replacing. For example, silicon—presently the backbone of the semiconductor industry—has four valence electrons. Silicon can be n-doped
with phosphorous (five valence electrons) to form an n-type semiconductor or p-doped with boron (three valence electrons) to form a p-type
semiconductor. These impurities either add occupied orbitals in the band
gap (n-doping) or empty orbitals in the band gap (p-doping). Schematic
densities of states for n-type and p-type semiconductors are shown in
Figure 9.6. For either type of doping, the probability that an electron can
be excited is greatly increased compared to an undoped semiconductor.
Note that molecular materials and nanomaterials may have densities of
states that resemble a doped system and may be intrinsically n-type or ptype without further modification; the ability control electronic structure
via molecular design or particle size is greatly beneficial for nanomaterials
in contrast with conventional bulk semiconductors.
In an n-type material, electrons in the conduction band are the mobile
charge carrier. However, in a p-type system, the mobile charge carriers are
unoccupied orbitals in the valence band, which are called holes (h
+
) and
are treated as having a positive charge since they represent the absence
of an electron. The difference in electronic structure charge carrier type
between different types of semiconductors can be exploited to build
functional devices such as solar cells, light emitting diodes, and transistors.
9.2.3 Solar cells and light-emitting diodes
One illustrative example of a class of semiconductor device that uses charge
transfer to convert energy from an external source to useful electrical work
is solar cells, also known as photovoltaic cells. Solar cells consist of a layer
of a p-type material in contact with a layer of an n-type material (Figure
9.7). The difference in the band energies creates an electric field in the
region between the materials. If light at a wavelength sufficient to excite an
electron in the p-type material from the valence band to the conduction
band is shone on the device, electrons are excited to the conduction band,
then are pulled into the conduction band of the n-type material by the
electric field induced by difference in band energies between the two
materials. A hole is left behind in the valence band of the p-type material.
The electron and hole each migrate to electrodes and pass through an
electric circuit where they recombine and deliver their energy (Figure 9.7).
The maximum voltage (called the open-circuit voltage, or V OC ) of an
individual photovoltaic cell is limited by the energy difference of the
CHAPTER 9: Introduction to Functional Nanomaterials
344
with phosphorous (five valence electrons) to form an n-type semiconductor or p-doped with boron (three valence electrons) to form a p-type
semiconductor. These impurities either add occupied orbitals in the band
gap (n-doping) or empty orbitals in the band gap (p-doping). Schematic
densities of states for n-type and p-type semiconductors are shown in
Figure 9.6. For either type of doping, the probability that an electron can
be excited is greatly increased compared to an undoped semiconductor.
Note that molecular materials and nanomaterials may have densities of
states that resemble a doped system and may be intrinsically n-type or ptype without further modification; the ability control electronic structure
via molecular design or particle size is greatly beneficial for nanomaterials
in contrast with conventional bulk semiconductors.
In an n-type material, electrons in the conduction band are the mobile
charge carrier. However, in a p-type system, the mobile charge carriers are
unoccupied orbitals in the valence band, which are called holes (h
+
) and
are treated as having a positive charge since they represent the absence
of an electron. The difference in electronic structure charge carrier type
between different types of semiconductors can be exploited to build
functional devices such as solar cells, light emitting diodes, and transistors.
9.2.3 Solar cells and light-emitting diodes
One illustrative example of a class of semiconductor device that uses charge
transfer to convert energy from an external source to useful electrical work
is solar cells, also known as photovoltaic cells. Solar cells consist of a layer
of a p-type material in contact with a layer of an n-type material (Figure
9.7). The difference in the band energies creates an electric field in the
region between the materials. If light at a wavelength sufficient to excite an
electron in the p-type material from the valence band to the conduction
band is shone on the device, electrons are excited to the conduction band,
then are pulled into the conduction band of the n-type material by the
electric field induced by difference in band energies between the two
materials. A hole is left behind in the valence band of the p-type material.
The electron and hole each migrate to electrodes and pass through an
electric circuit where they recombine and deliver their energy (Figure 9.7).
The maximum voltage (called the open-circuit voltage, or V OC ) of an
individual photovoltaic cell is limited by the energy difference of the
CHAPTER 9: Introduction to Functional Nanomaterials
344