3 Solar Cells: Basics
41
Fig. 3.5 Absorption coefficient α and penetration depth d pen (of monochromatic light), as a function
of wavelength λ and of photon energy hν, for three semiconductor materials commonly used in solar
cells. The penetration depth d pen is thereby defined as the depth at which the light has decreased
to 1/e of its original value; e being Euler’s number (The number e = 2.718… is a mathematical
constant that is the base of the natural logarithm: the unique number whose natural logarithm is
equal to one). Adapted from [11]
3.2.3 Spectrum of the Incoming Light
The quantity of light absorbed by a semiconductor depends on the bandgap energy (as
previously discussed), but also on the spectrum of the light (i.e. the energy distribution
of the incident radiation as a function of wavelength). Figure 3.6 is a reproduction of
Fig. 2.1 in Chap. 2: It shows the spectrum of sunlight outside the earth’s atmosphere
(AM 0) and that of sunlight on the surface of the earth (AM 1.5), under the precise
conditions defined in Chap. 2. The AM 1.5 spectrum is considered to be the reference
spectrum for all terrestrial solar modules, whereas the AM 0 spectrum is used for
space applications of solar cells.
3.2.4 Relationship Between Light Spectrum
and Semiconductor Bandgap
A semiconductor with a lower value of bandgap energy will be able to absorb a
broader range of the solar spectrum (i.e. more low-energy photons) compared to a
semiconductor with a higher bandgap. However, in the first case, a substantial part
of the incident energy will be lost by thermalisation (i.e. by the energy difference
41
Fig. 3.5 Absorption coefficient α and penetration depth d pen (of monochromatic light), as a function
of wavelength λ and of photon energy hν, for three semiconductor materials commonly used in solar
cells. The penetration depth d pen is thereby defined as the depth at which the light has decreased
to 1/e of its original value; e being Euler’s number (The number e = 2.718… is a mathematical
constant that is the base of the natural logarithm: the unique number whose natural logarithm is
equal to one). Adapted from [11]
3.2.3 Spectrum of the Incoming Light
The quantity of light absorbed by a semiconductor depends on the bandgap energy (as
previously discussed), but also on the spectrum of the light (i.e. the energy distribution
of the incident radiation as a function of wavelength). Figure 3.6 is a reproduction of
Fig. 2.1 in Chap. 2: It shows the spectrum of sunlight outside the earth’s atmosphere
(AM 0) and that of sunlight on the surface of the earth (AM 1.5), under the precise
conditions defined in Chap. 2. The AM 1.5 spectrum is considered to be the reference
spectrum for all terrestrial solar modules, whereas the AM 0 spectrum is used for
space applications of solar cells.
3.2.4 Relationship Between Light Spectrum
and Semiconductor Bandgap
A semiconductor with a lower value of bandgap energy will be able to absorb a
broader range of the solar spectrum (i.e. more low-energy photons) compared to a
semiconductor with a higher bandgap. However, in the first case, a substantial part
of the incident energy will be lost by thermalisation (i.e. by the energy difference