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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.1 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
2

 

 

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.3 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). 4

 

 

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.540 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%).6 An area of
research that is currently of major interest is the use of 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. 78

 

Fluorescence vs Phosphorescence

Fluorescence
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. 9

Phosphorescence
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 phosphorescence9

 

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. 101112

 

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 facilitates
larger display sizes at higher resolutions.13

                                                                                                                                 

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. 14

 

Fig.
3: The structure of an OLED14

 

 

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).15

Fig. 4: Monomer
of poly(p-phenylene
vinylene)

 

 

In 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 as 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.
5: A diagram depicting how OLEDs emit
light14

 

 

 

 

 

 

 

 

Anodes
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 16. 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
17.

 

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.18 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.19 This
architecture varies the configuration of electron/hole transport materials
within the emissive layer utilizing a dopant emitter.
This
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.20

 

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. 212223

 

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. 242526 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. 27

 

Fig. 6: Anthracene
structure                  Fig. 7: Tetracene
structure

 

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.28

 

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. 29

 

Fig. 8: N-vinylcarbazole
monomer

 

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.4

Phosphorescent
OLEDs

Rather like OLEDs, PHOLEDs produce light by virtue electroluminescence of
an organic semiconductor layer in the presence of an electric current
via the mechanisms previously discussed. The main in aspect in which PHOLEDs
differ from OLEDs is that they produce light from triplet and singlet states
which means they can potentially have an internal quantum efficiency of 100%31

This can be accomplished by using an organometallic complex to
dope a host molecule. The organometallic complexes contain a heavy metal at the
centre like platinum or iridium31. These organometallic complexes
experience spin-orbit interaction which allows intersystem crossing to occur as
depicted in Fig. 2. As a result of
this, phosphorescence is observed. Polymers like
as poly(N-vinylcarbazole) , depicted in Fig. 8, can be used as a host molecules.

 

White light emitting platinum(II) complexes used in PHOLED
design

Pt(II)
complexes have been utilized as phosphorescent emitters in small-molecule
OLEDs. Since M. A Baldo first reported the use PtOEP (2,3,7,8,12,13,17,18octaethyl-21H,23H-porphine
platinum(II)) as a red emissive dopant in PHOLED design,  other Pt(II) containg organometallic complexes
have been utilized to prepare OLEDs that emit red, green and white light
displaying quantum efficiencies of up to 16.5%.32

Fig. 9: Structure of PtOEP

It has
been suggested that platinum based phosphorescent complexes have the highest
ceiling in regards to efficiency because of their triplet quantum yield, short
state lifetime and variable emission colour 33. There are 3 main
categories of chelating ligands used in Pt(II) organometallic complexes:
tetradentate ligands, tridentate ligands and bidentate ligands. Each of these
categories offer interesting variations in the functioning of the respective
complexes as WOLEDs (White Organic Light-Emitting Diode).

A
promising tridentate complex which showed promise was synthesised by Rausch. A.
F., et. al. in 2011. A WOLED utilizing 8 wt% PtL11Cl doped in
1,3-bis(N-Carbazoyloyl)benzene emitted nearly purely white light at the CIE
coordinates of 0.33,0.35 at 1300 cd m-2 which extremely close to the
CIE coordinates of pure light white33.

 

Fig 10: PtL11Cl structure33

There are
many strategies for fabricating WOLEDs. One of the most efficient and popular
ways of accomplishing this is by combing the emission of multiple emissive
compounds within one layer to produce white light.

It is
possible to fabricate WOLEDs that exploit excimer emission in the red/orange
region as result of interaction along molecules of blue-emitting phosphors by
using a blue-orange complementary colour strategy. In 2001 Adachi. C., et. al.
synthesized a system that co-dopes Ptppy-2 and FIrpic in a single layer. This
enables the orange excimer emission of Ptppy-2 and the blue monomer emission of
FIrpic to combine to produce white light with a single EML (Emissive layer).34

­Fig. 11:
Structure of Ptppy-8               Fig. 12: Structure of FIrpic

Because
this strategy employs more than one emissive material, there are a few
associated drawbacks. Chief among which being colour-aging effects due to the different
rates of degradation of the individual materials over time as well as
voltage-dependent emission.33

The
performance of this system could be improved upon by selecting more suitable
host materials, optimisation functional layer’s energy levels and by adding an
electron blocking layer (EBL).34

WOLED
device architectures can potentially be simplified by employing the excimer
properties of square-planar complexes.36

In 2013,
Fleetham. T.; et. al. produced a WOLED that used Pt(II) bis(methyl-imidazolyl)benzene
chloride (Pt-16). This WOLED managed to achieve CIE colour coordinates of
(0.33,0.33), an improvement of the maximum EQE at 20.1% and a CRI of 8037.

Fig. 12: Structure of Pt-16 and it’s respective
emission profile36

Despite
the promising nature of Pt-16, PLQY (photoluminescent quantum yield)
measurements have shown that it is inefficient and a compromise of efficiency
vs colour quality must be considered. This leads to the need for development of
square-planar Pt(II) which can exhibit high levels of efficiency in both
monomer and excimer emission.38

In 2014, Fleetham. T.; et. al. produced 2 deep-blue
platinum emitters (PtOO7 and PtON7) that could potentially be used in a
combination of emissive materials to create a WOLED. These complexes are
tetradentate, halogen-free cycolmetaling ligands and are based on
phenyl-carbene ligands. 35

Fig. 13
& 14: Structures
of PtOO7 and Pt7O7 and their corresponding chart depicting their
                       electrophosphorescent profiles 35

 

 

 

PtON7
exhibits a peak EQE of 23.7% and CIE coordinates of (0.15,0.14) which is
superior to other platinum and iridium analogues.

Fig. 15: Graph depicting the EQE of Pt707

Unfortunately,
these complexes do not show excimer emission at higher concentrations and are
not suitable candidates for use in white single-dopent systems. This research
does however provide an avenue of exploration as they show that square-planar
Pt(II) complexes  can be used in
blue-emitting devices and sufficiently stable and efficient for eventual use in
excimer white PHOLEDs. 39  

 

Conclusion

OLED
technology is an ever-expanding field with research efforts being directed in many
different areas. One of potent areas is the development of
electrophosphorescent Pt(II) complexes as they offer a host of significant
advantages. However, there are still pitfalls that need to be navigated in
order for PHOLED devices to be viable option for use in lighting and displays.
Significant advancements have been made in the past 20 or so years and if the
rate of development continues as it is, PHOLED devices may be commercially
available in the near future.

 

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