Page 37 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR CELL ELECTRONICS 7

Figure 1.3 Semiconductor depletion region formations.

In light-emitting diodes (LEDs), a controlled amount and type of doping material in
a PN-type device connected across a dc voltage source converts the electric energy
to visible light with differing frequencies and colors, such as white, red, blue,
amber, and green.
In solar cells, when a PN junction is exposed to sunshine, the device converts the
stream of photons (packets of quanta) that form the visible light into electrons (the
reverse of the LED function), making the device behave like a minute battery with
a unique characteristic voltage and current, which is dependent on the material
dopants and PN-junction physics. This is shown in Figure 1.3.
The bundles of photons that penetrate the PN junction randomly strike silicon atoms
and give energy to the outer electrons. The acquired energy allows the outer electrons
to break free from the atom. Thus, the photons in the process are converted to electron
movement or electric energy as shown in Figure 1.4.
It should be noted that the photovoltaic energy conversion efficiency is dependent
on the wavelength of the impinging light. Red light, which has a lower frequency,
produces insufficient energy, whereas blue light, which has more energy than needed
to break the electrons, is wasted and dissipates as heat.

Solar Cell Electronics

An electrostatic field is produced at a PN junction of a solar cell by impinging photons
that create 0.5 V of potential energy, which is characteristic of most PN junctions and
all solar cells. This miniscule potential resembles in function a small battery with