Organic Light Emitting Diodes

UNIVERSITY OF LJUBLJANA
FACULTY OF MATHEMATIC AND PHYSICS
DEPARTMENT OF PHYSICS
SEMINAR
OLEDs
Organic Light Emitting Diodes
AUTHOR : Simon Rankel
ADVISOR : Dr. Dragan Mihailović
Ljubljana, May 2004

Abstract
An OLED is an electronic device made by placing a series of organic thin films between two
conductors. When electrical current is applied, a bright light is emitted. OLEDs are lightweight,
durable, power efficient and ideal for portable applications. OLEDs have fewer process steps and also
use both fewer and lower-cost materials than LCD displays. More and more people believe that
OLEDs can replace the current technology in many applications.
First part of this seminar is a physical overview of semiconducting properties of organic polymers
and takes a deeper insight in their electron structure. Second part of seminar deals with structural
OLED properties and phosphorescence material doping phenomena as improvement of efficiency.
Then it offers some technical facts, the question of stability of OLEDs and so present and future
aspects of use in actual products.
Contents
1 Introduction 3
2 The physics behind OLEDs 4
2.1 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . 4
2.2 SSH model of electronic structure . . . . . . . . . . . . . . . . . 5
2.3 Doping, charge transport . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Light emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 OLED device structure 10
3.1 Basic PLED - Charge injection . . . . . . . . . . . . . . . . . . . 10
3.2 Basic SMOLED. . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 Phosphorescent doping (Singlet, triplet excitons) . . . . . . . . . 12
4 Technical facts, Stability 13
5 Matrix types of OLEDs, Products 14
6 Conclusion 16
7 References 17
2

1 Introduction
The basic idea of organic LED (OLED) operation is fairly simple and similar to that of inorganic
LED's. Electrons and holes are injected into an "active" region. There, they recombine resulting in
photon emission. Electrons are injected in the conduction band of the appropriate organic material and
holes are injected into the valence band. These carriers then move by diffusion until they meet and
recombine to form excitons. When these excitons decay to their ground state, photon emission occurs.
This is the same process as in fluorescence, but here caused by induced electricity and thus called
electroluminiscence.
In the 1960s the first LEDs on the basis of semi-conducting metals were developed and rapidly
found their way to applications. At the same time, the phenomenon of electroluminescence was also
observed for organic materials (anthracene single crystals).
But it was not until 1987 the research group at Eastman Kodak Research Laboratories reported an
organic electroluminescent diode with some remarkable characteristics [1]. By choosing organic
instead of inorganic semiconductors in a novel device resembling a conventional p-n diode, they
produced intensive electroluminescence while using a low-drive voltage. This efficiency indicated
great potential for display applications, and intense research and development efforts on OLED ensued
in numerous industrial laboratories, particularly in Japan. A little more than a decade later, the first
OLED displays have been commercialized, and the technology is poised to challenge the dominance
of liquid-crystal displays (LCDs) in many applications.
OLEDs do not have to be manufactured in semiconductor factories (like LEDs). Nor are they
limited to relatively small sizes. Organic LEDs can potentially be made with a low-cost printing line,
much like you print a newspaper, so very high resolution of displays which consume little power is
achievable.
OLED devices can be divided into two classes: depending on the type of organic layer [8]:
• Small molecule devices (SMOLED)
• Organic polymer devices (PLED or LEP).
Small-molecule devices are fabricated using vacuum evaporation techniques, whereas polymer
structures can be applied using spin-casting or even ink-jet printing techniques. Originating at Eastman
Kodak Co. (Rochester, NY), the small-molecule technology has achieved commercialization first.
There has been a big advance recently also on PLEDs first discovered by researchers at Cambridge
University in 1989. OLEDs based on “small molecules” have tris (8-hydroxyquinolinato) aluminum
(Alq3) as the prototype (Figure 1), and only a few derivatives have been proposed so far.
Organic materials used for two different types of OLED :
Figure 1 : SMOLED prototype – Alq3 PLED prototype - PPV
OLEDs based on polymers have poly-paraphenylene-vinylene (PPV) as the prototype (Figure 1),
but over the years a number of derivatives of PPV or other polymers have been proposed and used.
3

2 The physics behind OLEDs
2.1 Organic semiconductors
SMOLEDs are constructionally more similar to LEDs than PLEDs are, thus we first take a look at
conductivity properties of polymer materials used in PLEDs. The question arrising is: How polymer
becomes conductive? How can plastic conduct electricity?
Polymers are made of long chain molecules entangled between each other. The polymer chains are
formed by connecting many small molecular units called monomers (Fig. 2)[9].
Figure 2: polymer chain building
Most polymers are organic compounds, which means they are composed mostly of carbon chains with
hydrogen, oxygen and nitrogen atoms. These atoms form covalent bonds where the electrons are
localized in the low energy bonding orbitals of the chain molecule. Hence polymers typically do not
conduct electricity and are used in electronic application as insulator.
The prototypical conducting polymer is polyacetylene (PA), (CH)n (Fig. 3). Every bond contains a
localized “sigma” (σ) bond which forms a strong chemical bond. In addition, every double bond also
contains a less strongly localised “pi” (π) bond which is weaker. A π molecular orbital is thus formed
when two carbon atoms form a double bond and the 2pz orbitals have the same symmetry. The
electrons in this π orbital have equal probability of being around each carbon nucleus.
Figure 3: 2pz orbitals interact and form a “conductive” electron cloud
Morover, π-bonding, in which the carbon orbitals are in the sp²pz configuration and in which the
orbitals of successive carbon atoms along the backbone overlap, leads to electron delocalization along
the backbone of the polymer. (Delocalized electrons are electrons that are shared by more than two
atoms). This electronic delocalization provides the “highway” for charge mobility along the backbone
of the polymer chain. [2]
As a result, therefore, the electronic structure in conducting polymers is determined by the chain
symmetry (i.e. the number and kind of atoms within the repeat unit), with the result that such polymers
can exhibit semiconducting or even metallic properties.
Actually because of the Peierls Instability with two carbon atoms in the repeat unit, the π-band is
divided into π- and π* bands (Fig. 4) [10]. What Peierls showed is that due to the coupling between
electronic and elastic properties the polymer develops a structural distortion such as to open a gap in
the electronic excitation spectrum.
4

Figure 4: Bonding and anti-bonding (*) π orbitals
Since each band can hold two electrons per atom (spin up and spin down), the π-band is filled and the
π*-band is empty. The energy difference between the highest occupied state in the π-band and the
lowest unoccupied state in the π*-band is the π-π* energy gap, Eg. The bonding orbital (π) corresponds
to the valence “band” of the semiconductor and the antibonding orbital (π*) corresponds to the
conduction “band”. Each conjugated part of a molecule in a conjugated material is characterized by a
band-like energy distribution in the electronic density of states with an energy gap between the
Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital
(LUMO). This energy band structure is similar to a semiconductor. In this form polymer is a poor
electrical conductor. In a diatomic molecule, a molecular orbital (MO) diagram can be drawn showing
a single HOMO and LUMO, corresponding to a low energy π orbital and a high energy π* orbital.
Figure 5: molecular orbital diagram
Every time an atom is added to the molecule, a further MO is added to the MO diagram (Fig. 5).
Thus for a PPV chain which consists of ~1300 atoms involved in conjugation, the LUMOs and
HOMOs will be so numerous as to be effectively continuous. And this results in two bands we
mentioned above. They are separated by a band gap which is typically 0-10eV and depends on the
type of material. PPV has a band gap of 2.5eV.
2.2 SSH model of electronic structure
The Su-Schrieffer-Heeger (SSH) model is a simple tight-binding model for polyacetylene [11]. In
this model, the coordinates and motion of the atomic nuclei is treated classically, but the π -electron
dynamics is treated fully quantummechanically in a tight-binding approximation. The crucial aspect of
the model is that the coupling between the nuclear coordinates is taken into account via the distancedependent
hopping terms. The idea behind this is simply that the overlap integrals between
neighboring atomic orbitals increase when the distance between neighboring atoms decreases.
Conjugated polymers are π-bonded macromolecules in which the fundamental monomer unit is
repeated many times. Thus, N, the Staudinger index as in (CH)N, is large. Since the end-points are not
important when N is large, the π-electron transfer integral t tends to delocalize the electronic
wavefunction over the entire macromolecular chain. This tendency toward delocalization is limited by
5

disorder (which tends to localize the wavefunctions) and by the Coulomb interaction, which binds
electrons when transferred to a nearby repeat unit to the positive charge left behind (a hole).
The construction of the remarkably successful SSH Hamiltonian is based on two assumptions [2]:
(a) The π-electronic structure can be treated in the tight-bonding approximation with a transfer integral
t ≈ 2.5 eV, and (b) The chain of carbon atoms is coupled to the local electron density through the
length of the chemical bonds.
tn, n+
1 = to
+ α ( un
+ 1 − un
)
(1)
where tn , n+
1 is the bond-length dependent hopping integral from site n to n +1 and n is the
displacement from equilibrium of the carbon atom. The first assumption defines the lowest order
hopping integral, , in the tight-binding term that forms the basis of the Hamiltonian (Eqn. 2). The
second assumption provides the first-order correction to the hopping integral. This term couples the
electronic states to the molecular geometry, giving the electron-phonon (el-ph) interaction where α is
the el-ph coupling constant. The precise form of Eqn. (1), in which the dependence of the hopping
integral on the C-C distance is linearized for small deviations about , is the first term in a Taylor
expansion.The resulting SSH Hamiltonian is then written as the sum of three terms:
u
th
n
to
to
H
∑
2
+
+
pn
1
2
[ −to
+ α ( un+
1 − un
)]( cn+
1,
σ cn,
σ + cn,
σ cn+
1.
σ ) + ∑ + K∑(
un
1 − un
) (2)
2m
2
SSH = +
n,
σ
n n
where pn are the nuclear momenta, n are the displacements from equilibrium, m is the carbon mass,
and K is an effective spring constant. The and are the fermion creation and annihilation
operators for site n and spin σ. The last two term are, respectively, a harmonic »spring constant« term
which represents the increase in potential energy that results from displacement from the uniform
bonds lenghts in (CH)x and a kinetic energy term for the nuclear motion.
u
+
cn, σ cn,
σ
Figure 6: Electronic structure of semiconducting PA; left - Band structure, right - Density of
states. The energy opens at k = π/2a as a result of Peierls distortion
The spontaneous symmetry breaking due to the Peierls instability implies that for the ground state
of a pristine chain, the total energy is minimized for u n > 0 .Thus to describe the bond alternation in
the ground state, we use:
u )
n
n → un
= (−1
uo
(3)
With this mean-field approximation, the value uo
which minimizes the energy of the system can be
calculated as a function of the other parameters in the Hamiltonian. Qualitatively, however, one sees
6

that uo and - o both minimize the energy for trans-polyacetylene since the bonds all make the same
angle with respect to the chain axis. Hence, the energy as a function of u has a double minimum at
± , as shown in Fig. 7 [2,12].
u
u
o
Figure 7: Total energy of the dimerized polyacetylene chain
Strictly speaking, the SSH model is directly applicable only to polyacetylene; however, recent
work has shown that the primary excitation in luminescent polymers like PPV can also be described
within the linear chain model. In PPV and its derivatives, the lowest excitonic wave function extends
over several repeat units; the properties of excitons are therefore not very sensitive to the delicate
structure within the unit.
2.3 Doping, charge transport
In conjugation, the bonds between the carbon atoms are alternately single and double. However,
conjugation is not enough to make the polymer material conductive. In addition – and this is what the
dopant does – charge carriers in the form of extra electrons or holes have to be injected into the
material. Before a current can flow along the molecule one or more electrons have to be removed or
inserted. If an electrical field is then applied, the electrons constituting the π bonds can move rapidly
along the molecule chain. The conductivity of the plastic material, which consists of many polymer
chains, will be limited by the fact that the electrons have to "jump" (hop) from one molecule to the
next. Hence, the chains have to be well packed in ordered rows.
There are two types of doping, oxidation or reduction. In the case of PA the reactions are written:
Oxidation with halogen (p-doping): [CH]n + 3x/2 I2 → [CH]n x+ + x I3 -
Reduction with alkali metal (n-doping): [CH] n + x Na → [CH]n x- + x Na +
However, it is not the iodide or sodium ions that move to create the current, but the electrons from the
conjugated double bonds.
The mechanisms of conductivity in doped polymers are based on the motion of charged defects
within the conjugated framework. In solid state physics these charged excitations are usually described
as positive or negative solitons (ion defects) and polarons (radical ion defects) respectively [13].
The trans-structure of polyacetylene possesses a two-fold degenerate ground state (Fig. 8) and
single and double bonds can be interchanged without changing energy. A break in pattern of bond
alternation separates degenerate ground-state structures.
Figure 8: degenerate phases in trans-polyacetylene
7

This break leads to a free radical defect, a so-called neutral soliton which is relatively stable
(Figure 9). Addition of an acceptor removes an electron and creates a positive soliton (or a neutral one
if the electron removed is not the free electron). The resulting carbocation is stabilised by having the
charge spread over several monomer units and the charged solitons are responsible for making
polyacetylene a conductor
Figure 9: Charge defects in polyacetylene and oxidative doping.
A further type of charge storage occurs when the generated charge and the radical are coupled to
each other via local resonance of the charge and the radical. This type of charge transport is present in
polymers like PPV. This combination of a charge site and a radical dependent of each other is called a
polaron. A new localised electronic state is created in the band gap, with the lower energy states being
occupied by a single unpaired electron. Unlike the soliton, the polaron cannot move without first
overcoming an energy barrier so movement is by a hopping motion.
FIGURE 10: Radical cation (”polaron”) formed by removal of one electron on the 5th carbon
atom of a undecahexaene chain (a ―> b). The polaron migration shown in c ―> e.
If a second electron is removed from an already oxidised section of the polymer, either a second
independent polaron may be created or, if it is the unpaired electron of the first polaron that is
removed, a bipolaron is formed (with lower energy than 2 polarons) (Figure 11). The two positive
charges of the bipolaron are not independent, but move as a pair, like the Cooper pair in the theory of
superconductivity. While a polaron, being a radical cation, has a spin of 1/2, the spins of the
bipolarons sum to S = 0.
8

Figure 11: Band theory model in polymers. At high doping level, the soliton regions tend to
overlap and create new mid-gap energy bands that may merge with the valence and conduction
bands allowing freedom for extensive electron flow. However, for most heavily doped conjugated
polymers it is conceivable that the upper and the lower bipolaron bands will merge with the
conduction and the valence bands respectively to produce partially filled bands and metallic like
conductivity. Conduction occurs because the mean free path of a charge carrier extends over a
large number of lattice sites. The residence time on each site is small compared with the time it
would take for a carrier to become localized.
The mechanisms of charge transport in polymers are still not fully elucidated. One example is that a
stretched polyacetylene shows a better conductivity than the same material without orientated
morphology and it remains under investigation how inter-chain charge transfer takes place.
2.4 Light emission
Conjugated polymers can show photoluminescence as well as electroluminescence. The first being
obvious from the fluorescent colour a typical light emitting polymer like PPV exhibits. This is due to
the relaxation of a singlet excited state generated [13] by the absorption of a photon (Figure 12). A
similar process is responsible for the electroluminescence of a conjugated polymer but the generation
of the excited state is different. Instead of excitation by a photon, a negative electrode injects electrons
(generation of radical anions) and a positive electrode injects holes (generation of radical cations)
respectively. Electrons and holes can combine as they are attracted to each other by Coulomb
interactions. This leads to the formation of neutral bound excited states termed excitons. The spin
wavefunction of an exciton can be triplet or singlet as in the case of photoluminescence. The
decomposition of them leads to light emission. The emitted wavelength of an electrically excited
conjugated polymer depends crucially on its band gap
c
Eg = h
(4)
λ
where h is Planck’s constant and c is the speed of light. PPV produces yellow-green luminescence (Eg
= 2.5 eV).
9

Figure 12: Photoluminiscence and electroluminiscence in conjugated polymers. a) Irradiation can
excite an electron from the LUMO in the HOMO and two new energy states are generated. Both
are filled with an electron of opposite spin (singlet exited state). Relaxation to the ground state
leads to the emission of light of smaller frequency. b) In order to show electroluminescence,
radical ions have to be produced in the polymer by the application of an electric field. When
radical ions of opposite charge combine, so-called excitons (singlet or triplet excited state) are
formed and the decomposition of this neutral excited state (recombination) leads to radiative
emission.
3 OLED device structure
An OLED device consists of one or more semiconducting organic thin films sandwiched between two
electrodes – one of which must be transparent. (Fig. 13).
3.1 Basic PLED
Figure 13: basic (two-layer) OLED structure
The energy level diagram of a typical single layer PLED is shown in Figure 14. The device utilizes
~100 nm of PPV with an ITO anode and calcium cathode [3,14].
Figure 14: PLED device operation (energy diagram)
10

When a forward bias is applied, electrons are injected from the cathode into the LUMO of the
polymer and holes are injected from the anode into the HOMO of the polymer. Thus, the electrons
must overcome the barrier (electron injection barrier) between the Ca Fermi level and the LUMO level
of the polymer. Low work function metals such as Mg or Ca are typically used to minimize this barrier
and provide an ohmic contact. A good energy match between cathode and LUMO means that not
much energy is lost when electrons are injected. Similarly, to ensure ohmic injection of holes from the
ITO Fermi level into the HOMO of the polymer, the ITO may be treated in various ways (e.g.,
exposure to an ultraviolet-ozone cleaning) to lower its Fermi level. (barrier is here called hole injection
barrier.)
3.2 Basic SMOLED
The p-n diode structure proves to be a key feature for the OLED device. The basic structure of
SMOLED consists of two layers of organic thin films - a hole transport layer – HTL (p-layer by LED)
and an electron transport layer - ETL (n-layer by LED) - sandwiched between an anode and a cathode
(Fig. 15). These two organic layers, each on the order of about 500 Å thick, provide the appropriate
media for transporting charge carriers, toward the interface formed between the two layers [15].
Figure 15: A basic OLED device structure consists of a hole-transport layer and an electron
transport layer sandwiched by a cathode and anode.
One of the most basic SMOLED device structures uses an organic material called NPB (naphthyl
substituted benzidine derivative) as HTL and Alq3 as ETL. In this typical structure we use indium tin
oxide (ITO) as the transparent anode and magnesium-doped silver (Mg:Ag) as the cathode.
When voltage is applied, charge injection of electrons through the cathode and hole through the
anode occurs. Electrons are transported to the LUMO of the ETL, and holes to the HOMO of the HTL.
Recombination of these charges occurs across the barriers, with holes primarily moving into alq3 (Fig.
16). Excitons formed in Alq3 in this case emit green fluorescence. See Flash movie [16].
Figure 16: SMOLED device operation (energy diagram)
11

This two-layer design also is important because it provides the necessary energetic barriers at the
interface to effectively localize the recombination of the oppositely charged carriers at or near the
interface region. As a result, this organic interface region, on the order of 100 to 200 Å thick, is also
primarily responsible for the light generation from the SMOLED device.
3.3 Phosphorescent dopping (Singlet, Triplet excitons)
The process of charge injection and recombination in OLEDs results in the generation of singlets and
triplets. Quantum statistics limits the direct generation of singlets to 25%, while the rest of the
excitons, 75 % are triplets (Figure 17). In traditional OLEDs, which are fluoroscence based, only the
singlet excitons contribute directly to the light generation process. Taking into consideration the
device plannar geometry, and other factors, the external quantum efficiency is limited to about 5% [5].
Significant increases in the device quantum efficiency can be brought upon by allowing the triplets to
contribute to the light-emission process. This can be achieved by using phosphorescent dyes in an
appropriate host material [15].
Figure 17: singlet and triplet exciton light emission
Even though highly fluorescent dyes increase EL efficiencies for small-molecule OLEDs, they
only harness a fraction of all electrically generated excitons. Two types of excitons are formed when
electrically injected carriers recombine: singlet excitons with total spin S = 0 and triplet excitons with
total spin S = 1. Since the ground state of organic molecules has S = 0, and the relaxation of a
molecule through the radiative recombination of an exciton must conserve spin, fluorescent emission
from singlet excitons is the only allowed process that generates photons. Hence, for typical
fluorescent-based OLEDs, all triplet excitons are wasted. For small-molecule devices, it is believed
that only 25% of the emissive singlet excitons are formed during electrical excitation. However, some
materials do exhibit light emission from triplet excitons. In these materials, the singlet and triplet
states are mixed and hence the excited triplet states share some singlet character and radiative decay to
the ground state is allowed. This process is known as phosphorescence. Adding a heavy metal atom
such as iridium to an organic molecule increases the spin-orbit coupling that mixes singlet and triplet
excited states allowing for efficient radiative decay of triplet excitons.
The energy level schematic of an OLED employing an iridium-based phosphorescent small
molecule is shown in Figure 18. Here, two ETL layers are used - one (CBP) hosts the phosphorescent
iridium complex and one (BCP) acts solely as a hole (and exciton) blocking layer. Upon injection,
holes are transported in the HTL and recombine with the electrons that have been injected into the hole
blocking layer and have drifted to the CBP ETL. Both singlet and triplet excitons are formed in the
CBP host and then both types of excitons are transferred nonradiatively to the emissive state of the
iridium complex. This state then emits light through phosphorescence. The net effect is that both the
singlet and triplet excitons created in CBP are utilized for light emission. And this clearly
demonstrates the potential of high efficiency OLEDs based on phosphorescence.
12

Figure 18: Schematic energy level diagram of an optimized small molecule OLED employing the
phosphorescent complex: Ir(ppy)3 doped into a CBP host. HTL is a thin film of (α-NPD). A BCP
molecule with a large HOMO-LUMO spacing is used here as both a carrier and exciton blocking
layer. A bilayer cathode consisting of a thin (

5 Matrix types of OLEDs, Products
An OLED is a current-driven device. That is, the intensity of the output light is directly
proportional to the electrical current flow through the device. An OLED display, therefore, requires
the control and modulation of electrical current levels through individual elements (pixels) in order to
display text or graphic images.
There are two types of OLED display architectures: passive matrix and active matrix. In a
passive-matrix OLED display, the columns provide the data signal, and the rows are addressed one at
a time. The current flow through a selected row is necessarily pulsed to a level that is proportional to
the total number of rows in the display. This instantaneous current requirement places a restriction on
the size and resolution of the passive matrix design. This restriction is removed in an active-matrix
OLED display, where each individual pixel can be designed to switch on or off within a frame time.
As a result, the device does not suffer the resolution limitations or the high-instantaneous-current
requirements of a passive-matrix design. The size and resolution of these displays are determined by
practical considerations such as the constraints of the substrates rather than the OLED component.
Because the pixel architecture and electrode geometry for the OLED element are already defined on
the substrate, the fabrication of the OLED component is straightforward. The cathode is continuous
over the entire display area, requiring no patterning.
In 1998 Pioneer put on the market first commercially available passive matrix OLED displays in
car radio CD-players (Fig. 20).
Figure 20: 2004 range of Pioneer car radios ; left - Pioneer DEH-P 6600 with blue OLED
display (280$), right - Pioneer DEH-P 8600 with full color OLED display (550 $)
These small molecule based displays were also found in Motorola cellular phones by 2000. Recently
also mobile phones from other manufactorers came on market, with external 256 colors OLEDs as
complement to bigger internal LCDs like in Samsung presented below (Figure 21). In 2003 Kodak
introduced a digital camera incorporating the first commercially available active matrix OLED
display. After Motorola
Figure 21: a) Samsung SGH-E 700 b) Kodak EasyShare LS633 (120.000 SIT)
14

Nearby Future
a) Big screens - The technology will provide competitive full-size computer displays and flat-panel
TV screens that consume less power than possible with flat panel technologies available today.
A prototype of world's largest (till now) - 20"
OLED full color display, WXGA (1280x768)
with Low power consumption driven by
Amorphous Silicon TFTs has been presented in
2003 (Figure 22) by Chi Mei Optoelectronics
Corporation (CMO) from Taiwan.
Figure 22: Technical details: The 20-inch prototype is a top-emitting full-color active-matrix
display. It has WXGA resolution (1280 x 768 pixels) and a power consumption of 25 Watt at a
brightness of 300 cd / m² (its desktop display brightness, can exceed 500 cd/m2) [19].
b) FOLEDs - flexible OLEDs [7] are organic light emitting devices built on flexible substrates (Fig.
23). Flat panel displays have traditionally been fabricated on glass substrates because of structural
and/or processing constraints. Flexible materials have significant performance advantages over
traditional glass substrates.
For the first time, FOLEDs may be made on a wide
variety of substrates that range from optically-clear
plastic films to reflective metal foils. These materials
provide the ability to conform, bend or roll a display into
any shape. This means that a FOLED display may be
laminated onto a helmet face shield, a military uniform
shirtsleeve, an aircraft cockpit instrument panel or an
automotive windshield [20].
Figure 23: FOLED for DARPA, courtesy Universal Display
FOLEDs will also generally be less breakable, more impact resistant and more durable compared to
their glass-based counterpart [7].
c) Lightning applications - The rapid progress in efficiency and performance demonstrated by OLED
technology over the last decade has caused many companies to consider OLEDs as a potential solidstate
light source for lighting applications. In terms of application potential, OLEDs nicely
complement inorganic LEDs - a technology more often associated with solid-state lighting (SSL). In
particular, since inorganic LEDs are bright point sources of light, they are naturally suited for
applications such as spot or task lighting that require spatial control over the illuminating beam. In
contrast, OLEDs represent a diffuse source of light and so are naturally suited to large area generallighting
and signage applications where, for instance, fluorescent lighting is used today.
15

6 Conclusion
The future prospect looks bright for OLED flat panel displays. OLED displays have been used in
aftermarket car audio for the last several years. They are starting to find use in cell phones as
secondary displays and are expected to enter the market this year in the form of full color displays for
cell phones. Many industry analysts predict OLED displays for laptops and computer monitors will
start to enter the market as soon as 2006. Other applications, such as flexible displays for outdoor
advertising signage and billboards, are further out in time. This application requires over 50 times
performance improvement in OLED materials in brightness and lifetime, and also requires a complex
active matrix drive scheme, inkjet deposition for low cost manufacturing, and flexible substrates.
Therefore, given the requirements and capabilities of billboard application, it may not be realized
within the next five years. Another advanced application for OLED materials includes replacement for
fluorescent room lighting, which again depends upon substantial improvement in the performance of
materials, in particular, energy efficiency. Meanwhile, in the short term, OLED displays have started
and will continue to penetrate the $30 billion dollar display market and have begun to realize their
bright future ahead.
16

7 References
[1] C.W.Tang and S.A. Van Slyke, Organic electroluminiscent diodes; Appl. Phys. Lett. 51, 913
(1987)
[2] Alan J.Heeger, Semiconducting and Metallic Polymers: The fourth generation of polymeric
materials, Nobel Lecture, December 8, 2000
[3] C.W. Tang, S.A. VanSlyke and C.H. Chen, Electroluminiscence of doped organic thin films;
J. Appl. Phys. 65, 3610 (1989)
[4] G.Parthasarathy et al., Organic Light Emiiting Devices; The Electrochem. Soc. Int, Summer 2003
[5] G.E.Jabbour et al., High-efficieny organic electrophosphorescent devices through balance of
charge injection; Appl. Phys. Lett. 80, 2026 (2002)
[6] Ioannidis et al., C-V characteristic of OLEDs; Appl. Phys. Lett. 72, 3038 (1998)
[7] Anna. B. Chwang et al., Thin film encapuslated flexible organic electroluminiscent displays;
Appl. Phys. Lett. 83, 413 (2003)
Internet:
[8] http://www.calpoly.edu/~drjones/chem447/Polymers%20CD/Files/LED/pledsoleds.htm
[9] http://classes.engr.arizona.edu/mse110/Lab/CP.pdf
[10] http://www.colby.edu/chemistry/CH145/CH145Lab/MO%20Lab%20-CH1452001.pdf
[11] http://www.ilorentz.org/~saarloos/Correlateds/soundvel.html
[12] http://www.nobel.se/chemistry/laureates/2000/public.html
[13] http://www.chemonaut.de/dateien/CHEM364.pdf
[14] http://www.electrochem.org/publications/interface/summer2003/IF6-03-Pages42-47.pdf
[15] http://oemagazine.com/fromTheMagazine/feb01/brightness.html
[16] http://www.optics.arizona.edu/oled/INTRO.HTM
[17] http://oemagazine.com/fromTheMagazine/jun02/pdf/polymers.pdf
[18] http://www.tu-darmstadt.de/fb/ms/fg/em/OLED.pdf
[19] http://www.idtech.co.jp/en/news/press/20030312.html
[20] http://www.universaldisplay.com/foled.php - FOLED movie
other links:
www.osram-os.com/
http://www.erc.arizona.edu/Education/REU/Student%20Reports%2003/chris%20report.pdf
http://www.research.ibm.com/journal/rd/451/curioni.html
http://www.eurekalert.org/pub_releases/2003-03/giot-nta032103.php
http://www.rochester.edu/college/workshop/presentations/MarkThompson/MT_2003.pdf
http://www.usc.edu/org/techalliance/Anthology2003/Final_Crawford.pdf
http://www.physicstoday.org/pt/vol-54/iss-12/p42.html
http://www-mtl.mit.edu/MEngTP/John_Ho_Proposal.pdf
17