To create practical electronic components two types of materials must be prepared
from a base of a pure intrinsic semiconductors, such materials are type n (negative)
and type p (positive),these materials are the base of all semiconductor devices
industry, including solar cells.
Material Type n
Materials type n are created by adding a controlled number of impurities atoms to
the semiconductors. On semiconductors materials type n elements with 5 valence electrons
or pentavalent such as Antimony (Sb), Arsenic (As) or Phosphorus (P) are added, these
types of elements are called electron donors or just donors. The additions of these
elements create a surplus electron in each atom that has no covalent bond in the
crystal lattice, it is very weakly attached to the parent atom easing the moving
of the electron in the new doped material n. The preferred doping element for materials
type n on solar cells is Phosphorus.
Materials Type p
Materials type p are formed by adding atoms with only 3 valence electrons such as
the Boron (b), Gallium (Ga) or Indium (In) as a mean to create a electron deficit
in the covalence bond with the Silicon atoms, this deficit of electrons creates a
positive charge or hole in the semiconductor material, these types of elements are
called electron acceptors or just acceptors, the preferred doping element for material
type p on solar cells is Boron.
It was learned that the solar cells are formed by two types of semiconductor materials
p and n, it was also learned that to increase the efficiency of the solar cells to
a practical level some small percentof impurities of other elements are added to
increase the majority carriers and conductivity.
On the n side of the junction just in the border there are very few free electrons
and in the p side just in the junction border are very few holes or positive charges,
these majority carriers attract each other producing a “neutral zone” where no more
positive or negative charge carriers will exist, after all the majority carrier have
been neutralized each other in the junction boundary a electrostatic field is formed,
the impurity elements donors now have lost their electrons and become positive, while
acceptors have lost their holes and now become negative, due to these loses the atoms
are now electrically charged and are called ions.
As seen on the figure when a photon hits the n material, an electron is displaced
making that atom positive, other electron in the lattice immediately tray to fill
that hole, now the atom that provided the electron will have a new hole to fill,
this behavior starts a chain reaction in which the hole is moving trough the atoms
in the material until no more holes are available, once all holes have been “used”
in the n material, the electrons on the p side will fill the holes trough the barrier
which has a potential between 0.4 to 0.7 volts, this complete the electrical circuit
started by the photon.