Anatomy of a Solar Cell

B­efore now, our silicon was all electrically neutral. Our
extra electrons were balanced out by the extra protons in the
phosphorous. Our missing electrons (holes) were balanced out
by the missing protons in the boron. When the holes and
electrons mix at the junction between N-type and P-type
silicon, however, that neutrality­ is disrupted. Do all the
free electrons fill all the free holes? No. If they did, then
the whole arrangement wouldn't be very useful. Right at the
junction, however, they do mix and form a barrier, making it
harder and harder for electrons on the N side to cross to the
P side. Eventually, equilibrium is reached, and we have
an electric field separating the two sides.

This electric field acts as a diode, allowing (and even
pushing) electrons to flow from the P side to the N side, but
not the other way around. It's like a hill -- electrons can
easily go down the hill (to the N side), but can't climb it
(to the P side).

So we've got an electric field acting as a diode in which
electrons can only move in one direction.

When light, in the form of photons, hits our solar cell, its
energy frees electron-hole pairs.

Each photon with enough energy will normally free exactly one
electron, and result in a free hole as well. If this happens
close enough to the electric field, or if free electron and
free hole happen to wander into its range of influence, the
field will send the electron to the N side and the hole to
the P side. This causes further disruption of electrical
neutrality, and if we provide an external current path,
electrons will flow through the path to their original side
(the P side) to unite with holes that the electric field sent
there, doing work for us alo­ng the way. The electron flow
provides the current, and the cell's electric field causes
a voltage. With both current and voltage, we have power,
which is the product of the two.

There are a few more steps left before we can really use our
cell. Silicon happens to be a very shiny material, which
means that it is very reflective. Photons that are reflected
can't be used by the cell. For that reason, an antireflective
coating is applied to the top of the cell to reduce
reflection losses to less than 5 percent.

The final step is the glass cover plate that protects the
cell from the elements. PV modules are made by connecting
several cells (usually 36) in series and parallel to achieve
useful levels of voltage and current, and putting them in
a sturdy frame complete with a glass cover and positive and
negative terminals on the back.

How much sunlight energy does our PV cell absorb?
Unfortunately, the most that our simple cell could absorb is
around 25 percent, and more likely is 15 percent or less. Why
so little?