40
A. Shah
3.2.2 Direct, Non-direct and Indirect Band-Gaps
The terms “direct”, “non-direct” and “indirect” refer here to the interaction between
incoming light and the semiconductor itself:
The semiconductor is considered to have a direct gap, when the absorption of an
incoming photon is directly possible, without the interaction of a phonon
2 within the
semiconductor. GaAs, CdTe and CuInSe 2 are typical examples of semiconductors
with a direct gap.
The semiconductor is considered to have an indirect gap, when the absorption of
an incoming photon is only possible with the simultaneous interaction of a phonon
within the semiconductor. Here, the probability of absorption is significantly reduced,
as compared to the case of a semiconductor with a direct bandgap.
3 Si, Ge and C
are typical examples of semiconductors with indirect gaps. These materials have
absorption coefficients that are relatively low (see Fig. 3.5
4 ).
The semiconductor is considered to have a non-direct gap, if it is an amorphous
semiconductor, and if it has an indirect bandgap in its crystalline form. Due to its
amorphous structure, the interaction with a suitable phonon becomes “easier” (i.e.
it has a higher probability), and therefore the absorption coefficient is increased, by
about a factor of ten, with respect to the crystalline form. On the other hand, the
values of the bandgap are modified (increased) by the amorphous structure.
5
Therefore, if silicon is used as a solar cell material, we basically have to use
relatively thick silicon wafers; this is in contrast to the case of GaAs, CdTe and
CuInSe 2 , where thin photo-absorbing layers are already sufficient. For this reason,
sophisticated light-trapping techniques have been developed for silicon solar cells;
especially for thin-film silicon solar cells.
2 A phonon is a quantum of mechanical energy, given by the vibration of the atoms within the
material.
3 In semiconductors with an indirect gap, one needs the presence of a specific, suitable phonon, at
the time and at the place where the light is absorbed. This requirement reduces the probability of
absorbing the light. Thus, the probabilities of absorption, as well as the absorption coefficient are
both significantly reduced.
4 One notes from this figure that as the wavelength of the light increases, the absorption coefficient
becomes smaller—and the corresponding penetration depth d pen of the light becomes higher: This
means that, in all cases (for all solar cell materials and solar cell designs), light with long wavelengths,
i.e. infrared light, is not properly absorbed: This part of the incoming light spectrum penetrates
deeply into the solar cell, with only a small part of it being absorbed and used by the solar cell. The
majority of such light just passes through the solar cell, without rendering any contribution to the
solar cell current. Take, as an example a crystalline silicon wafer with a thickness d wafer = 100 µm;
Light with a wavelength over about 1000 nm will only be very insufficiently absorbed, as here d pen
> d wafer .
5 This effect is up to now not fully understood.
A. Shah
3.2.2 Direct, Non-direct and Indirect Band-Gaps
The terms “direct”, “non-direct” and “indirect” refer here to the interaction between
incoming light and the semiconductor itself:
The semiconductor is considered to have a direct gap, when the absorption of an
incoming photon is directly possible, without the interaction of a phonon
2 within the
semiconductor. GaAs, CdTe and CuInSe 2 are typical examples of semiconductors
with a direct gap.
The semiconductor is considered to have an indirect gap, when the absorption of
an incoming photon is only possible with the simultaneous interaction of a phonon
within the semiconductor. Here, the probability of absorption is significantly reduced,
as compared to the case of a semiconductor with a direct bandgap.
3 Si, Ge and C
are typical examples of semiconductors with indirect gaps. These materials have
absorption coefficients that are relatively low (see Fig. 3.5
4 ).
The semiconductor is considered to have a non-direct gap, if it is an amorphous
semiconductor, and if it has an indirect bandgap in its crystalline form. Due to its
amorphous structure, the interaction with a suitable phonon becomes “easier” (i.e.
it has a higher probability), and therefore the absorption coefficient is increased, by
about a factor of ten, with respect to the crystalline form. On the other hand, the
values of the bandgap are modified (increased) by the amorphous structure.
5
Therefore, if silicon is used as a solar cell material, we basically have to use
relatively thick silicon wafers; this is in contrast to the case of GaAs, CdTe and
CuInSe 2 , where thin photo-absorbing layers are already sufficient. For this reason,
sophisticated light-trapping techniques have been developed for silicon solar cells;
especially for thin-film silicon solar cells.
2 A phonon is a quantum of mechanical energy, given by the vibration of the atoms within the
material.
3 In semiconductors with an indirect gap, one needs the presence of a specific, suitable phonon, at
the time and at the place where the light is absorbed. This requirement reduces the probability of
absorbing the light. Thus, the probabilities of absorption, as well as the absorption coefficient are
both significantly reduced.
4 One notes from this figure that as the wavelength of the light increases, the absorption coefficient
becomes smaller—and the corresponding penetration depth d pen of the light becomes higher: This
means that, in all cases (for all solar cell materials and solar cell designs), light with long wavelengths,
i.e. infrared light, is not properly absorbed: This part of the incoming light spectrum penetrates
deeply into the solar cell, with only a small part of it being absorbed and used by the solar cell. The
majority of such light just passes through the solar cell, without rendering any contribution to the
solar cell current. Take, as an example a crystalline silicon wafer with a thickness d wafer = 100 µm;
Light with a wavelength over about 1000 nm will only be very insufficiently absorbed, as here d pen
> d wafer .
5 This effect is up to now not fully understood.