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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.
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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.
==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 were conducted in the late 1950s and early 1960’s at places like RCA. These studies were driven by the hypothesis that organic materials would act as efficient light sources and would also serve as organic photoconductors.  
    
    
See W. Helfrich & W.G.  Sneider Phys. Rev. Lett. 14(7), 229 (1965)
See Helfrich <ref>W.Helfrich & W.G.  Sneider Phys. Rev. Lett. 14(7), 229 (1965)</ref>


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.
Scientists used fine slices of highly purified anthracene (C<sub>14</sub>H<sub>10</sub>), approximately 5mm thick, connect them to electrodes, and apply 100-1000 volts. If the system did not short there would be faint blue light emission. Often there would be a lightning bolt-like discharge and graphite was produced instead of light. The high voltages were needed to overcome the internal resistance to charge migration (Rs) in the organic solid.  The relatively large thickness of these slices was an experimental necessity – pin-hole-free ultrathin film deposition technology was not yet available for organic materials.  Emission was seen occurring near the cathode, suggesting that charge recombination occurred closer to this electrode than the center of the device.


Next they brought the thickness down to 50 microns and used a saw emission from a semitransparent gold electrode.
<embed_document width="55%" height="400">http://depts.washington.edu/cmditr/media/OLED4_firstexperiments.pdf</embed_document>
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 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.
Subsequent experiments used thinner slices of anthracene (50 microns) with a semitransparent gold electrode and a silver paste cathode. Lower drive voltages were required, and light emission through the semitransparent anode was observed. Despite the relatively high external quantum efficiency of these devices, it was clear that substantial materials development would be necessary before real display devices would be realized.  The cited review by Dresner shows a substantial understanding of the physics of these light emitting organic thin films, which extrapolates directly to our understanding of OLEDs today.
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.
 
In these early devices, the rate of transport of the positive cation radical (or hole state) and the transport of the electron rich state were not equal, causing the recombination to occur near one of the contact electrodes. This phenomenon is now known to inhibit device efficiency.
 
See Dresner <ref>J. Dresner, RCA Rev. 30, 322 (1969)</ref>
See Pope 1963 <ref>Electroluminescence in organic crystals M. Pope ert al J. Chem. Phys.38, 2042 (1963)</ref>
 
==How is Light Generated?==
*Electrons are removed at the anode, creating a cation radical state.
*Electrons are added at the cathode, creating the anion radical state.  
*These charges move through the organic crystal, and meet each other near the center of the device.  
*An electron transfer between the reduced donor and the oxidized acceptor takes place.  
*Enough excess free energy is generated to create the emissive state of anthracene, thus light is emitted.  
   
   
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.
==The First Practical OLEDs==
[[Image:OLED4_heterojunction.jpg|thumb|400px]]
The first practical OLEDs were built in the late 1980’s by Ching Tang and Andy Van Slyke at Kodak.
 
C.W. Tang and coworkers at Kodak used spin casting and sublimation deposition of the organic components of the device, with patterned electrodes on the top and bottom. This was an important technological advance over slicing single-crystal anthracene as thin as possible with a razor knife. Vapor depositing molecules in a vacuum system on top of a transparent conductor enabled the layers to be much thinner, and have almost none of the pinholes that cause shorts. Transparent conducting oxide (TCO) thin films on glass served as the anode and allowed light to be emitted in a usable fashion for displays.
 
The keys to successful development of these first devices was control of organic film deposition to create a pin-hole free film (with high RP) which was thin enough to yield a lower series resistance than the first anthracene devices, producing light emission with drive voltages under 50 volts.  These devices produced about 15 candelas per square meter: dim but visible light. (A typical CRT computer screen operates with 100-200 candelas per square meter.)
 
==Anatomy of Early OLEDs==
*The '''base''' of the device was a glass or plastic substrate.
*A '''transparent conductor''' such as indium tin oxide (ITO).
*A 50 nm '''hole transport layer''' (HTL) of an easily oxidized spin-coated polymer such as triarylamine or poly vinyl carbazol.
*A 50 nm '''electron transport layer''' (ETL) of aluminum tris quinalate.
*A '''cathode''' made from a metal such as aluminum, magnesium-silver alloy or even calcium. 
 
See Tang  C.W. <ref>Tang and S.A. VanSlyke Appl. Phys. Lett. 51, 913(1987)</ref>
<ref>C.W. Tang, U.S. Patent # 4,356,429 (1980)</ref>
 
== References ==


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.
<references/>
[[category:organic LED]]


See  C.W. Tang and S.A. Van Slyke Appl. Phys. Lett. 51, 913 (1987)
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Latest revision as of 14:05, 23 July 2009

Previous Topic Return to OLED Menu Next Topic

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 were conducted in the late 1950s and early 1960’s at places like RCA. These studies were driven by the hypothesis that organic materials would act as efficient light sources and would also serve as organic photoconductors.

See Helfrich [1]

Scientists used fine slices of highly purified anthracene (C14H10), approximately 5mm thick, connect them to electrodes, and apply 100-1000 volts. If the system did not short there would be faint blue light emission. Often there would be a lightning bolt-like discharge and graphite was produced instead of light. The high voltages were needed to overcome the internal resistance to charge migration (Rs) in the organic solid. The relatively large thickness of these slices was an experimental necessity – pin-hole-free ultrathin film deposition technology was not yet available for organic materials. Emission was seen occurring near the cathode, suggesting that charge recombination occurred closer to this electrode than the center of the device.

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

Subsequent experiments used thinner slices of anthracene (50 microns) with a semitransparent gold electrode and a silver paste cathode. Lower drive voltages were required, and light emission through the semitransparent anode was observed. Despite the relatively high external quantum efficiency of these devices, it was clear that substantial materials development would be necessary before real display devices would be realized. The cited review by Dresner shows a substantial understanding of the physics of these light emitting organic thin films, which extrapolates directly to our understanding of OLEDs today.

In these early devices, the rate of transport of the positive cation radical (or hole state) and the transport of the electron rich state were not equal, causing the recombination to occur near one of the contact electrodes. This phenomenon is now known to inhibit device efficiency.

See Dresner [2] See Pope 1963 [3]

How is Light Generated?

  • Electrons are removed at the anode, creating a cation radical state.
  • Electrons are added at the cathode, creating the anion radical state.
  • These charges move through the organic crystal, and meet each other near the center of the device.
  • An electron transfer between the reduced donor and the oxidized acceptor takes place.
  • Enough excess free energy is generated to create the emissive state of anthracene, thus light is emitted.

The First Practical OLEDs

OLED4 heterojunction.jpg

The first practical OLEDs were built in the late 1980’s by Ching Tang and Andy Van Slyke at Kodak.

C.W. Tang and coworkers at Kodak used spin casting and sublimation deposition of the organic components of the device, with patterned electrodes on the top and bottom. This was an important technological advance over slicing single-crystal anthracene as thin as possible with a razor knife. Vapor depositing molecules in a vacuum system on top of a transparent conductor enabled the layers to be much thinner, and have almost none of the pinholes that cause shorts. Transparent conducting oxide (TCO) thin films on glass served as the anode and allowed light to be emitted in a usable fashion for displays.

The keys to successful development of these first devices was control of organic film deposition to create a pin-hole free film (with high RP) which was thin enough to yield a lower series resistance than the first anthracene devices, producing light emission with drive voltages under 50 volts. These devices produced about 15 candelas per square meter: dim but visible light. (A typical CRT computer screen operates with 100-200 candelas per square meter.)

Anatomy of Early OLEDs

  • The base of the device was a glass or plastic substrate.
  • A transparent conductor such as indium tin oxide (ITO).
  • A 50 nm hole transport layer (HTL) of an easily oxidized spin-coated polymer such as triarylamine or poly vinyl carbazol.
  • A 50 nm electron transport layer (ETL) of aluminum tris quinalate.
  • A cathode made from a metal such as aluminum, magnesium-silver alloy or even calcium.

See Tang C.W. [4] [5]

References

  1. ↑ W.Helfrich & W.G. Sneider Phys. Rev. Lett. 14(7), 229 (1965)
  2. ↑ J. Dresner, RCA Rev. 30, 322 (1969)
  3. ↑ Electroluminescence in organic crystals M. Pope ert al J. Chem. Phys.38, 2042 (1963)
  4. ↑ Tang and S.A. VanSlyke Appl. Phys. Lett. 51, 913(1987)
  5. ↑ C.W. Tang, U.S. Patent # 4,356,429 (1980)
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