74
S. Leu and D. Sontag
4.1.1 Preliminary Remarks: Reflection, Refraction,
Absorption and Transmission
In Chap. 3, we learned that electricity is generated when a photon is absorbed by a
solar cell. The basic materials for solar cells are semiconductors. Semiconductors are
characterized by having an energetic bandgap (forbidden band or forbidden zone)
between the valence band and the conduction band. By absorbing photons, electrons
can be lifted from the valence band into the conduction band. The condition for
this is that the photon energy is equal to or greater than the bandgap energy of
the semiconductor.
1 If this condition is fulfilled, positively and negatively charged
electric charge carriers can be generated by the incoming sunlight. The conductivity
of the semiconductor plays an important role in the separation of the charge carriers
and letting them flow out of the solar cell. Doping can increase the conductivity. In
this Sect. 4.1 we want to learn how we can absorb as many photons as possible.
Light that strikes the surface of a material will partly be reflected; the rest of the
light will penetrate into the material. The light, which penetrates into the material,
is refracted. By “refraction” one designates the bending of the light rays when they
enter into a material. Light rays are refracted at each interface as they transit from one
material, to another material that has different optical properties. In photovoltaics,
one has very many interfaces, between regions, which have different optical properties—either because they are doped in a different way or because their chemical
composition is different.
These are the 4 basic phenomena in Optics: reflection, refraction, absorption
and transmission. Newton already thought about the phenomenon of partial reflection. Indeed, at the boundary (interface) between two different materials, it is not
clear how a photon can “decide” whether to penetrate into the second material or to
be reflected back into the first material. Quantum Physics give us the answer: it tells
us that we can only state the probability with which photons are reflected. We do
not know which photon will be reflected and which one will be transmitted. But we
know that always the same proportion of photons is reflected; we also have precise
data on the optical properties of different materials.
Another interesting phenomenon of optics, which is important to us, was found
by Pierre de Fermat: When light is refracted at a boundary layer or “interface”, it
will always take the fastest path, not the shortest path. This property also gives rise
to puzzles: How can the light at the boundary layer “know” which path is the fastest
one? And how can the photons “know” which one of them should be reflected and
which one may penetrate into the next material, so that always the same proportion
of photons are reflected? Quantum Physics provides the following explanation: the
light simply “tries out” all possible ways at the same time. All alternative paths (i.e.
all paths except the one that is selected) are cancelled out by destructive overlapping.
1 The bandgap of a silicon crystal is 1.12 eV.
S. Leu and D. Sontag
4.1.1 Preliminary Remarks: Reflection, Refraction,
Absorption and Transmission
In Chap. 3, we learned that electricity is generated when a photon is absorbed by a
solar cell. The basic materials for solar cells are semiconductors. Semiconductors are
characterized by having an energetic bandgap (forbidden band or forbidden zone)
between the valence band and the conduction band. By absorbing photons, electrons
can be lifted from the valence band into the conduction band. The condition for
this is that the photon energy is equal to or greater than the bandgap energy of
the semiconductor.
1 If this condition is fulfilled, positively and negatively charged
electric charge carriers can be generated by the incoming sunlight. The conductivity
of the semiconductor plays an important role in the separation of the charge carriers
and letting them flow out of the solar cell. Doping can increase the conductivity. In
this Sect. 4.1 we want to learn how we can absorb as many photons as possible.
Light that strikes the surface of a material will partly be reflected; the rest of the
light will penetrate into the material. The light, which penetrates into the material,
is refracted. By “refraction” one designates the bending of the light rays when they
enter into a material. Light rays are refracted at each interface as they transit from one
material, to another material that has different optical properties. In photovoltaics,
one has very many interfaces, between regions, which have different optical properties—either because they are doped in a different way or because their chemical
composition is different.
These are the 4 basic phenomena in Optics: reflection, refraction, absorption
and transmission. Newton already thought about the phenomenon of partial reflection. Indeed, at the boundary (interface) between two different materials, it is not
clear how a photon can “decide” whether to penetrate into the second material or to
be reflected back into the first material. Quantum Physics give us the answer: it tells
us that we can only state the probability with which photons are reflected. We do
not know which photon will be reflected and which one will be transmitted. But we
know that always the same proportion of photons is reflected; we also have precise
data on the optical properties of different materials.
Another interesting phenomenon of optics, which is important to us, was found
by Pierre de Fermat: When light is refracted at a boundary layer or “interface”, it
will always take the fastest path, not the shortest path. This property also gives rise
to puzzles: How can the light at the boundary layer “know” which path is the fastest
one? And how can the photons “know” which one of them should be reflected and
which one may penetrate into the next material, so that always the same proportion
of photons are reflected? Quantum Physics provides the following explanation: the
light simply “tries out” all possible ways at the same time. All alternative paths (i.e.
all paths except the one that is selected) are cancelled out by destructive overlapping.
1 The bandgap of a silicon crystal is 1.12 eV.