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

 

Introduction

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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 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.

 

History

 

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

 

Fig
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

 

How OLEDs
work

 

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 produced. 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, 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

 

Fig 5: The
structure of an OLEDH

 

Fig 6: A
diagram depicting how OLEDs emit lightH

 

One 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
vinylene).Z

 

Fig 6: Monomer
of poly(p-phenylene
vinylene)

However, the manufacturing OLEDs that employ multiple layers is
possible which generally leads to better efficiency. As well as conductive
properties, different materials may be chosen to aid charge injection at
electrodes by providing a more gradual electronic profile,26 or
block a charge from reaching the opposite electrode and being wasted.27 Many
modern OLEDs incorporate a simple bilayer structure, consisting of a conductive
layer and an emissive layer. More recent developments in OLED architecture
improves quantum efficiency (up to 19%) by using a graded heterojunction.28 In the graded
heterojunction architecture, the composition of hole and electron-transport
materials varies continuously within the emissive layer with a dopant emitter.
The graded heterojunction architecture combines the benefits of both
conventional architectures by improving charge injection while simultaneously
balancing charge transport within the emissive region.29

During operation, a voltage is applied across the OLED such that
the anode is positive with respect to the cathode. Anodes are picked based upon
the quality of their optical transparency, electrical conductivity, and
chemical stability.30 A current of electrons flows through the device from
cathode to anode, as electrons are injected into the LUMO of the organic layer
at the cathode and withdrawn from the HOMO at the anode. This latter process
may also be described as the injection of electron holes into the HOMO.
Electrostatic forces bring the electrons and the holes towards each other and
they recombine forming an exciton, a bound state of
the electron and hole. This happens closer to the emissive layer, because in
organic semiconductors holes are generally more mobile than
electrons. The decay of this excited state results in a relaxation of the
energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this
radiation depends on the band gap of the
material, in this case the difference in energy between the HOMO and LUMO.

As electrons and holes are fermions with half integer spin, an exciton may either be in a singlet state or a triplet state depending on how the spins
of the electron and hole have been combined. Statistically three triplet
excitons will be formed for each singlet exciton. Decay from triplet states (phosphorescence) is spin forbidden, increasing
the timescale of the transition and limiting the internal efficiency of
fluorescent devices. Phosphorescent
organic light-emitting diodes make use of spin–orbit
interactions to facilitate intersystem crossingbetween
singlet and triplet states, thus obtaining emission from both singlet and
triplet states and improving the internal efficiency.

Indium tin oxide (ITO)
is commonly used as the anode material. It is transparent to visible light and
has a high work function which
promotes injection of holes into the HOMO level of the organic layer. A typical
conductive layer may consist of PEDOT:PSS31 as
the HOMO level of this material generally lies between the work function of ITO
and the HOMO of other commonly used polymers, reducing the energy barriers for
hole injection. Metals such as barium and calcium are often used for the cathode as
they have low work functions which
promote injection of electrons into the LUMO of the organic layer.32 Such
metals are reactive, so they require a capping layer of aluminium to avoid degradation.

Experimental research has proven that the properties of the
anode, specifically the anode/hole transport layer (HTL) interface topography
plays a major role in the efficiency, performance, and lifetime of organic
light emitting diodes. Imperfections in the surface of the anode decrease
anode-organic film interface adhesion, increase electrical resistance, and
allow for more frequent formation of non-emissive dark spots in the OLED
material adversely affecting lifetime. Mechanisms to decrease anode roughness
for ITO/glass substrates include the use of thin films and self-assembled
monolayers. Also, alternative substrates and anode materials are being
considered to increase OLED performance and lifetime. Possible examples include
single crystal sapphire substrates treated with gold (Au) film anodes yielding
lower work functions, operating voltages, electrical resistance values, and
increasing lifetime of OLEDs.33

Single carrier devices are typically used to study the kinetics and charge transport mechanisms
of an organic material and can be useful when trying to study energy transfer
processes. As current through the device is composed of only one type of charge
carrier, either electrons or holes, recombination does not occur and no light
is emitted. For example, electron only devices can be obtained by replacing ITO
with a lower work function metal which increases the energy barrier of hole
injection. Similarly, hole only devices can be made by using a cathode made
solely of aluminium, resulting in an energy barrier too large for efficient
electron injection.343536

 

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

 

Phosphorescent
OLEDs

Rather like OLEDs, PMOLEDs 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.

Electrons and holes are both fermions with half integer spin. An exciton is formed by the coulombic attraction between
the electron and the hole, and it may either be in a singlet state or a triplet state, depending on the spin states of
these two bound species. Statistically, there is a 25% probability of forming a
singlet state and 75% probability of forming a triplet state.23 Decay of the excitons
results in the production of light through spontaneous emission.

In OLEDs using fluorescent organic molecules only, the
decay of triplet excitons is quantum mechanically forbidden by selection rules, meaning that the lifetime of
triplet excitons is long and phosphorescence is not readily observed. Hence it
would be expected that in fluorescent OLEDs only the formation of singlet
excitons results in the emission of useful radiation, placing a theoretical
limit on the internal quantum efficiency (the
percentage of excitons formed that result in emission of a photon) of 25%.4

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 quantumefficiencies as high as
16.5%.15 – Platinum Binuclear Complexes as Phosphorescent Dopants for
Monochromatic and White Organic Light-Emitting Diodes

A
Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. (2010). “Recent Advances in
White Organic Light-Emitting Materials and Devices (WOLEDs)”. Advanced
Materials. 22 (5): 572–582.

 

B D’Andrade, B. W.; Forrest, S. R. (2004). “White Organic Light-Emitting Devices
for Solid-State Lighting”. Advanced Materials. 16 (18): 1585–1595.

 

C
Chang, Yi-Lu; Lu, Zheng-Hong (2013). “White Organic Light-Emitting Diodes
for Solid-State Lighting”. Journal of Display Technology. PP (99):
1.

 

D
Bernanose, A.; Comte, M.; Vouaux, P. (1953). “A new method of light
emission by certain organic compounds”. J. Chim. Phys. 50:
64.

 

E
Bernanose, A.; Vouaux, P. (1953). “Organic electroluminescence type of
emission”. J. Chim. Phys. 50: 261.

 

F Bernanose,
A. (1955). “The mechanism of organic electroluminescence”. J.
Chim. Phys. 52: 396.

 

G “PMOLED vs AMOLED – what’s the difference? | OLED-Info”. www.oled-info.com,  
Published: 31/07/2017, Retrieved: 05/01/2018.

 

H “The OLED Structure” https://newhavendisplay.com/pkc_oledtechnology.html,
Published 18/06/2017, Retrieved: 05/01/2018.

 

I Fukagawa, H; Shimizu, T; et a. (2015). “Highly
efficient and stable organic light-emitting diodes with a greatly reduced
amount of phosphorescent emitter”. Scientific
Reports. 5 (9855): 1-7.

 

J Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.;
Colaneri, N.; Heeger, A. J. (1992). “Flexible light-emitting diodes made
from soluble conducting polymers”. Nature. 357 (6378):
477–479. 

 

K “Samsung Flexible OLED Display
Foreshadows Rollable Tablets And Smartphones”. https://hothardware.com/news/samsung-flexible-oled-smartphone-displays-foreshadows-rollable-tablets-and-smartphones, Published: 26/05/2016, Retrieved: 05/01/2018

M “LG
Unveils Expanded OLED TV Lineup at CES 2015”. http://www.lg.com/ae/press-release/lg-unveils-expanded-oled-tv-lineup-at-ces-2015, Published 01/06/2015, Retrieved 05/01/2018

N “Comparison of OLED and LCD”.
Fraunhofer IAP: OLED Research. https://web.archive.org/web/20100413172135/http://oled-research.com/oleds/oleds-lcd.html
Published 24/07/2007, Retrieved 05/01/2018

 

O Tang, C. W.; Van Slyke, S. A. (1987).
“Organic electroluminescent diodes”. Applied Physics Letters. 51 (12):
913. 

 

P “Organic Light-Emitting Diodes Based on
graded Heterojunction Architecture Has Greater Quantum Efficiency”. https://web.archive.org/web/20131209061543/http://license.umn.edu:80/technologies/20100200_organic-light-emitting-diodes-based-on-graded-heterojunction-architecture-has-greater-quantum-efficiency,
Published: 09/12/2013, Retrieved: 06/01/2018

 

Q Baldo, M. A. et al. (1998). “Highly
efficient phosphorescent emission from organic electroluminescent devices”. Nature 395, 151–154

R Kallmann,
H.; Pope, M. (1960). “Positive Hole Injection into Organic
Crystals”. The Journal of
Chemical Physics. 32: 300. 

 

S Kallmann, H.;
Pope, M. (1960). “Bulk Conductivity in Organic Crystals” Nature. 186 (4718): 31–33. 

 

T Mark, P; Helfrich, W. (1962). “Space-Charge-Limited
Currents in Organic Crystals”. Journal of Applied Physics. 33: 205.

 

U Pope, M.; Kallmann, H. P.; Magnante, P. (1963).
“Electroluminescence in Organic Crystals”. The Journal of Chemical Physics. 38 (8): 2042. 

 

V Endo, A. et
al. (2009). “Thermally activated delayed fluorescence from Sn41–porphyrin
complexes and their application to organic light emitting diodes – a novel
mechanism for electroluminescence.” Adv.
Mater. 21, 1–5

 

W Endo, A. et al. (2011). “Efficient up-conversion of
triplet excitons into a singlet state and its application for organic light emitting
diodes.” Appl. Phys. Lett. 98, 083302

 

X Sano, Mizuka; Pope, Martin; Kallmann, Hartmut (1965).
“Electroluminescence and Band Gap in Anthracene”. The Journal of Chemical Physics. 43 (8): 2920. 

 

Y Partridge,
R (1983). “Electroluminescence from polyvinylcarbazole films: 3.
Electroluminescent devices”. Polymer. 24 (6): 748–754.

 

Z
Burroughes J. H. et. Al (1990). “Light-emitting diodes based on conjugated polymers”.
Nature. 347: 539-541

 

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