7.19 Production of Visible Light
237
If the intermediate state is ‘metastable’, i.e. it has a relatively low probability of
making a transition to a lower state, then the material is called ‘phosphorescent’.
Metastable decay times for various materials range from a few thousandth of a
second to hours.
7.19.2 Solid-State Generation of Light
In solid materials, many of the electrons are tightly bound to atoms and molecules.
The least tightly held are referred to as valence electrons, as they can take part
in the chemistry of bonding. Weakly held electrons may jump between atoms, or
even become delocalized across the material, making that material a good electrical
conductor. The possible electron quantum states for the valence electrons of all the
atoms are numerous. If a large number of quantum states have nearby energies,
they are said to be in an “energy band”. The valence electrons are in the ‘valence
band’. With the Pauli exclusion principle at work, the valence electrons fill the
possible quantum states with only one per state. At room temperatures, the lowest
of these quantum states will be just about completely filled. Quantum states with
energy far above k B T will likely be unoccupied. The energy at which the occupancy
probability takes the value of one half is called the ‘Fermi energy level’,
If the electrons near the Fermi level are free to roam throughout the material,
they are called conduction electrons, and occupy the ‘conduction band’. These are
the electrons which carry current in the metal. They also cause polished metals to
appear shinny (by reflecting light), and to be good conductors of heat (which is
carried quickly by the ‘gas’ of electrons in the metal).
In insulators and semiconductors, there is an energy gap between the valence
band and the conduction band. (Good conductors have no such gap.) In semiconductors, the gap is less than about 4 eV, so that a few volts placed across the
semiconductor can pull electrons from the valence band into the conduction band.
For silicon, the gap is 1.2 eV, close to the energy of photons at the solar intensity
peak, 1.4 eV. Thus, silicon is useful for generating electric energy from sunlight as
a photoelectric solar cell. 30
Similarly, as described in Sect. 7.17.7, light-emitting diodes (LEDs) use semiconductors to generate visible light.
30 Photoelectric solar cells with frequency filters above optimized semiconducting surfaces can
reach energy conversion efficiencies of 35% (Zhao et al., A solar photovoltaic system with ideal
efficiency close to the theoretical limit, Opt Express 20(1) A28–A38 (2012)). Plant photosynthesis
can be up to 2% efficient.
237
If the intermediate state is ‘metastable’, i.e. it has a relatively low probability of
making a transition to a lower state, then the material is called ‘phosphorescent’.
Metastable decay times for various materials range from a few thousandth of a
second to hours.
7.19.2 Solid-State Generation of Light
In solid materials, many of the electrons are tightly bound to atoms and molecules.
The least tightly held are referred to as valence electrons, as they can take part
in the chemistry of bonding. Weakly held electrons may jump between atoms, or
even become delocalized across the material, making that material a good electrical
conductor. The possible electron quantum states for the valence electrons of all the
atoms are numerous. If a large number of quantum states have nearby energies,
they are said to be in an “energy band”. The valence electrons are in the ‘valence
band’. With the Pauli exclusion principle at work, the valence electrons fill the
possible quantum states with only one per state. At room temperatures, the lowest
of these quantum states will be just about completely filled. Quantum states with
energy far above k B T will likely be unoccupied. The energy at which the occupancy
probability takes the value of one half is called the ‘Fermi energy level’,
If the electrons near the Fermi level are free to roam throughout the material,
they are called conduction electrons, and occupy the ‘conduction band’. These are
the electrons which carry current in the metal. They also cause polished metals to
appear shinny (by reflecting light), and to be good conductors of heat (which is
carried quickly by the ‘gas’ of electrons in the metal).
In insulators and semiconductors, there is an energy gap between the valence
band and the conduction band. (Good conductors have no such gap.) In semiconductors, the gap is less than about 4 eV, so that a few volts placed across the
semiconductor can pull electrons from the valence band into the conduction band.
For silicon, the gap is 1.2 eV, close to the energy of photons at the solar intensity
peak, 1.4 eV. Thus, silicon is useful for generating electric energy from sunlight as
a photoelectric solar cell. 30
Similarly, as described in Sect. 7.17.7, light-emitting diodes (LEDs) use semiconductors to generate visible light.
30 Photoelectric solar cells with frequency filters above optimized semiconducting surfaces can
reach energy conversion efficiencies of 35% (Zhao et al., A solar photovoltaic system with ideal
efficiency close to the theoretical limit, Opt Express 20(1) A28–A38 (2012)). Plant photosynthesis
can be up to 2% efficient.