34
A. Shah
• Section 3.5 describes the limits for solar cell conversion efficiency, and, also,
how these limits are affected by operating conditions: by temperature, and by the
intensity of the incoming light.
• Section 3.6 introduces two important “tools”, which are used to evaluate solar
cells: the measurement of the cell’s “Spectral Response” and of its “Quantum
Efficiency”. Section 3.6 will explain the difference between these two methods,
as well as their practical use.
This chapter is based on concepts from Solid-State Physics and from Quantum
Theory. Unfortunately it is not possible to explain, in a precise manner, the functioning of a solar cell without using these concepts. We have therefore thought it useful
to summarize these concepts in the following “Preamble”.
Preamble: Some basic terms from semiconductor physics
1. Materials. There are basically three kinds of solid materials
(a) Insulators—they do not conduct electric current (Example: most textiles,
glass, porcelain and “plastics”).
(b) Conductors—they can easily conduct electric current (Example: Metals,
like gold, silver, copper, iron, mercury etc.). (Here, we are considering the
whole range of conductors, from those with very high conductivity, like
silver, copper, gold and aluminium, over those with medium conductivity,
like nickel, iron and tin, to those with low conductivity, like lead, stainless
steel and mercury.)
(c) Semi-Conductors—they can conduct an electric current only under certain circumstances (Example: photoconductors—conducting current only
if illuminated).
2. Band diagrams of semiconductors (and also of other solid materials)
(a) The band diagrams characterize the behaviour of electrons within the
semiconductor.
(b) The band diagram (as we use it in this book) is a plot with the Energy E on
the vertical axis and the location x on the horizontal axis.
(c) There exists a “valence band”. When the valence band is fully occupied, the
electrons are not free to move around—they are tied to an atom. However,
if an electron “goes missing”, it leaves a “hole” behind. The “hole” behaves
just like an electron but it has a positive charge, whereas the electron has
a negative charge. To understand a little better what a hole is like, let us
consider the following analogy: Imagine a large hall with many chairs.
Imagine also that most of the chairs are occupied—just very few of them
are free. Now if one of the persons gets up from his chair at position A, and
sits on a free chair, at position B—it looks as if the free chair has moved from
B to A. The free chairs correspond to the holes and the persons correspond
to the electrons.
A. Shah
• Section 3.5 describes the limits for solar cell conversion efficiency, and, also,
how these limits are affected by operating conditions: by temperature, and by the
intensity of the incoming light.
• Section 3.6 introduces two important “tools”, which are used to evaluate solar
cells: the measurement of the cell’s “Spectral Response” and of its “Quantum
Efficiency”. Section 3.6 will explain the difference between these two methods,
as well as their practical use.
This chapter is based on concepts from Solid-State Physics and from Quantum
Theory. Unfortunately it is not possible to explain, in a precise manner, the functioning of a solar cell without using these concepts. We have therefore thought it useful
to summarize these concepts in the following “Preamble”.
Preamble: Some basic terms from semiconductor physics
1. Materials. There are basically three kinds of solid materials
(a) Insulators—they do not conduct electric current (Example: most textiles,
glass, porcelain and “plastics”).
(b) Conductors—they can easily conduct electric current (Example: Metals,
like gold, silver, copper, iron, mercury etc.). (Here, we are considering the
whole range of conductors, from those with very high conductivity, like
silver, copper, gold and aluminium, over those with medium conductivity,
like nickel, iron and tin, to those with low conductivity, like lead, stainless
steel and mercury.)
(c) Semi-Conductors—they can conduct an electric current only under certain circumstances (Example: photoconductors—conducting current only
if illuminated).
2. Band diagrams of semiconductors (and also of other solid materials)
(a) The band diagrams characterize the behaviour of electrons within the
semiconductor.
(b) The band diagram (as we use it in this book) is a plot with the Energy E on
the vertical axis and the location x on the horizontal axis.
(c) There exists a “valence band”. When the valence band is fully occupied, the
electrons are not free to move around—they are tied to an atom. However,
if an electron “goes missing”, it leaves a “hole” behind. The “hole” behaves
just like an electron but it has a positive charge, whereas the electron has
a negative charge. To understand a little better what a hole is like, let us
consider the following analogy: Imagine a large hall with many chairs.
Imagine also that most of the chairs are occupied—just very few of them
are free. Now if one of the persons gets up from his chair at position A, and
sits on a free chair, at position B—it looks as if the free chair has moved from
B to A. The free chairs correspond to the holes and the persons correspond
to the electrons.