Data Sheet

Structure and Function
of a Solar Cell
Chapter 9 Theory
58
Fuel Cell 10
manner (below). Boron has
three electrons in its outer
shell, each of which enters
into a bond with the neighbor-
ing silicon atom.
There is an electron missing
for a fourth bond, however.
This kind of defect or gap is
known as an “electron hole.”
Even at room temperature, an
electron from a neighboring Si
atom can “jump over” into this
hole, making the hole seem
to move. The conductivity of
the silicon doped in this man-
ner, in other words, depends
on the mobility of the “holes”
(positive charges). This zone
is known as a p-doped layer.
Both the p-doped and the n-
doped layers conduct well,
and are neutral on their own.
So there is no voltage.
This is how the silicon is manip-
ulated to make it photoelectri-
cally sensitive. A look into the
solar cell’s crystal lattice will
show us how it works.
Creating the
Voltage Potential
In the area where the p-
and n-doped layers touch,
a boundary layer known as
a p–n junction forms, where
a few electrons from the n-
doped layer wander over into
the p-doped layer. There, they
replace electrons that are
missing for covalent bonding.
The movement of the elec-
trons from the n-doped layer
Doping the Silicon
To create solar cells from the
highly pure silicon wafers,
they have to be somewhat
“impurified” again. This pro-
cess is known as doping. It
involves vapor-deposition of
tiny doses of pure elemental
phosphorus on one surface of
the wafer, and boron on the
other surface. The proportion
of these doping elements to
the silicon is about equiva-
lent to one drop of water in a
swimming pool.
enter into a covalent bond
with four silicon atoms in the
crystal lattice. That is because
silicon is usually tetravalent,
meaning it has four “bonding
arms.”
So the phosphorus atom’s fifth
electron cannot find a bond-
ing partner, and it is there-
fore very loosely attached to
the phosphorus atom. Even at
room temperature, the bond
will be easily broken. So silicon
doped in this manner has free
electrons (negative charges)
and is therefore called an n-
doped layer.
The solar cell’s lower layer is
doped with boron in a similar
Silicon Atom
Silicon Crystal Lattice
Doped with Phosphorus
Silicon Crystal Lattice
Doped with Boron
Silicon Crystal Lattice
In the upper layer, a silicon
atom will be replaced by a
phosphorus atom in a few
spots (above). In its outer
shell, phosphorus has five
electrons. There is one elec-
tron left over, since it can only