lights with big potential: light emitting diodes & organic light
Commercial History (1960s - Today)
The LED is a light source which uses semiconductors and electroluminescence
to create light. There are two major kinds of light emitting diodes:
LED and OLED. The LED is different than EL
lamp in that it uses a small semiconductor crystal with reflectors
and other parts to make the light brighter and focused into a single
point. The OLED is very similar to the EL lamp in
design, using a flat sandwich of materials. It is different than the
LED and EL lamp in that it uses organic (carbon) molecules in the layer
that emits light.
credits and sources are located at the bottom of each lighting page
on LEDs and OLEDs, click the bracket icon on the lower right to expand
the LED lamp is popular due to it's efficiency and many believe it is
a 'new' technology. The LED as we know it has been around for over 50
years. The recent development of white LEDs is what has brought it into
the public eye as a replacement for other white light sources.
uses: indication lights on devices, small and large lamps,
traffic lights, large video screens, signs, street lighting(although
this is still not widespread)
LED array designed for use as a street lamp. A massive aluminum
heat sink is needed with the high wattage LEDs
-Energy efficient source of light for short
distances and small areas. The typical LED requires only 30-60
milliwatts to operate
-Durable and shockproof unlike glass bulb lamp types
-Directional nature is useful for some applications like reducing
stray light pollution on streetlights
be unreliable in outside applications with great variations
in summer/winter temperatures, more work is being done now to
solve this problem
-Semiconductors are sensitive to being damaged by heat, so large
heat sinks must be employed to keep powerful arrays cool, sometimes
a fan is required. This adds to cost and a fan greatly reduces
the energy efficient advantage of LEDs, it is also prone to
failure which leads to unit failure
-Circuit board solder and thin copper connections crack when
flexed and cause sections of arrays to go out
-Rare earth metals used in LEDs are subject to price control
monopolies by certain nations
-Reduced lumen output over time
per watt: 28 - 150 (depends on environment)
*Lamp life: 25,000 - 100,000 hours
(White LEDs) - 70
*Color Temperature (White LEDs) - 2540
*Available in 0.01 - 3 W
White LEDs became an affordable commercial product in the 2000s
1. HOW THEY WORK
create light by electroluminescence in a semiconductor material.
Electroluminescence is the phenomenon of a material emitting light when
electric current or an electric field is passed through it - this happens
when electrons are sent through the material and fill electron holes.
An electron hole exists where an atom lacks electrons (negatively
charged) and therefore has a positive charge. Semiconductor materials
like germanium or silicon can be "doped" to create and control
the number of electron holes. Doping is the adding of other elements
to the semiconductor material to change its properties. By doping a
semiconductor you can make two separate types of semiconductors in the
same crystal. The boundary between the two types is called a p-n junction.
The junction only allows current to pass through it one way, this is
why they are used as diodes. LEDs are made using p-n junctions. As electrons
pass through one crystal to the other they fill electron holes. They
emit photons (light). This is also how the semiconductor
A 5 Watt LED, one of the most powerful LEDs available.
A laser also creates light, but through a different construction. Read
more about semiconductor devices used in
understand p-n junctions and semiconductors better you will need to
invest a good amount of time in a lecture, it is not a simple phenomena
and far too lengthy to cover here. See a 59 minute introduction lecture
to solid state (semiconductors) here.
are used to help filter the light output of the LED. They create a more
pure "harsh" color.
had to figure out how to control the angle the light escapes the semiconductor,
this "light cone" is very narrow. They figured out how to
make light refract or bounce off all surfaces of the semiconductor crystal
to intensify the light output. This is why LED displays traditionally
have been best viewed from one angle.
various colors of LEDs on display at the Edison Tech Center. The metal
tabs on the sides of each help distribute the heat away from the LED.
Peter Heppner at the MTV music awards, Bucharest, Romania
A "Jumbotron" or full color LED display. This type of display
is only usable for large area applications and decorative backgrounds
in small spaces. The human eye can only effectively perceive the image
at more than 6 meters distance. The tricolor array is arranged in the
close-up at the top right.
For more details
on elements used for each color go here.
Two different types of LEDs, both in a strip mount configuration
The Edison Tech Center was able to run an array
of bright LED's in the remote Amazon for a video production using
only 6 AA batteries. To get the same lumen output from an incandescent
for the same duration would have required a heavy car battery. The LED
is more robust than a bulb with filament, this makes it excellent for
in the field applications. It is resistant to vibration and sudden impacts.
2. INVENTORS AND DEVELOPMENTS
The First LED:
Above: this experiment of a tunnel diode atop a GaAs semi-insulating
substrate convinced Pittman and Biard that there must be light
emission going on, and resulted in further experimentation.
early years of the 1960s consisted of a 'race' in the field of
semiconductors. Gallium arsenide and germanium were some of the
first semiconductors uses before silicon became the preferred
material in the industry. Engineers were experimenting with p/n
junctions. These devices were being developed as diodes since
they can pass current in one direction by not the other. GE, Bell
Labs,Lincoln Labs, RCA research labs, and Texas Instruments worked
to develop semiconductors for power control and laser technology.
It was in this race that the LED was 'discovered' in the Fall
of 1961 by James R. Biard and Gary Pittman. Gary had been working
in the related field of solar cells since 1958. In their efforts
to try to make an X-band GaAs varactor diode they created tunnel
diodes (which had been developed first at Esaki). They placed
the tunnel diode on a GaAs substrate and discovered that there
must be light production going on during forward bias operation.
Using an infrared detector just brought in from Japan they tested
it and discovered that the devices lit up brightly!
after this others made discoveries in the field, however TI was
the first to get a patent and sell the first LED for $130 each.
The SNX-100 was the first LED sold (summer of 1962). The LEDs
were first used with IBM computers to replace tungsten bulbs that
controlled punch card readers (infrared light was sent through
the holes, or blocked by the card). Today there is a myriad of
applications for the LED.
The First LED
You can clearly
see the p/n junction in the patent drawing. The p/n junction
is a single crystal with two types of semiconductors created
* 1972: Herbert P. Maruska
& Jacques Pankove developed the violet LED which set the stage for
development of a bright blue LED in 1993
Randy Lamb, UC Santa Barbara / Semicon West 2012 / PD-USGOV / Bob Biard
- H.J. Round discovered
electroluminescence when using silicon carbide and a cats whisker.
Oleg Losev independently discovered the phenomena the same year.
London, United Kingdom
- Oleg V. Losev studied
the phenomena of light emitting diodes in radio sets. His first
work on 'LEDs' involved a report on light emission from SiC.
In 1927 he published a detailed report but his work was not
well known until the 1950s when his papers resurfaced.
Saint Petersburg, Russia
- James R. Biard. "Bob"
Biard and Gary Pittman
developed the Infrared LED at Texas instruments. This was the
first modern LED. It was discovered by 'accident' while TI tried
to make an X-band GaAs varactor diode. The discovery was made
during a test of a tunnel diode using a zinc diffused area of
a GaAs (Gallium Arsenide) semi-insulating substrate.
- Gary Pittman worked together
with James R. Biard. He had started working in 1958 with semiconductor
GaAs for the creation of early solar cells. He discovered and
developed the infrared LED with James R. Biard.
- Nick Holonyack Jr. develops
the red LED, the first LED of visible light. He used GaAsP (Gallium
Arsenide Phosphide) on a GaAs substrate. General Electric.
Syracuse, New York
- M. George Craford creates
the first yellow LED at Monsanto using GaAsP. He also develops
a brighter red LED.
St. Louis, Missouri
Semicon West 2012
- Herbert Maruska and Jacques Pankove
develop the violet LED using Mg-doped
GaN films. The
violet LED is the foundation for the true blue LED developed
Labs , New Jersey
- Shuji Nakamura develops
the world's first bright blue LED using GaN (Gallium nitride).
It wouldn't be until the 1990s that the blue LED would become
low cost for commercial production.
Randy Lamb, UC Santa Barbara
- Thomas P. Pearsall develops
special high brightness LEDs for fiber optic use. This improves
communications technology worldwide. Paris,
OLED: Organic Light Emitting Diodes
is an OLED?
Organic LED is made of a layer of organic electroluminescent material
with p/n junction sandwiched between to electrodes. At least one of
the electrodes is transparent so the photons can escape. Similar to
an EL lamp, current is passed
through a semiconductor (like the phosphor in an EL lamp), however the
difference is that an OLED uses a p/n junction were there is a recombination
of p and n carriers. EL (TDFEL, TFEL,
powder EL) technology only uses a material excited by current to
semiconductor in an OLED is organic which means it contains carbon.
The OLED uses one of two kinds of compounds: polymers or 'small molecule'.
Read more about how it works below.
short distance indoor lamps (produces a diffused light)
Displays - small: phones and media devices and large: televisions,
units are lighter than traditional LEDs and can be made thinner as well
-OLEDs can provide a more energy efficient alternative to LCD computer
and television monitors
-Can be used in a myriad of new applications in which lighting technology
has never been used before
cost of OLEDs is still high and each unit produces less lumens than
a normal LED
-The technology is still under development so the life of the OLED is
being researched as new materials are used and tested each year. Until
more research is done we will not know how these lamps with new materials
compare with established technology.
per watt: up to 50 in lamps (as of 2/2012)
*Lamp life: still under research
(White OLEDs) - still under research
*Color Temperature (White LEDs) - various
whites are in development
*Available wattage: N/A
Patent by OLED
co-inventor Stephen Van Slyke
(computer monitors, televisions, mobile phone screens):
OLED display is made by using multiple layer construction along with
transistors which control whether each pixel is on or off. This is very
similar to EL displays. The OLED display has the potential to be more
efficient and thinner than the LCD. One advantage is that does not need
a cold cathode fluorescent backlight
like an LCD. The lack of a backlight means it can better display blacks
(the back light always seeps through in black areas of the screen).
The OLED display can also provide better contrast ratios than an LCD.
The OLED display may also be made into a thin flexible material
which could roll up like a newspaper. Currently the OLED is not
as bright as EL or LCD displays it works better in areas with less ambient
light. That may change as engineers work to increase luminosity.
Below: our simple video on OLED use in monitors and
diagram above is a simple modern OLED. There are a many new ways to
construct the OLED using a variety of layer configurations. Displays
will have additional layers such as an active matrix TFT (thin film
transistor) which control pixel regions.
the OLED Works:
OLEDs had one layer of organic material between two electrodes.
Modern OLEDs are bi-layer, they have an emissive layer and conductive
layer sandwiched between two electrodes (see diagram above).
Electric current passes from the cathode to the anode. It passes through
two layers of organic molecules.
The first layer the electrons pass into what is called the emissive
layer. Electrons leave the conductive layer making 'holes'
(positive charge). Meanwhile in the emissive layer there are excessive
electrons (negative). The 'holes' jump to the emissive layer along the
border of the two layers where they recombine with electrons (this place
is the p/n junction). When the electrons join the holes light is emitted.
color is dependent on the materials used in the organic or polymer layers
Wikipedia: Tobias G.
Types of OLEDs:
LEC - Electrochemical Cell - this has ions added to the OLED
PMOLED - Passive-matrix OLED - the first display technology,
developed in the mid 90s
AMOLED - Active-matrix OLED - used in displays, it has a switch
built into it in the form of a thin film transistor backplane. The transistor
allows the unit to be switched on and off.
PLED - polymer LED
Polymer LEDs use a plastic to emit light. They have the properties of
semiconductors yet are versatile and low cost to produce. The layers
that emit light are similar to an ink and will be very cheap to manufacture
once stable compounds and processes are developed.
understanding of these improvements requires a basic background in chemistry
and physics, you also can read more detail here.
will allow for thinner TV and computer displays, transparent "heads
up" displays, flexible displays, flat roll-on surface lights on
the sides of buildings or vehicles, changing camouflage displays for
military vehicles, new photovoltaic applications, and much more. We
can expect a lower production cost compared to LEDs due to less part
assembly. At the moment OLEDs need more lab work to reach full potential.
Carbon nanotube technology is being developed for use
with the OLED.
OLED Inventors and Developments:
1987- Ching Tang
discovers that he can create light by sending current through
a carbon material. Steven Van Slyke and Tang built the first
OLED at Kodak in 1987. Later he works on an OLED displays. His
first light was a bright green light at 10 volts. Ching Tang
continues to work on OLEDs at the University of Rochester.
Rochester, New York
University of Rochester
1987- Steven Van Slyke
worked with Ching Tang on the first OLEDs at Kodak. Kodak becomes
a patent holder of SMOLED (small-molecule OLED) technology.
SMOLED requires depositing organic molecules in a vacuum. This
is a very expensive process at the time.
Rochester, New York
Steven Van Slyke
- Chihaya Adachi invented
electron transport materials and constructed a double heterostructure
OLED which has been widely used in present OLEDs. Since 1988,
his research has been focusing on material development and device
physics. Recently he invented 3rd generation light emitting
materials, TADF, enabling efficient EL. Read
from his papers
- Tetsuo Tsutsui contributed
much to the development of high-performance OLEDs through the
proposals of design concept on multilayer OLEDs in 1988, the
concept of carier balance in 1992, and the usage of the optical
microcavity effect in 1993. Testuo Tsutsui with Prof.
Shogo Saito made many contributions to the commercialization
of small-size OLED displays by Pioneer, TDK and Sanyo Electric
, Japan Photo:
Prof. Tetsuo Tsutsui
with Richard Friend and Donal
Bradley discover polymer based
OLEDs, this reduced cost of production to marketable
levels. PLED (polymer LED) technology competes with SMOLED for
the future of OLEDs. Cambridge University Cavendish Laboratory.
Cambridge, United Kingdom
- Richard Friend - PLED
developer at Cambridge University Cavendish Laboratory. The
polymer LED is cheaper to produce than SMOLED because it does
not use vacuum deposition. Thin films can be deposited on larger
sheets. This is a step towards the later development of OLED
- Donal Bradley -
PPV, ~100 nm thick between indium oxide and aluminum layers
to create the conjugated polymer LED reported in Nature in 1990.
This had a yellow-green colored light. Quantum efficiency: 0.05%.
Subsequently developed high efficiency polyfluorene PLEDs at
Sheffield University with Dow Chemical Company. Donal
Bradley now leads the Centre for Plastic Electronics at
Imperial College London, working in collaboration with Sumitomo
Chemical Company on PLEDs for displays, lighting and lasers.
See useful links on Bradley's work on Useful Links below.
London, United Kingdom
Imperial College, London
- Teruo (Ted) Tohma developed
the first commercial OLED displays using PMOLED (passive matrix
organic LED) technology at Pioneer. He also envisioned the move
from PMOLED to AMOLED and later developed a p-Si active matrix
displays. The AMOLED is first sold in 2007.
- Stephen R. Forrest advanced
OLED research, and continues to work on development. In 1998
Forrest and Thompson built the first phosphorescent OLED.
University of Michigan
- Mark E. Thompson and Stephen
Forrest develop the first phosphorescent OLED (PHOLED) by using
an emissive later of platinum(II) octaethylporphyrin. The PHOLED
is more efficient than a normal fluorescent LED. They developed
green and red OLEDs using iridium. The PHOLED is still not commercialized.
University of Southern California.
Los Angeles, California
Mark E. Thompson
- You - Choose engineering
for your career! Work on fascinating new technologies, make
new breakthroughs in lighting and change our world for the better.
LEARN MORE HERE
of major events:
1979 - Discovery of organic electroluminescent
1987 - First organic light emitting
diode built at Kodak
1990 - First PLEDs discovered, Cambridge
1997 - First commercial PMOLED displays
produced at Pioneer
1998 - First phosphorescent OLED
2007 - Samsung Mobile Display develops
the first commercial AMOLED (active matrix organic LED) display.
are presented in the order of chronological development
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Links, More Resources:
The PLED: Donal Bradley, Jeremy Burroughes and Richard Friend:
Nature paper highlighted in: www.nature.com/physics/looking-back
(This is the most highly cited paper in the field. DDCB was corresponding
Royal Society Bakerian Lecture: http://royalsociety.org/royalsociety.tv/?from=footer
(Freely viewable on-line)
Another RS video on plastic electronics: http://www.youtube.com/watch?v=0D_W_q1a0vU
Centre for Plastic Electronics Homepage: http://www3.imperial.ac.uk/plasticelectronics
DDCB Home page: http://www3.imperial.ac.uk/people/d.bradley
Website, graphics and article by M. Whelan
Thanks to assistance from Bob Biard, Chihaya Adachi, Chris King, Donal
Bradley, Tetsuo Tsutsui and Ted Tohma
University of Rochester
Wikipedia: LED, Electron Hole, YAK, OLEDs
Edison Tech Center: Interview with Robert N. Hall
Interview with John D. Harnden Jr.
Also thanks to information provided by Jeremy Burroughes, Robert Biard
US Patents: 67794676 Pakbaz
Contributions of Prof. Tetsuo Tsutsui in R&D of OLED technology
James R. Biard
Edison Tech Center
UC Santa Barbara
University of Rochester
University of Michigan
J. Robert Biard
Steve Van Slyke
Imperial College, London
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