3 Solar Cells: Basics
69
Fig. 3.27 IQE curve of a heterojunction crystalline silicon solar cell. Courtesy of Mathieu Boccard,
PV-Lab, IMT Neuchâtel, EPFL
range of long wavelengths have a much lower energy per photon, than the photons
in the range of short wavelengths. In fact, according to the Preamble to this chapter,
the energy of a photon depends on the wavelength λ of the light according to the
expression E photon = (h × c)/λ, with h being the constant of Planck (≈ 6.6 × 10
−34
kg m
2 s
−1 ) and c the speed of light (≈ 3 × 10
8 m s
−1 ). Thus, for high values of
λ (long wavelength region) E photon has a small value, whereas for low values of λ
(short wavelength region) E photon has a high value.
If we now compare Figs. 3.26 and 3.27, these are both Quantum Efficiency Curves,
so we are, for both figures, counting the incoming photons—however to plot Fig. 3.26,
which is the EQE curve, we are counting all the photons, which reach the solar cell,
whereas in Fig. 3.27, which is the IQE curve, we count only those photons, which
reach the photoactive region of the cell.
3.6.3 Practical Consequences
In Chap. 2 of this book, various curves of Spectral Irradiance were presented (with
the units [W/m
2 /nm]) and for different conditions: (a) different times of the day (one
noted there, that in the mornings and in the evenings, there was comparatively more
red light coming in than at noon); (b) for different weather conditions (one noted
there that if there was snow and sun, there was comparatively more red light coming
in than if one had a cloudy day); (c) for indoor lighting with various lamps (one noted
there that for most LED lamps, there was comparatively more blue light coming in
than if one had direct sunlight).
Thus, if the ambient conditions to which the solar cell or module is exposed
changes, not only the intensity of the light is modified—even drastically reduced in
69
Fig. 3.27 IQE curve of a heterojunction crystalline silicon solar cell. Courtesy of Mathieu Boccard,
PV-Lab, IMT Neuchâtel, EPFL
range of long wavelengths have a much lower energy per photon, than the photons
in the range of short wavelengths. In fact, according to the Preamble to this chapter,
the energy of a photon depends on the wavelength λ of the light according to the
expression E photon = (h × c)/λ, with h being the constant of Planck (≈ 6.6 × 10
−34
kg m
2 s
−1 ) and c the speed of light (≈ 3 × 10
8 m s
−1 ). Thus, for high values of
λ (long wavelength region) E photon has a small value, whereas for low values of λ
(short wavelength region) E photon has a high value.
If we now compare Figs. 3.26 and 3.27, these are both Quantum Efficiency Curves,
so we are, for both figures, counting the incoming photons—however to plot Fig. 3.26,
which is the EQE curve, we are counting all the photons, which reach the solar cell,
whereas in Fig. 3.27, which is the IQE curve, we count only those photons, which
reach the photoactive region of the cell.
3.6.3 Practical Consequences
In Chap. 2 of this book, various curves of Spectral Irradiance were presented (with
the units [W/m
2 /nm]) and for different conditions: (a) different times of the day (one
noted there, that in the mornings and in the evenings, there was comparatively more
red light coming in than at noon); (b) for different weather conditions (one noted
there that if there was snow and sun, there was comparatively more red light coming
in than if one had a cloudy day); (c) for indoor lighting with various lamps (one noted
there that for most LED lamps, there was comparatively more blue light coming in
than if one had direct sunlight).
Thus, if the ambient conditions to which the solar cell or module is exposed
changes, not only the intensity of the light is modified—even drastically reduced in