|SEMICONDUCTOR NON-TRADITIONAL ENERGY SOURCES|
The internal electric field in pn-junction of solar element separate holes and electrons and the generation of photocurrent takes place. A total current of the illuminated solar element has two components: a photocurrent and a dark current. The electrical equivalent circuit of a solar element can be presented as consisting from a nonlinear element - pn junction and generator of a photocurrent (fig 1 без резисторов). According to the circuit for I-V characteristics of a solar elements the next equation can be written down:
Whence, if to put a current equal to zero, it is possible to receive expression for an voltage of the open circuit:
That in loading the maximal power was allocated, it is necessary that its resistance was coordinated with solar cell I-V characteristic. The maximal power, generating in the load - Pm will be equal to the area of the maximal rectangular, which can be entered in space covered I-V curve of a solar element (see the bottom right figure - a curve 1). The relation of the maximal power in loading to current-voltage product (area covering I-V curve of a rectangular) is designated FF, and refers to as as factor of filling
FF = Pm / (UХХ·IКЗ).
Efficiency of the solar element equals to the relation of loading electrical power to power of falling radiation:
As it is visible from the given formula, the efficiency depends on a kind I-V curve than closer I-V curve to rectangular, than higher efficiency. The real solar cells always have dissipated power resistance of contacts and the semiconductor volume, which are presented by Rs at the equivalent circuit. If we take into consideration this resistor, so as Rsh (fig 2) the equation for IV-curve is the next:
In ideal case J0 = 0, Rsh infinetly large ¥.
The series and shunt resistors decreas FF and consistently output power )curve 2 upper figure).This equation take into consideration generation-recombination current, which in solar cells on the base of wide band gap materials surpasses diffusion current of pn-junction. On the basis of the equivalent circuit it is possible to write down the equation connecting parameters of a solar element at the given meaning of a photocurrent IL: