3
The working principle of a solar cell
In this chapter we present a very simple model of a solar cell. Many notions presented in
this chapter will be new but nonetheless the general idea of how a solar cell works should
be clear. All the aspects presented in this chapter will be discussed in greater detail in the
following chapters.
The working principle of solar cells is based on the photovoltaic effect, i.e. the
generation of a potential difference at the junction of two different materials in response to
electromagnetic radiation. The photovoltaic effect is closely related to the photoelectric
effect, where electrons are emitted from a material that has absorbed light with a
frequency above a material-dependent threshold frequency. In 1905, Albert Einstein
understood that this effect can be explained by assuming that the light consists of well
defined energy quanta, called photons. The energy of such a photon is given by
where h is Planck’s constant and ν is the frequency of the light. For his explanation of the
photoelectric effect Einstein received the Nobel Prize in Physics in 1921 [23].
The photovoltaic effect can be divided into three basic processes:
1. Generation of charge carriers due to the absorption of photons in the materials
that form a junction
Absorption of a photon in a material means that its energy is used to excite an electron
from an initial energy level E i to a higher energy level E f , as shown in Figure 3.1 (a).
Photons can only be absorbed if electron energy levels E i and E f are present so that their
difference equals the photon energy, hν = E f − E i . In an ideal semiconductor electrons can
populate energy levels below the so-called valence band edge, E V , and above the so-called
conduction band edge, E C . Between those two bands no allowed energy states exist which
could be populated by electrons. Hence, this energy difference is called the bandgap, E G =
E C − E V . If a photon with an energy smaller than E G reaches an ideal semiconductor, it will
not be absorbed but will traverse the material without interaction.
The working principle of a solar cell
In this chapter we present a very simple model of a solar cell. Many notions presented in
this chapter will be new but nonetheless the general idea of how a solar cell works should
be clear. All the aspects presented in this chapter will be discussed in greater detail in the
following chapters.
The working principle of solar cells is based on the photovoltaic effect, i.e. the
generation of a potential difference at the junction of two different materials in response to
electromagnetic radiation. The photovoltaic effect is closely related to the photoelectric
effect, where electrons are emitted from a material that has absorbed light with a
frequency above a material-dependent threshold frequency. In 1905, Albert Einstein
understood that this effect can be explained by assuming that the light consists of well
defined energy quanta, called photons. The energy of such a photon is given by
where h is Planck’s constant and ν is the frequency of the light. For his explanation of the
photoelectric effect Einstein received the Nobel Prize in Physics in 1921 [23].
The photovoltaic effect can be divided into three basic processes:
1. Generation of charge carriers due to the absorption of photons in the materials
that form a junction
Absorption of a photon in a material means that its energy is used to excite an electron
from an initial energy level E i to a higher energy level E f , as shown in Figure 3.1 (a).
Photons can only be absorbed if electron energy levels E i and E f are present so that their
difference equals the photon energy, hν = E f − E i . In an ideal semiconductor electrons can
populate energy levels below the so-called valence band edge, E V , and above the so-called
conduction band edge, E C . Between those two bands no allowed energy states exist which
could be populated by electrons. Hence, this energy difference is called the bandgap, E G =
E C − E V . If a photon with an energy smaller than E G reaches an ideal semiconductor, it will
not be absorbed but will traverse the material without interaction.