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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
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Page 1: OLEDs - University of Ljubljanamafija.fmf.uni-lj.si/seminar/files/2003_2004/OLEDs.pdf · use both fewer and lower-cost materials than ... and takes a deeper insight in their electron

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

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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

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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.

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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.

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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 distance-dependent 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

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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.

)( 11, nnonn uutt −+= ++ α (1)

where is the bond-length dependent hopping integral from site n to n +1 and 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:

1, +nnt nuthn

ot

ot

∑ ∑∑ −+++−+−= ++++

++n n

nnn

nn

nnnnnoSSH uuKm

pccccuutH 2

1

2

.1,

,,,11 )(21

2))](([ σ

σσσσα (2)

where are the nuclear momenta, 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.

np nu+σ,nc σ,nc

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 0>nu .Thus to describe the bond alternation in the ground state, we use:

on

nn uuu )1(−=→ (3)

With this mean-field approximation, the value which minimizes the energy of the system can be calculated as a function of the other parameters in the Hamiltonian. Qualitatively, however, one sees

ou

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that and - 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].

ou ou

ou

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]nx- + 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

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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.

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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

λchEg = (4)

where h is Planck’s constant and c is the speed of light. PPV produces yellow-green luminescence (Eg = 2.5 eV).

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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).

Figure 13: basic (two-layer) OLED structure

3.1 Basic PLED 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)

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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)

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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.

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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 (<10 Å) layer of LiF, followed by a thicker layer of Al, is used to efficiently inject electrons. The emission between the two dashed lines within the CBP energy level gap schematically depicts radiative recombination of both singlet and triplet excitons on the Ir(ppy)3 molecule.

4 Technical facts, Stability

Already first OLEDs had very remarkable characteristics such as: brightness around 1000 cd/m² and external luminous efficiency 1.5 lm/W achievable already at voltages from 2-3 V [1]. Nowadays even greater values have been achieved: 40 lm/W, contrast ratio 3000:1, brightness: 10000 cd/m². OLED pixels response is only 5ms (200 frames per second). And that's quite a remarkable difference compared to good LCDs with contrast ratio: 500:1, brightness 400 cd/m² and response time: 16-25 ms. There have been reported also extreme achievements in brightness as high as 140.000 cd/m². Outstanding are also wide viewing angles of OLEDs - up to 170 degrees [6].

The reliability of OLED devices has been the major concern for practical applications. It has, however, improved substantially over the years. Using various sets of organic materials, many laboratories have reported achieving luminance life (half decay) on the order of 10,000 hours in devices of all colors using various sets of organic materials (Fig. 19) [17] (for comparison today inorganic LEDs achieve life times of 100,000 hours and more). The materials currently used in OLEDs are very sensitive to moisture and oxygen. So preventing moisture and oxygen diffusion into the display area are two major challenges in development of long-lived OLEDs.

Figure 19 : characteristics of different color OLEDs

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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)

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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.

Figure 22: Technical details: The 20-inch pdisplay. It has WXGA resolution (1280 x 76brightness of 300 cd / m² (its desktop displa b) FOLEDs - flexible OLEDs [7] are organic23). Flat panel displays have traditionally beeand/or processing constraints. Flexible matetraditional glass substrates.

Figure 23: FOLED for DA

FOLEDs will also generally be less breakable,their glass-based counterpart [7].

c) Lightning applications - The rapid progresstechnology over the last decade has caused mastate light source for lighting applications.complement inorganic LEDs - a technology mparticular, since inorganic LEDs are bright applications such as spot or task lighting that contrast, OLEDs represent a diffuse source of lighting and signage applications where, for ins

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.

rototype is a top-emitting full-color active-matrix 8 pixels) and a power consumption of 25 Watt at a y brightness, can exceed 500 cd/m2) [19].

light emitting devices built on flexible substrates (Fig. n fabricated on glass substrates because of structural rials have significant performance advantages over

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].

RPA, courtesy Universal Display

more impact resistant and more durable compared to

in efficiency and performance demonstrated by OLED ny companies to consider OLEDs as a potential solid- In terms of application potential, OLEDs nicely ore often associated with solid-state lighting (SSL). In point sources of light, they are naturally suited for require spatial control over the illuminating beam. In light and so are naturally suited to large area general-tance, fluorescent lighting is used today.

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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.

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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.pdfhttp://www.research.ibm.com/journal/rd/451/curioni.htmlhttp://www.eurekalert.org/pub_releases/2003-03/giot-nta032103.phphttp://www.rochester.edu/college/workshop/presentations/MarkThompson/MT_2003.pdfhttp://www.usc.edu/org/techalliance/Anthology2003/Final_Crawford.pdfhttp://www.physicstoday.org/pt/vol-54/iss-12/p42.htmlhttp://www-mtl.mit.edu/MEngTP/John_Ho_Proposal.pdf

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