12-06-2013, 02:37 PM
Organic light-emitting diode (OLED) technology: materials, devices and display technologies
Organic light-emitting.pdf (Size: 370.51 KB / Downloads: 73)
Abstract:
Since the breakthrough by Kodak in 1987, organic light-emitting diodes (OLEDs) have been seen
as one of the most promising technologies for future displays. A number of materials have been developed
and improved in order to fulfil the requirements of this application. Thematerials differ from one another
by their structure but also by the mechanism involved in the electroluminescence produced (fluorescence
versus phosphorescence). When properly stacked, these materials result in a device that can achieve the
required high efficiency and long lifetime. Such red, green and blue devices can then be combined in
matrices to become the core of a display. Building up these structures onto a display backplane is one
of the challenges facing the industry. The circuitry for driving the pixels can be adapted to the OLED,
sometimes at the expense of the simplicity of the display, but bearing in mind that the fabrication process
must remain industrially viable.
INTRODUCTION
During the last two decades, organic light-emitting
diodes (OLEDs) have attracted considerable interest
owing to their promising applications in flat-panel
displays by replacing cathode ray tubes (CRTs) or
liquid crystal displays (LCDs). Electroluminescence is
the emission of light from materials in an electric
field, and in the 1960s1 this phenomenon was
observed from single crystals of anthracene. Despite
the high quantum efficiency obtained with such
organic crystals, no application has emerged owing
to the high working voltage required as a result
of the large crystal thickness and poor electrical
contact quality. Nevertheless, these studies have
led to a good understanding of the basic physical
processes involved in organic electroluminescence,
i.e. charge injection, charge transportation, exciton
formation and light emission.
Materials and efficiencies
The main requirements for OLED materials are high
luminescence quantum yield in the solid state, good
carrier mobility (both n and p type), good filmorming
properties (pinhole free), good thermal and
oxidative stability, and good colour purity (adequate
CIE coordinates). The first generation of efficient
devices, pioneered by Tang and Van Slyke2 from
Eastman Kodak, was based on fluorescent materials.
In this case, the emission of light is the result
of the recombination of singlet excitons, but the
internal quantum efficiency is limited to 25 %. Typical
examples of fluorescent RGB materials are shown in
Table 1.
Device structures
The historical evolution of OLED architectures is
shown in Fig. 3. For SM-OLEDs an increase in
the complexity of the devices has been reported in
the literature. The first studies on anthracene in
the 1960s used a simple monolayer structure, and
since the breakthrough of the Kodak group, more
and more layers have been used with specialized
functions such as the hole injecting layer, hole
transporting layer, hole blocking layer, emitting layer,
and electron transporting layer. It has been shown
that the electroluminescence efficiency of OLEDs
can be increased by carrier or exciton confinement
within a multilayer device.35,36 The confinement of
charge carriers can increase the capture of carriers, and
the confinement of excitons can improve the energy
transfer from the host to the guest.
Lifetime and device stability
Device stability is an important issue for an emissive
technology such as OLEDs, and particularly differential
ageing of the three primary colours. Despite the
absence of any standardized measurementmethod, the
device lifetime is usually defined as the mean time to
half-brightness. It is generally assumed that for display
applications, except probably for portable electronics,
a lifetime of over 20 000 h with a reasonable brightness
level of at least 100 cd m−2 is necessary.
DISPLAY TECHNOLOGY
Since the mid-1980s, these organics materials have
been seen as a key component of a promising display
technology likely to challenge liquid-crystal displays
(LCDs). One of the characteristics of an LCD or
an OLED display compared with a cathode-ray tube
(CRT) is the layout of its active area consisting of
pixels which form the images in an off or on state. The
main feature of an OLED pixel is that it is an emissive
device which can be switched off and be completely
black, whereas a liquid crystal pixel is a transmissive
device which does not allow complete occultation
of backlight. Nevertheless, these two devices have a
number of similarities that can pave the way toward
OLED industrialization.
Passive-matrix displays
In the simplest case, a pixel is defined by the crossover
area of linear electrodes deposited on each side
of the liquid crystal or of the emissive material in
the case of an OLED. In such a configuration, the
electrodes are oriented 90 ◦ from each other as shown
in Fig. 4.
In the addressing method used for such matrix
displays, each line is selected during a period of T/N,
where T stands for the frame time and N for the
number of lines of the display. During this period
all the necessary pixels are activated according to
the image content. Then, the next line is selected.
In the case of a transducer without memory, it is
necessary to refresh the image at a rate of at least
50 Hz in order to avoid any flickering effect. The
frame time should then be less than 20ms. The
electrodes of a given pixel are shared with all the
pixels of the same line and of the same column,
and the voltage applied to it is controlled during
the period in which the line is selected.
OLED deposition
Depositing the organic materials onto the substrate to
obtain red, green and blue pixels is a major challenge
facing the industry. The requirements include, among
others, accurate positioning and uniformity of the
deposition. Small molecules are currently deposited
by evaporation through a shadow mask, and polymers
are mostly dispensed by inkjet printing.
Colour generation
A number of approaches have been tried for producing
full-colour OLED displays, i.e. in fabricating red,
green and blue pixels. The challenge is not only
in patterning the pixels but also in having them
constantly emitting light in a given ratio corresponding
to a satisfactory white colour. This latter point relies
mainly on the intrinsic performance of the materials
themselves (typically their lifetime), but is the purpose
of the particular approach described below.
CONCLUSIONS
At the end of the 1990s, OLEDs were seen as a
disruptive technology for the display industry. Some of
their advantages have now proven to be a breakthrough
compared with LCDs. Its thickness, currently less than
2mm (against 4–6mm for LCDs), is one of them.
This will decrease further as thin-film encapsulation
will replace the glass or metallic lid currently used.