Environmental Engineering Reference
In-Depth Information
Conduction band
Electron
Forbidden
band
Photon
Valence band
Figure 4.4
The Lifting of Electrons from the Valence Band to the Conduction
Band Caused by Light Energy in a Semiconductor (Inner Photo Effect)
conduction band and the valence band can also overlap. Electrons can move
within the solid and contribute to the electron conductivity in a partially filled
conduction band. The specific electrical resistance of conductors is very low
(
ρ
<10
-5
m). Most conductors are metallic materials.
The specific electrical resistance of isolators is high (
Ω
m). The
conduction band is totally empty and a relatively large amount of energy is
needed to elevate electrons from the valence band to the conduction band due
to the high band gap (
E
g
ρ
>10
7
Ω
5 eV) in isolators.
Semiconductors
have the most relevance for photovoltaics. The specific
electrical resistance is between 10
-5
m. The conduction
bands of semiconductors are empty, as in the case of isolators. However, due
to the lower band gap (
E
g
< 5 eV), electrons can be more easily be lifted to
the conduction band (Figure 4.4). The elevation of electrons into the
conduction bands by photons is called the inner or
internal photo effect
.
An incident photon can have several effects: photon energy lower than the
band gap will not elevate an electron because it cannot bridge the band gap.
Photons with energy larger than the band gap can elevate electrons into the
conduction band with a part of its energy. Surplus energy is lost, because the
electron falls back to the edge of the conduction band.
Photoresistors, which change their resistance depending on the irradiance,
use the internal photo effect. Photovoltaic cells also use the internal photo
effect for generating current.
Ω
m and 10
7
Ω
Principle of solar cells
Photovoltaic systems employ semiconductors. They have four electrons in the
outer shell, or orbit, on average. These electrons are called valence electrons.
Elementary semiconductors are elements of group IV of the periodic table of
elements, for instance silicon (Si), germanium (Ge) or tin (Sn). Compounds of
two elements containing one element from group III and one from group V
(so-called III-V compounds) and II-VI compounds or combinations of various
elements also have four valence electrons on average. An example of a III-V
semiconductor is gallium arsenide (GaAs) and an example of a II-VI
