1588 Chapter 44
44.8.5.2 OLED Works in the Following Manner
- The basic OLED cell structure consists of a stack of
thin organic layers sandwiched between a trans-
parent anode and a metallic cathode. - The organic layers comprise a hole-injection layer, a
hole-transport layer, an emissive layer, and an
electron-transport layer. - When an appropriate voltage (typically a few volts) is
applied to the cell, the injected positive and negative
charges recombine in the emissive layer to produce
light (electroluminescence). - The structure of the organic layers and the choice of
anode and cathode are designed to maximize the
recombination process in the emissive layer, thus
maximizing the light output from the OLED device.
OLEDs are typically fabricated on a transparent
substrate on which the first electrode (usually
indium-tin-oxide which is both transparent and conduc-
tive) is first deposited.
Then one or more organic layers are coated by either
thermal evaporation in the case of small organic dye
molecules, or spin coating of polymers. In addition to
the luminescent material itself, other organic layers may
be used to enhance injection and transport of electrons
and/or holes.
The total thickness of the organic layers is of order
100 nm.
Lastly, the metal cathode (such as magnesium-silver
alloy, lithium-aluminum, or calcium) is evaporated on
top.
The two electrodes add perhaps 200 nm more to the
total thickness of the device. Therefore the overall
thickness (and weight) of the structure is mostly due to
the substrate itself.
OLEDs can be manufactured in several different
types, classified by the size of molecule they use, and
the type of substrate they are manufactured on. Some
examples are:
- TOLED—Transparent OLED. This is manufactured
on a clear substrate suitable for applications such as
heads-up displays. - FOLED—Flexible OLED. This type of OLED is
manufactured into a sealed flexible substrate that can
be curved, rolled, or bent.
44.8.6 Light Emitting Diode
Light emitting diodes are popping up everywhere due to
their high light output and relatively low power con-
sumption. They are finding uses in homes, automo-
biles, and of course high brightness outdoor displays.
Their newest application is as backlight illumination for
LCD flat panel displays and as a light source for small
pico projectors utilizing DLP and LCoS chips.44.8.6.1 LED Characteristics- Extremely high brightness.
- Relatively low maintenance.
- Long life, >50,000 hours.
- Outdoor/indoor capability.
- Modular construction with scalable display sizes.
- 3 to 25 mm pixel pitches available.
44.8.6.2 LED Operates in the Following MannerThe phenomenon of electroluminescence was discov-
ered in 1907 by Henry Joseph Round.
British experiments in the 1950s led to the first
modern red LED, which appeared in the early 1960s.
By the mid-1970s LEDs could produce a pale green
light. LEDs using dual chips (one in red and one in
green) were able to emit yellow light.
The early 1980s brought the first generation of super
bright LEDs, first in red, then yellow, and finally green,
with orange-red, orange, yellow, and green appearing in
the 1990s.
The first significant blue LEDs also appeared at the
start of the 1990s, and hig-intensity blue and green in
the mid-1990s.
The ultra bright blue chips became the basis of white
LEDs, in which the light emitting chip is coated with
fluorescent phosphors.
This same technique has been used to produce virtu-
ally any color of visible light and today there are LEDs
on the market, which can produce previously exotic
colors, such as aqua and pink.
Light emitting diodes (LEDs) are source of contin-
uous light with a high efficiency.- At the heart of a light emitting diode is a semicon-
ductor chip, containing several very thin layers of
material that are sequentially deposited onto a
supporting substrate. - The first semiconductor material that is deposited
onto the substrate is doped with atoms containing
excess electrons, and a second doped material,
containing atoms having too few electrons, is then
deposited onto the first semiconductor to form the
diode. The region created between the doped semi-
conductor materials is known as the active layer.