By David Herres
Solar cells, the building blocks of solar arrays, are simple two-wire diodes packaged and configured to produce electrical current when exposed to electromagnetic radiation of suitable frequency. This implies the cell must be flat, with a broad side situated as close to perpendicular as possible to the incoming radiation. It is also essential that this front surface is transparent so radiation can enter the cell.
A solar array consists of a large number of solar cells, wired in series and parallel. The connection to the cell can be problematic because the lead, as in any diode, must attach to electrodes at both sides of the semiconducting junction. But conductive materials are opaque and transparent materials such as glass are insulators. So how can the outer front surface serve as an electrode and also admit sunlight? (The back side doesn’t get illuminated and the electrode can be a metal plate.)
The challenge has been partially solved by designing solar cells in which the front surface contains a conductive grid to which the lead is attached. This limits the light that can enter the cell, which is a drawback. Making the conductive members of the grid smaller will admit more light, but at the expense of boosting the impedance of the cell. This accumulated impedance across the total array results in a significant loss of power generation.
One method for mitigating both types of loss is to form the conductors that cross the face of the cell as rectangular bars, the greater dimension perpendicular to the face of the cell. That way the blocking effect is to some extent lessened while maintaining conductivity. Another approach to the same dilemma is to extend two or more heavier bus bars across the face of the cell. They serve as distribution lines for more numerous slender conductors oriented perpendicular to them. The smaller cross section can be tolerated because the length is less.
When solar radiation strikes any object that is not totally reflective or totally transparent, the object absorbs some of the solar energy so there is increased molecular, atomic and subatomic activity. This is natural, because the energy must go somewhere. Unfortunately, this energy does not take the form of a useful voltage that would produce current through an external load. That is because the atomic motion is random, so any voltage potentials would cancel out.
The solar cell creates usable dc at its output because it is a diode. In essence the random and chaotic electrical energy is rectified in the cell and given a path .