Introduction the drawbacks of LCD displays such as not



Display technology is increasingly required to
adapt and evolve in order to meet the demands of today’s society. One of the
most promising display technologies, in development and in use currently, is
OLED display technology.

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The desire for efficient OLED displays is
warranted as they offer a range of advantages when compared to other
technologies. For example, OLED displays can be produced on flexible plastic substrates
which enables the manufacturing of flexible OLED displays which gives host to a
wide gamut of potential applications.J Non-flat OLED displays have
already seen use consumer technology in the production of curved OLED TV’s and
smartphones in Samsung’s “Edge” range of devices.


Fig 1: Samsung’s
flexible OLED display technology K



Another advantage of OLED displays is that
they offer better picture quality via greater contrast ratios and viewing
angles which can be attributed to direct light that OLEDs emit. Because OLED
displays do not employ a backlight, they do not suffer from some of the
drawbacks of LCD displays such as not able to display true blacks correctly and
generally being thicker than their OLED counterparts. OLEDs, when inactive, do
not consume power or emit any light which means they are able to deliver true
blacks.N OLED displays are also lighter than traditional LCD which
can again be attributed to the lack of a backlight. OLED displays also have
significantly faster response times than LCD displays. LCD displays can
facilitate a refresh of down to 1ms and a refresh rate of 240Hz, however LG
have claimed that OLED displays could potentially reach a stage where they have
a response time that is 1,000 times faster than conventional LCD displays (0.001ms). M



However, OLEDs are not
without their drawbacks. Recently, the efficiency of OLEDs have been under
scrutiny in an attempt to reduce the energy usage of OLED devices like lighting
systems and displays. O Fluorescent OLED displays have reached the
stage where they are reliable for practical uses, however, because of the
nature of electrofluorescene, they can only have a maximum quantum efficiency
of 25% which is the calculated as the amount of photons created per injected
carrier. This is because, of all the excited-state populations, only the singlet spin states are fluorescent and only make up a minor portion
(around 25%).Q With phosphorescent molecule containing heavy metals
and TADF (Thermally Activated Delayed Fluorescence) materials, a quantum
efficiency of 100% is achievable. VW


It is necessary to understand the working
principle of OLED displays to properly assess the advantages and disadvantages
of PHOLED (Phosphorescent Oragnic Light-Emitting Diodes)  displays.




Electroluminescence in organic materials was
first observed by André Bernanose and his colleagues at the French university
Nancy-Université in 1953. High alternating voltages in air were applied to
compounds like alcidine orange. The compounds were either dissolved in or
deposited on thin cellophane films or cellulose. The initial observations made
attributed the electroluminescence to excitation of electrons or direct
excitation of the dye molecules. DEF


Martin Pope and his colleagues at New York
University developed ohmic dark-injecting electrode contacts to
organic crystals in 1960. They also defined the required energetic requirements
for electrode contacts and electron and hole injection. RST The
electrode contacts are utilized as the foundation of electron and hole injection
in today’s OLED devices. In 1963, they also managed to observe DC (direct
current) electroluminescence on a solitary crystal of anthracene and on tetracene-doped
anthracene crystals using a silver electrode at 400 volts. U


3: Antrhacene                                  Fig 4: Tetracene


Popes group’s research continued and in
1965 they observed that when an external electric field is not supplied, electroluminescence
in anthracene can be attributed to the conducting energy level being higher than
excitation level and to the recombination of thermalized hole and electron.X


The first reported observation of
electroluminescence in polymers was reported by Roger Partridge at the National
Physical Laboratory and the paper was published in 1983. A 2.2 µM thick poly(N-vinylcarbazole) film between two charge injecting electrodes
made up the device. Y


The first practical OLED was
made in 1987 by Steven Van Slyke and Ching W. Tang for the Eastman Kodak
company and utilized conventional fluorescent materials.O




An OLED (organic light-emitting diode) is an
LED that utilizes an organic material as the electroluminescent layer that
produces light as a response to an electric current. This layer sits between
two electrodes where one of the electrodes is typically transparent. OLEDs can
be used as a light source in many devices such as computer monitors, television
screens, mobile phones and smart watches, among many other devices. Research
into the development of white OLEDs for use in solid-state lighting is a
particular area of research which is of major interest. ABC


Two main types of OLEDs exist; OLEDs that use
small males and OLEDs that utilize polymers. Mobile ions can be added to OLEDs
to create LECs (light-emitting electrochemical cell) which have a different
mechanism of operation. There are two main schemes that are used to control
OLED displays and result in the creation of either active-matrix OLEDs (AMOLED)
and passive-matrix OLEDs (PMOLED). With active-control, a thin-film transistor
backplane is used which allows direct access to each OLED in the display which means
they can be switched on and off independently. A passive-matrix control scheme controls
each row and line of the display sequentially. AMOLED offers more advantages
than PMOLED as it allows for facilitates larger display sizes at higher


Conventional OLEDs consist of an organic layer
placed in between two electrodes, all of this structure is situated on a
substrate. As a consequence of the delocalization of pi electrons, the organic
molecules are able to conduct electricity. The materials used in the OLED are
regarded as organic semiconductors as they have various levels of conductivity,
from conductors to insulators. P