3 Solar Cells: Basics
35
The highest energy level of the valence band is given by the edge of the
valence band. It is designated by the symbol E v .
(d) There exists a “conduction band”. Here, the electrons are free to move
around because the band is not fully occupied. The lowest energy level
of the conduction band is given by the edge of the conduction band. It is
designated by the symbol E c
(e) Between valence band and conduction band, there is a “forbidden zone”,
also called “gap” or, to be more precise “bandgap E g ”.
3. Elementary particles in quantum physics
(a) Electron → elementary particle of electric current;
i. Charge of the electron: ≈ 1.6 × 10
−19 C
ii. Mass of the electron: ≈ 9 × 10
−31 kg
iii. Energy of the electron: depends on the state of the electron.
(b) Photon → elementary particle of light
i. Charge of the photon: 0
ii. Mass of the photon: 0
iii. Energy of the photon: depends on the wavelength λ of the light according to the expression E photon = (h × c)/λ, with h Planck’s constant (≈
6.6 × 10
−34 kg m
2 s
−1 ) and c the speed of light (≈ 3 × 10
8 m s
−1 ).
(c) Phonon → it is not really an elementary particle, but an “excitation quantum” given by the mechanical vibrations within a crystal. However, for the
purposes of this book, we can treat it as if it were an elementary particle
i. Charge of the phonon: 0
ii. Energy of the phonon: depends on the state of vibrations within the
crystal.
4. Momentum
This is a term, which is often difficult to grasp by the beginner. A good way to
understand the concept of momentum is to think of a ball, which is being thrown
by a child. The ball has a certain mass (corresponding to its weight), say 1 kg. The
ball has also a certain velocity, say 2 m per second. Velocity is a quantity with a
direction—in the case of our ball, the direction may be horizontal, if the child is
throwing the ball forward, OR it may be vertical, if the child is throwing the ball
upwards. Now the momentum of the ball is simply the product of velocity times
mass.
5. The dual nature of light
(a) Light is a propagating wave. Its characteristic properties are the wave length
and the amplitude of the wave—the latter being related to the intensity of
the light.
Interference effects can be explained by the wave properties of light.
(b) A light wave is also composed of particles, called photons. Their energy
depends on the wavelength of the light. The number of photons in a light
wave corresponds to its amplitude and, thus, to its intensity.
35
The highest energy level of the valence band is given by the edge of the
valence band. It is designated by the symbol E v .
(d) There exists a “conduction band”. Here, the electrons are free to move
around because the band is not fully occupied. The lowest energy level
of the conduction band is given by the edge of the conduction band. It is
designated by the symbol E c
(e) Between valence band and conduction band, there is a “forbidden zone”,
also called “gap” or, to be more precise “bandgap E g ”.
3. Elementary particles in quantum physics
(a) Electron → elementary particle of electric current;
i. Charge of the electron: ≈ 1.6 × 10
−19 C
ii. Mass of the electron: ≈ 9 × 10
−31 kg
iii. Energy of the electron: depends on the state of the electron.
(b) Photon → elementary particle of light
i. Charge of the photon: 0
ii. Mass of the photon: 0
iii. Energy of the photon: depends on the wavelength λ of the light according to the expression E photon = (h × c)/λ, with h Planck’s constant (≈
6.6 × 10
−34 kg m
2 s
−1 ) and c the speed of light (≈ 3 × 10
8 m s
−1 ).
(c) Phonon → it is not really an elementary particle, but an “excitation quantum” given by the mechanical vibrations within a crystal. However, for the
purposes of this book, we can treat it as if it were an elementary particle
i. Charge of the phonon: 0
ii. Energy of the phonon: depends on the state of vibrations within the
crystal.
4. Momentum
This is a term, which is often difficult to grasp by the beginner. A good way to
understand the concept of momentum is to think of a ball, which is being thrown
by a child. The ball has a certain mass (corresponding to its weight), say 1 kg. The
ball has also a certain velocity, say 2 m per second. Velocity is a quantity with a
direction—in the case of our ball, the direction may be horizontal, if the child is
throwing the ball forward, OR it may be vertical, if the child is throwing the ball
upwards. Now the momentum of the ball is simply the product of velocity times
mass.
5. The dual nature of light
(a) Light is a propagating wave. Its characteristic properties are the wave length
and the amplitude of the wave—the latter being related to the intensity of
the light.
Interference effects can be explained by the wave properties of light.
(b) A light wave is also composed of particles, called photons. Their energy
depends on the wavelength of the light. The number of photons in a light
wave corresponds to its amplitude and, thus, to its intensity.