Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 234

Within the energy gap model electrons are energetically lifted beyond the con-
duction band so that they are no longer considered as a part of the solid body. For
individual atoms the corresponding energetic borderline is the ionisation energy;
whereas it is referred to as vacuum level for solid bodies. It is defined as the ener-
getic border at which the energy of the electron detached from the solid body,
inside the vacuum, equals zero. The vacuum level is thus identical to the upper
edge of the conduction band or the upper edge of all bands above the valence
band.
Internal photo effect. The internal photo effect describes also absorption of elec-
tromagnetic radiation within a solid body. The electrons are in this case not de-
tached from the solid body. They are only lifted from the valence band up to the
conduction band. Therefore, electron-hole-pairs are created which enhance the
electric conductivity of the solid body (Chapter 6.1.3)
The internal photo effect is the basis for the photovoltaic effect and thus of the
solar cell. However, the photovoltaic effect requires an additional boundary layer,
for instance a metal-semiconductor junction, a p-n-junction or a p-n-hetero-
junction (i.e. an interface between two different materials with different types of
conductivity; Chapter 6.1.6).
6.1.5 P-n-junction
By well defined addition of donors and acceptors (diffusion, alloying, ion implan-
tation) adjacent p- and n-regions are created inside a semiconductor crystal
(Fig. 6.4). Especially abrupt transitions from one type of conductivity to the other
one are obtained by epitaxy. Here, the layer by layer growth of a semiconductor
enables a transition within almost one atomic layer to the subsequent one.
If p- and n-doped materials brought into contact, holes from the p-doped side
diffuse into the n-type region and vice versa. First a strong concentration gradient
is formed at the p-n-junction, consisting of electrons inside the conduction band
and holes inside the valence band. Due to this concentration, gradient holes from
the p-region diffuse into the n-region while electrons diffuse from the n- to the p-
area. Due to the diffusion, the number of majority carriers are reduced on both
sides of the p-n-junction. The charge attached to the stationary donors or accep-
tors then creates a negative space charge on the p-side of the transition area and a
positive space charge on the n-side.
As a result of the equilibrated concentration of free charge carriers an electrical
field is built up across the border interface (p-n-junction). The described process
creates a depletion layer in which diffusion flow and reverse current compensate
each other. The no longer compensated stationary charges of donors and acceptors
define a depletion layer whose width is dependent on the doping concentration
(Fig. 6.4, Fig. 6.5).