Surface and bulk passivation of multicrystalline silicon solar cells by ...

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Figure 1.2 A schematic of photogeneration effect.
In order to produce a solar cell, the semiconductor has to be contaminated or
doped. Pure semiconductors (also called "intrinsic") do not conduct electricity at
room temperature and are thus not useful for the fabrication of electronic devices.
Semiconductors conduct electricity if they are doped with a small amount of impurity
atoms. In the case of silicon (Si), these would be atoms from group III and group V
of the periodic table. Impurities in semiconductors are divided into two broad
categories: donors and acceptors. A donor is an element with typically one more
valence electron than the group IV semiconductor. Each Si atom has four electrons in
its outermost shell; these electrons are involved in forming chemical bonds with the
neighboring Si atoms. For example, an arsenic (As) atom has five electrons in its
outermost shell. The extra electron in As can be easily removed from the As donor on
which it was originally localized. If this happens, the electron can move freely in the
semiconductor material and conduct electric current. A semiconductor doped with
donors is known as n-type material. An acceptor is an element with typically one
fewer valence electron than the element that forms the semiconductor. A boron (B)
atom has three electrons in its outermost shell. The missing electron in the chemical