4. LUMINESCENCE
Luminous simply means giving off light; most things in our world produce light because
they have energy that originally came from the Sun, which is the biggest, most luminous
thing we can see.
although the Moon appears to give off light, it's not actually luminous because it's simply
reflecting light from the Sun like a giant mirror made of rock.
Luminous is quite a vague word really. Arguably, even a flashlight bulb is luminous,
because it turns electricity (electrical energy) into light and shines it toward us. But
bulbs like this are incandescent and make light by making heat.
Luminescent things, by contrast, make light when their atoms become excited in a
process that needs little or no heat to make it happen.
5. Bioluminescence-
Chemiluminescence-
Crystalloluminescence –
Electroluminescence-
Cathodoluminescence-
Mechanoluminescence
luminescence generated by a living organism.
results from some chemical or electrochemical
reactions.
is produced during crystallization .
generates light in response to an electric current
passing through some materials
Cathodoluminescence occurs when an electron
beam impacts on a luminescent material such as a
"phosphor
resulting from any mechanical action on a solid,
can be subdivided into:
6. TYPES OF LUMINESCENCE
Photoluminescence- is caused by moving electrons to
energetically higher levels through the absorption of photons.
Radioluminescence - is generated when some materials are exposed
to ionizing radiation like a, b or g rays.
Sonoluminescence- is the emission of short bursts of light from
imploding bubbles in a liquid when excited by sound.
Thermoluminescence- describes the phenomenon that certain
crystalline materials emit light when heated that is not black body radiation or
incandescence
7. ELECTROLUMINESCENCE
Electroluminescence (EL)- is an optical phenomenon and
electrical phenomenon in which generates light in response to
an electric current passing through some material.
This is distinct from black body light emission resulting from heat
(incandescence), from a chemical reaction (chemiluminescence),
sound (sonoluminescence), or other mechanical action
(mechanoluminescence).
In essence, electroluminescence results radiative recombination of
electrons and holes; typically in semiconductors.
It is the basis for LED's and semiconductor Lasers and thus of prime
importance in the context of light sources.
8. MECHANISM IN EL
Electroluminescence-
is the result of radiative recombination of electron
and holes in a material, usually a semiconductor.
The excited electrons release their energy as photons -
light.
Prior to recombination, electrons and holes may be
separated either by doping the material to form a p-n
junction such as light-emitting diodes, or
through excitation by impact of high-energy electrons
accelerated by a strong electric field (as with
the phosphor in electroluminescent displays).
9. Electroluminescent Materials
The most common electroluminescent (EL) devices are composed of
either powder (primarily used in lighting applications) or thin films (for
information displays.)
Electroluminescent devices are fabricated using either organic or
inorganic electroluminescent materials. The active materials are
generally semiconductors of wide enough bandwidth to allow exit of the
light.
The most typical inorganic thin-film EL (TFEL) is ZnS:Mn with
yellow-orange emission. Examples of the range of EL material include:
Powdered zinc sulfide doped with copper (producing greenish
light) or silver (producing bright blue light)
Thin-film zinc sulfide doped with manganese (producing orange-
red color.
Semiconductors containing Grou III and Group V elements, such
as indium phosphide (InP), gallium arsenide (GaAs), and gallium nitride
(GaN).
12. ORGANIC LIGHT EMITTING DIODEs
An organic light-emitted diode (OLED) consists of a single
layer or double layer or multiple layer of organic materials
sandwiched between two electrodes, at least one of which is
transparent.
In recent years, OLEDs have attracted world-wide attention as
a candidate for next generation of flat-panel displays and solid
state lighting devices.
The OLEDs consisting of hole-and electron-transporting
molecular materials or of spin- coated thin polymers films
have been fabricated with active device thickness of less than
one hundred nanometer
For an applied voltage of about 3 to 5V, the OLEDs emit light
brighter than conventional TV screen with much higher
efficiencies, brilliant colours, large viewing angle, switching
times fast enough for video real time image displays, and
lifetime well above 100,000 hours.
13. Though electroluminescence (EL) in organic materials was observed
long time ago, bright organic EL at low voltage was first announced by
C.W.Tang and S.A.VanSlyke of Corporate Research laboratories,
Rochester, New York, USA in 1987 on 8-hydroxyquinoline aluminium
(Alq3).
Another breakthrough in organic EL came in 1990 through the
publication of J.H.Burroughes and his co-workers of Cavendish
Laboraories. Cambridge, United Kingdom on light emitting diodes
based on conjugated polymer, poly (p-phenylene vinylene) (PPV).
Since these reports on low voltage organic EL, organic light emitting
diodes have attracted worldwide attention of a large number of
scientists, technologists and engineers.
20. Conceptual illustration of the flow of light emission
The flow of light emission can be illustrated by the concept of a person going down
the big slide. The state of sitting on the top of the slide corresponds to excitation, and
the act of sliding down corresponds to light emission. To emit light continuously for
10,000 hours means that the organic material of the emissive layer repeats this act
without interruption over several hundred million of times.
21. Working of OLEDs
In OLEDs, the light emission involves the following steps:
(i) Injection of charge carriers from electrodes.
(ii) Recombination of injected charge carriers.
(iii) Generation of singlet and triplet excitions.
(iv) Radiative decay of singlet excitions.
The injection of charge carriers from electrodes takes place by
Richardson-Schottky thermonic emission model(at low voltage) or
by Fowler-Nordheim tunneling model(at high voltage).
Richardson-Schottky thermonic
emission model
Fowler-Nordheim tunneling
model
23. 23
Electron spin = 1/2 , Hole spin = 1/2
Exciton spin = 0 (Singlet)
Total spin of exciton (electron-hole bound state)
1 (Triplet)
24. Figure :- The spin vectors for singlet and triplet states. S and M s are the
total and magnetic spin quantum numbers, respectively, and α and β are
the spin "up"and "down," respectively.
34. The Nobel Prize in Physics 2014
The Nobel Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura "for the
invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources".
Isamu Akasaki Japan
Hiroshi Amano
Japan
Shuji Nakamura,
USA
Lighting plays a major role in our quality of life. The development of light-emitting diodes (LEDs) has made more
efficient light sources possible. Creating white light that can be used for lighting requires a combination of red,
green, and blue light. Blue LEDs proved to be much more difficult to create than red and green diodes. During the
1980s and 1990s Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura successfully used the difficult-to-handle
semiconductor gallium nitride to create efficient blue LEDs.
36. Drawbacks of OLEDs
The biggest technical problem left to overcome has been the
limited lifetime of the organic materials. Particularly, blue OLEDs
typically have lifetimes of around 5,000 hours when used for flat
panel displays, which is lower than typical lifetimes of LCD or
Plasma technology.
However , recent experimentation has shown that it is possible
to swap the chemical component for a phosphorescent one, if
the subtle differences in energy transitions are accounted for,
resulting in lifetimes of up to 20,000 hours for blue PHOLEDs.
The intrusion of water into displays can damage or destroy the
organic materials. Therefore , improved sealing processes are
important for practical manufacturing and may limit the
longevity of more flexible displays.
Cost
