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OLED Device Applications

2009-03-13T18:40:26Z

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Organic Light Emitting Diodes (OLED) are just are just beginning to appear in the commercial market. These products represent the fruition of 50 years of research building first on the principles of silicon LEDS.

The first OLED devices include TVs, computer monitors, electronic control displays, cameras, phones, and lighting devices.

==The advantages of OLEDs==
*Superior viewing angle- For monitors and TVs the screen is visible from the side rather than just face on as many LCD monitors are.
*Color Rendition- New dopants and dyes are being developed to give OLED a bigger range and flexibility of color rendition.
*Brightness- The OLED pixels produce light rather than block light with polarizers as an LCD display does.
*Faster Response- OLED devices typically have response time of .01 ms compared to 2 ms for LEDs.
*Energy Efficiency- The OLED is an efficient, low heat light source
*Cost- New polymers and coatings will allow LEDs to be produced by printing and spin coating techniques
*Flexibility- Polymer backing and thin coatings permit OLED to flex without breaking.
*Thin- A OLED display could be paper thin.

==Device construction==
An OLED consists of a thin transparent electrode, two or more organic transport/ emitting layers and metal cathode. When power is applied to the electrodes light is emitted from the central layer.

Individual red, green and blue emitting OLEDs are arranged in a grid with individual power supplies for each pixel. This is called a passive display. This is being replaced with active thin film transistor display that uses a transistor to control each pixel. This is called an active matrix display.
==Commercial OLED Products==
[http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644539854| Sony OLED TV]

http://www.universaldisplay.com/

http://www.kodak.com/eknec/PageQuerier.jhtml?pq-path=1473&pq-locale=en_US&_requestid=204

http://www.cdtltd.co.uk/

http://www.novaled.com/

[http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf Osram Opto Semiconductors]

Fluorescent/Phosphorescent Dopants

2009-03-13T18:40:09Z

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==Example OLED applications==
The Pioneer car stereo was one of the first OLEDs to reach the market with green and blue OLED.

The polymer OLEDs use an emissive layer based on derivatives of polyphenylenevinylene which where spin-cast or deposited with an inkjet printing process. The first of those consisted of two different polmer materials with different ionization potentials and electron affinities to create a heterojunction. These were capped with calcium or magnesium and more recently with aluminum or other more stable metals.
==Jablonski Diagram==
The Jablonski diagram shows a dopant molecule that can be excited from its singlet state to its first excited singlet state by overlap of its absorption with the emission from the host dye such as aluminum quinolate. The first dopants were primarily fluorescent dopants which shift the energy of the emission of the device slightly more to the red and improve the efficiency and stability of the device. The holy grail is top capture 75% of the energy which exists in the triplet energy. So we need to find dopants that can tap the triplet energy.
==Device Efficiency==
External power efficiency is a product of the light out-coupling efficiency and the internal efficiency. With OLEDs only about 20% the light that is created is emitted in the forward direction. The remaining energy is lost in substrate or waveguide modes which go to the side. This has intriguing possibilities for sensors. Internal efficiency is related to the efficiency of all the components in the OLED.

*γ The gamma is the ratio of electrons to holes, typically less than on. So there is an energy loss due this imbalance.
*<phi> Phi is the quantum efficiency of the molecule that is doing the emitting. You want to find molecules with the highest possible quantum efficiency for luminescence.

*nex is the fraction of luminescent excitations that are harvested based on spin statistics.
Fluorescent OLED have external efficiency of about 5% while phosphorescent OLEDs can go up to 20% efficiency.
==How to Increase Performance==
How do you increase performance? First you can balance charge injection. Single anthracene crystal devices had an efficiency of less than 1%. By going to a two layer device the external efficiency jumps to 1%. The first of doped devices use DCM2 a cumarin dye that is used in some dye lasers. It was doped at a high level of 10% works as a energy acceptor from the Alq3 host and it boosted the external efficiency up to 2.5%. It is an orange–red emitter which gets move your emission from green to the red part of the spectrum.
==Dynamics of Förster Energy Transfer==
Consider the dynamics of Förster energy transfer. The spin statistic dictate that 75% of the electron transfer is in the triplet state. In the case of emission the singlet state gives you fluorescence and phosphorescence comes from the decay of the triplets state back to the ground state. It would be best to capture both types of forms in the devices. Energy efficiency is strongly dependent on the overlap of donor emission with acceptor absorbance.

Because of the weak probability of intersystem crossing from fluorescent to the phosphorescent triplet state these molecules have lifetimes that are rather long, milliseconds and longer. OLEDs need to have lifetimes that are shorter.
Phosphorescent dopants such as the heavy metal porphyrin PtOEP pushes the efficiency to 5%. The heavy metal increases the probability of intersystem crossing.

The mechanism of triplet energy transfer is Dexter energy transfer involving the exchange of an electron. This has a different distance dependence than Forster transfer, its much closer in. The right phosphorescent dyes create triplet states at relatively low concentrations. The efficiency is strongly dependent on the orbital overlap between the donor and acceptor system. Heavy metal atoms such as platinum and iridium help to mix the singlet and triplet states by spin-orbit coupling.

==Excited States of Organometallic Complexes==
There are a couple of considerations when you work with organometallic complexes. There are ligand to metal transitions in the absorption of these molecules. There are ligand centered, metal to ligand centered and as well as metal centered transitions as well. You have to be careful to understand which of the molecular orbitals you are going to use to maximize the triplet state and the phosphorescence.

See Balzani, V. and Scandola, F. Supramolecular Photochemistry, Ellis Harwood, England 1991, for a review of the rules for design of phosphorescent dopants.

Platinum octoethyl porphyrin (Pt OEP) was the first phosphorescent dopant. The graph shows the host emission from a aluminum quinolate diode. By the time you get to 20% PtOEP there is virtually no emission from the Alq3 and nice red emission from the PtOEP.
[[Image:PtOEP.png|thumb|300px|Platinum octaethylporphyrin]]
http://www.chemspider.com/ImageView.aspx?id=21169877&mode=3d

It is important to look at the quantum efficiency for these devices as a function of the applied current density. You want the most light out for least amount of current injected into the system. Pick a threshold luminance of 100 candelas per square meter which is about the about the intensity of a computer screen display. For PtOEP there is a quantum efficiency of 1% at 100 candelas and current density of 10 mA per square centimeter. This is not bad but could be better. Also PtOEP might not be the best choice for an emitter because of its long lifetime of 100 microseconds. We will lose some energy from this phosphorescence because it lasts so long and there are other non-radiative decay processes that suck energy away.

Organic Heterojunctions

2009-03-13T18:39:27Z

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==Organic Herterojunctions==
The mobility for electrons passing into the HTL is much lower than the mobility of holes passing into the ETL. In recent years there has success in balancing these mobilities but even in polymer light emitting diodes it is still a problem.
To make a practical OLED there are these considerations:
*First minimize the injection barriers for both hole injection and electron injection. This decreases the drive voltage required to create light from the device.
*Its desirable to match the charge mobilities in HTL and ETL layers. Some researchers have tried to slow down the hole injection so as to keep the arrival of the holes matched with the arrival of the electrons. This approach can be used up to a point.
*You want localize the emission of zone to the center of the device. It possible to inject a hole from the HTL into the ETL. However, it is better to have the charges collect at the interface and undergo a cross reaction to generate the emissive state.
This simplified diagram shows the change in band position-- potential energy as a function of distance across the device when the devices is under a positive bias. This assumes a common vacuum level which is not quite correct. Electrons and holes build up at the heterojunction.

Holes can cross the barrier or holes can encounter an electron at the interface. They are energetically separated from each other but spatially seeing each other across the interface.
The recombination process creates enough excess free energy to create an emissive state of the ETL molecular system and light is emitted towards the HTL layer.

==Typical components of a single heterojunction OLED==
[[Image:OLED7_heterjunct_chems.JPG|thumb|400px]]
These are the typical components of a heterojunction OLED that are used in many labs. TBD is a well known Bis triarylamine which is easy to oxidize and forms a very stable cation radical because of the resonance stabilization between the two nitrogen centers. Its energy level is shown missing an electron. These can have lifetimes in dry solvents of up seconds to minutes so they extremely stable. They are very strongly colored. It is used in the hole transport layer.

Aluminum tris quinolate is a typical emissive material and the electron transport material with a very high quantum yield for emission. It is reducible. The lifetime in solution of the one electron reduced state is short. It is unstable and quite reactive.
When these two species encounter each other across the interface of an OLED an electron transfer event takes place. The resultant excess free energy is deposited in the lower bandgap of the two materials which in this case is the ETL material. This is what creates the green emission in the LED.
 
[[Image:OLED7_voltagrams.JPG|thumb|400px]]

These are voltamograms for various components that might be of use in building OLEDs.
Following TBD, as we sweep towards the positive is the first one electron oxidation and then a second, and then the reduction of the di-cation radical and then the reduction of the cation radical back to the neutral state. In an OLED we only worry about the first of the two one electron transfer processes.

For aluminum quinolate if we scan at very high sweep rates at a microelectrode we see a one electron reduction process and then the oxidation of the anion radical product. This does not remain stable at low sweep rates, it has a half life on the order of microseconds compared with second for TPD. Even with very clean solvents once you make the radical anion it will go out and find something else to reduce such as traces of oxygen or water. In the condense phase environment we can anticipate it will be much more stable.

If you go far enough positive Alq3 it will be undergo oxidation but it is chemically irreversible. This is why you want to avoid hole injection to this layer. On the other had it very hard to reduce TBD, therefore we are only concerned with one electron oxidation.

So in an experimental cell with both components in the cell at the same time we rapidly pulse the electrode forming the radical anion state of ALq3, reverse the charge and form the radical cation of the TBD. The two molecule diffuse together near the electron solution interface where they undergo electron transfer reactions they emit light which can leaves the cell through the window.

We developed Aluminum tris suphonamide quinolate (Al(qs)3) at U of A. By adding the sulphonamide substituent to the 8 hydroxy quinolin ring we made the molecule much easier to reduce. The diagram show three one electron reductions however only the first is relevant for OLED production.

Polyvinyl Carbazole (PVK) is harder to oxidize than TPD.

DiQA , a quinacridone derivative is related to one of the first dopants used by Kodak to create the green OLED, has a very stable one electron reduction product and a very stable oxidation product as shown in the voltamograms. We are going to use this molecule to OLEDs and let it do both energy and charge capture to create a green emissive state.

There is adequate excess free energy in the cross reaction between TPD+ and Alq3- to create an emissive state Alq3* which gives the green light. The number of emissive states generated per unit time is related to the rate coefficient for electron transfer which is turn related to the excess free energy in the cross reaction and the reorganization energy involved in undergoing that electron transfer. Marcus theory does apply.

There is the problem of spins statistics. We are going generation both the 24% singlet states and 75% triplet states. This will have an impact on the optimization of OLEDs.

OLED Charge Mobilities

2009-03-13T18:38:32Z

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==Fundamentals of Charge Transport in Organic Solids==
[[Image:OLED6_transportrates.JPG|thumb|300px|Comparative Charge Mobilities]]
The concept of charge transport was developed first for crystal. Crystals have period structures, the band model applies and there is delocalization of electrons in the electron conduction band and holes in the valence band. This is the case with doped silicon.
Amorphorous organic materials have charge localized in the form of an radical ion and it moves by interstitial hopping, that is hopping from molecular site to molecular site. This is intrinsically a less efficient process than in crystalline inorganic semiconductors. This is reflected in the mobility of charge carriers.
Hole conduction and electron conduction occur in opposite directions as a result of an applied electric field. They do not occur at the same rate. In general electron conduction is a slower process than hole transport.
In the early day there was an amorphous organic state, the liquid crystal state and the single crystal state. The benchmark mobility for amorphous silicon is 0.5 x 10 -1 cm2/Vs. Charge mobility is measured as a flux of charge over and area of one square centimeter per volt per second. If you get an organic materials to come anywhere close to that you have a real winner. Most organic materials have charge mobility of 10-3 cm2/Vs up to 1 cm2/Vs. This is important because it ultimately limits how thick you can make an organic device. OLEDs are made quite thin because of these low mobilities. Over the last two decades the mobility has risen, for example single crystal rubrene has been have shown to have mobilities around 10-15 cm2/Vs. There are liquid crystalline systems which good charge mobilities along the stacking axis. So the conventional wisdom that organic materials are low mobility does not have to be true.

==Transfer Mobility Experiments==
[[Image:OLED6_tof_exp.JPG|thumb|300px|Time of flight]]
To measure mobility you create a thick film of the organic material, place it over the top of a transparent electrode and on the side place a collection electrode. Then fire a laser at a wavelength that will photogenerate charge carriers. There may be a sensitizer or the laser can be tuned to one of the absorption bands for the organic material. Suppose you use a nitrogen laser in the UV with a short pulse length which launches a sheet of charges into the organic layer. Then we measure the amount of time it takes for the charges to arrive at the collection electrode and then fall off. This is the time of flight or TOF. The vertical axis is the compensation current in the external circuit. We measure the transit time measured at the inflection point of the curve. Notice that as you increase the voltage of the system the transit time goes down. Electron mobilities are therefore field dependent. It is best to have non-dispersive transport in which charges start as sheet of charges and then move across the material as a sheet resulting in a very well defined transition. If the motion of the charges is more randomized, if there is lots of disorder in the solid and a lots of traps, then you get a less clear transition point. This is dispersive transport.
[[Image:OLED6_fieldandtemperature.JPG |thumb|300px| Field and Temp Dependence]]

The second series of plots shows that the process is temperature dependent indicating that it is an activated process. This is activation of a charge from one molecular center, up and over a barrier and into another molecular center. This like the problem of electron transfer that Marcus first described and it is dependent on the internal and external reorganization energy that involves charge transfer from one species to another. It can be described in much the same manner that Marcus first described homogeneous electron transfer.

 
==The disorder formalism (Bassler and Borsenberger)==
[[Image:OLED6_disorderformalism.jpg|thumb|400px]]

There have been many efforts to describe this process formally. The disorder formalism of Bassler and Borsenberger has done the best job of describing what we see. Transport occurs by hopping through a manifold of localized states that are energetically and positionally disordered. This describes transport for most organic systems. Mobility can be expressed in terms of intrinsic mobility and a Boltzman-like expression that accounts for the energy barriers and the degree of disorder in the system. This accounts for the temperature dependence and the field dependence of transfer.

[[Image:OLED6_fieldependance.jpg|thumb|400px|This graph shows the mobility of charge carriers in several transport materials as a function of the applied voltage. It varies as the square root of the field. These show a charge mobility of less than 10-4 and for some systems it can get worse than that.]]
 
[[Image:OLED6_disorder-dipolar.JPG|thumb|400px]]
There are several contributions to energetic disorder including molecular dipole, Van der Waals or dispersive interactions and matrix interactions. Random distribution of dipoles generates fluctuations in electrostatic potential because local dipole fields could either help or hinder you from moving charge from one system to another. Most of the really good OLED materials are non-polar and have not intrinsic local dipoles.

 

==Hole and Electron Mobility in Non-Crystalline Materials==
[[Image:OLED6_noncrystalinemobility.JPG|thumb|400px]]
Here are actual mobilities for non-crystaline materials. TPD is a bis aral amine which at the time was one of the better compounds with a mobility of 10-3. TPD when 50% diluted into polycarbonate (TPD:PC) results in mobility drop of two orders of magnitude. Poly vinyl carbazol (PVK) has been around for long while and it’s mobility is poor. Aluminu quinalate (AlQ), DPQ and PBD are electron transporter and emissive materials which also have low mobility. If you make an OLED with one of these hole transport materials and another electron transport material there will be an asymmetry in their rates.

Organic/Organic Heterojunctions in OLEDs

2009-03-13T18:37:52Z

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==Single-Layer OLEDs Band-Edge Offsets vs. Contact Electrodes==
This simplistic energy diagram shows what happens to an organic film that is sandwiched between two contacting electrodes. One side is indium tin oxide and the other is aluminum. We are making the assumption that there is constant vacuum level for all components but this is not precisely correct. The ITO has a work function represented by the vertical arrow. For clean ITO this is typically 4.7 - 4.8ev. For dirty ITO it can be a low as 4- 4.2ev. The work function for AL is about 4.1ev. We need a work function difference between the layers because we want the aluminum to inject electrons into the layer while the ITO layer inject holes into the layer. This difference ultimately helps dictate the voltage that the device turns on.
[[Image:OLED5-singlelayer.jpg |thumb | 400px|Band Edge Offsets]]
There is an energy offset which the ionization potential of the organic layer, the energy required to take an electron out to vacuum. This something that can be measured with an UV photoelectron spectrometer. We typically do not get a good energy match between the ITO bottom contact and the organic layer. There is small energy barrier that must be overcome in order to inject holes into the organic layer. If is small it’s not a big problem.

By the same token the electron affinity EA is the distance between the vacuum level to the lumo state, that is the energy required to acquire an electron. Cathode materials have a bigger offset between the work function of the electrode and the electron affinity level. This is the energy barrier for injecting electrons into the organic layer. In an electrochemical system with a concentrated electrolyte solution the same situation occurs but it can be overcome by setting up an electrical double layer at the interface. The steep potential gradient at the interface makes it possible to get an electron injection regardless of the electrode work function. However in condensed phase environments, which have very low dielectric constants and no added ionic species in them, these energy levels are significant. Ultimately we have to go to high voltages and then tunnel charges through the energy barriers or do thermionic emission to lower drive voltages.

In the early experiments with single crystal anthracene the charge does not move at the same rate. In this case the positive charge electrode which injects holes created a charge carrier that moves faster than the electron carrier. As a result most of the recombination occurs closer to the negative electrode. That is critical problem for display devices because the metal or metal like electrodes tend to quench the emissive states before they can give off their light. They act as an energy sink. This minimized the efficiency of the devices no matter how hard they were driven. That was the state of the art before the two layer OLED were developed.

==Organic/Organic’ Heterojunction Devices==
[[Image:OLED5-organic_heterojunction.jpg |thumb||400px | Organic Heterojunction]]
This is the ubiquitous way of building a light emitting thin film system. They all involve at least one heterojunction. The transport layer is designed to be easy to oxidize typically using bis tri aral amines. The electron transport layer is somewhat easier to reduce that the hole transport layer and more difficult to oxidize than the hole transport layer. In electrochemical vernacular we want to find chemicals that are easy to oxidize and form stable cation radicals, and other side chemicals that are easier to reduce and form stable radical anions. This results in an energy offset between the two organic layers so that electrons once injected into the ETL layer have a large energy barrier to surmount in order to move into the HTL. The holes injected into the HTL have a large barrier to move into ETL layer. As a consequence, at low applied fields the holes and electrons build up at or near the interface. That is the site of recombination and this keeps the process away from the surface of the contacting electrodes. This was a critically important advancement.

The First OLEDs

2009-03-13T18:36:41Z

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The first OLEDs were green as seen in this early example from the University of Arizona. The image on the right is so bright that is appears white.
==The First (“Heroic”) Experiments to Generate Light From Organic Materials:Top and Bottom Electrodes/Rectifying Junctions==

The first experiments to generate light from organic materials can be traced back to the late 1950s and early 1960’s at places like RCA.

See W. Helfrich & W.G. Sneider Phys. Rev. Lett. 14(7), 229 (1965)

People would take highly purified anthracene and cut it very fine, approximately 5mm thick. They would connect these to electrodes and apply between 100 and 1000 volts. If the system did not short there would be light emission. Often there would be a lightning bolt like discharge and you produce graphite instead of light.

<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED4_firstexperiments.pdf</embed_document>

Next they brought the thickness down to 50 microns and used a saw emission from a semitransparent gold electrode.
How is light generated? At the anode there is the removal of an electron to create a cation radical state. At the cathode is the addition of an electron to create the anion radical state. These charges need to hop through the organic crystal in order to meet each other somewhere near the center of the device. They undergo an electron transfer between this reduced donor and the oxidized receptor. There is enough excess free energy generated to create the emissive state of anthracene. The key piece is that in the early devices the rate of transport of the positive cation radical or hole state and the transport of the electron rich state where not equal so the recombination occurred near one of the contacting electrodes. This proves to be a real problem for the efficiency of the device.

==The First “Practical” Organic Light Emitting Diodes==
[[Image:OLED4_heterojunction.jpg|thumb|400px]]
The first practical OLEDs was in the late 1980’s by Ching Tang and Andy Van Slyke at Kodak. This was revolution for the technology. They started with a glass or plastic substrate with a transparent conductor such as indium tin oxide which can be highly doped and be highly conductive yet maintain transparency in the visible wavelength region.
They deposited a hole transport layer typically some tri aral amine or in the early versions a polymer called poly vinyl carbazol; something that is easily oxidized at this transparent conductor.

Next we add an electron transport layer and the total thickness has been reduced down to 100 nm. This was a technological advance because you were no longer having to slice single crystal anthracene really thin with a razor knife instead you are vapor depositing molecules in a vacuum system on top of this transparent conductor. This also solved another problem. When you deposit two successive layers there are fewer pin holes that create a dead short circuit between the anode and cathode.

Finally the top electrode is something like aluminum, magnesium, silver alloy or even an electropositive metal such as calcium. The hole transparent layer was a spin coated polymer. This produced about 15 candelas per square meter. A typical CRT computer screen operates with 100-200 candelas per square meter. So this was a dim but visible light. Aluminum tris quinalate was used as both the electron transport and emissive layer.

See C.W. Tang and S.A. VanSlyke Appl. Phys. Lett. 51, 913(1987)
C.W. Tang, U.S. Patent # 4,356,429 (1980)

Vacuum deposition enabled thin electron transport layer
Hole transport layer was spin-coated polymer: 10 – 20 V, 15cd/m2 brightness
All vacuum device: 10 – 20 V, 100 cd/m2 using Alq3 emission layer

What is a Light Emitting Diode?

2009-03-13T18:35:47Z

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==p-type and n-type materials==
Inorganic Light emitting diodes have become very common in the last decade. They are used for traffic lights and taillights. If you look inside you see a very small inorganic multi-layer diode material that is driven in forward bias. That light is emitted isotropically and a cone is used to focus the light in the forward direction. In the last decade we gone from red being readily available to green and blue. You can get these quite inexpensively. The diode laser is ubiquitous.

Here is a simple model for diode. We take a piece of p type semiconductor material that is created with excess dopants such as boron. This is assembled with an type semiconductor which is created with silicon that has been doped with phosphorous. When you bring these two together you have the potential for charge transfer between these two materials. This creates a PN diode. You deplete the n part of interfacial area of electrons and you deplete the p portion of the interface of holes. This creates a built-in potential, Vbi.

==Properties of Diodes==
Diodes provide rectification. Current only flows in one direction. When apply voltage and pass current through a resistor you get a linear plot known as Ohms law. When you apply voltage and pass current through a diode you get an asymmetry in the voltage current response. If we add a positive bias the current is exponentially increasing. With a reverse bias there is very little current flowing. A good diode should have very low turn on voltage Vd with exponential increase, and very low current with negative bias.
The Shockley equation describes the current through the diode at any bias. The reverse saturation current Js is related to the purity of the device and should be as low as possible. The applied potential, the diode quality factor Boltzman constant, absolute temperature and charge on the electron. This results in current flowing across the depletion layer region. Interesting things happen when you go from bias to bias, especially in systems that can produce light.

This is an energy level diagram as a function of distance across a PN diode. The system is at equilibrium therefore the Fermi level in both the N region and the P region are exactly the same. This is the definition of equilibrium in the condensed phase. We can think of the Fermi level as the average level of an electron entering or leaving the solid. There is significant energy difference as we move from the N region to the P region for both electrons and holes because the depletion of the majority carriers that have occurred in that system.

In the case of reverse bias we make that depletion even more significant. We increase the built-in voltage by Vd . The deplete region thickness increases. As long as it is a purified material we see very little current flowing.

In reverse bias the Fermi levels are no longer aligned because the system is not in equilibrium. There is an energy barrier that makes it very difficult for electrons in the N region to transit to the P region and similarly it is difficult for the holes to move from the P region to the N region. This is exactly the region in a organic photovoltaic cell where the application of light causes absorbs energy and causes the charge to move. However in LEDs or OLEDs you do not want current flow when there is reverse bias.

==Forward Bias Diode==
In the forward biased diode the current flows and the depletion region is narrowed or eliminated. Majority carriers move from one region to the other. The energy bands are not in equilibrium and the energy level of the P region is lower than the N region. Electrons move from the N region to the P region and holes move from P region to N region. Forward bias in an LED moves the electron and holes. Recombination events occur at the junction and excess free energy is dissipated as light coming out of the center region. The color of light from an inorganic diode is controlled by the band gap energy for those semiconductors. The first LED were gallium arsenside that has a low bandgap that gives the red light of early calculators. As people began to tailor the bandgap of 3-5 semiconductors they have achieved orange, green and most recently blue color emission.

What is a Light Emitting Diode?

2009-03-13T18:35:35Z

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==p-type and n-type materials==
Inorganic Light emitting diodes have become very common in the last decade. They are used for traffic lights and taillights. If you look inside you see a very small inorganic multi-layer diode material that is driven in forward bias. That light is emitted isotropically and a cone is used to focus the light in the forward direction. In the last decade we gone from red being readily available to green and blue. You can get these quite inexpensively. The diode laser is ubiquitous.

Here is a simple model for diode. We take a piece of p type semiconductor material that is created with excess dopants such as boron. This is assembled with an type semiconductor which is created with silicon that has been doped with phosphorous. When you bring these two together you have the potential for charge transfer between these two materials. This creates a PN diode. You deplete the n part of interfacial area of electrons and you deplete the p portion of the interface of holes. This creates a built-in potential, Vbi.

==Properties of Diodes==
Diodes provide rectification. Current only flows in one direction. When apply voltage and pass current through a resistor you get a linear plot known as Ohms law. When you apply voltage and pass current through a diode you get an asymmetry in the voltage current response. If we add a positive bias the current is exponentially increasing. With a reverse bias there is very little current flowing. A good diode should have very low turn on voltage Vd with exponential increase, and very low current with negative bias.
The Shockley equation describes the current through the diode at any bias. The reverse saturation current Js is related to the purity of the device and should be as low as possible. The applied potential, the diode quality factor Boltzman constant, absolute temperature and charge on the electron. This results in current flowing across the depletion layer region. Interesting things happen when you go from bias to bias, especially in systems that can produce light.

This is an energy level diagram as a function of distance across a PN diode. The system is at equilibrium therefore the Fermi level in both the N region and the P region are exactly the same. This is the definition of equilibrium in the condensed phase. We can think of the Fermi level as the average level of an electron entering or leaving the solid. There is significant energy difference as we move from the N region to the P region for both electrons and holes because the depletion of the majority carriers that have occurred in that system.

In the case of reverse bias we make that depletion even more significant. We increase the built-in voltage by Vd . The deplete region thickness increases. As long as it is a purified material we see very little current flowing.

In reverse bias the Fermi levels are no longer aligned because the system is not in equilibrium. There is an energy barrier that makes it very difficult for electrons in the N region to transit to the P region and similarly it is difficult for the holes to move from the P region to the N region. This is exactly the region in a organic photovoltaic cell where the application of light causes absorbs energy and causes the charge to move. However in LEDs or OLEDs you do not want current flow when there is reverse bias.

==Forward Bias Diode==
In the forward biased diode the current flows and the depletion region is narrowed or eliminated. Majority carriers move from one region to the other. The energy bands are not in equilibrium and the energy level of the P region is lower than the N region. Electrons move from the N region to the P region and holes move from P region to N region. Forward bias in an LED moves the electron and holes. Recombination events occur at the junction and excess free energy is dissipated as light coming out of the center region. The color of light from an inorganic diode is controlled by the band gap energy for those semiconductors. The first LED were gallium arsenside that has a low bandgap that gives the red light of early calculators. As people began to tailor the bandgap of 3-5 semiconductors they have achieved orange, green and most recently blue color emission.

What is a Light Emitting Diode?

2009-03-13T18:35:12Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]] |[[The first OLEDs|Return to OLED Menu]]

==p-type and n-type materials==
Inorganic Light emitting diodes have become very common in the last decade. They are used for traffic lights and taillights. If you look inside you see a very small inorganic multi-layer diode material that is driven in forward bias. That light is emitted isotropically and a cone is used to focus the light in the forward direction. In the last decade we gone from red being readily available to green and blue. You can get these quite inexpensively. The diode laser is ubiquitous.

Here is a simple model for diode. We take a piece of p type semiconductor material that is created with excess dopants such as boron. This is assembled with an type semiconductor which is created with silicon that has been doped with phosphorous. When you bring these two together you have the potential for charge transfer between these two materials. This creates a PN diode. You deplete the n part of interfacial area of electrons and you deplete the p portion of the interface of holes. This creates a built-in potential, Vbi.

==Properties of Diodes==
Diodes provide rectification. Current only flows in one direction. When apply voltage and pass current through a resistor you get a linear plot known as Ohms law. When you apply voltage and pass current through a diode you get an asymmetry in the voltage current response. If we add a positive bias the current is exponentially increasing. With a reverse bias there is very little current flowing. A good diode should have very low turn on voltage Vd with exponential increase, and very low current with negative bias.
The Shockley equation describes the current through the diode at any bias. The reverse saturation current Js is related to the purity of the device and should be as low as possible. The applied potential, the diode quality factor Boltzman constant, absolute temperature and charge on the electron. This results in current flowing across the depletion layer region. Interesting things happen when you go from bias to bias, especially in systems that can produce light.

This is an energy level diagram as a function of distance across a PN diode. The system is at equilibrium therefore the Fermi level in both the N region and the P region are exactly the same. This is the definition of equilibrium in the condensed phase. We can think of the Fermi level as the average level of an electron entering or leaving the solid. There is significant energy difference as we move from the N region to the P region for both electrons and holes because the depletion of the majority carriers that have occurred in that system.

In the case of reverse bias we make that depletion even more significant. We increase the built-in voltage by Vd . The deplete region thickness increases. As long as it is a purified material we see very little current flowing.

In reverse bias the Fermi levels are no longer aligned because the system is not in equilibrium. There is an energy barrier that makes it very difficult for electrons in the N region to transit to the P region and similarly it is difficult for the holes to move from the P region to the N region. This is exactly the region in a organic photovoltaic cell where the application of light causes absorbs energy and causes the charge to move. However in LEDs or OLEDs you do not want current flow when there is reverse bias.

==Forward Bias Diode==
In the forward biased diode the current flows and the depletion region is narrowed or eliminated. Majority carriers move from one region to the other. The energy bands are not in equilibrium and the energy level of the P region is lower than the N region. Electrons move from the N region to the P region and holes move from P region to N region. Forward bias in an LED moves the electron and holes. Recombination events occur at the junction and excess free energy is dissipated as light coming out of the center region. The color of light from an inorganic diode is controlled by the band gap energy for those semiconductors. The first LED were gallium arsenside that has a low bandgap that gives the red light of early calculators. As people began to tailor the bandgap of 3-5 semiconductors they have achieved orange, green and most recently blue color emission.

The OLED Test Cell

2009-03-13T18:34:21Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]] |
[[What is a Light Emitting Diode?|Next Topic]]
==Solution electrogenerated chemiluminescence example molecules==
[[Image:OLED2 ECL.PNG|thumb|300px]]
We are going to focus on Diphenylanthracene since so much is known about its electrogenerated chemiluminescence and it has strong literature. All these molecules have the same common components. They have absorbance spectra in solution with a vibronic fine structure, they have small Stokes shifts, and luminescence spectra with corresponding vibronic fine structure.

 
==Prototype Emissive Devices==
[[Image:OLED2 prototype.JPG|thumb|300px]]
The interest in ECL was so strong that in the 1970s several investigators decided this might be a way to create light in a usable way. An electrochemical cell can be built with two electrodes separated by a narrow space filled with solution in which both A an D are located. A and D could be two different chemicals or one in the case of diphenylanthracene (DPA). You independently control the potential of the two electrodes using a potentiostat so that at the anode you generate the cation radical form of A at a diffusion-controlled rate. At the cathode you generate the radical anion form of the species D at a diffusion controlled rate. These molecules will diffuse away from the electrode at which they are produced into the solution volume between the electrodes. Where they meet at the center of the device the electron transfer reaction between the donor and the acceptor will occur and there will be an emissive state. This is heroic science because it’s very hard to produce.

*Layers must be very thin so they can be filled electrochemically in a very short period of time
*The possibility of interaction for the two molecules to be high.
*You need molecules that have long lifetimes for the both the cation radical and the anion radical space so that means some careful purification has to be done.
*One of these electrodes has to allow you get light out of the system otherwise there is no display.

[[image:OLED2_energetics.JPG|thumb|400px]]
The solution ECL experiment is simple to construct – a flowing solution containing both D and A is passed in front of a Pt or Au microelectrode surrounded by a reference and counter electrode in a small solution cavity. The potential of the Au or Pt electrode is alternatively pulsed positive (to generate A+.) and negative (to generate D-.) with a frequency typically up to a few KHz. The products of these heterogeneous electron transfer reactions diffuse away from the electrode, where they are likely to encounter each other (on the time scale of microseconds). The resultant recombination reactions generate the emissive state of one pair of these molecules, the light from that emission event is coupled out to a multi-channel detector spectrophotometer and the emission spectrum recorded.

Consider the non- display application which was used to look at the dynamics of electron transfer and light emission. This system was used by Neal Armstrong and Mark Wightman and the UNC , a group that had the world’s expertise in this area at the time.
This involves a very small cavity with solvent in it. A window in the center positioned opposite a photomultiplier tube or some photodetector.

There are three electrodes. The center one is the working electrode, typically a small microband or microdisc electrode with a diameter of 5-50 microns. The counter and reference electrodes are poised on either side. The solution cavity is able to flow in fresh solution periodically, this is important.

 
==Characterization of the Energetics of Charge Recombination==
You modulate the potential of the working electrode with respect to the reference electrode between two extremes. At one extreme it generates the cation radical of A and at the other extreme it generates the anion radical of D.
You cycle the potential of the these two extremes at frequencies of a kilohertz or more. By flipping the potential back and forth so fast you generate both species in the diffusion layer volume next to the electrode surface where they can interact and give off a photon which is counted with the detector. Wightman was able to monitor single photons and single electron transfer events with this system. The results can be further studied with spectroscopy of the light emitting process to confirm that the results match or differ from what you see in a device.

==Cross Reactions==
[[Image:OLED2_ecl_cross.JPG|thumb|400px]]
This an example of a voltamogram of the obtained at the micro electrode for DPA in a low dielectric constant solvent. When you scan negatively you the microelectrode response for a one electron reduction of the DPA to its radical anion. When you scan positively this give the one electron oxidation of the DPA to its radical cation form. The different between the midpoint potentials are called the halfway potentials. This give an indication of the energy difference of the two forms and a clue to the amount of excess free energy that will be generated by the redox reaction that follows. In this reaction there is more than 3 electron volts of excess free energy. DPA exciting state easily forms and re-emits with a peak wavelength of about 450 nm. If you calculate the energy this is 3-3.1 eV. So the electo chemical event generated more than enough excess free energy to create the excited singlet state of DPA.

Now you compare the yield the DPA excited states as you systematically change the redox event that occurs. First generating the DPA anion with one pulse and the DPA cation with another pulse the ΔE_(½ ) is about 3.33 electron volts. This forms the excited singlet state known as the “S-route” .

Next you put a second species Methoxybenzophonone which reduced at a slightly less negative potential, generating its anion radical at 3.24 electron volts. When you do spectroscopy you see that you are still forming the DPA exciting state and its slightly less efficient. It is still an S-route process.

Using Benzophonone it has a less negative potential with an excess free energy of 3.15 volts and still is an S-route.

<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED2_ecl_redoxpairs.pdf</embed_document>

==Marcus Theory for Electron Transfer==
<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED2_marcustheory.pdf</embed_document>

The Marcus theory for electron transfer provides some of the needed predictability to describe both probability of light emission from an ECL cross reaction, and the probability of light emission in an OLED at the interface between two dissimilar charge transporting layers.

All electron transfer reactions proceed through a transition state whose energy is defined primarily by the “reorganization energy” (λT). This energy can be divided into an “internal reorganization” energy (λi -- the energy needed to reorganize the internal molecular environment in going from the initial to the final state), and the “external reorganization energy (λo – the energy needed to reorganize the solvent or condensed phase environment surrounding the molecule in proceeding from the initial to the final state).

In general as the excess Gibbs free energy (ΔE α ΔG) in the system is increased (Points a, b, c) so that ΔE (ΔG) is close to or exceeds λT = λi + λo in magnitude, the reaction rate accelerates exponentially. For light-emitting electrochemical processes this reaction rate enhancement leads directly to greater production of emissive states, i.e. the output of the ECL process is controlled by controlling the difference in reduction and oxidation potentials of the two reacting components.

This provides some guidance in the design of two-layer OLEDs (see below), where it is clear that you want to maximize the excess free energy in the critical charge recombination process.

You go from a situation where you maximized the free energy at point C. Then by changing the identity of the two pairs of the redox reaction you go to less free energy at point B and then finally point A. You still generate the excited state but with lower overall rate.

The rate is proportional to the number of photons you get out of the experiment.
This gives an underlying principal for the design of OLED systems.
The idea is maximize the free energy and minimize the reorganization energies for those redox events. Maximize the rate of electron transfer and therefore maximize the rate of light output. You want to me at position C rather position A in the design of the systems.

Unfortunately the design is never that simple.

*The DPA+ + BP- system is results in point A.

*The DPA+ + MOPA- puts us at point B

*The DPA+ + DPA- system is results in point C.

You have changed which species gets reduced and as a consequence changed the excess free energy in the redox reaction.
It is possible to create an excess free energy which exceeds the reorganization energy (point d) in which case you enter the “inverted region” and reaction rate decreases. A few examples of this phenomenon have been observed in solutions and glasses, but it has not been, to date, reported in OLEDs.

It would be possible to go too far and get into what’s called “Marcus inverted region” where the excess free energy larger by far than the reorganization energy. In this case you might see the rate of reaction come down. To date we have not seen examples of this in OLEDs. It may have occurred but it difficult to prove. It’s really a question if the excess free energy is much less than the reorganization energy, comparable or greater than the reorganization energy.

The OLED Test Cell

2009-03-13T18:34:00Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]]
[[What is a Light Emitting Diode?|Next Topic]]
==Solution electrogenerated chemiluminescence example molecules==
[[Image:OLED2 ECL.PNG|thumb|300px]]
We are going to focus on Diphenylanthracene since so much is known about its electrogenerated chemiluminescence and it has strong literature. All these molecules have the same common components. They have absorbance spectra in solution with a vibronic fine structure, they have small Stokes shifts, and luminescence spectra with corresponding vibronic fine structure.

 
==Prototype Emissive Devices==
[[Image:OLED2 prototype.JPG|thumb|300px]]
The interest in ECL was so strong that in the 1970s several investigators decided this might be a way to create light in a usable way. An electrochemical cell can be built with two electrodes separated by a narrow space filled with solution in which both A an D are located. A and D could be two different chemicals or one in the case of diphenylanthracene (DPA). You independently control the potential of the two electrodes using a potentiostat so that at the anode you generate the cation radical form of A at a diffusion-controlled rate. At the cathode you generate the radical anion form of the species D at a diffusion controlled rate. These molecules will diffuse away from the electrode at which they are produced into the solution volume between the electrodes. Where they meet at the center of the device the electron transfer reaction between the donor and the acceptor will occur and there will be an emissive state. This is heroic science because it’s very hard to produce.

*Layers must be very thin so they can be filled electrochemically in a very short period of time
*The possibility of interaction for the two molecules to be high.
*You need molecules that have long lifetimes for the both the cation radical and the anion radical space so that means some careful purification has to be done.
*One of these electrodes has to allow you get light out of the system otherwise there is no display.

[[image:OLED2_energetics.JPG|thumb|400px]]
The solution ECL experiment is simple to construct – a flowing solution containing both D and A is passed in front of a Pt or Au microelectrode surrounded by a reference and counter electrode in a small solution cavity. The potential of the Au or Pt electrode is alternatively pulsed positive (to generate A+.) and negative (to generate D-.) with a frequency typically up to a few KHz. The products of these heterogeneous electron transfer reactions diffuse away from the electrode, where they are likely to encounter each other (on the time scale of microseconds). The resultant recombination reactions generate the emissive state of one pair of these molecules, the light from that emission event is coupled out to a multi-channel detector spectrophotometer and the emission spectrum recorded.

Consider the non- display application which was used to look at the dynamics of electron transfer and light emission. This system was used by Neal Armstrong and Mark Wightman and the UNC , a group that had the world’s expertise in this area at the time.
This involves a very small cavity with solvent in it. A window in the center positioned opposite a photomultiplier tube or some photodetector.

There are three electrodes. The center one is the working electrode, typically a small microband or microdisc electrode with a diameter of 5-50 microns. The counter and reference electrodes are poised on either side. The solution cavity is able to flow in fresh solution periodically, this is important.

 
==Characterization of the Energetics of Charge Recombination==
You modulate the potential of the working electrode with respect to the reference electrode between two extremes. At one extreme it generates the cation radical of A and at the other extreme it generates the anion radical of D.
You cycle the potential of the these two extremes at frequencies of a kilohertz or more. By flipping the potential back and forth so fast you generate both species in the diffusion layer volume next to the electrode surface where they can interact and give off a photon which is counted with the detector. Wightman was able to monitor single photons and single electron transfer events with this system. The results can be further studied with spectroscopy of the light emitting process to confirm that the results match or differ from what you see in a device.

==Cross Reactions==
[[Image:OLED2_ecl_cross.JPG|thumb|400px]]
This an example of a voltamogram of the obtained at the micro electrode for DPA in a low dielectric constant solvent. When you scan negatively you the microelectrode response for a one electron reduction of the DPA to its radical anion. When you scan positively this give the one electron oxidation of the DPA to its radical cation form. The different between the midpoint potentials are called the halfway potentials. This give an indication of the energy difference of the two forms and a clue to the amount of excess free energy that will be generated by the redox reaction that follows. In this reaction there is more than 3 electron volts of excess free energy. DPA exciting state easily forms and re-emits with a peak wavelength of about 450 nm. If you calculate the energy this is 3-3.1 eV. So the electo chemical event generated more than enough excess free energy to create the excited singlet state of DPA.

Now you compare the yield the DPA excited states as you systematically change the redox event that occurs. First generating the DPA anion with one pulse and the DPA cation with another pulse the ΔE_(½ ) is about 3.33 electron volts. This forms the excited singlet state known as the “S-route” .

Next you put a second species Methoxybenzophonone which reduced at a slightly less negative potential, generating its anion radical at 3.24 electron volts. When you do spectroscopy you see that you are still forming the DPA exciting state and its slightly less efficient. It is still an S-route process.

Using Benzophonone it has a less negative potential with an excess free energy of 3.15 volts and still is an S-route.

<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED2_ecl_redoxpairs.pdf</embed_document>

==Marcus Theory for Electron Transfer==
<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED2_marcustheory.pdf</embed_document>

The Marcus theory for electron transfer provides some of the needed predictability to describe both probability of light emission from an ECL cross reaction, and the probability of light emission in an OLED at the interface between two dissimilar charge transporting layers.

All electron transfer reactions proceed through a transition state whose energy is defined primarily by the “reorganization energy” (λT). This energy can be divided into an “internal reorganization” energy (λi -- the energy needed to reorganize the internal molecular environment in going from the initial to the final state), and the “external reorganization energy (λo – the energy needed to reorganize the solvent or condensed phase environment surrounding the molecule in proceeding from the initial to the final state).

In general as the excess Gibbs free energy (ΔE α ΔG) in the system is increased (Points a, b, c) so that ΔE (ΔG) is close to or exceeds λT = λi + λo in magnitude, the reaction rate accelerates exponentially. For light-emitting electrochemical processes this reaction rate enhancement leads directly to greater production of emissive states, i.e. the output of the ECL process is controlled by controlling the difference in reduction and oxidation potentials of the two reacting components.

This provides some guidance in the design of two-layer OLEDs (see below), where it is clear that you want to maximize the excess free energy in the critical charge recombination process.

You go from a situation where you maximized the free energy at point C. Then by changing the identity of the two pairs of the redox reaction you go to less free energy at point B and then finally point A. You still generate the excited state but with lower overall rate.

The rate is proportional to the number of photons you get out of the experiment.
This gives an underlying principal for the design of OLED systems.
The idea is maximize the free energy and minimize the reorganization energies for those redox events. Maximize the rate of electron transfer and therefore maximize the rate of light output. You want to me at position C rather position A in the design of the systems.

Unfortunately the design is never that simple.

*The DPA+ + BP- system is results in point A.

*The DPA+ + MOPA- puts us at point B

*The DPA+ + DPA- system is results in point C.

You have changed which species gets reduced and as a consequence changed the excess free energy in the redox reaction.
It is possible to create an excess free energy which exceeds the reorganization energy (point d) in which case you enter the “inverted region” and reaction rate decreases. A few examples of this phenomenon have been observed in solutions and glasses, but it has not been, to date, reported in OLEDs.

It would be possible to go too far and get into what’s called “Marcus inverted region” where the excess free energy larger by far than the reorganization energy. In this case you might see the rate of reaction come down. To date we have not seen examples of this in OLEDs. It may have occurred but it difficult to prove. It’s really a question if the excess free energy is much less than the reorganization energy, comparable or greater than the reorganization energy.

The OLED Test Cell

2009-03-13T18:33:23Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]]
[[What_is_a_Light_Emitting_Diode|Next Topic]]
==Solution electrogenerated chemiluminescence example molecules==
[[Image:OLED2 ECL.PNG|thumb|300px]]
We are going to focus on Diphenylanthracene since so much is known about its electrogenerated chemiluminescence and it has strong literature. All these molecules have the same common components. They have absorbance spectra in solution with a vibronic fine structure, they have small Stokes shifts, and luminescence spectra with corresponding vibronic fine structure.

 
==Prototype Emissive Devices==
[[Image:OLED2 prototype.JPG|thumb|300px]]
The interest in ECL was so strong that in the 1970s several investigators decided this might be a way to create light in a usable way. An electrochemical cell can be built with two electrodes separated by a narrow space filled with solution in which both A an D are located. A and D could be two different chemicals or one in the case of diphenylanthracene (DPA). You independently control the potential of the two electrodes using a potentiostat so that at the anode you generate the cation radical form of A at a diffusion-controlled rate. At the cathode you generate the radical anion form of the species D at a diffusion controlled rate. These molecules will diffuse away from the electrode at which they are produced into the solution volume between the electrodes. Where they meet at the center of the device the electron transfer reaction between the donor and the acceptor will occur and there will be an emissive state. This is heroic science because it’s very hard to produce.

*Layers must be very thin so they can be filled electrochemically in a very short period of time
*The possibility of interaction for the two molecules to be high.
*You need molecules that have long lifetimes for the both the cation radical and the anion radical space so that means some careful purification has to be done.
*One of these electrodes has to allow you get light out of the system otherwise there is no display.

[[image:OLED2_energetics.JPG|thumb|400px]]
The solution ECL experiment is simple to construct – a flowing solution containing both D and A is passed in front of a Pt or Au microelectrode surrounded by a reference and counter electrode in a small solution cavity. The potential of the Au or Pt electrode is alternatively pulsed positive (to generate A+.) and negative (to generate D-.) with a frequency typically up to a few KHz. The products of these heterogeneous electron transfer reactions diffuse away from the electrode, where they are likely to encounter each other (on the time scale of microseconds). The resultant recombination reactions generate the emissive state of one pair of these molecules, the light from that emission event is coupled out to a multi-channel detector spectrophotometer and the emission spectrum recorded.

Consider the non- display application which was used to look at the dynamics of electron transfer and light emission. This system was used by Neal Armstrong and Mark Wightman and the UNC , a group that had the world’s expertise in this area at the time.
This involves a very small cavity with solvent in it. A window in the center positioned opposite a photomultiplier tube or some photodetector.

There are three electrodes. The center one is the working electrode, typically a small microband or microdisc electrode with a diameter of 5-50 microns. The counter and reference electrodes are poised on either side. The solution cavity is able to flow in fresh solution periodically, this is important.

 
==Characterization of the Energetics of Charge Recombination==
You modulate the potential of the working electrode with respect to the reference electrode between two extremes. At one extreme it generates the cation radical of A and at the other extreme it generates the anion radical of D.
You cycle the potential of the these two extremes at frequencies of a kilohertz or more. By flipping the potential back and forth so fast you generate both species in the diffusion layer volume next to the electrode surface where they can interact and give off a photon which is counted with the detector. Wightman was able to monitor single photons and single electron transfer events with this system. The results can be further studied with spectroscopy of the light emitting process to confirm that the results match or differ from what you see in a device.

==Cross Reactions==
[[Image:OLED2_ecl_cross.JPG|thumb|400px]]
This an example of a voltamogram of the obtained at the micro electrode for DPA in a low dielectric constant solvent. When you scan negatively you the microelectrode response for a one electron reduction of the DPA to its radical anion. When you scan positively this give the one electron oxidation of the DPA to its radical cation form. The different between the midpoint potentials are called the halfway potentials. This give an indication of the energy difference of the two forms and a clue to the amount of excess free energy that will be generated by the redox reaction that follows. In this reaction there is more than 3 electron volts of excess free energy. DPA exciting state easily forms and re-emits with a peak wavelength of about 450 nm. If you calculate the energy this is 3-3.1 eV. So the electo chemical event generated more than enough excess free energy to create the excited singlet state of DPA.

Now you compare the yield the DPA excited states as you systematically change the redox event that occurs. First generating the DPA anion with one pulse and the DPA cation with another pulse the ΔE_(½ ) is about 3.33 electron volts. This forms the excited singlet state known as the “S-route” .

Next you put a second species Methoxybenzophonone which reduced at a slightly less negative potential, generating its anion radical at 3.24 electron volts. When you do spectroscopy you see that you are still forming the DPA exciting state and its slightly less efficient. It is still an S-route process.

Using Benzophonone it has a less negative potential with an excess free energy of 3.15 volts and still is an S-route.

<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED2_ecl_redoxpairs.pdf</embed_document>

==Marcus Theory for Electron Transfer==
<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED2_marcustheory.pdf</embed_document>

The Marcus theory for electron transfer provides some of the needed predictability to describe both probability of light emission from an ECL cross reaction, and the probability of light emission in an OLED at the interface between two dissimilar charge transporting layers.

All electron transfer reactions proceed through a transition state whose energy is defined primarily by the “reorganization energy” (λT). This energy can be divided into an “internal reorganization” energy (λi -- the energy needed to reorganize the internal molecular environment in going from the initial to the final state), and the “external reorganization energy (λo – the energy needed to reorganize the solvent or condensed phase environment surrounding the molecule in proceeding from the initial to the final state).

In general as the excess Gibbs free energy (ΔE α ΔG) in the system is increased (Points a, b, c) so that ΔE (ΔG) is close to or exceeds λT = λi + λo in magnitude, the reaction rate accelerates exponentially. For light-emitting electrochemical processes this reaction rate enhancement leads directly to greater production of emissive states, i.e. the output of the ECL process is controlled by controlling the difference in reduction and oxidation potentials of the two reacting components.

This provides some guidance in the design of two-layer OLEDs (see below), where it is clear that you want to maximize the excess free energy in the critical charge recombination process.

You go from a situation where you maximized the free energy at point C. Then by changing the identity of the two pairs of the redox reaction you go to less free energy at point B and then finally point A. You still generate the excited state but with lower overall rate.

The rate is proportional to the number of photons you get out of the experiment.
This gives an underlying principal for the design of OLED systems.
The idea is maximize the free energy and minimize the reorganization energies for those redox events. Maximize the rate of electron transfer and therefore maximize the rate of light output. You want to me at position C rather position A in the design of the systems.

Unfortunately the design is never that simple.

*The DPA+ + BP- system is results in point A.

*The DPA+ + MOPA- puts us at point B

*The DPA+ + DPA- system is results in point C.

You have changed which species gets reduced and as a consequence changed the excess free energy in the redox reaction.
It is possible to create an excess free energy which exceeds the reorganization energy (point d) in which case you enter the “inverted region” and reaction rate decreases. A few examples of this phenomenon have been observed in solutions and glasses, but it has not been, to date, reported in OLEDs.

It would be possible to go too far and get into what’s called “Marcus inverted region” where the excess free energy larger by far than the reorganization energy. In this case you might see the rate of reaction come down. To date we have not seen examples of this in OLEDs. It may have occurred but it difficult to prove. It’s really a question if the excess free energy is much less than the reorganization energy, comparable or greater than the reorganization energy.

Light Emitting Electrochemical Processes

2009-03-13T18:31:32Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]] | [[The OLED test cell|Next Topic]]

This article serves as an introduction to the design and chemistry of organic light emitting diodes (OLED).

== Light emission in the OLED arises from recombination (electron transfer ) reactions of cation and anion radical of conjugated aromatic molecules. ==

Several decades ago it was noted that poly(acenes) and related poly-aromatic hydrocarbons, in very dry nonaqueous (non-polar) solvents can be reduced by one electron (chemically or electrochemically) to produce an energetic radical anion state (D-.). These same molecules can often be oxidized by one electron to produce a cation radical state (A+.).
Should A+. and D-. encounter each other in solution, a “recombination” electron transfer reaction can occur – the excess free energy in this reaction can be deposited in one of the molecular species, to form its lowest excited state (singlet), or in some cases, its lowest triplet excited state – states which are the same as those created by photoexcitation of the molecule. Emission from this state occurs with a lifetime of ca. nanoseconds, with quantum yields approaching 100% in some cases.
These “electrogenerated chemiluminescence” (ECL) processes are direct analogues of the charge recombination processes which occur in the condensed phase in an OLED. They are also closely related to the [[wikipedia:chemiluminescence|chemiluminescence]] and bioluminescence processes which occur in living organisms, the most easily recognized example being the luminescence processes which occur in fireflys.

It was quickly released that in order to create the emissive state by injection charge the following processes took place:
<gallery widths=300px heights=200px perrow=3>
Image:Oled1_3_eclredox.png‎|First the loss of an electron by this conjugated aromatic system to form a cation radical species.
Image:Oled1_4_eclredox.png‎‎|
Image:Oled1_5_eclredox.png‎‎|Then the addition of electron to a similar species at the opposite electrode to give us an anion radical.
Image:Oled1_6_eclredox.png‎‎|These two species react and do an electron transfer reaction from the donor species to an acceptor species. There is a lot of excess free energy that is given off in this process which is can be predicted by the formal potentials for the formation of these two species.
Image:Oled1_7_eclredox.png‎‎|Finally one of the molecules harvests this energy in the form of an excited state in a singlet or triplet state or both. This is the species that is used to emit light.
</gallery>

[[Image:Ecl-redox.gif‎|400px|The complete sequence.]]
 

We can write this from the point of view of a homogenous electrochemical process. At the same time this was being done in the condensed phase people were beginning to explore this processes in solution. Rudy Marcus used this as a central tenant in his development of electron transfer theory between small molecule systems.

== The Jablonski diagram is used to describe the energy (wavelength) of absorbance and luminescence for aromatic molecules.==
The [[Jablonksi]] diagram is our way of describing what is happening with small molecules and small conjugated aromatic systems.
<gallery heights=200px widths=300px perrow =3>
Image:Oled1 8 jablonski.png|First we have the absorption of a photon and the creation of excited singlets states with different vibronic excited levels.
Image:Oled1 10 absorbance.png|This shows the absorption spectra with its associated fine structure.This is the lowest energy of the absorption bands these are the vibronic levels associated with the population of the the ν=1 , ν =2 levels.
Image:Oled1_9_relaxation.png|That is followed very fast non-radiative relaxation.
Image:Oled1_13_fluoresence.png|Finally there is fluorescent decay to give back the energy in form of an emissive state.The lifetimes for fluorescence is on the order of nanoseconds typically.
Image:Oled1_14_abs-lum-graph.png|The spectral response for this molecule looks like a mirror image of the absorption spectrum. There is a small shift called the Stokes shift which very typical for planar aromatic compounds such as anthracene and related molecules.
Image:Oled1_15_phosphor.png|Once the excited singlet state has been formed there is the possibility of doing intersystem crossing to a triplet state. This change in spin is a forbidden process and as a consequence triplet states tend to be much longer lived.
</gallery>
Most molecules have lifetimes of the order of 1-100 microseconds. Those compounds that are most useful for organic light emitting diodes will have lifetimes closer to 1 microsecond. Most people are familiar with molecules that [[phosphorescence]] with much longer lifetime and these which are less useful. Vibronic excited states and a ground state. These tend to have emission events at much longer wavelengths.

This a summary taken from a Florida state website that gives all the different lifetime ranges associated with excitation, nonradiative relaxation an extremely fast process, with decay, fluoresces or via intersystem crossing and then phosphorescence back to the ground state.

== The color of absorption and emission in simple molecular systems is controlled by the structure of the molecule and by the degree of conjugation in the aromatic system. ==

In general as the number of aromatic rings increases in these molecular systems the energy for both the absorption and emission events goes down, ie they shift to the red. The same can be said for the carotenoid like assemblies where just increasing the number of double bonds in the system changes both the energy of absorption and emission.
To summarize for most polyacine- like systems which were among the first to be examined by this process. There is an absorption and emission event, a small Stokes shift, and a change of wavelength of these two depending on the degree of conjugation in the aromatic system.

== The ratio of singlet state to triplet state formation helps to determine the OLED efficiency. ==
[[Image:Oled1_18_spinstatics.png|thumb|left|400px|right|Singlet recombination]]
[[Image:Oled1_19_spinstatics.png|thumb|right|400px|right|Triplet recombination]]
 
<gallery widths=400 heights=300>

Image:Oled1_15_phosphor.png|Phosphorescence with linked systems
Image:Oled1_19_jablonski ratio.png|When see electrochemical excitation of these systems we tend to find that the number singlet state and triplet states partitions with 25% of energy deposited as singlet states and 75%of energy deposited in triplet states. This has a significant impact the optimization of organic light emitting diodes that either going to use fluorescent molecules to create the light to produce the light or phosphorescent dopants to create the light. The advent of the use of phosphorescent dopants has increased the efficiency to the near fluorescent lighting levels.Ratio of energy available.
</gallery>

How does this happen? The spin statistics of recombination can be described in a number of ways. This is the simplest way but there are some short comings to this simplistic description. We start with a molecule in which an electron has been removed the spin of the remaining electron in the two level diagram can either be up or down. The electron in the donor molecular can either be spin up or spin down. During the process of electron transfer we create either an excited singlet state of the acceptor and the neutral donor or excited singlet state of the donor and the neutral acceptor.

The details of this are somewhat complicated. During this electron transfer event there forms a contact ion pair or solvent separated ion pair if it is in solution. It is single entity and by this means we can exchange electrons and create either a singlet of the donor or singlet of the acceptor. Typically it the molecule with the lowest emissive energy that winds up having the excess energy from this process. In this case you can’t really tell because they were both identical to begin with. In actual organic light emitting diode it is the lowest band gap molecule that ends up being the emissive species.

Finally, to form the triplet state you can have three types of triplets. Thus it is three times as likely to form a triplet as it is a singlet which accounts for the 3:1 triplet to singlet ratio in systems of this type. It is debated whether or not these ratios apply in the condensed phases. They apply in solutions and early indications are that it also applies in the condensed state. The consequence of this is that if you are going to optimize a light source you must harvest as much of energy of the triplet state as possible.
 

== Small molecules are used in electrogenerated chemiluminescence (ECL) studies to help elucidate these light emitting processes in “condensed phases” ==
[[Image:Oled1 23 threeECLmolecules.png|center|400px]]
Several other molecular species are now known to provide for stable one-electron reduced states and one-electron oxidized states, in dry, non-polar solvents. Their recombination charge transfer processes produce blue-emitting states (diphenylanthracene, [[DPA]]), green-emitting states (di-isoamylquinacridone, DIQA (and other N,N’ dialkyl derivatives of quinacridone)), yellow emitting states (rubrene), and red-emitting states (ruthenium-trisbypryidine, Ru(bpy)+3). In all of these molecules their emissive states can be produced by charge transfer reactions between their reduced and oxidized forms. The separation in formal potentials for oxidation by one electron and reduction by one electron, expressed in electron volts, slightly exceeds the energy needed to directly excite the molecule to its lowest singlet state (S0 → S1).

OLED Device Applications

2009-03-13T18:28:58Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]] |
[[Light Emitting Electrochemical Processes|Next OLED Topic]]

Organic Light Emitting Diodes (OLED) are just are just beginning to appear in the commercial market. These products represent the fruition of 50 years of research building first on the principles of silicon LEDS.

The first OLED devices include TVs, computer monitors, electronic control displays, cameras, phones, and lighting devices.

==The advantages of OLEDs==
*Superior viewing angle- For monitors and TVs the screen is visible from the side rather than just face on as many LCD monitors are.
*Color Rendition- New dopants and dyes are being developed to give OLED a bigger range and flexibility of color rendition.
*Brightness- The OLED pixels produce light rather than block light with polarizers as an LCD display does.
*Faster Response- OLED devices typically have response time of .01 ms compared to 2 ms for LEDs.
*Energy Efficiency- The OLED is an efficient, low heat light source
*Cost- New polymers and coatings will allow LEDs to be produced by printing and spin coating techniques
*Flexibility- Polymer backing and thin coatings permit OLED to flex without breaking.
*Thin- A OLED display could be paper thin.

==Device construction==
An OLED consists of a thin transparent electrode, two or more organic transport/ emitting layers and metal cathode. When power is applied to the electrodes light is emitted from the central layer.

Individual red, green and blue emitting OLEDs are arranged in a grid with individual power supplies for each pixel. This is called a passive display. This is being replaced with active thin film transistor display that uses a transistor to control each pixel. This is called an active matrix display.
==Commercial OLED Products==
[http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644539854| Sony OLED TV]

http://www.universaldisplay.com/

http://www.kodak.com/eknec/PageQuerier.jhtml?pq-path=1473&pq-locale=en_US&_requestid=204

http://www.cdtltd.co.uk/

http://www.novaled.com/

[http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf Osram Opto Semiconductors]

OLED Device Applications

2009-03-13T18:28:09Z

69.91.158.17:

[[Main_Page#I.29_Organic_Light_Emitting_Diodes_-_OLED|Return to OLED Menu]]
[[Light Emitting Electrochemical Processes|Next OLED Topic]]

Organic Light Emitting Diodes (OLED) are just are just beginning to appear in the commercial market. These products represent the fruition of 50 years of research building first on the principles of silicon LEDS.

The first OLED devices include TVs, computer monitors, electronic control displays, cameras, phones, and lighting devices.

==The advantages of OLEDs==
*Superior viewing angle- For monitors and TVs the screen is visible from the side rather than just face on as many LCD monitors are.
*Color Rendition- New dopants and dyes are being developed to give OLED a bigger range and flexibility of color rendition.
*Brightness- The OLED pixels produce light rather than block light with polarizers as an LCD display does.
*Faster Response- OLED devices typically have response time of .01 ms compared to 2 ms for LEDs.
*Energy Efficiency- The OLED is an efficient, low heat light source
*Cost- New polymers and coatings will allow LEDs to be produced by printing and spin coating techniques
*Flexibility- Polymer backing and thin coatings permit OLED to flex without breaking.
*Thin- A OLED display could be paper thin.

==Device construction==
An OLED consists of a thin transparent electrode, two or more organic transport/ emitting layers and metal cathode. When power is applied to the electrodes light is emitted from the central layer.

Individual red, green and blue emitting OLEDs are arranged in a grid with individual power supplies for each pixel. This is called a passive display. This is being replaced with active thin film transistor display that uses a transistor to control each pixel. This is called an active matrix display.
==Commercial OLED Products==
[http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644539854| Sony OLED TV]

http://www.universaldisplay.com/

http://www.kodak.com/eknec/PageQuerier.jhtml?pq-path=1473&pq-locale=en_US&_requestid=204

http://www.cdtltd.co.uk/

http://www.novaled.com/

[http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf Osram Opto Semiconductors]