23.3 Light-Emitting Diodes
657
Fig. 23.24 a Schematic cross-section of QD LED. Current is fed to a single QD via an oxide aperture. b Plan-view
SEM image of QD LED. c Electroluminescence (EL) spectrum (T = 10 K, U = 1.65 V, I = 0.87 nA) of single
InGaAs/GaAs QD LED (diameter of oxide aperture 0.85 µm, thickness 60 nm). The single line is due to (neutral) exciton
recombination. The inset shows dependence of EL spectrum on injection current; at higher currents a second peak due
to biexciton recombination (XX→X) appears. Adapted from [1969]
Fig. 23.25 Electroluminescence
spectrum of a quantum dot
LED designed for broad
spectral range (at
5 kA/cm 2 ). Adapted
from [1971]
Ultrabroad Spectral Emission
An LED based on the emission from a quantum dot ensemble exhibits a fairly broad spectrum because of
inhomogeneous broadening due to size fluctuations of the quantum dots (cmp. Fig. 14.47). Additionally
several ensembles of QDs with different mean emission wavelength can be incorporated in a device, e.g.
in stacked layers [1970]. This way ultrabroad electroluminescence spectra can be realized (Fig. 23.25).
Also emission on the ground and excited state can be used for broad spectral emission.
23.3.7 Organic LEDs
An organic light emitting diode (OLED) is made from organic semiconductors. The pioneering work
was made by Tang and Van Slyke [1789, 1972]. Typical layer sequences are depicted in Fig. 21.62.
The light emission occurs through the anode (and the transparent ITO layer) while the metal cathode
is opaque. Two major configurations are possible, emission through the transparent substrate (glass)
or top emission (Fig. 23.26).
657
Fig. 23.24 a Schematic cross-section of QD LED. Current is fed to a single QD via an oxide aperture. b Plan-view
SEM image of QD LED. c Electroluminescence (EL) spectrum (T = 10 K, U = 1.65 V, I = 0.87 nA) of single
InGaAs/GaAs QD LED (diameter of oxide aperture 0.85 µm, thickness 60 nm). The single line is due to (neutral) exciton
recombination. The inset shows dependence of EL spectrum on injection current; at higher currents a second peak due
to biexciton recombination (XX→X) appears. Adapted from [1969]
Fig. 23.25 Electroluminescence
spectrum of a quantum dot
LED designed for broad
spectral range (at
5 kA/cm 2 ). Adapted
from [1971]
Ultrabroad Spectral Emission
An LED based on the emission from a quantum dot ensemble exhibits a fairly broad spectrum because of
inhomogeneous broadening due to size fluctuations of the quantum dots (cmp. Fig. 14.47). Additionally
several ensembles of QDs with different mean emission wavelength can be incorporated in a device, e.g.
in stacked layers [1970]. This way ultrabroad electroluminescence spectra can be realized (Fig. 23.25).
Also emission on the ground and excited state can be used for broad spectral emission.
23.3.7 Organic LEDs
An organic light emitting diode (OLED) is made from organic semiconductors. The pioneering work
was made by Tang and Van Slyke [1789, 1972]. Typical layer sequences are depicted in Fig. 21.62.
The light emission occurs through the anode (and the transparent ITO layer) while the metal cathode
is opaque. Two major configurations are possible, emission through the transparent substrate (glass)
or top emission (Fig. 23.26).