Luminescent to meet the demands of today’s society.

Luminescent Pt (II) complexes and their application in white OLEDs



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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.


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



Another advantage of OLED displays is that
they offer better picture quality via greater contrast ratios and viewing
angles which can be attributed to the 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 being able to display true blacks
correctly and generally being thicker than their OLED counterparts. This is
because 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 or refraction panel. 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



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 Whilst
fluorescent OLED displays have reached the stage where they are reliable for
practical uses, however, because of the nature of fluorescence, 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
An area of research that is currently of major interest is the use
phosphorescent complexes in OLED devices. These devices are known as PHOLEDs
(Phosphorescent Organic Light-Emitting Diode) and offer significant advantages
over current OLED devices which are already seen as a major step forward in
display technology when compared to consumer LCD devices. With phosphorescent molecule containing heavy metals and TADF
(Thermally Activated Delayed Fluorescence) materials, a quantum efficiency of
100% is achievable. VW


Fluorescence vs Phosphorescence

is the core principal by which current consumer OLED devices operate.
Fluorescence can described as the absorption of photons by a molecule in the
singlet ground state which are then promoted to a singlet excited state. As the
molecule relaxes to the ground state, it release a photon of a lower
wavelength, and therefore lower energy. AD

offers a variation of this principle. A phosphorescent material gradually emits
the photons it absorbs over a longer period of time than fluorescence which is
typically around 10 nS. This can be attributed to electrons undergoing
intersystem crossing into an excited triplet state from which the emission of
light via phosphorescence occurs.

Fig 2: Jablonski diagram depicting fluorescence and phosporescenceAD




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 primary schemes that can be used to
control OLED displays and, depending on which one is used, result in either
active-matrix OLEDs (AMOLED) or passive-matrix OLEDs (PMOLED) being manufactured.
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 resolutions.G


Conventional OLEDs consist of an organic layer
placed in between two electrodes which 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. AB


Fig 5: The
structure of an OLEDH



of the most simple polymer OLED systems only contained one organic layer. This
was created by J. H. Burroughes and
his colleagues in 1990 and utilized a solitary layer of poly(p-phenylene

Fig 6: Monomer
of poly(p-phenylene



an OLED, an electric current flows from the cathode to the anode, injecting electrons
in the LUMO of cathode which are then withdrawn from the HOMO of the anode
which is also referred to hole injection. The hole and the electron are brought
together via electrostatic forces and combine to form an exciton. Because holes
move more freely in organic semiconductors than electrons, this process occurs more
closely to the emissive layer. When the exciton relaxes it releases radiation in
the visible spectrum producing light which is where OLEDs function as a light
emitting device originates. The difference in HOMO
and LOMO energy levels determines the frequency of the light emitted.  

Fig 7: A
diagram depicting how OLEDs emit lightH









are selectively chosen based on a few key criteria including their chemical
stability optical transparency and their electrical conductivity. A
popular material that is used for this is indium tin oxide it’s high work
function encourages the injection of holes in the HOMO of the organic layer and
it is transparent AE. Barium and calcium are common cathode materials because of the
low work functions they possess as they encourage the injection of electrons in
the organic layer AF.


Manufacturing of OLEDs that employ multiple layers is possible which
generally leads to better efficiency. A range of materials can be used to influence
conductive properties or to potentially improve charge injection at the
electrodes by offering a more contoured electronic profile.AA Most
OLED  devices in production today use a
bilayer structure which constituted of a conductive and emissive layer as
depicted in Fig 5. There are a few different OLED architectures which offer different advantages. An
interesting development in OLED technology that has been shown to improve
internal quantum efficiency, is the implementation of a graded heterojunction
architecture. A graded heterojunction acts an interface between the conductive
and emissive layers of an OLED.P This architecture varies the configuration of electron/hole transport
materials within the emissive layer utilizing a dopant emitter.
approach to device architecture is particularly advantageous as it improves
charge injection and balances charge transport in the emissive region. This
approach to device architecture could potentially yield an internal quantum
yield double that of conventional OLED systems.AC

An interesting area of research is the use of
electrophosphorescent Pt(II) complexes as a substitute to traditional
fluorescent compounds which are common today.


Early history of OLED technology


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



Rather like OLEDs, PHOLEDs produce light via electroluminescence of
an organic semiconductor layer
in an electric current. Electrons and holes are injected into the organic layer
at the electrodes and form excitons, a bound state of
the electron and hole.

However, phosphorescent OLEDs generate light from both triplet
and singlet excitons, allowing the internal quantum efficiency of such devices
to reach nearly 100%.5

This is commonly achieved by doping a host molecule with
an organometallic complex.
These contain a heavy metal atom at the centre of the molecule, for example
platinum6 or iridium, of which the
green emitting complex Ir(mppy)3 is
just one of many examples.1 The large spin-orbit
interaction experienced by the molecule due to this heavy metal
atom facilitates intersystem crossing,
a process which mixes the singlet and triplet character of excited states. This
reduces the lifetime of the triplet state,78 therefore phosphorescence
is readily observed.

Typically, a polymer such as poly(N-vinylcarbazole) is used as a
host material to which an organometallic complex is added
as a dopant. Iridium complexes54 such as
Ir(mppy)352 are
currently the focus of research, although complexes based on other heavy metals
such as platinum53 have also
been used.

 Platinum(II) complexes have
been used as phosphorescent emitters in small-molecule OLEDs.4,12,20–25 Since
the first phosphorescent OLED was reported with
2,3,7,8,12,13,17,18octaethyl-21H,23H-porphine platinum(II) (PtOEP) as a red
emissive dopant,4 platinum(II) complexes have been used to prepare OLEDs that
give green, red, and even white EL with external quantum efficiencies as high
as 16.5%.15 – Platinum Binuclear Complexes as Phosphorescent Dopants for
Monochromatic and White Organic Light-Emitting Diodes

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