Photovoltaics - Understanding Renewable Energy Systems - page 119

Ionization boundary

Single
atom

Diatomic
molecule

Traitomic
molecule

k atoms
(solid)

Figure 4.2 Energy States of Electrons in Atoms, Molecules and Solids

internal photo effect determines the effect of light in a solar cell, and is
described in the following sections.
Whereas electrons in single atoms take clearly defined energy states, this is
not the case in molecules with multiple electrons. Interactions between
electrons of molecules with several atoms smear the energy states to narrow
neighbouring levels. In a solid with k atoms, these levels are so close to each
other that it is no longer possible to separate them. Here the single energy
states of the electron orbits become so-called energy bands (see Figure 4.2).
However, these energy bands can also carry only a limited number of electrons.
In the energy band model, electrons fill the bands one after another starting
with the first, lowest energy band. The highest fully occupied band is called
the valence band (VB). The next highest band, which can be partially occupied
or totally empty, is called the conduction band (CB). The space between the
valence band and conduction band contains forbidden energy states and is
therefore called the forbidden band (FB). The energy gap between the bands is
called the band gap E g .
Solids are subdivided into conductors, semiconductors and isolators
depending on the arrangement and occupation of the energy bands (see Figure
4.3). Electrons in conductors only fill the conduction band partially. The

E

CB

CB CB

Conductor band (CB)

E g ≥ 5 eV FB

FB

Forbidden band (FB) VB E g < 5 eV

Valence band (VB) VB VB

Conductor Semiconductor Isolator

Figure 4.3 Energy Bands of Conductors, Semiconductors and Isolators