (electrons) within the
photovoltaic device. The holes are swept into the positive or p-layer and the
electrons are swept into the negative or n-layer. Although these opposite
charges are attracted to each other, most of them can only recombine by passing
through an external circuit outside the material because of the internal
potential energy barrier.
Therefore if a circuit is made power can be produced from the cells under illumination, since the free electrons have to pass through the load to recombine with the positive holes. The amount of power available from a PV device is determined by:
Single crystal silicon solar cells, for example cannot currently convert more than 25% of the solar energy into electricity, because the radiation in the infrared region of the electromagnetic spectrum does not have enough energy to separate the positive and negative charges in the material. Polycrystalline silicon solar cells have an efficiency of less than 20% at this time and amorphous silicon cells, are presently about 10% efficient, due to higher internal energy losses than single crystal silicon.
A typical single crystal silicon PV cell of 100 cm2 will produce about 1.5 watts of power at 0.5 volts DC and 3 amps under full summer sunlight (1000Wm-2). The power output of the cell is almost directly proportional to the intensity of the sunlight. (For example, if the intensity of the sunlight is halved the power will also be halved). An important feature of PV cells is that the voltage of the cell does not depend on its size, and remains fairly constant with changing light intensity. However, the current in a device is almost directly proportional to light intensity and size. When people want to compare different sized cells, they record the current density, or amps per square centimeter of cell area.
The power output of a solar cell can be increased quite effectively by using a tracking mechanism to keep the PV device directly facing the sun, or by concentrating the sunlight using lenses or mirrors. However, there are limits to this process, due to the complexity of the mechanisms, and the need to cool the cells. The current output is relatively stable at higher temperatures, but the voltage is reduced, leading to a drop in power as the cell temperature is increased. More information on PV concentrators can be found later in this information file. (reference 18)