9.8 Electroluminescence 217
When this device, depicted in Figure 9.41, is connected to a direct electric
current source, the cathode, aluminum, emits electrons e
– , jumping from the
conduction band of the metal into the lowest unoccupied band of the luminescence material. The anode, ITO, releases holes h
+ into the luminescent material. In reality, an electron jumps from the highest occupied band of the
luminescence material into the conduction band of the anode, releasing a positively charged hole, in the luminescence layer. The energy necessary for this
process is U e at the side of the cathode and U h at the anode’s side. The anode
material must be selected in such a way that the energy to emit electrons, the
work function, is significantly larger than that of the cathode material. Radiative
recombination of the hole in the highest occupied band of the electroluminescent material and the electron in the conduction band leads to the emission of
light. In the simplest case, the wavelength of the emitted light λ can be
calculated using the energy of the bandgap E g : λ =
ch
E g
(c is the velocity of light,
h is the Planck constant) To emit light, one of the electrodes must be made of
transparent or translucent material, usually ITO. Doping of the luminescence
material may modify the process without changing the basic principles. From
this idea, the device shown in Figure 9.41 is a direct current system; however,
experimental experience shows that the application of an alternating current
source increases the efficiency of the system.
A summary of the electric properties of electroluminescence cells was given
by Nelson and Fothergill [21].
The concept to apply quantum dots as emitters in electroluminescence devices
has a series of advantages. The main advantage may be found in the fact that the
emission wavelength can be adjusted by the particle size. This allows the design
of multicolor screens using only one type of particle. Therefore, the chemistry of
the system is independent of the color. Furthermore, using appropriate intermediate layers it is possible to design systems with a multitude of layers to improve the
efficiency of the system. These additional organic layers, charge-carrier emitter
layers, are placed between the different layers of nanoparticles. This provision is
necessary to repeat the active layer system a few times to increase the efficiency
of the system.
The light emitter in electroluminescence devices based on charge transfer may
be set up with with particles and with nanorods. There are indications that the
application of nanorods may increase the efficiency. This phenomenon is displayed in Figure 9.42. Furthermore, this figure contains a juxtaposition of experimental spectra obtained by photo- and electroluminescence.
Figure 9.42 displays the spectral intensity distribution of ZnS nanorods with a
length of 400 nm and a diameter of roughly 35 nm in comparison to ZnS nanoparticles with a mean diameter of 4 nm synthesized by an equivalent process. To
obtain a broad emission band, the particles and the rods were doped with 0.13%
When this device, depicted in Figure 9.41, is connected to a direct electric
current source, the cathode, aluminum, emits electrons e
– , jumping from the
conduction band of the metal into the lowest unoccupied band of the luminescence material. The anode, ITO, releases holes h
+ into the luminescent material. In reality, an electron jumps from the highest occupied band of the
luminescence material into the conduction band of the anode, releasing a positively charged hole, in the luminescence layer. The energy necessary for this
process is U e at the side of the cathode and U h at the anode’s side. The anode
material must be selected in such a way that the energy to emit electrons, the
work function, is significantly larger than that of the cathode material. Radiative
recombination of the hole in the highest occupied band of the electroluminescent material and the electron in the conduction band leads to the emission of
light. In the simplest case, the wavelength of the emitted light λ can be
calculated using the energy of the bandgap E g : λ =
ch
E g
(c is the velocity of light,
h is the Planck constant) To emit light, one of the electrodes must be made of
transparent or translucent material, usually ITO. Doping of the luminescence
material may modify the process without changing the basic principles. From
this idea, the device shown in Figure 9.41 is a direct current system; however,
experimental experience shows that the application of an alternating current
source increases the efficiency of the system.
A summary of the electric properties of electroluminescence cells was given
by Nelson and Fothergill [21].
The concept to apply quantum dots as emitters in electroluminescence devices
has a series of advantages. The main advantage may be found in the fact that the
emission wavelength can be adjusted by the particle size. This allows the design
of multicolor screens using only one type of particle. Therefore, the chemistry of
the system is independent of the color. Furthermore, using appropriate intermediate layers it is possible to design systems with a multitude of layers to improve the
efficiency of the system. These additional organic layers, charge-carrier emitter
layers, are placed between the different layers of nanoparticles. This provision is
necessary to repeat the active layer system a few times to increase the efficiency
of the system.
The light emitter in electroluminescence devices based on charge transfer may
be set up with with particles and with nanorods. There are indications that the
application of nanorods may increase the efficiency. This phenomenon is displayed in Figure 9.42. Furthermore, this figure contains a juxtaposition of experimental spectra obtained by photo- and electroluminescence.
Figure 9.42 displays the spectral intensity distribution of ZnS nanorods with a
length of 400 nm and a diameter of roughly 35 nm in comparison to ZnS nanoparticles with a mean diameter of 4 nm synthesized by an equivalent process. To
obtain a broad emission band, the particles and the rods were doped with 0.13%