OLED technology nuts and bolts

OLED technology nuts and bolts

OLED the most likely technology for nextgeneration
displays
OLED is one next-generation display technology. Light is generated by applying a
current to organic materials deposited on a substrate. Devices that use OLED are
often called OLEDs (organic light-emitting diodes).

One key advantage of OLED is impressive display performance. As the organic
materials generate light themselves, OLED displays offer a wide viewing angle, high
contrast, rapid response, and wide-ranging color reproduction, resulting in highly
realistic images. No light generation is needed to display the color black, so power
consumption tends to be lower than traditional display technologies. What is more,
OLED devices are simple in structure, with a thin layer of organic material deposited
on a substrate, resulting in thinner displays that are also more shock-resistant.
OLED displays are expected to be used in flexible displays moving forward.

OLED light generation has basis in quantum mechanics
Light generation via OLED has its basis in quantum mechanics. When a current is
applied, injected electrons and electron holes recombine inside the OLED device,
producing energy that is emitted as light. The luminescent materials accept the
recombination energy, shifting them from a ground state to an excited state. The
energy generated in the process of returning to a ground state is emitted as light
(Figure 1).

Light emitted by OLED devices differs in color depending on what luminescent
materials are used. It is known that as the strength of light energy (energy level)
increases, the color of emitted light changes from red to green, yellow, and then
blue. By using luminescent materials appropriate for a specific level of energy, one
can control the color of the emitted light, so OLED devices enable light in a wide
range of colors.

The light emitted via OLED is divided into two categories (fluorescence and
phosphorescence) based on the state of the excitement of the luminescent
materials. When electrons recombine with electron holes in the luminescent layer,
excitons are created in singlet and triplet states with a ratio of 1:3. Light produced
by singlet states is called fluorescence, while light produced by triplet states is
called phosphorescence. Theoretically, 25% of the excitation energy is fluorescent
and 75% is phosphorescent making phosphorescence more important in terms of
improving luminescent efficiency.

OLED device structure
The basic structure of an OLED device is one in which an organic luminescent layer
is sandwiched between organic conductive layers deposited on an electrode. The
first OLED device was a simple structure in which the hole transport layer and
luminescent layer was sandwiched between electrodes. However, to improve
connectivity between the electrode and organic layer and to improve electron
injection efficiency and luminescent efficiency, OLED devices are now adopting a
multiple organic layer structure (Figure 2).

The main organic layers are: 1) the electron injection layer (EIL), the layer that
electrons are injected into from the cathode; 2) the electron transport layer (ETL),
the layer that transports electrons to the emissive layer; 3) the hole injection layer
(HIL) the layer that electrons are injected into from the anode; 4) the hole transport
layer (HTL), the layer that; the layer that transports holes to the emissive layer; and
5) the emissive layer (EML), the layer that emits light (luminance).

Active OLEDs are divided into bottom emission devices, which use a TFT to emit
light through a transparent substrate, and top emission devices, which use a
transparent electrode to emit light directly. In bottom emission, the luminescent
surface area is restricted by the TFT. Top emission has the benefit of not having this
restriction.

OLED full color technologies
When using OLED in displays full color can be achieve using various technologies,
including 1) three color emission, 2) the filter method, 3) color conversion, and 4)
the stacked organic LED (SOLED) (Figure 3). Currently almost all OLEDs adopt
three color emission or the filter method.

Three color emission achieves full color by coating displays with red, green, and
blue (RGB) luminescent materials. Bright colors are a feature of this technology.
However, RGB coating requires high-level manufacturing technologies and
differences in the lifespan of luminescent materials means that uneven coloring is
an issue over the long term.

The filter method achieves full color by using a filter to convert white OLED into red,
green, and blue colors. Luminescent material coating is not required, which makes
the manufacturing process relatively straight forward. However, light intensity is only
one third of that achieved using three-color emission.

The color conversion method was developed by Idemitsu Kosan. First, a blue
emitting laminate is made, and then a fluorescent conversion membrane is used to
convert blue light to red and green. As blue has a higher excitation energy level than
red and green, blue light can be used to excite red and green fluorescent materials
and produce light. The color conversion manufacturing process is simple but color
conversion efficiency is low.

SOLED (transparent stacked OLED) uses a pixel architecture that stacks sub-pixels
(red, green, and blue elements in each pixel) vertically rather than side by side. A
transparent electrode is used to control luminance. SOLED has high display
capability but the manufacturing technology is difficult and we believe hurdles to
commercialization are high.

OLED drive methods
OLEDs can use either passive matrix or active matrix addressing schemes. Passive
matrix OLEDs are driven by the application of a current to a through electrode. A
simple structure is one of the strengths of a passive matrix. Active matrix OLEDs
use a TFT circuit to switch pixels on and off. They are becoming the mainstream
drive method as they have a longer life and allow for higher resolution than passive
matrix OLEDs.

The TFT circuitry in active matrix OLED is a lot more complicated than in LCD as
drive circuits not only turn pixels on and off but also control the strength of
luminance. Low temperature polysilicon (LTPS) is currently used for OLED TFT.
LTPS 1) allows electrons to move at faster speeds and 2) has a stable threshold
voltage (converting voltage into electric current is key in OLEDs), making it better
suited to OLED drive circuits (Figure 4).

Research is advancing on oxide semiconductors as a next-generation TFT
technology. We think oxide semiconductors will be suitable for large OLED because
they have sufficiently high electron transfer speeds, can use amorphous films
(making it easier to expand screen sizes), and the manufacturing process is
comparatively simple.

Comparison of OLED and LCD technologies
The display capability of OLED is superior to that of existing LCD in terms of
viewing angle, brightness, and response speed. Because OLED is a self light
emitting device, it offers a wide viewing angle, high contrast ratio, and fast response.
A LCD, by contrast, transmits light from a backlight, making it difficult to improve the
viewing angle, brightness, and response speed. Also, the resolution of small and
medium-size OLEDs using the most recent OLED technologies has improved to be
on a par with LCD (Figure 5).

The LCD structure combines host of technologies, including backlight unit (BLU),
liquid crystal materials, and optical technologies, whereas OLED has a simple
structure, with organic materials layered on a substrate. This makes OLED better
suited to thinner display designs and gives it greater shock resistance qualities.
OLED development has a short history compared with mature LCDs and related
technologies are a work-in-progress. While LCD materials, device structure, and
manufacturing process technologies have been commoditized, OLEDs do not yet
have recognized standards, with the development and application of various
technologies and techniques still at the investigational stage.

As a result, OLEDs have the following shortcomings: 1) they are expensive
compared with LCDs because of yield problems and the need for major investment;
2) they have shorter lifespans than LCDs because of problems with luminescent
materials; and 3) mass-production technologies for large displays have not yet been
perfected. We believe the resolution of these issues will require improvement in
mass-production technology and the commercialization of next-generation
technologies for printing, etc.

CRTs and PDPs have numerous weaknesses compared with LCDs and OLEDs.
The picture quality of CRTs, which for many years were used in CRT TVs, is
exceptionally good, but because of high energy consumption and the difficulty of
making larger and thinner displays, CRTs have dropped of the scene as FPD TVs
have spread. PDP demand is peaking, reflecting such problems as high energy
consumption and high manufacturing costs compared with LCD (reducing costs is
difficult).

OLED display manufacturing process
The OLED manufacturing process comprises the following steps: 1) front-end
processing (drive circuit fabrication, ITO electrode patterning, etc), 2) deposition
(OLED element formation), 3) sealing, and 4) back-end processing (modularization,
inspection, etc) (Figure 6). While OLED and LCD manufacturing have many
common processes, centering on the front-end, deposition, sealing and other
processes are unique to OLEDs. Below we explain the standard manufacturing
process for active-matrix OLEDs using three-color emission.

Front-end processing involves the fabrication of a TFT backplane on a glass
substrate. Currently, TFT circuits are made from a process that adopts LTPS.
Because expanding the size of substrates using LTPS is technically difficult, we
understand makers use processes that take 40% of a 5.5G glass substrate and use
laser annealing. Thereafter, the ITO electrodes are attached.

Deposition involves applying organic materials to the glass substrate with the TFT
circuit and ITO electrodes to form the OLED element. Vacuum deposition or
vacuum thermal evaporation is the standard method. In a vacuum chamber, the
organic materials are heated (evaporated) and allowed to condense as thin films
onto cooled substrates. After the hole injection layer (HIL) and the hole transport
layer (HTL) are formed, the RGB luminescent materials are coated using a metal
mask to form the emissive layer (EML). Finally, the electron injection layer (EIL) and
the electron transport layer (ETL) are formed.

The substrate is then sealed to protect the OLED element from oxygen and
moisture. Once the OLED element has been deposited in the vacuum chamber, a
glass cover is placed on the glass substrate and hermetically sealed with a laser.
The sealed glass substrate then passes through back-end processing (substrate
cutting, modularization, inspection) and shipped as an OLED.

Development of next generation OLED volume
production technologies advancing
RGB coating using vacuum deposition has a low yield and is difficult to apply for
large displays. For this reason laser transfer and printing technologies are being
developed as next generation coating technologies (Figures 7 and 8). The
commercialization of the thin-film encapsulation method, which seals with a thin
layer, and of flexible displays as next-generation technologies is also underway.

The print method uses various printing technologies to manufacture OLEDs by
coating OLED materials dissolved in an organic solvent onto the substrate. It has
not yet been adopted in mass production because materials technologies suitable
for printing have not yet been developed. However, printing has major advantages:
1) it is simpler than the vacuum process and is done with mainstream equipment,
making it easier to control capex; 2) materials efficiency is high; and 3) it is easier to
adapt more complex circuitry and larger displays. If new printing techniques are
commercialized we think mass production of low-cost OLEDs would become
possible and we thus view this significant development for the future of OLEDs.

In contrast, the laser transfer method is a technology for coating RGB in separate
colors by transferring luminescent materials on a toner film selectively onto the
substrate using laser beams. The advantages of the laser transfer method are that
1) colors are separated with great precision and the technology is suited to higher
definitions, 2) the technology works with large screens, and 3) it is easy to improve
luminescent strength and efficiency because the aperture ratio is high.

Development of the laser transfer method is being pushed by Samsung Electronics,
with its laser-induced thermal imaging (LITI) approach, and by Sony with its laserinduced
pattern-wise sublimation (LIPS) approach. Demo models of high-resolution
OLEDs have been made with these approaches. We understand that Samsung
Electronics is looking into bringing in the laser transfer method in film-making
processes for some luminescent materials in order to manufacture high-definition
panels.

In sealing processes, too, development is ongoing of a next-generation technology,
thin film encapsulation (TFE), which covers the surface of the OLED with a thin
coating of organic and metal material. The advantages of TFE are as follows: 1)
sealant cover glass is unnecessary, so the glass substrate loses a layer, which
leads to reduced variable costs, 2) production is efficient, because film layering and
sealing processes are undertaken sequentially via deposition, and 3) OLEDs can be
made thinner.

Commercialization of flexible displays
We expect flexible displays to be commercialized as a next-generation technology.
OLEDs can be adapted for flexible displays; this is because they can be made at
low temperatures, which makes it possible to use plastic substrates, and because
OLED elements are formed from organic compounds. As LCDs are made by
inserting liquid crystal materials into a glass substrate, it will be difficult to adopt
them for flexible displays.

Flexible displays are thin and light and are strong enough that they will not break
even if hit with a hammer. The application and design potential of flexible displays is
another attraction; they can be used for products that have a curved shape and
eventually it may be possible to make displays that can be hung on a wall like a
poster.

Sony and Samsung have been developing flexible OLED displays (Figure 9). When
Samsung announced 2011 Q3 results it said it would unveil a smartphone with a
flexible display at the start of 2012. While details are unclear, we expect the product
will leverage the aforementioned design strengths of flexible displays.

To commercialize flexible OLED displays it is necessary to adapt substrates,
transparent electrodes, TFTs, and other materials as well as manufacturing
processes. In the field of substrate materials, Ube is developing plastic substrates,
and Asahi Glass, Nippon Electric Glass, and others are developing ultra-thin glass.
For transparent electrodes, we expect a development shift from existing ITO
electrodes to flexible electrodes that employ carbon nanotubes, conductive
polymers, and silver nanowire. We expect oxide semiconductors and organic
semiconductors to be used for TFT circuits. As oxide semiconductors are
manufactured at low temperatures they can be adapted for plastic substrates and
used with amorphous membranes, which should make it relatively easy to adapt
them for flexible circuits.

Looking further ahead, we think roll-to-roll production using new printing
technologies and organic TFTs will provide a huge boost for flexible displays. While
development is only at the basic research stage, if organic semiconductor TFT and
printing technologies are commercialized, the production of TFT circuits and organic
EL elements could be integrated using roll-to-roll technology, which should
significantly improve productivity.

OLED materials
OLED materials are classified according to the shape of their molecules and
molecular mass into low-molecular-weight materials and high-molecular-weight
materials. They are also classified according to the properties of their organic layers
into luminescent materials and electron/hole transport materials, etc. OLED
materials are designed artificially using the methods of synthetic organic chemistry
and there are a wide variety of materials.

Many materials makers have focused their materials development on lowmolecular-
weight materials. This is because it is easier to engage in materials
development with them as 1) molecular weights are low and the design, synthesis,
and refining of organic molecules is relatively easy and 2) it is enough to design
organic materials with specific functions. In contrast, the development of highmolecular-
weight materials has lagged, as 1) the synthesis and refining of them is
technically difficult and 2) it is difficult to control the performance of organic
materials as variances in molecular weights occur easily.

Low-molecular-weight materials are OLED materials that use organic materials with
molecular weights of a few hundred. Organic low-molecular weight molecules with
the properties necessary for each organic layer (electron injection layer, electron
transport layer, hole injection layer, hole transport layer, luminescent layer, etc.) are
used as materials. OLED devices are often formed by the accumulation of lowmolecular-
weight materials with different properties via deposition through the
application of heat.

Conductive organic low-molecular-weight molecules are used as transport and
injection materials for electrons and holes. For transport materials, materials with
superior electron and hole carrier mobility properties are used, while for injection
materials, materials that bind well to both inorganic and organic matter are used, as
the injections occur in the interlayer between the electrode and the organic layer. In
both cases, materials development and selection occurs after consideration of
electrical and chemical compatibility of materials.

For luminescent materials, organic low-molecular-weight materials that differ in
terms of 1) fluorescence and phosphorescence and 2) luminescent color are used.
This is because the physical mechanisms by which light is produced in fluorescence
and phosphorescence is different and because the energy levels of luminescent
materials changes according to the luminescent color. Leading fluorescent materials
are aluminum and other metal complexes, anthracene derivatives, and dye
materials (coumarin, rubrene, etc.) By contrast, phosphorescent materials emit light
in triplet states, so iridium and other high-period metal complexes are mainly used
as luminescent materials.

High-polymer materials are OLED materials that use polymer macromolecules.
Polymer groups that have been readied with luminescent and hole/electron carrier
properties are brought into the high-polymer material and are used as OLED
materials by forming a polymer through polymerization. There are many technical
problems in the development of high-polymer materials but hopes are high that they
can be mass produced at low cost in the future, as 1) it is possible to form OLED
devices with a simple layer structure and 2) they are suited to production using the
printing method as they exhibit superior solubility in organic solvents.

Together with low- and high-molecular weight molecules, the biggest development
issues are presented by 1) long-life blue luminescent materials and 2) blue
phosphorescent materials. Energy levels in blue luminescence are higher than in
red or green luminescence, so blue luminescent materials tend to have short
lifespans. Short blue lifespans lead to screen burn-in or issues with the white
balance, as well as limitations on display lifespans. The development of blue
phosphorescent materials has been slower than it has for red and green ones and
this is becoming a major issue in the way of improving the luminescent efficiency of
OLEDs.

History of OLEDs
The occurrence of electroluminescence in organic materials has been recognized
since the 1950s, but it has been a long road to develop practical uses due to issues
of luminescence intensity, efficiency, and useful life. Development was stalled for
some time before Dr. Tang, working at Eastman-Kodak, made his technological
breakthroughs. Dr. Tang and his team succeeded in producing a high-intensity light
for several minutes by applying voltage to a device formed by organic film materials
on an electrode. This opened the door to R&D into OLEDs at numerous companies
and institutions, in Japan as well as overseas (Figure 10).

The 1990s saw the development of a number of basic technologies that advanced
OLEDs. Light-emitting polymer (P-OLED) technologies were developed at
Cambridge University in 1990. In 1993, Professor Junji Kido of Yamagata University
developed the world’s first white OLED panels. Active-matrix OLEDs were
developed by TDK in 1996. Cambridge Display Technology (CDT), a venture firm
established as a result of research results at Cambridge University, continued to
develop P-OLEDs. In 1999, Pioneer unveiled products incorporating passive matrix
OLEDs.

In the first decade of the 21st century, Japanese companies spearheaded efforts to
commercialize OLEDs. In 2001, NEC developed a FOMA handset using OLEDs for
the main display. Kodak and Mitsubishi Electric introduced digital cameras with
OLED displays in 2003. And then in 2007, Sony started to produce TV sets with
OLED displays.

From the latter half of the last decade, South Korean and other manufacturers have
been making advances in making OLED products practical and commercial.
Samsung has started full-scale mass production of small OLED panels through
subsidiary Samsung Mobile Display, and the shift to OLED displays for
smartphones continues.

Among Japanese makers, 2011 saw Sony announce a number of products
incorporating OLEDs, including DSLR cameras, head-mounted displays, and mobile
devices such as the PS Vita. In addition, the commercialization of OLED lighting is
getting up to speed, as companies like Panasonic Idemitsu OLED Lighting and
Lumiotec started shipping OLED panels in September 2011.