Of all the leading display technologies, none has generated
more excitement as the display technology of the future
than organic light-emitting diodes (OLEDs). OLEDs possess
all of the positive attributes of any current display technology
with little or no negative features—at least not yet.
For example, they don’t require any backlighting like other
displays, such as liquid-crystal displays (LCDs). OLEDs present
bright, clear video and images (brightness levels of more
than 1000 candelas/m2 and contrast ratios greater than 10,000:
1) that are easy to see at almost any angle. They also dissipate
low amounts of power and have fast switching rates. Their
response times are in the range of a few microseconds, which
together with their color-producing capability (over 16 million
colors), makes them ideal candidates for TVs. NTSC-compatible
TVs have already been demonstrated.
Furthermore, they’re lightweight and extraordinarily thin.
At this year’s Display 2008 Conference, Sony showed off a
0.2-mm prototype—the thinnest OLED yet, according to
Sony. Its manufacturing costs have the potential to be lower
than other displays. Work on that front is ongoing and looks
very promising. Several companies have tried roll-to-roll manufacturing
with various levels of success.
But there are some drawbacks. A big one is a limited lifetime,
particularly for blue and green colors. Part of the reason
is the need to keep out water, which can damage an OLED’s
organic materials, necessitating very tight sealing levels during
their manufacturing.
Experimental green OLEDs with lifetimes of nearly 200,000
hours have been obtained. To date, the best lifetimes achieved
for experimental blue OLEDs have been about 62,000 hours.
A joint development by Toshiba, Matsushita, and Idemitsu
Kosan yielded similar results using a thin-film transistor
(TFT) substrate. Their work concentrated on a 2.2-in., 240-
by 320-pixel quarter video graphics array (QVGA) for mobile
phones, achieving 100 mW of power consumption.
OLEDs also typically emit less light per unit area than
inorganic solid-state LEDs, which are usually designed for use
as point-light sources. In fact, Epson Co. developed OLED
materials that contributed to longer lifetimes by eliminating
some early-stage deterioration of the organic materials.
Given these facts and the bright outlook researchers predict
for OLED displays, design engineers should get to know more
about OLEDs—how they work, how to apply them, what performance
levels can be expected, and the status of this exciting
technology, which is sure to satisfy a range of future displays.
THE BASIC STRUCTURE
An OLED consists of a metal cathode (typically aluminum
or calcium) and an anode (typically indium tin oxide, or ITO)
located on a glass substrate. Between these electrodes lie deposited
emissive and conductive layers of organic molecules or
polymers (Fig. 1). The deposition process occurs in rows and
columns on a flat carrier by a “printing” process, forming a
matrix of pixels that emit light of different colors, like red, green,
blue, or white. Several layers
can be stacked on top of one
another.
OLEDs operate on the
attraction between positively
charged (holes) and negatively
charged (electrons)
particles. When voltage is
applied, one layer becomes
negatively charged relative to
another transparent layer. As
energy passes from the negatively
charged (cathode) layer
to the other (anode) layer, it
stimulates organic material
between the two, which emits
light visible through the outermost
layer of glass.
Electrostatic forces bring the electrons
and holes toward each other and they
recombine. The recombination occurs closer
to the emissive layer, because in organic
semiconductors, holes are more mobile
than electrons. The recombination causes
a drop in the energy levels of the electrons,
accompanied by an emission of radiation
whose frequency is in the visible region.
Should the anode have a negative potential
with respect to the cathode, the OLED
won’t work. In this condition, holes move
to the anode and electrons to the cathode,
so they move away from each other and
thus don’t recombine.
Doping or enhancing organic material
helps control the brightness and color of
light. The organic materials can consist of
small single structures or molecules, or complex
chains of molecules (polymers), to best
suit the manner in which they are produced.
The original OLEDs, developed by
Eastman-Kodak in the late 1980s, made
use of small organic molecules. Although small molecules
emitted bright light, they had to be made
in a costly vacuum deposition process.
More recently, larger polymer molecules
have been used, which can be made less
expensively and in large sheets, suiting
them for large-screen displays.
Continue on Page 2