3 Solar Cells: Basics
37
3.1 The Photovoltaic Effect: Interaction of Light
and Matter
The photovoltaic effect is the operating principle of the solar cell: it is the creation
of voltage or electric current in a material upon exposure to light. A young French
physicist, Alexandre-Edmond Becquerel (24 March 1820–11 May 1891) was the
first to experimentally demonstrate the photovoltaic effect: In 1839, at the age of 19,
whilst working in his father’s laboratory, he was able to demonstrate, for the first
time, the photovoltaic effect, i.e. the generation of an electric current, when light is
absorbed in a material [1, 2].
The theoretical basis to understand the photovoltaic effect was given by Albert
Einstein in 1905: to obtain an idea of Einstein’s arguments, let us now look, at the
nature of light:
Light has, in fact, a dual nature; on one hand, it can be described as consisting of
electromagnetic waves, with a given wavelength λ, and a given frequency ν = c/λ,
where c is the speed of light. On the other hand, it can also be viewed as consisting of
“particles of light”, or “quanta of light”, or (as we call them nowadays), of “photons”,
with a given energy E ph . Einstein was the first to use this second nature of light—i.e.
to view light as composed of particles, or “photons”. Based upon Max Planck’s theory
of black-body radiation, Einstein postulated that the energy E ph in each quantum of
light was equal to its frequency ν multiplied by a constant: E ph = hν (where h was later
called “Planck’s constant”). Einstein was trying to explain experiments carried out
by Hertz in 1887: Hertz had accidentally stumbled upon the “photoelectric” effect,
1
while doing experiments with radio waves. His observations on the interaction of
light and matter could not be explained until Einstein published his paper [3], thereby
postulating that only photons with energy E ph above a certain threshold (given by the
material used) possess the required energy to eject a single electron, thus creating
the effect observed by Hertz. This discovery led to the quantum revolution in physics
and earned Einstein the Nobel Prize in Physics in 1921.
1 The “photoelectric” effect is similar to the “photovoltaic” effect: In both cases light is absorbed by
a material, leading to the excitation of electrons within the material. In the “photovoltaic” effect, the
electrons are collected via electrodes connected to the material and create an external current: this
is the case of solar cells. In the “photoelectric” effect, the material is located in a vacuum chamber;
the electron is emitted out of the material and collected on an electrode placed within the vacuum
chamber.
37
3.1 The Photovoltaic Effect: Interaction of Light
and Matter
The photovoltaic effect is the operating principle of the solar cell: it is the creation
of voltage or electric current in a material upon exposure to light. A young French
physicist, Alexandre-Edmond Becquerel (24 March 1820–11 May 1891) was the
first to experimentally demonstrate the photovoltaic effect: In 1839, at the age of 19,
whilst working in his father’s laboratory, he was able to demonstrate, for the first
time, the photovoltaic effect, i.e. the generation of an electric current, when light is
absorbed in a material [1, 2].
The theoretical basis to understand the photovoltaic effect was given by Albert
Einstein in 1905: to obtain an idea of Einstein’s arguments, let us now look, at the
nature of light:
Light has, in fact, a dual nature; on one hand, it can be described as consisting of
electromagnetic waves, with a given wavelength λ, and a given frequency ν = c/λ,
where c is the speed of light. On the other hand, it can also be viewed as consisting of
“particles of light”, or “quanta of light”, or (as we call them nowadays), of “photons”,
with a given energy E ph . Einstein was the first to use this second nature of light—i.e.
to view light as composed of particles, or “photons”. Based upon Max Planck’s theory
of black-body radiation, Einstein postulated that the energy E ph in each quantum of
light was equal to its frequency ν multiplied by a constant: E ph = hν (where h was later
called “Planck’s constant”). Einstein was trying to explain experiments carried out
by Hertz in 1887: Hertz had accidentally stumbled upon the “photoelectric” effect,
1
while doing experiments with radio waves. His observations on the interaction of
light and matter could not be explained until Einstein published his paper [3], thereby
postulating that only photons with energy E ph above a certain threshold (given by the
material used) possess the required energy to eject a single electron, thus creating
the effect observed by Hertz. This discovery led to the quantum revolution in physics
and earned Einstein the Nobel Prize in Physics in 1921.
1 The “photoelectric” effect is similar to the “photovoltaic” effect: In both cases light is absorbed by
a material, leading to the excitation of electrons within the material. In the “photovoltaic” effect, the
electrons are collected via electrodes connected to the material and create an external current: this
is the case of solar cells. In the “photoelectric” effect, the material is located in a vacuum chamber;
the electron is emitted out of the material and collected on an electrode placed within the vacuum
chamber.