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공학박사 학위논문
Fluorescent Organic Light-Emitting
Diodes using Exciplex Host and
Phosphorescent Sensitizer
엑시플렉스 호스트와 인광 감광제를 이용한
형광유기발광소자
2019년 8월
서울대학교 대학원
재료공학 전공
김 현 구
i
Abstract
Fluorescent Organic Light-Emitting
Diodes Utilizing Phosphorescent
Sensitizer and Exciplex Host
Hyun-Gu Kim
Department of Materials Science and Engineering
The Graduate School
Seoul National University
While the display industry is expanding with the development of various
electronic devices, OLEDs are replacing conventional displays such as liquid
crystal displays due to their various advantages. In addition, OLEDs have been
applied in the field of solid-state lighting. However, in order for OLEDs to be
widely commercialized in the future technology, both efficiency and device
stability problems must be solved.
The external quantum efficiency (EQE), which is an index of efficiency
in OLEDs, is composed of out-coupling efficiency and internal quantum
efficiency (IQE). Many researchers working on OLEDs and organic
ii
semiconductors have increased the out-coupling efficiency through the
development of light extraction structures, emitters with horizontal dipole
orientation, host and charge transporting materials with low refractive indices.
In order to increase the IQE of OLEDs, the development of emitters with high
photoluminescence quantum yield, bipolar host materials, and device structure
with a charge balance such as an exciplex-based device has been reported.
On the other hand, it is also important to utilize 75% of triplet excited
states as light-emitting process to increase the IQE of OLEDs. Many
researchers have developed phosphorescence and thermally activated delayed
fluorescence (TADF) materials which utilize the spin-mixing process between
singlet and triplet excited states to improve the efficiency of OLEDs. However,
the device stability of the OLEDs has been a problem due to slow decay rate
caused by spin-mixing process in the emitters. In particular, fluorescent OLEDs
are mainly commercialized even though they exhibit low EQE due to a problem
of operational lifetime in the blue region.
In order to solve the problem, researches on harvesting triplet excited
states while using conventional fluorescent emitters have been performed.
However, the conventional fluorescent dye-based OLEDs showed lower
efficiency than those of phosphorescent and TADF OLEDs due to the
inefficient triplet harvesting. Therefore, it is necessary to develop a novel
system which can solve both the efficiency and the operational lifetime problem
of the OLEDs.
iii
This thesis deals with the research topic of efficient triplet harvesting by
stable conventional fluorescent emitters to fabricate fluorescent OLEDs with
high efficiency and long operational lifetime.
Chapter 2 contains a system which can utilize both the sensitizing effect
and the induced heavy atom effect to efficiently harness triplet excited states by
conventional fluorescent dyes. We used an exciplex host and a phosphorescent
dopant to increase the singlet fraction of the fluorescent emitter by using spin-
mixing process in the phosphorescent dopant and increased reverse intersystem
crossing caused by induced heavy atom effect. Utilizing a newly designed
system, we realized a fluorescent OLEDs with EQE of 23.7%, which was the
world's highest efficiency based on the conventional fluorescent dye-based red
OLEDs. Using optical simulation and experimental results, we have found that
a fluorescent OLEDs with a maximum external quantum efficiency close to 30%
can be obtained when all the singlet excitons on the fluorescent emitter emit
light.
Based on the idea, Chapter 3 shows the results for highly efficient
fluorescent OLEDs with long operational lifetime using an exciplex host and a
phosphorescent sensitizer. By using a conventional fluorescent emitter with
high photoluminescence quantum yield, fluorescent OLEDs with EQE more
than 25.0% were reported. The energy transfer process reduced the decay
lifetime of the exciplex and the phosphorescent material, which exhibit long
decay process by spin-mixing process. As a result, the fluorescent OLEDs
iv
showed operational lifetime (LT50) over 99 hrs at 1000 cd m-2, which are longer
than those of phosphorescent OLEDs and fluorescent OLEDs with exciplex
host.
Based on the results of Chapter 2 and 3, we tried to extend the
combination of exciplex host and phosphorescent sensitizer to the blue
fluorescent OLEDs to improve the efficiency and device stability of the blue
OLEDs, which are key issues in the OLEDs. However, the sensitization by the
phosphorescent dopant in the blue region has been regarded as a difficult area
due to the necessity for the development of ultraviolet-emitting material for
efficient energy transfer. Chapter 4 contains the results for a blue fluorescent
OLEDs using a phosphorescent sensitizer. According to the photophysical
properties, we found that spin-mixing in a deep-blue phosphorescent sensitizer
followed by energy transfer process to blue fluorescent emitter can occur even
though the phosphorescent sensitizer exhibits small energy difference from the
blue fluorescent emitter. As a result, a blue fluorescent OLED with a maximum
EQE of 15.3% was developed through triplet harvesting using a phosphorescent
sensitizer.
Keywords: Fluorescent organic light-emitting diodes, exciplex,
phosphorescent sensitizer, conventional fluorescent emitter, triplet harvesting,
energy transfer, operational lifetime
Student Number: 2013-23037
v
Contents
Abstract .................................................................................. i
Contents ................................................................................. v
List of Tables ....................................................................... vii
List of Figures .................................................................... viii
Introduction ................................................... 1
1.1 Brief introduction to Organic Light-Emitting Diodes .................. 1
1.2 Determining factors in efficiency of OLEDs ................................ 4
1.3 Exciplex-based OLEDs ................................................................. 9
1.4 Triplet harvesting by a conventional fluorescent emitter in
OLEDs .............................................................................................. 11
1.5 Outline of the thesis .................................................................... 17
Harnessing Triplet Excited States by
Fluorescent Dopant Utilizing co-doped Phosphorescent
Dopant in Exciplex Host for Efficient Fluorescent
Organic Light-Emitting Diodes ........................................ 19
2.1 Introduction ................................................................................. 19
2.2 Experimental ............................................................................... 22
vi
2.3 Result and discussion .................................................................. 25
2.4 Conclusion .................................................................................. 42
Highly Efficient, Conventional Fluorescent
OLEDs with Extended Operational Lifetime .................. 43
3.1 Introduction ................................................................................. 43
3.2 Experimental ............................................................................... 45
3.3 Result and discussion .................................................................. 48
3.4 Conclusion .................................................................................. 61
Triplet Harvesting by a Fluorescent Emitter
Using a Phosphorescent Sensitizer for Blue Organic
Light-Emitting Diodes........................................................ 62
4.1 Introduction ................................................................................. 62
4.2 Experimental ............................................................................... 65
4.3 Result and discussion .................................................................. 68
4.4 Conclusion .................................................................................. 84
Summary and Conclusion .......................... 85
Bibliography ....................................................................... 88
초 록 .................................................................................... 99
vii
CURRICULUM VITAE .................................................. 103
Awards ............................................................................... 104
List of Publications ........................................................... 105
List of Presentations ......................................................... 106
List of Patents ................................................................... 109
List of Tables
Table 2.1 The decay rate of Ir(ppy)2tmd (PDk ) and the energy transfer rate from
Ir(ppy)2tmd to DCJTB (/ET PD FDk
) in TCTA:B4PYMPM:Ir(ppy)2tmd films
with and without DCJTB................................................................................ 32
Table 2.2 Maximum EQE of total and DCJTB emission, and the radiative
( ,
,
S FD
r EL ) and total (,S FD
EL ) singlet fractions of the exciton on DCJTB in the
devices, according to Ir(ppy)2tmd concentration ........................................... 36
Table 3.1 Device performances of OLEDs .................................................... 55
Table 4.1 Device characteristics of the fluorescent OLEDs ........................... 80
Table 4.2 Bond dissociation energies of host materials ................................. 82
viii
List of Figures
Figure 1.1 Commercial products of OLEDs; (a) Curved OLED Panel in
Samsung Galaxy S10 (Samsung Electronics 2019), (b) LG OLED Falls (LG
Electronics 2019), (c) Flexible OLED lighting (LG Display 2016). (d) OLED
lighting in automobile (Mercedes-Benz 2017). ................................................ 3
Figure 1.2 Schematic diagram of excited-states processes; (a) Fluorescent
OLEDs, (b) Phosphorescence OLEDs (Organo-metallic compounds), and (c)
Thermally activated delayed fluorescence OLEDs. ......................................... 8
Figure 1.3 Schematic illustrations for excited states formation and energy
transfer processes in OLEDs based on conventional fluorescent emitter; (a)
Phosphorescent-sensitized OLEDs, (b) TADF-sensitized OLEDs
(hyperfluorescene OLEDs), and (c) Fluorescent OLEDs based on TADF or
exciplex hosts. ................................................................................................ 13
Figure 2.1 Schematic illustration of the emission mechanism of a fluorescent
emitter in an exciplex-forming co-host with an iridium (Ir) complex. ........... 26
Figure 2.2 (a) Molecular structures of the host materials, phosphorescent dopant
and fluorescent acceptor. (b) Photoluminescence (PL) spectra of
TCTA:B4PYMPM exciplex, Ir(ppy)2tmd, and DCJTB doped in
TCTA:B4PYMPM mixed film; and absorption spectra of DCJTB and
Ir(ppy)2tmd in methylene chloride. (c) PL spectra of DCJTB doped in
TCTA:B4PYMPM mixed films with and without Ir(ppy)2tmd ...................... 28
ix
Figure 2.3 (a–b) Transient PL of TCTA:B4PYMPM:0.5 wt% DCJTB film at
various Ir(ppy)2tmd concentrations; (a) integrated from 460 to 500 nm
(exciplex), and (b) integrated from 580 to 610 nm (DCJTB), and (c) transient
PL of TCTA:B4PYMPM:8 wt% Ir(ppy)2tmd with and without DCJTB
integrated from 510 to 534 nm (Ir(ppy)2tmd). ............................................... 30
Figure 2.4 (a) Transient PL of Ir(ppy)2tmd in TCTA:B4PYMPM:Ir(ppy)2tmd
films at various Ir(ppy)2tmd concentrations, (b–c) Transient PL of Ir(ppy)2tmd
in TCTA:B4PYMPM:Ir(ppy)2tmd with and without DCJTB; (b) 2 wt%, (c) 4
wt% ................................................................................................................ 31
Figure 2.5 (a) Current density-voltage-luminance (J-V-L) characteristics, (b)
electroluminescence (EL) spectra of OLEDs at 10 mA cm-2, (c-d) external
quantum efficiencies (EQEs) of OLEDs versus current density; (c) total
emission, and (d) DCJTB emission. ............................................................... 34
Figure 2.6 (a) J-V-L characteristics, (b) EL spectra of OLEDs at 10 mA cm-2, (c)
EQEs of OLEDs versus current density at different doping concentrations of
DCJTB............................................................................................................ 37
Figure 2.7 Decay and energy transfer processes in TCTA:B4PYMPM:DCJTB
system ............................................................................................................. 39
Figure 3.1 (a) Schematic illustration of the excited-state process. (b) Molecular
structure of the exciplex-forming host, phosphorescent materials, and
fluorescent emitter (c) Absorption spectrum of TBRb in methylene chloride
x
solution and the normalized PL spectra of the TCTA:B4PYMPM-mixed film,
Ir(ppy)3 and Ir(ppy)2tmd doped in the TCTA:B4PYMPM-mixed films, and the
TBRb solution. (d) PL spectra of 2 wt% TBRb in the TCTA:B4PYMPM-mixed
film without an Ir complex, with Ir(ppy)3, and with Ir(ppy)2tmd................... 49
Figure 3.2 (a) Transient PL of the TCTA:B4PYMPM mixed film, and the
TCTA:B4PYMPM:2 wt% TBRb film with and without an Ir complex at the
exciplex emission (integrated from 450 to 485 nm). and (b) Transient PL of the
TCTA:B4PYMPM:8 wt% Ir complex film with and without TBRb at the Ir
complex emission (integrated from 510 to 535 nm). ..................................... 51
Figure 3.3 Transient PL of the TCTA:B4PYMPM:2 wt% TBRb film with and
without an Ir complex at the TBRb emission (integrated from 540 to 710 nm).
........................................................................................................................ 52
Figure 3.4 (a) Schematic diagram of organic light-emitting diodes (OLEDs) and
the energy level of materials. (b) Current density–voltage–luminance (J–V–L)
characteristics. (c) Electroluminescence (EL) spectra at 1,000 cd m–2. (d)
External quantum efficiencies (EQEs) of OLEDs as a function of luminance.
........................................................................................................................ 54
Figure 3.5 Angle-dependent PL of 2 wt% TBRb in TCTA:B4PYMPM with and
without 8 wt% of the Ir complexes. ............................................................... 56
Figure 3.6 Device lifetimes of the OLEDs at an initial luminance (L0) of 1,000
cd m-2. ............................................................................................................. 59
xi
Figure 4.1 (a) Molecular structures of the host (mCBP, TSPO1), iridium (Ir)
complex sensitizer [(dfpysipy)2Ir(mpic)], and fluorescent emitter (TBPe) (b)
Absorption and PL spectra of (dfpysipy)2Ir(mpic) and TBPe in methylene
chloride with the PL spectra of mCBP and TSPO1 films. (c) PL spectra of films
in the mCBP:TSPO1 mixed host doped with 1 wt% TBPe (black square), 1 and
5 wt% (dfpysipy)2Ir(mpic) (red circle and blue triangle), and 1 and 5 wt%
(dfpysipy)2Ir(mpic) with 1 wt% TBPe (dark cyan down triangle and magenta
diamond). ....................................................................................................... 69
Figure 4.2 (a) Transient PL of three films in mCBP:TSPO1 mixed host doped
with 5 wt% (dfpysipy)2Ir(mpic) (red), and 5 wt% (dfpysipy)2Ir(mpic) + 1 wt%
TBPe (blue). The inset shows the transient PL of 1 wt% TBPe in mCBP:TSPO1
mixed film. (b) Time-resolved PL spectra of the three films. ........................ 71
Figure 4.3 Absorption spectra of mCBP and TSPO1 ..................................... 72
Figure 4.4 Transient PL of the mCBP:TSPO1:1 wt% TBPe film co-doped with
(dfpysipy)2Ir(mpic) integrated from 435 to 620 nm. ...................................... 73
Figure 4.5 (a) Schematic diagram of fluorescent organic light-emitting diodes
(OLEDs) (b) Current density–voltage–luminance (J–V–L) characteristics, (c)
Electroluminescence (EL) spectra at 1,000 cd m-2, and (d) External quantum
efficiencies of OLEDs as a function of luminance. ........................................ 76
Figure 4.6 (a) Current efficiency, and (b) Power efficiency of OLEDs according
to luminance. .................................................................................................. 77
xii
Figure 4.7 Angle-dependent PL intensity of mCBP:TSPO1:1 wt% TBPe film
measured at 463 nm and the fitting with the horizontal emitting dipole
orientation of 76%. ......................................................................................... 78
Figure 4.8 Device lifetime of OLEDs at an initial luminance of 400 cd m-2 . 81
1
Introduction
1.1 Brief introduction to Organic Light-Emitting Diodes
Organic light-emitting diodes (OLEDs) are devices composed of organic
thin films and electrodes to convert electricity into light. Contrary to liquid
crystal display (LCD), which were widely used as conventional displays,
OLEDs do not require a back light due to self-luminescent property. Therefore,
OLEDs have light weight and high contrast ratio compared with LCD. In
addition, OLEDs have several advantages over conventional displays such as
low power consumption, high color purity, and fast repetition rate. Another
advantage of OLEDs is that they can be fabricated on various substrates
including flexible or transparent substrates. Because of these advantages,
OLEDs have been considered as a next-generation display, and are widely used
as displays for various electronic devices. As a solid-state lighting, OLEDs
have several advantages over conventional lightings. OLEDs are eco-friendly
light source which have higher efficiency compared to incandescent lamp and
light bulbs. In addition, OLEDs have high color temperature and color
rendering index, and they can be manufactured as a large-area and flexible
surface light source. Therefore, OLEDs are expanding their applications not
only in displays but also in lighting. Figure 1.1 shows the several examples of
the commercial products using OLEDs such as a smart phone, TV with a large-
area, OLED lighting with warm white color, and back light of automobiles.
2
Since M. Pope et al. first observed electroluminescence (EL) in
anthracene single crystal,1 several EL from perylene-based material or
polymers were reported.2,3 Then in 1987, C. W. Tang and S. A. Van Slyke
reported representative organic electroluminescent device using multi-layered
organic thin films.4 They constructed the OLED with following structure;
indium tin oxide/hole transporting layer (HTL)/electron transporting layer
(ETL)/Mg:Ag.4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]
(TAPC) and tris(8-hydroxyquinolinato)aluminum (Alq3) were used as HTL and
ETL, respectively, and light emission takes place in Alq3 layer. The green-
emitting OLEDs showed external quantum efficiency (EQE) of about 1% and
brightness over 1,000 cd m-2. After 3 years, C. W. Tang et al. developed host-
dopant system in emitting layer to tune emission color and improve the EL
efficiency of OLEDs.5 They used Alq3 as host material, and small concentration
of Coumarin 540 and DCM were used as the fluorescent dopant. Emission from
the fluorescent dopants were observed due to energy transfer from Alq3 to the
fluorescent dopants, and the efficiencies of the OLEDs were improved about
2.5 times compared to that non-doped Alq3 layer.
The introduction of the multi-layer structure and the host-dopant system
has greatly contributed to the state-of-art OLEDs. Along with the development
of emitters, host and charge transporting materials, and device structures, the
efficiency and color purities of OLEDs have been improved to the level of
commercialization.
3
Figure 1.1 Commercial products of OLEDs; (a) Curved OLED Panel in
Samsung Galaxy S10 (Samsung Electronics 2019), (b) LG OLED Falls (LG
Electronics 2019), (c) Flexible OLED lighting (LG Display 2016). (d) OLED
lighting in automobile (Mercedes-Benz 2017).
4
1.2 Determining factors in efficiency of OLEDs
EQE is the representative parameters for the efficiency of OLEDs, which
is defined as the number of photons per injected electrons in OLEDs. The EQE
can be expressed as Equation (1.1),6
int / ( , ) ( , )EQE out S T eff PL outq (1.1)
where int is the internal quantum efficiency (IQE), and
out is out-
coupling efficiency of OLEDs. out depends on the horizontal ratio of
emitting dipole ( ) and the geometric factor ( ) which is related to the
position of recombination zone in EML and the device structure, and this can
be enhanced if the emitters have horizontal dipole orientation.7–9 There has been
a report that maximum EQE of green OLEDs can increase around 26% to 46%
without any light extraction structure when the value of emitter which has
photoluminescence (PL) quantum yield (PLQY) of 1 (PL =1) changes from
0.67 (isotropic) to 1 (horizontally oriented).6 In addition, OLEDs with
maximum EQE over 40% can be realized in red and blue colors if the emitter
has PL =1 and =1.10,11 Based on these ideas, many researchers have
successfully increased the out of OLEDs by designing emitters with high
values. Meanwhile, the use of host or transporting materials with low refractive
indices can further increase the out due to the reduced substrate and wave-
guided modes.12,13 If the organic materials constituting OLEDs have a low
refractive index of 1.5, OLEDs with EQE over 60% can be achieved.14
5
Therefore, several efforts have been performed to increase the out through
the development of organic materials with low refractive indices or design of
device structure considering the refractive indices of materials.14,15
int can be expressed as the products of effective quantum yield
( ( , )eff PLq ), charge-balance factor ( ), and singlet-triplet ratio (/S T ).
( , )eff PLq expresses the radiative quantum efficiency of emitter in a cavity
structure, and it depends on PL and . is unity when the same number of
injected electrons and holes are recombined in OLEDs. Many researchers have
adopted charge injection layer,16 p- or n-doped charge transporting layer,17,18 or
charge blocking layer19 in OLEDs to confine injected electrons and holes in
EML and control injection and transport properties. In addition, mixed-host and
bipolar host materials have been developed for balanced charge transport in
EML.20,21 Recently, OLEDs based on exciplex co-host have been reported for
well-matched charge balance, which will discussed in Chapter 1.3.
/S T has been also considered as the important factor influencing int
of OLEDs. According to the spin-statistics, injected electrons and holes in
OLEDs form singlet and triplet excited states with a ratio of 1:3. The first-
generation OLEDs including representative devices mentioned in Chapter 1.1
were based on fluorescent emitters. In case of the conventional fluorescent
molecules, only the singlet excited states can participate in light-emission and
the triplet excited states decay nonradiatively. Therefore, maximum int of
6
conventional fluorescent OLEDs was limited to 25%. (Figure 1. 2a) For this
reason, many researchers have focused on the methods to utilize 75% of triplet
excited states in light-emitting process.
M. Baldo et al., reported phosphorescence emission at room temperature
in organo-metallic compounds.22 Large spin-orbit coupling in the organo-
metallic compounds due to heavy atom leads to intersystem crossing (ISC)
from singlet to triplet excited states and phosphorescence emission from the
lowest triplet excited states to ground states. Therefore, both singlet and triplet
excitons in the heavy metal complex can participate the radiative transition,
resulting in int of 100%. (Figure 1.2b) The OLEDs based on
phosphorescence with heavy metal complex are called phosphorescent OLEDs,
and this is considered as second-generation OLEDs. The development of the
phosphorescent emitter has contributed to the commercialization of OLEDs.
H. Uoyoma et al., developed thermally activated delayed fluorescence
(TADF) materials to harvest triplet excited states without heavy metal.23 Due to
the small orbital overlap between highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO), TADF molecules show
small energy difference between singlet and triplet excited states (∆EST).
Therefore, efficient reverse intersystem crossing (RISC) to singlet excited
states takes place and theoretical int of 100% can be possible by using the
TADF materials. The fluorescent OLEDs based on TADF emitters are called as
third-generation OLEDs. (Figure 1.2c)
7
The development of organo-metallic compounds and TADF materials
has enabled the realization of highly efficient phosphorescent and fluorescent
OLEDs by using spin-mixing process between singlet and triplet excited states.
However, the operational stability of these OLEDs has been issued due to the
slow decay process in the emitters caused by spin-mixing process. Especially
in blue region, recent commercial products are still relied on fluorescent
OLEDs despite their low efficiencies.
8
Figure 1.2 Schematic diagram of excited-states processes; (a) Fluorescent
OLEDs, (b) Phosphorescence OLEDs (Organo-metallic compounds), and (c)
Thermally activated delayed fluorescence OLEDs.
9
1.3 Exciplex-based OLEDs
Exciplex is generated by the partial charge transfer (CT) between excited
states of M (M*) and ground state of N, where M and N are different molecules.
When M* and N are close, the complex between M* and N (M*N) is stabilized
with decreased energy. Contrary to excited states, the ground states complex
between M and N (MN) is unstable with increased energy since the interaction
between two molecules is not substantial. Following Franck-Condon principle,
exciplex emission occurs from the minimum of excited state complex to the
repulsive point of ground states which has indefinite vibrational states.
Therefore, the exciplex shows red-shifted and featureless emission specrum.24
Like TADF material, HOMO and LUMO of exciplex are distributed separately
in donor and acceptor molecules, respectively. Therefore, efficient RISC
process due to small ∆EST takes place also in exciplex.
In the past, the formation of exciplex was considered as the detrimental
factor in the efficiency of OLEDs.25,26 However, various fluorescent and
phosphorescent OLEDs based on exciplex have been reported in recent year
due to the advantage of using exciplex.10,14,27–33 If hole transporting and electron
transporting material are used as the constituents of exciplex-forming co-host
in OLEDs, negligible charge injection barrier from the transporting layer to
EML can be achieved. In addition, bimolecular recombination in the exciplex
host followed by energy transfer process to dopant can lower trapped charge
density in the dopant, thereby preventing trap-induced efficiency reduction
10
such as exciton-polaron quenching.29,34 Furthermore, bipolar characteristics of
the exciplex host can balance the number of electrons and holes in EML with
broad emission zone. These advantages facilitate OLEDs with low driving
voltages and efficiency roll-off at high current density. In addition, the
experimental EQE of recently reported fluorescent and phosphorescent OLEDs
based on exciplex co-host are well matched with the theoretical EQE of OLEDs,
indicating negligible electrical loss in the devices.6,10,29,30,35–38 These results
indicate that the use of exciplex in OLEDs seems to be a promising approach
for realizing efficient OLEDs.
11
1.4 Triplet harvesting by a conventional fluorescent emitter in
OLEDs
Triplet harvesting by conventional fluorescent emitter can be a candidate
to achieve efficient OLEDs with long operational lifetime due to short exciton
lifetime and high stability of the conventional fluorescent emitter. Two methods
have been reported to harvest triplet excited states by conventional fluorescent
emitters in OLEDs.
One method is the use of spin-mixing mediator, called sensitizer. Many
researchers have focused on the fluorescent OLEDs using organo-metallic
compounds, TADF materials, and intermolecular CT complex (exciplex) as
sensitizer. The excited states formed in the sensitizer are transferred to the
fluorescent dopant after spin-mixing process. Since Fӧrster energy transfer
from the sensitizer to singlet excitons of fluorescent emitter can take place
longer range than Dexter energy transfer to triplet excitons of fluorescent
emitter,39,40 the singlet fraction of the fluorescent emitter can be increased.
M. Baldo et al. firstly reported the red fluorescent OLED by using
organo-metalic compounds as sensitizer.41 Heavy metal in the organo-metalic
compounds induces large spin-orbit interaction, resulting in spin-mixing
process. In addition, Fӧrster energy transfer from triplet to singlet fluorescent
emitter can occur via dipole-dipole coupling. (Figure 1.3a) Based on two
processes, fluorescent OLEDs with int over 25% have been reported.
12
In 2014, H. Nakanotani et al. reported fluorescent OLEDs based on
TADF sensitizer, called hyperfluorescence.42 Triplet excitons in the TADF
sensitizers can be up-converted to singlet excitons, followed by the Fӧrster
energy transfer to singlet of the fluorescent emitter. (Figure 1.3b) Since the
TADF materials show broad and featureless emission spectrum because of CT
characteristics, the TADF-sensitized fluorescent OLEDs can also have higher
color purity than that of TADF OLEDs.
The use of TADF material or exciplex as a host for fluorescent OLEDs
was also reported.43,44 (Figure 1.3c) In addition to the advantages of
hyperfluorescence OLEDs, bipolar characteristics of host materials can
facilitate fluorescent OLEDs with good charge balance in EML.
13
Figure 1.3 Schematic illustrations for excited states formation and energy
transfer processes in OLEDs based on conventional fluorescent emitter; (a)
Phosphorescent-sensitized OLEDs, (b) TADF-sensitized OLEDs
(hyperfluorescene OLEDs), and (c) Fluorescent OLEDs based on TADF or
exciplex hosts.
14
Three conditions have been required to efficiently harvest triplet excited
states by sensitization. First, excited states should be formed mainly on the
sensitizers rather than the fluorescent emitter to utilize spin-mixing process. For
hyperfluorescence OLEDs, high doping concentration of TADF sensitizer over
15% has been used for direct exciton formation in the TADF materials.42 The
use of exciplex co-host has been considered as a good candidate since Langevin
recombination is dominant in the OLEDs based on exciplex co-host.34,44 To
minimize direct exciton formation on the fluorescent emitter, many researchers
have used fluorescent emitter with small doping concentration around 1%.42,45–
47 In addition to low doping concentration of the fluorescent emitter, various
approaches such as interfacial exciplex,48,49 alignment of HOMO and LUMO
level between the sensitizers and the fluorescent emitters have been
suggested.45 Second, Dexter energy transfer from the triplet of sensitizer to the
triplet of fluorescent emitter should be prevented. Dexter energy transfer is the
electron exchange process with spin conservation between donor (excited state)
and acceptor (ground state), and the transfer rate ( D
ETk ) can be expressed as
Equation (1.2),
e x p ( 2 / )D
ETk KJ R L (1.2)
where K is related to the specific orbital interaction, J is the spectral
overlap integral between donor and acceptor, R and L are the distance between
donor and acceptor and van der Waals radii, respectively.40 According to
Equation (1.2), low doping concentration of the fluorescent emitter can
15
facilitate not only minimizing charge trapping in the emitter but also reducing
Dexter energy transfer by increasing R value. Besides the low doping
concentration, various approaches have been suggested to reduce triplet Dexter
energy transfer in fluorescent OLEDs; (1) Separated electron-hole pairs with
interlayer with high triplet energy,47 (2) Multi-EML consisting of an alternating
sensitizer and fluorescent emitting layers for spatial separation between the
sensitizer and the fluorescent emitter,41,50 (3) Design of fluorescent emitter with
electronically inert terminal substituent to increase R and reduce J between
active core of the fluorescent emitter with adjacent molecule,46,51 (4) TADF
materials with high RISC rate to lower triplet Dexter energy transfer
efficiency.43,52 Third, efficient Fӧrster energy transfer from the singlet of
sensitizer to the singlet of fluorescent emitter is required. Fӧrster energy
transfer occurs by dipole-dipole interaction between donor and acceptor, and
the Fӧrster transfer rate is proportional to PLQY of donor and spectral overlap,
and inversely proportional to R6.39 Therefore, large PLQY of sensitizer is
required for efficient Fӧrster energy transfer to the fluorescent dopant. In
addition, since a small concentration of fluorescent dopant is used to prevent
Dexter energy transfer, large spectral overlap between the PL spectra of the
sensitizers and the absorption spectra of the fluorescent emitters is also required
to satisfy both efficient Fӧrster energy transfer with reduced Dexter energy
transfer. Especially for blue fluorescent OLEDs, many researchers have
considered that small Stokes shift of fluorescent emitter can be advantageous
for efficient Fӧrster energy transfer from the sensitizer to the fluorescent
16
emitter.53,54
The other method for harvesting triplet excitons in conventional
fluorescent emitter is the use of induced heavy atom effect for the spin
conversion from triplet to singlet in the CT state of host molecules before
forming an exciton. In 2007, M. Segal et al. reported the increased fraction of
singlet exciton due to enhanced spin-mixing process in CT states caused by
heavy metal in organo-metallic compounds.55 The increased singlet excited
states in host molecules can be transferred to the singlet of fluorescent emitter,
resulting in efficient fluorescent OLEDs. Not only in the fluorescent OLED
with conventional fluorescent emitter, the induced heavy atom effect have been
also successful in efficient triplet harvesting in TADF OLEDs.56–58
Based on the two methods, various red, green, and blue fluorescent
OLEDs based on conventional fluorescent emitter have been reported.
However, the efficiency of the fluorescent OLEDs are not comparable to those
of phosphorescent or TADF OLEDs. Novel strategy to harvest triplet excited
states should be developed to improve the efficiency of fluorescent OLEDs
comparable to that of phosphorescent or TADF OLEDs. Especially,
phosphorescent-sensitized blue fluorescent OLEDs have not been reported
because of the difficulty in the development of efficient deep-blue or
ultraviolet-emitting organo-metallic compounds for efficient Fӧrster energy
transfer.
17
1.5 Outline of the thesis
In this thesis, we describe the fluorescent OLEDs based on exciplex
cohost and phosphorescent sensitizer. In addition, we report the phosphorescent
dye-sensitized blue fluorescent OLEDs. Besides the device characteristics of
the fluorescent OLEDs, we investigated the excited state dynamics in the
OLEDs.
The use of both sensitization and induced heavy atom effect would be
valuable for efficient triplet harvesting by a conventional fluorescent emitter.
Therefore, we developed fluorescent OLEDs based on exciplex cohost and
phosphorescent sensitizer. As a result, the red fluorescent OLED with
maximum EQE of 23.7% was achieved. The fraction of singlet excitons on
fluorescent emitter was increased to 65.9%, and we predicted that the maximum
EQE of fluorescent OLEDs would be expected to nearly 30% if all the singlet
excited states in the fluorescent emitter decays radiatively (Chapter 2).
Based on the combination of sensitization and induced heavy atom effect
reported in Chapter 2, we successfully developed efficient fluorescent OLEDs
with improved device stability. The yellow fluorescent OLEDs using exciplex
cohost and phosphorescent sensitizer exhibited EQE exceeding 25.0% by using
conventional fluorescent emitter with high photoluminescence quantum yield.
In addition, the fluorescent OLEDs showed operational lifetimes of 99.4 and
312.7 hrs, which are longer than those of phosphorescent OLEDs and
fluorescent OLED with exciplex host. We confirmed that energy transfer
18
processes reduce the lifetime of the exciplex and triplet excitons in
phosphorescent material, resulting in increased device stability of the OLEDs
(Chapter 3).
According to the Chapter 2 and 3, we confirmed that the use of exciplex
cohost and phosphorescent sensitizer in fluorescent OLEDs facilitated high
efficiency and long operational lifetime. Since the development of efficient blue
OLEDs with high stability has been a challenging issue, it would be valuable if
the combination of exciplex cohost and phosphorescent sensitizer can be
applied to blue OLEDs. However, the application of phosphorescent sensitizer
to blue fluorescent OLEDs has been considered to be difficult because of the
requirement for ultraviolet-emitting phosphorescent dye for efficient energy
transfer to the blue-emitting fluorescent emitter. Contrary to common belief, we
confirmed that triplet harvesting by blue fluorescent emitter via sensitization is
possible with deep-blue-emitting phosphorescent material which has small
energy difference from the fluorescent emitter. With the development of deep-
blue phosphorescent material, we successfully developed a blue fluorescent
OLED with EQE of 15.3% using phosphorescent sensitization (Chapter 4).
19
Harnessing Triplet Excited States by
Fluorescent Dopant Utilizing co-doped
Phosphorescent Dopant in Exciplex Host for
Efficient Fluorescent Organic Light-Emitting
Diodes
2.1 Introduction
Since Tang and VanSlyke reported electroluminescence (EL) using
fluorescent molecules,4 harnessing triplet excited states resulting from
electrical pumping has been an important research topic in organic light-
emitting diodes (OLEDs). One of the most successful approaches is to use
heavy metal complexes as emitters, known as phosphorescent OLEDs.29,59–61
More recently, delayed fluorescence from intra- and intermolecular excited
state charge transfer complexes, known as thermally activated delayed
fluorescence (TADF), has also been demonstrated as an effective way to
convert triplet to singlet utilizing the small energy difference between singlet
and triplet excited states in the charge transfer states.23,27,28,30,33,62–66 Both
approaches are based on the idea of spin mixing of singlet and triplet excited
states, expressed by Equation (2.1)24
so
ST
H
E
(2.1)
20
where λ is the first-order mixing coefficient between singlet and triplet states,
soH is the spin-orbit interaction and STE is the singlet-triplet energy gap.
The large soH of heavy metal complexes in phosphorescent OLEDs, and the
small STE in TADF OLEDs, are utilized to increase spin mixing. Both
approaches have been very successful, demonstrating 100% internal quantum
efficiencies in recent years.29,30,33,60–62,64–67 Because of these mixing processes,
however, decay processes in phosphorescence and TADF are much slower than
in fluorescence without spin mixing, invoking instability and efficiency roll-off
at high current density.
One way of overcoming the drawbacks of the previous approaches is to
use fluorophores as the final emitter, by separating the spin mixing and light
emitting processes. Either phosphorescent or TADF molecules are used for the
mixing process; these are known as sensitizers and are doped with fluorescent
dyes in a host. Energy transfer from the sensitizers to the fluorescent dopant is
utilized to connect the two processes. Longer range Fӧrster energy transfer from
the triplet (phosphorescent dyes) or singlet excitons (TADF molecules) of
sensitizers to singlet excitons of fluorophores than the Dexter energy from
triplet to triplet increases the singlet portion of the fluorophores higher than the
statistical ratio of singlet excitons to triplet excitons. Phosphorescent dyes used
as the sensitizer41,68,69 have yielded an external quantum efficiency (EQE) of
9%.68 Recently, TADF materials were used as the sensitizer for spin reversal
from triplet to singlet,42–45,70 to demonstrate a fluorescent OLED with an EQE
21
of 18%.42 Either excitons directly formed on sensitizers or energy transferred
from host excitons can be utilized in both cases.
Another method for harvesting triplet excitons using fluorescent dopants
is to increase the spin reversal of triplet to singlet in the charge transfer state in
the host, before forming an exciton via the induced heavy metal effect of the
phosphor to increase the singlet to triplet ratio (/s t ), followed by energy
transfer to fluorescent dopant molecules. An EQE of 3.4% has been realized
using this approach.55
It would be useful to find a method for combining the sensitizing effect
and the induced heavy atom effect to realize fluorescent OLEDs with even
higher efficiency than reported thus far. An exciplex host seems to be an ideal
system for utilizing the induced heavy atom effect, based on Equation (2.1),
because an exciplex has intermolecular charge transfer characteristics with
small STE . It has also been reported that bimolecular recombination
dominates over trap-assisted recombination in phosphorescent OLEDs based
on an exciplex host.34
Here, we demonstrate an efficient fluorescent OLED using a
phosphorescent dye as a sensitizer in an exciplex co-host system co-doped with
a fluorescent emitter. A fluorescent OLED with a maximum EQE of 23.7% is
realized, which is the highest value reported thus far for a conventional
fluorescent red dye-based OLED.
22
2.2 Experimental
Fabrication of organic film samples: Before deposition of organic film,
fused silica substrates were cleaned by using piranha solution. The piranha
solution was prepared by mixing sulfuric acid (H2SO4) and hydrogen peroxide
(H2O2) with a ratio of 3:1. The cleaning process were carried out by dipping the
fused silica substrates in the piranha solution about 2 hrs then rinsing with
deionized water. The organic layers were deposited on the fused silica by using
thermal vacuum evaporation under a pressure lower than 5 × 10-7 Torr. Total
deposition rate for molecules was set to 1 Å s-1, and host molecules were
deposited to have a molar ratio of 1:1.
Fabrication of OLEDs: 100 nm-thick indium tin oxide (ITO) coated
glass substrates were rinsed with deionized water. After rinsing, acetone
dipping and isopropylalcohol dipping were carried out (1 min for each process),
followed by isopropylalcohol boiling for 5 min. Then, UV-ozone cleaning of
the substrates were performed for 10 min, followed by vacuum deposition
process. All organic and metal layers were also deposited under a pressure
lower than 5 × 10-7 Torr. The deposition rate of organic materials and metal
electrode were 1 and 4 Å s-1, respectively.
Photoluminescence (PL) spectra, transient PL, and PL quantum
yields (PLQYs) of organic films: PL spectra of organic films were measured
using a continuous wave He-Cd laser (325 nm) as the excitation source and a
fiber optic spectrometer (Ocean Optics Maya 2000) as the optical detection
23
system. The transient PL was measured using a nitrogen laser (337 nm, Usho
Optical Systems Co., Ltd.) as the excitation light source and a streak camera
(Hamamatsu Photonics) as an optical detection system. The PL efficiency of
the emitter was measured using an integrating sphere 6 inches in diameter
(Labsphere Co.). The continuous wave He-Cd laser (325 nm) was used as the
excitation source. A monochromator (Acton Research Co.) attached to a
photomultiplier tube (Hamamatsu Photonics K.K.) was used as the optical
detector system.
Measurement of device characteristics: Current density, luminance,
and EL spectra of devices were measured using a programmable source meter
(Keithley 2400) and a spectrometer (Spectrascan PR650; Photo Research). The
characteristics were measured with 0.3 V intervals. The angle-dependent
emission patterns were measured using the programmable source meter
(Keithley 2400), a rotating stage and a fiber optic spectrometer (Ocean Optics
S2000). The patterns were determined by measuring the EL intensity from 0°
(normal direction) to 90° with 5° intervals. According to the patterns,
Lambertian correction factors of devices were calculated as Equation 2.2.14
24
0
/2 760 2
0 380 0
Lambertian correction factor
# of photons emitted from a device / unit area
# of photons in Lambertian surface with intensity of at normal direction / unit area
( , )sin
/
P
Pd d d
hc
/2 760 2
0
0 380 0
76018
0 380
760180
0 380
( )cos sin
/
( , )2 sin
/
( )2 cos sin
/
( , ) 36 36
ii
i
i i
i
i
Pd d d
hc
Pd
hc
Pd
hc
i
(2.2)
where θ, φ, and λ are the emitting angle, azimuthal angle, and wavelength (nm),
respectively. P(λ, θ) and P0(λ) are spectral intensity (W nm-1 sr-1 m-2) at θ and
the normal direction. h and c are Planck constant and the speed of light.
The EQE of the OLEDs were calculated from current density-voltage-
luminance (J-V-L) characteristics, EL spectra, and the Lambertian correction
factor, and this can be expressed as following equation.14
760
0
380
9
( )
(%) Lambertian correction factor 1010
P dq
EQEhc J
(2.3)
where q and J are the electric charge and current density (mA cm-2), respectively.
25
2.3 Result and discussion
The basic concept of harnessing the triplet exciplex in an exciplex host
co-doped with phosphorescent and fluorescent dyes is explained schematically
in Figure 2.1. Solid arrows represent the excited state processes without the
phosphorescent material, and the additional processes taking place in the
presence of the phosphorescent material are added as dashed lines. Intersystem
crossing (ISC) and reverse intersystem crossing (RISC) processes are also
included in the exciplex and phosphorescent material. In a system without a
phosphorescent dopant, the triplet exciplex can be utilized by RISC and the
conversion efficiency (RISC ) is described by the following equation.
,
/
RISCRISC T ex D
nr RISC ET E FD
k
k k k
(2.4)
where RISCk and ,T ex
nrk are the RISC and non-radiative transition rate of the
triplet exciplex, respectively, and /
D
ET E FDk is the Dexter energy transfer rate
from the triplet exciplex to the fluorescent dye. In a system with a
phosphorescent dopant, however, not only can the triplet exciplex be converted
to singlet exciplex, but it can also be transferred to the triplet phosphorescent
dopant, with an efficiency (Con ) given by the following equation
/
,
/ /
D
RISC ET E PDCon T ex D D
nr RISC ET E FD ET E PD
k k
k k k k
(2.5)
26
Figure 2.1 Schematic illustration of the emission mechanism of a fluorescent
emitter in an exciplex-forming co-host with an iridium (Ir) complex.
27
where /
D
ET E PDk is the Dexter energy transfer rate from the triplet exciplex to
the phosphorescent material. This equation clearly indicates that the RISCk and
/
D
ET E PDk values of the triplet exciplex result in a decrease in triplet fluorescent
emitter transferred from the triplet exciplex. Therefore, the singlet portion of
the excitons in the fluorescent dopant will increase as a result. In addition, spin
mixing between the triplet and singlet exciplex states, represented by RISCk ,
may be enhanced by the presence of the phosphorescent material due to the
induced heavy atom effect.56,71
To prove this concept, a 1:1 mixture of 4,4',4''-tris(N-carbazolyl)-
triphenylamine (TCTA) and 4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-
methylpyrimidine (B4PYMPM) was used as an exciplex-forming co-host, with
bis(2-phenylpyridine)iridium(III) (2,2,6,6-tetramethylheptane-3,5-diketonate)
[Ir(ppy)2tmd] used as a phosphorescent dopant and 4-(dicyanomethylene)-2-
tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) used as a
fluorescent acceptor. The chemical structures of the materials are shown in
Figure 2.2a. 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane
(TAPC) was used as the hole injection layer in the OLED. The device structure
of the OLEDs was: ITO (100 nm) / TAPC (65 nm) / TCTA (10 nm) / Emitting
layer (TCTA:B4PYMPM:0.5 wt% DCJTB:x wt% Ir(ppy)2tmd (30 nm) /
B4PYMPM (55 nm) / LiF (0.7 nm) / Al (100 nm). The absorption spectra of
Ir(ppy)2tmd and DCJTB measured in solution are compared with the
normalized PL spectra of TCTA:B4PYMPM co-deposited film,
28
Figure 2.2 (a) Molecular structures of the host materials, phosphorescent dop
ant and fluorescent acceptor. (b) Photoluminescence (PL) spectra of TCTA:B4
PYMPM exciplex, Ir(ppy)2tmd, and DCJTB doped in TCTA:B4PYMPM mix
ed film; and absorption spectra of DCJTB and Ir(ppy)2tmd in methylene chlor
ide. (c) PL spectra of DCJTB doped in TCTA:B4PYMPM mixed films with a
nd without Ir(ppy)2tmd
29
8 wt% Ir(ppy)2tmd, and 0.5 wt% DCJTB doped in TCTA:B4PYMPM mixed
film in Figure 2.2b. The TCTA:B4PYMPM exciplex emission has a much
larger spectral overlap with the DCJTB absorption than the Ir(ppy)2tmd
absorption due to the low extinction coefficient of Ir(ppy)2tmd. Therefore, the
Fӧrster radius from the exciplex to DCJTB (4.53 nm) is much larger than that
from the exciplex to Ir(ppy)2tmd (2.76 nm). The absorption spectrum of DCJTB
also has a large spectral overlap with the emission spectrum of Ir(ppy)2tmd,
facilitating efficient energy transfer from Ir(ppy)2tmd to DCJTB with a Fӧrster
radius of 4.54 nm. Figure 2.2c shows the PL spectra of 0.5 wt%-doped DCJTB
in the TCTA:B4PYMPM mixed films with and without Ir(ppy)2tmd. Large
Fӧrster radii from the exciplex and Ir(ppy)2tmd to DCJTB contribute over 90%
of the total emission from the films. Less than 10% of the total emission comes
from the exciplex and Ir(ppy)2tmd. Increasing the concentration of Ir(ppy)2tmd
increases the Ir(ppy)2tmd emission at the expense of exciplex emission.
The transient PL of the films was measured to understand the emission
processes of the films shown in Figure 2.2c. Figure 2.3a shows the transient PL
of DCJTB integrated from 580 nm to 610 nm in the TCTA:B4PYMPM:0.5 wt%
DCJTB films with and without Ir(ppy)2tmd. DCJTB shows delayed
fluorescence in all films, and the delayed portion increases as the Ir(ppy)2tmd
concentration increases. Since exciplexes are mostly formed upon
photoexcitation of the films, the singlet excitons on DCJTB are formed through
energy transfer directly from the exciplex or via Ir(ppy)2tmd. Thus, the delayed
emission characteristics of DCJTB reflect the energy transfer not only from the
30
Figure 2.3 (a–b) Transient PL of TCTA:B4PYMPM:0.5 wt% DCJTB film at
various Ir(ppy)2tmd concentrations; (a) integrated from 460 to 500 nm
(exciplex), and (b) integrated from 580 to 610 nm (DCJTB), and (c) transient
PL of TCTA:B4PYMPM:8 wt% Ir(ppy)2tmd with and without DCJTB
integrated from 510 to 534 nm (Ir(ppy)2tmd).
31
Figure 2.4 (a) Transient PL of Ir(ppy)2tmd in TCTA:B4PYMPM:Ir(ppy)2tmd
films at various Ir(ppy)2tmd concentrations, (b–c) Transient PL of Ir(ppy)2tmd
in TCTA:B4PYMPM:Ir(ppy)2tmd with and without DCJTB; (b) 2 wt%, (c) 4
wt%
32
Table 2.1 The decay rate of Ir(ppy)2tmd (PDk ) and the energy transfer rate from
Ir(ppy)2tmd to DCJTB (/ET PD FDk
) in TCTA:B4PYMPM:Ir(ppy)2tmd films
with and without DCJTB
System Ir(ppy)2tmd
concentration (wt%) PDk (s-1) /ET PD FDk
(s-1)
TCTA:B4PYMPM
:Ir(ppy)2tmd
2 6.80×105 -
4 7.04×105 -
8 7.14×105 -
TCTA:B4PYMPM
:0.5 wt%
DCJTB:Ir(ppy)2tmd
2 3.80×106 3.12×106
4 3.97×106 3.27×106
8 4.18×106 3.47×106
33
exciplex but also from Ir(ppy)2tmd.
The transient decay of the exciplex integrated from 460 nm to 500 nm is
shown in Figure 2.3b. The prompt decay rates of the exciplex in
TCTA:B4PYMPM:0.5 wt% DCJTB are faster than those of the exciplex
without DCJTB, indicating that efficient energy transfer from exciplex to
DCJTB occurred. In addition, the decay rate of the exciplex was even faster
when the concentration of Ir(ppy)2tmd in TCTA:B4PYMPM:0.5 wt% DCJTB
increased, indicating that the singlet exciplex was efficiently transferred to
Ir(ppy)2tmd and DCJTB. The energy transfer from Ir(ppy)2tmd to DCJTB can
be illustrated by the comparison of the transient PL of Ir(ppy)2tmd from
TCTA:B4PYMPM:Ir(ppy)2tmd films with and without DCJTB, shown in
Figure 2.3c and Figure 2.4a–c. The dashed lines in Figure 2.3c and Figure 2.4b-
c indicate the slowest components of the multi-exponential decays from which
we could estimate the slowest decay rate of Ir(ppy)2tmd (PDk ) from
TCTA:B4PYMPM:Ir(ppy)2tmd films with DCJTB. The estimated slowest PDk
and the energy transfer rate from Ir(ppy)2tmd to DCJTB ( /ET PD FDk ) are
summarized in Table 2.1, indicating that at least 82~83% of Ir(ppy)2tmd
excitons were transferred to DCJTB.
To investigate the effect of RISCk and /
D
ET E PDk in the exciplex system
on OLED performance, fluorescent OLEDs were fabricated using the exciplex-
forming mixed host co-doped with Ir(ppy)2tmd and DCJTB as the emitting
layer. The doping concentration of Ir(ppy)2tmd was varied from 0 to 8 wt% in
34
Figure 2.5 (a) Current density-voltage-luminance (J-V-L) characteristics, (b)
electroluminescence (EL) spectra of OLEDs at 10 mA cm-2, (c-d) external
quantum efficiencies (EQEs) of OLEDs versus current density; (c) total
emission, and (d) DCJTB emission.
35
the device. The J-V-L and emission spectra of the devices are shown in Figure
2.5a and 2.5b, respectively. The EQEs of the total and the DCJTB portion of
the emission spectrum in the devices are shown as a function of current density
in Figure 2.5c and 2.5d, respectively. Addition of 8 wt% Ir(ppy)2tmd enhanced
the maximum EQE of the total and DCJTB emissions to 23.7% and 21.6%,
respectively, from 10.6% and 10.1% without Ir(ppy)2tmd, as previously
reported,44 which is the highest efficiency ever achieved in fluorescence
OLEDs, to the best of our knowledge. The maximum EQE of the total and
DCJTB emissions are shown in Table 2.2. Considering an outcoupling
efficiency of 41%, calculated using the classical dipole model with a horizontal
dipole portion of 86% for DCJTB,6,44,72 the radiative singlet fraction of the
exciton on DCJTB (,
,
S FD
r EL ) is 52.7% which is about 2.2 times the 24.5%
obtained when only the RISCk value of the exciplex is utilized without the
Ir(ppy)2tmd system. With the fixed doping ratio of 8 wt% with Ir(ppy)2tmd, the
device characteristics at different doping concentrations of DCJTB are shown
in Figure 2.6a-c. All the devices show the similar J-V characteristics (Figure
2.6a). As the concentration of DCJTB is increased, the Ir(ppy)2tmd portion of
EL spectra is reduced since the energy transfer rate from Ir(ppy)2tmd to DCJTB
is increased (Figure 2.6b). However, the EL efficiency of the OLEDs (Figure
2.6c) was reduced from 23.7% to 13.3% as the concentration of DCJTB
increased from 0.5 to 2 wt%, resulting from concentration quenching of
DCJTB.73
36
Table 2.2 Maximum EQE of total and DCJTB emission, and the radiative
( ,
,
S FD
r EL ) and total (,S FD
EL ) singlet fractions of the exciton on DCJTB in the
devices, according to Ir(ppy)2tmd concentration
Ir(ppy)2tmd
Concentration
(wt%)
EQE of total
emission (%)
EQE of DCJTB
emission (%)
,
,
S FD
r EL (%) ,S FD
EL (%)
0 10.6 10.1 24.5 30.6
2 22.3 19.6 47.8 59.8
4 23.5 20.7 50.5 63.1
8 23.7 21.6 52.7 65.9
37
Figure 2.6 (a) J-V-L characteristics, (b) EL spectra of OLEDs at 10 mA cm-2,
(c) EQEs of OLEDs versus current density at different doping concentrations
of DCJTB
38
To calculate the total singlet fraction of the exciton on DCJTB in the
devices (,S FD
EL ), we first analyzed the PLQY and the radiative singlet fraction
of the exciton on DCJTB in the device (,
,
S FD
r EL ) without the Ir(ppy)2tmd system.
We assumed that only host materials are photoexcited and that all electrons and
holes are recombined in the exciplex co-host by bi-molecular recombination,
due to the low concentration of DCJTB. Since the PLQY of
TCTA:B4PYMPM:0.5 wt% DCJTB ( PL ) is the summation of the radiative
singlet fraction of the exciplex (,
,
S ex
r PL ) and ,
,
S FD
r PL , it is expressed as Equation
2.6:
, , , ,
, , / = 1 11 1
S ex S FD S ex F S FDISC RISC ISC RISC
PL r PL r PL r ET E FD r
ISC RISC ISC RISC
(2.6)
where ISC , RISC ,
,S ex
r , /
F
ET E FD are the ISC, RISC, radiative
transition and Förster energy transfer efficiency of the exciplex, respectively,
and ,S FD
r is the efficiency of the radiative transition in the singlet DCJTB
excitons. For the TCTA:B4PYMPM:DCJTB system shown in Figure 2.7, the
efficiencies of radiative transition (,S ex
r ), ISC ( ISC ) of the singlet exciplex,
Fӧrster energy transfer from exciplex to DCJTB ( /
F
ET E FD ), and RISC ( RISC )
of the triplet exciplex are expressed as follows:
39
Figure 2.7 Decay and energy transfer processes in TCTA:B4PYMPM:DCJTB
system
40
,
,
, ,
/
S ex
S ex r
r S ex S ex F
r nr ISC ET E FD
k
k k k k
(2.7)
, ,
/
ISC
ISC S ex S ex F
r nr ISC ET E FD
k
k k k k
(2.8)
/
/ , ,
/
F
F ET E FD
ET E FD S ex S ex F
r nr ISC ET E FD
k
k k k k
(2.9)
,
/
RISC
RISC T ex D
nr RISC ET E FD
k
k k k
(2.10)
where ,S ex
rk , ,S ex
nrk , and ISCk are the radiative transition, non-radiative
transition, and ISC rates of the singlet exciplex, ,T ex
nrk and RISCk are the non-
radiative transition and RISC rates of the triplet exciplex, and /
F
ET E FDk and
/
D
ET E FDk are the Fӧrster and Dexter energy transfer rates from the exciplex to
DCJTB, respectively.
Meanwhile, the radiative singlet fraction of the exciton on DCJTB in the
device (,
,
S FD
r EL ) can be expressed using Equation 2.11.
, ,
, /
(0.25 0.75)0.25
1
S FD F S FDISC
r EL RISC ET E FD r
ISC RISC
(2.11)
By using rate constants of exciplex,33 experimentally obtained values of
PL of 73%, and of
,
,
S FD
r EL of 24.5%, Fӧrster energy transfer rates from
singlet exciplex to singlet DCJTB ( /
F
ET E FDk ) of 2.0 × 107 s-1,
5 -1
/ 3.8 10 sD
ET E FDk and ,
0.80S FD
r were calculated. Then, ,S FD
EL
can be calculated by dividing ,
,
S FD
r EL into ,S FD
r .
41
The values of ,S FD
EL and ,
,
S FD
r EL for the devices are shown in Table 2.2.
,S FD
EL increased as the concentration of Ir(ppy)2tmd increased and reached a
value of 65.9% for 8 wt% of Ir(ppy)2tmd doping in the device. This indicates
that triplet exciplexes are efficiently harvested by increasing the Dexter energy
transfer to Ir(ppy)2tmd with its intrinsic RISC characteristic. If a fluorescent
emitter with a,S FD
r of 1 is substituted for DCJTB, EQEs of total emission of
29.7% and fluorescent emitter emission of 27.0% are expected, demonstrating
that exciplexes with phosphorescent dye show good potential as a system for
enhancing the efficiency of conventional fluorescent OLEDs. The maximum
achievable efficiency (27.0%) is only a little lower than the one (32.3%) from
the phosphorescent OLEDs based on Ir(ppy)2tmd.74
42
2.4 Conclusion
In conclusion, the singlet fraction of excitons in DCJTB was increased
by doping phosphorescent material, to increase the efficiency of fluorescent
OLEDs. TCTA:B4PYMPM exciplexes and Ir(ppy)2tmd excitons were
efficiently transferred to singlet DCJTB because of the large spectral overlap
between the emission spectra of the exciplex host and Ir(ppy)2tmd, and the
absorption spectrum of DCJTB. In addition, the device showed a maximum
EQE of total emission of 23.7%, and of DCJTB emission of 21.6%, with 8 wt%
of Ir(ppy)2tmd doping. Furthermore, we calculated that 65.9% of excitons were
formed as singlet DCJTB in the device and predicted that an EQE of total
emission of nearly 30%, and a fluorescent emitter emission of 27.0%, may be
expected if all singlet excitons in the fluorescent emitter decay radiatively.
Therefore, the method proposed is a promising development for achieving
maximum-efficiency conventional fluorescent OLEDs.
43
Highly Efficient, Conventional
Fluorescent OLEDs with Extended Operational
Lifetime
3.1 Introduction
The involvement of triplet excited states in the light emitting process has
been an important issue for efficient organic light-emitting diodes (OLEDs).
Many researchers have developed emitters that use spin mixing between singlet
and triplet excited states. Large spin-orbit interactions in heavy metal
complexes,10,14,29,59,61,75–77 and small singlet-triplet energy gaps in thermally
activated delayed fluorescence (TADF) materials23,30,63,65,66,78–80 and
intermolecular charge transfer complexes (exciplexes)27,28,32,33,64,81,82 facilitate
efficient spin-mixing in emitters, resulting in efficient phosphorescent and
fluorescent OLEDs. However, device stability has been a concern because of a
slow decay process caused by the spin-mixing process in the emitter.
In this regard, using conventional fluorescent molecules with a spin-
mixing mediator, called a sensitizer, can be an alternative. The efficiency of
OLEDs can be enhanced by spin mixing in the sensitizer followed by energy
transfer to the emitter, and the stable conventional fluorescent emitter can
improve the stability of the OLEDs. There have been several reports of
fluorescent OLEDs based on a sensitizer and a conventional fluorescent
44
emitter;33,41–45,52,53,68,83 however, the efficiencies were still lower than those of
OLEDs based on spin-mixing in the emitter.
Recently, we demonstrated a method of using both sensitization and
induced heavy atom effects to harness triplet excited states for efficient
fluorescent OLEDs.84 The fluorescent OLED based on an exciplex co-host and
phosphorescent sensitizer showed a maximum external quantum efficiency
(EQE) of 23.7%, which was the highest among fluorescent OLEDs based on
conventional fluorescent emitter. The device efficiency would be even higher
if the fluorescent emitter had a higher photoluminescence quantum yield
(PLQY). However, the device lifetime for this type of OLED has not been
reported.
In this work, we report highly efficient yellow fluorescent OLEDs based
on an exciplex cohost and iridium (Ir) complex. The fluorescent OLEDs
showed maximum EQEs higher than 25.0% by using an efficient conventional
fluorescent emitter. Energy transfer from the exciplex to the Ir complex, and
finally to the fluorescent dopant, reduced the lifetime of the exciplex and
excitons on the Ir complexes, thereby resulting in increased device lifetime. We
confirmed that the device lifetime could be improved by shortening the excited
state lifetime in the sequence of exciplex, triplet excitons in the Ir complexes
and singlet exciton in the fluorescent dyes.
45
3.2 Experimental
Absorption spectrum: 2,8-di[t-butyl]-5,11-di[4-(t-butyl)phenyl]-6,12-
diphenylnaphthacene (TBRb) was diluted to 10–5 M in methylene chloride
solvent. A Cary 5000 ultraviolet-visible-near infrared (UV-vis-NIR)
spectrophotometer (Agilent Technologies) was used to measure the absorption
spectrum. Extinction coefficient of TBRb can be calculated by dividing
absorbance into molar concentration of the solution and the width of quartz
cuvette used in the measurement.
Fabrication of photoluminescence (PL) samples and OLEDs: To
fabricate PL samples, fused silica substrates were cleaned by soaking in piranha
solution about 2 hrs then rinsing with deionized water. The piranha solution
was prepared by mixing sulfuric acid (H2SO4) and hydrogen peroxide (H2O2)
with a ratio of 3:1. After cleaning process, organic layers were deposited on the
fused silica substrates under a pressure lower than 5 × 10–7 Torr.
For the fabrication of the OLEDs, 70-nm-thick ITO-coated glass
substrates rinsed with deionized water were dipped in acetone and
isopropylalcohol for 1 min and boiled in isopropylalcohol for 5 min. Then, the
substrates were exposed to a UV ozone flux for 10 min. All organic materials
and aluminum electrode were also deposited by thermal vacuum evaporation
under the same conditions in the preparation of the PL samples. The deposition
rates of organic materials and metal electrode were 1 and 4 Å s-1, respectively.
46
PL spectra, angle-dependent PL, and PL quantum yields (PLQYs)
of organic films: A continuous-wave He-Cd laser (325 nm) was used as the
excitation source for PL measurements. PL spectra and angle-dependent PL
were measured using a fiber optics spectrometer (Maya2000; Ocean Optics) as
the optical detection system. An experimental setup for measuring angle-
dependent PL comprises a rotation stage, a half cylinder lens composed of fused
silica, a dichroic mirror, and a polarizer. The rotation stage and the half cylinder
lens were used to measure angle-dependent PL intensity from 0° to 90°, and the
dichroic mirror was used to filter excitation beam. The polarizer to separate p-
polarized light from s-polarized light since the p-polarized light contains both
horizontal and vertical dipole components. Detailed experimental information
for the experimental setup is described in previous papers.6,14,85 The PLQYs of
the films were measured using a 6 inch diameter integrating sphere (Labsphere).
A monochromator (Acton Research) attached to a photomultiplier tube
(Hamamatsu Photonics K.K.) was used as the optical detection system.
Transient PL measurement: The transient PL was measured using a
pulsed nitrogen laser (337 nm, Usho Optical Systems) as the excitation light
source and a streak camera (C10627; Hamamatsu Photonics) as an optical
detection system.
Device characteristics: Current density, luminance, and
electroluminescence (EL) spectra were measured using a programmable source
meter (Keithley 2400) and a spectrometer (Spectrascan PR650; Photo
47
Research). The angle-dependent EL emission patterns were measured using the
programmable source meter (Keithley 2400), a rotating stage and a fiber optics
spectrometer (S2000; Ocean Optics). The Lambertian correction factors of the
devices can be calculated by the emission patterns determined by the EL
intensity from 0° (normal direction) to 90° with 5° intervals. The EQEs and
power efficiencies of the OLEDs were calculated from their current density-
voltage-luminance (J–V–L) characteristics, EL spectra, and the Lambertian
correction factors. The power efficiency (lm W-1) of devices can be calculated
according to the Equation (3.1).
Power Efficiency = Lambertian correction factor10
L
V J
(3.1)
where L, V, and J are luminance (cd m-2), voltage (V), and current density (mA
cm-2) of devices, respectively.
A lifetime measurement system (M6000T; Polaronix) was used to
measure the operational lifetimes of the OLEDs. A current corresponding to an
initial luminance of 1,000 cd m-2 was constantly applied to the OLEDs at room
temperature until the luminance of devices reached 50% of the initial luminance.
48
3.3 Result and discussion
The excited state processes shown in Figure 3.1a illustrate the increased
device stability in the fluorescent OLEDs achieved via energy transfer. For
efficient fluorescent OLEDs based on an exciplex cohost and a conventional
fluorescent emitter, a low doping concentration is required because the excited
states should not be trapped in the fluorescent dopant, and Dexter energy
transfer from triplet exciplex to triplet fluorescent dopant should be prevented.
As a result, incomplete energy transfer from the exciplex to the fluorescent
dopant occurs, and the excited states are present in both the exciplex host and
the fluorescent dopant. The existence of the exciplex results in low device
stability because of the spin-mixing process. When the Ir complex is co-doped,
however, the excited states exist as excitons because of energy transfer from
the exciplex to Ir complex. Even if the energy transfer from the exciplex to the
Ir complex occurs, the excitons are formed mainly in the fluorescent dopant
rather than in the Ir complex because of efficient energy transfer from the Ir
complex to the fluorescent dopant. Therefore, efficient fluorescent OLEDs with
long device lifetimes would be expected by doping the exciplex cohost with an
Ir complex.
To verify this concept, we selected 4,4',4''-tris(N-carbazolyl)-
triphenylamine (TCTA) and bis-4,6-(3,5-di-4-pyridylphenyl)-2-
methylpyrimidine (B4PYMPM) at a molar ratio of 1:1 as an exciplex-forming
cohost. Two Ir complexes, tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) and
49
Figure 3.1 (a) Schematic illustration of the excited-state process. (b) Molecular
structure of the exciplex-forming host, phosphorescent materials, and
fluorescent emitter (c) Absorption spectrum of TBRb in methylene chloride
solution and the normalized PL spectra of the TCTA:B4PYMPM-mixed film,
Ir(ppy)3 and Ir(ppy)2tmd doped in the TCTA:B4PYMPM-mixed films, and the
TBRb solution. (d) PL spectra of 2 wt% TBRb in the TCTA:B4PYMPM-mixed
film without an Ir complex, with Ir(ppy)3, and with Ir(ppy)2tmd.
50
bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)
(Ir(ppy)2tmd), were selected as phosphorescent dopants, and TBRb, known to
have high PLQY,42,86 was used as a conventional fluorescent emitter. The
chemical structures of the materials are shown in Figure 3.1b. Figure 3.1c
shows the absorption spectrum of TBRB in methylene chloride solution and the
PL spectra of TCTA:B4PYMPM co-deposited film, and 8 wt%-doped Ir(ppy)3
and Ir(ppy)2tmd in the TCTA:B4PYMPM films. The large spectral overlap
between the absorption spectrum of TBRb and the PL spectra of the
TCTA:B4PYMPM exciplex, Ir(ppy)3 and Ir(ppy)2tmd, facilitated Fӧrster
energy transfer to the singlet of TBRb. The Fӧrster radii from the exciplex,
Ir(ppy)3, and Ir(ppy)2tmd to TBRb were calculated as 3.64, 3.91, and 3.91 nm,
respectively. Figure 3.1d shows the PL spectra of 2 wt% TBRb doped in the
TCTA:B4PYMPM-mixed film without an Ir complex, and with 8 wt% Ir(ppy)3
and Ir(ppy)2tmd. Exciplex emission from 450 to 485 nm disappeared by doping
with the Ir complex, which indicated that energy transfer from the exciplex to
the Ir complex also occurred. Most excitons were formed in the TBRb because
of efficient Fӧrster energy transfer to TBRb, resulting in the emission peak at
565 nm. The exciplex and phosphorescent emissions, which represented less
than 7% of the total emissions, are shown as dashed lines. The PLQYs of the
three films were 94 (without an Ir complex), 87 (doped with Ir(ppy)3) and 93
(doped with Ir(ppy)2tmd) ± 2%.
51
Figure 3.2 (a) Transient PL of the TCTA:B4PYMPM mixed film, and the
TCTA:B4PYMPM:2 wt% TBRb film with and without an Ir complex at the
exciplex emission (integrated from 450 to 485 nm). and (b) Transient PL of the
TCTA:B4PYMPM:8 wt% Ir complex film with and without TBRb at the Ir
complex emission (integrated from 510 to 535 nm).
52
Figure 3.3 Transient PL of the TCTA:B4PYMPM:2 wt% TBRb film with and
without an Ir complex at the TBRb emission (integrated from 540 to 710 nm).
53
We measured the transient PL to identify the energy transfer processes.
Figure 3.2a shows the transient PL of the exciplex emission integrated from
450 to 485 nm. The decay rate of the exciplex emission was faster when 2 wt%
of TBRb was doped, and the rate was even faster when 8 wt% of the Ir
complexes were doped. This demonstrated that efficient energy transfer from
the exciplex to dopants occurred. The energy transfer from an Ir complex to the
TBRb was confirmed by comparing the transient PL of the
TCTA:B4PYMPM:8 wt% Ir complex with and without the TBRb film at the Ir
complex emission (Figure 3.2b). The energy transfer contributed the increased
delayed fluorescence of the TBRb (Figure 3.3).
To identify the effect of the Ir complexes on the performance of the
OLEDs, we fabricated fluorescent OLEDs with the following structures:
indium tin oxide (ITO) (70 nm)/1,1-bis(4-bis(4-
methylphenyl)aminophenyl)cyclohexane (TAPC) (75 nm)/TCTA (10
nm)/emitting layer (EML) (30 nm)/B4PYMPM (50 nm)/LiF (0.7 nm)/Al (100
nm). A schematic diagram of the OLEDs and energy levels of the organic
materials is shown in Figure 3.4a. The TCTA:B4PYMPM exciplex was used
as the host in the EML, and the doping concentration of TBRb and the Ir
complexes were 2 and 8 wt%, respectively. For comparison, we also fabricated
phosphorescent OLEDs doped with 8 wt% of the Ir complexes without TBRb
in the EML. Figure 3.4b shows the J–V–L curves of all of the devices.
54
Figure 3.4 (a) Schematic diagram of organic light-emitting diodes (OLEDs)
and the energy level of materials. (b) Current density–voltage–luminance (J–
V–L) characteristics. (c) Electroluminescence (EL) spectra at 1,000 cd m–2. (d)
External quantum efficiencies (EQEs) of OLEDs as a function of luminance.
55
Table 3.1 Device performances of OLEDs
Device
Voltage [V] EQE [%] Power Efficiency [lm W-1]
Turn-on 1,000 cd m-2
10,000 cd m-2 Max. 1,000 cd m-2
10,000 cd m-2 Max. 1,000 cd m-2
10,000 cd m-2
TBRb 2.5 3.7 5.9 8.6 7.9 5.3 32.6 21.7 9.2
Ir(ppy)3+TBRb 2.3 3.1 4.4 25.0 24.8 22.9 100.4 82.2 54.3
Ir(ppy)2tmd+TBRb 2.2 2.9 4.3 26.1 26.0 23.9 114.3 91.9 57.4
Ir(ppy)3 2.3 3.1 4.5 27.0 26.7 23.9 120.1 99.3 60.5
Ir(ppy)2tmd 2.2 2.8 4.1 30.4 30.3 27.4 152.5 126.5 79.5
56
Figure 3.5 Angle-dependent PL of 2 wt% TBRb in TCTA:B4PYMPM with and
without 8 wt% of the Ir complexes.
57
The devices displayed low operating voltages with turn-on voltages of ca. 2.2–
2.5 V, close to the emitting photon energy divided by the elementary charge,
and all devices except the TBRb-only doped device reached 10,000 cd m–2 at
4.1–4.5 V. The EL spectra of all the devices were similar to those of the PL
spectra (Figure 3.4c). Graphs of the EQEs as a function of current density are
shown in Figure 3.4d. The maximum EQEs of the fluorescent OLEDs increased
from 8.6% to 25.0% and 26.1% when Ir(ppy)3 and Ir(ppy)2tmd were doped,
respectively. The EQE of 26.1% was the highest value ever achieved for
fluorescent OLEDs based on the TBRb emitter.42,52,86 Detailed characteristics
of all of the devices are summarized in Table 3.1. The fluorescent OLEDs
doped with the Ir complexes showed low efficiency roll-off that was similar to
that of the phosphorescent OLEDs. The emitting dipole orientation was
analyzed using the angle-dependent PL intensity.6,85 The angle-dependent PL
of 2 wt% TBRb measured at 565 nm is shown in Figure 3.5, and the horizontal
dipole ratio of 2 wt% TBRb was 85%. By using the classical dipole model with
PLQY of films and the horizontal dipole ratio of TBRb,6,72,87 we calculated
theoretical EQE of the fluorescent OLEDs with the Ir complexes assuming
singlet-triplet factor (/S T ) is 1. The theoretical EQEs of the fluorescent OLEDs
with Ir(ppy)3 and Ir(ppy)2tmd were 35.3% and 33.4%, respectively. Comparing
the experimental EQEs with the theoretical EQEs, /S T values were
calculated to be 74.8% for the Ir(ppy)3-doped fluorescent device and 73.9% for
the Ir(ppy)2tmd-doped fluorescent device. This demonstrated that the triplet
excited states were efficiently harvested by doping with the Ir complexes.
58
Figure 3.6 shows the decay of the EL intensity of the OLEDs as a
function of operating time under constant current at an initial luminance (L0) of
1,000 cd m–2. The operational lifetime (LT50) of the fluorescent OLEDs with 2
wt% of TBRb significantly increased from 7 to 99.4 and 312.7 h by doping with
Ir(ppy)2tmd and Ir(ppy)3, respectively. Smaller portion of the triplet excitons
on the fluorescent emitter in the phosphorescent dye-sensitized devices (~30%)
than the device without the sensitizers (>70%) must contribute to the reduction
of the average (singlet and triplet) exciton lifetime and density in the fluorescent
emitter to reduce the degradation coming from the exciton-polaron quenching.
Lower current stress in the phosphorescent dye-sensitized devices than the
device without the sensitizers to get the same luminance (e.g., 1,000 cd m-2)
coming from the higher efficiencies can be another reason to improve the
device stability. In addition, the LT50 of the fluorescent OLEDs doped with
Ir(ppy)3 and Ir(ppy)2tmd were 2.8- and 3.7-times longer, respectively, than that
of the phosphorescent OLEDs. This indicates that the energy transfer from the
Ir complexes to the fluorescent emitter shown in Figure 3.2b reduces the
lifetime of the excited states (from ca. 1 μs for the triplet excitons of the Ir
complexes to a nanosecond-scale for the singlet excitons of TBRb) and exciton
density, and subsequently increases the device stability, because reduced
exciton-polaron quenching is expected.
59
Figure 3.6 Device lifetimes of the OLEDs at an initial luminance (L0) of 1,000
cd m-2.
60
However, the LT50 of the fluorescent OLEDs doped with the Ir
complexes were still shorter than those of OLEDs using the same fluorescent
emitter based on other host and transporting materials,52,86 probably because of
the unstable host and phosphorescent materials used in this study; TCTA was
reported as unstable because of homolytic dissociation of carbon-nitrogen
bonds.88–90 Recently, phosphorescent OLEDs based on bis-4,6-(3,5-
dipyridylphenyl)-2-methylpyrimidine (BPyMPM) derivative material
displayed short operational lifetimes. Additionally, we reported that proper
selection of exciplex host and transporting materials can achieve long
operational lifetime of red phosphorescent OLEDs as well as high efficiency
probably due to the reduced polaron density in the device when stable exciplex
forming materials were used.91 Therefore, the device lifetime will be even
longer if we can identify more stable hosts and phosphorescent materials by
using the device structure proposed in this paper due to reduced exciton density
coming from the reduced exciton lifetime.
61
3.4 Conclusion
In conclusion, we fabricated highly efficient yellow fluorescent OLEDs
with improved device stability by doping an Ir complex into an exciplex cohost.
The fluorescent OLED based on TCTA:B4PYMPM exciplex and Ir(ppy)2tmd
showed a maximum EQE of 26.1% with low efficiency roll-off. Energy transfer
processes caused by doping Ir(ppy)3 allowed excited states to exist in the TBRb
and increased device lifetime from 7 to 312.7 h. Furthermore, we found that the
device stability depended on where the excited states were formed. Our method
is a promising approach to realizing efficient fluorescent OLEDs with long
device lifetimes.
62
Triplet Harvesting by a Fluorescent
Emitter Using a Phosphorescent Sensitizer for
Blue Organic Light-Emitting Diodes
4.1 Introduction
The development of efficient blue organic light-emitting diodes
(OLEDs) with long operational lifetimes has been a challenging issue for
practical applications such as flat-panel displays and solid-state lighting. Heavy
metal complexes and thermally activated delayed fluorescence (TADF)
materials have yielded highly efficient blue OLEDs via harvesting of triplet
excitons.14,63,66,75,76,79,92–95 However, the blue OLEDs still suffer from limited
operational lifetime.
Rather than developing stable phosphorescent or TADF molecules,
using conventional fluorescent emitters with either phosphorescent or TADF
sensitizers has been proposed as a way of realizing a long operational lifetime
and high efficiency at the same time, where excited states are spin-mixed in
phosphorescent or TADF sensitizers followed by energy transfer (ET) to
fluorescent emitters.41,42,51,52,68,84,96,97 The use of TADF sensitizers has proven
highly successful in recent years, demonstrating efficient blue fluorescent
OLEDs with external quantum efficiencies (EQEs) up to 18.8% (achieved by
reducing triplet ET from the sensitizer to the fluorescent emitter),42,50,53,70,98 as
well as a highly efficient deep-blue fluorescent OLED with an EQE of 19.0%
63
and Commission Internationale de L’Eclairage (CIE) coordinates of (0.14,
0.15) (achieved by using a fluorescent emitter with a small Stokes shift).54
However, spin-mixing in the TADF sensitizers is not always efficient because
reverse intersystem crossing (RISC) of the TADF materials depends on the
charge transfer (CT) and locally excited states, which can be influenced by the
host polarity.99–101 Additionally, if incomplete ET from the sensitizer to the
fluorescent emitter occurs, blue fluorescent OLEDs with high color purity
would not be expected due to additional broad spectral emission from the
TADF materials, caused by the CT characteristics.
In terms of spin-mixing process, phosphorescent dyes as the sensitizer
would be more efficient than TADF dyes because of the large spin-orbit
coupling. Using phosphorescent dyes as the sensitizer was proposed many
years ago,41,68 and highly efficient conventional fluorescent OLEDs with long
lifetimes have been achieved using phosphorescent dyes as the sensitizers.97
However, efficient blue fluorescent OLEDs with long lifetimes would be much
more difficult to achieve than those of other colors and have not yet been
reported to our best knowledge. The realization of phosphorescent dye-
sensitized blue fluorescent OLEDs is difficult because of the challenges in
finding the ultraviolet (UV)-emitting phosphorescent dyes required for efficient
ET from the phosphorescent dye to the blue emitting fluorescent emitter.
In this paper, we report an efficient blue fluorescent OLED using a
phosphorescent sensitizer and a conventional fluorescent emitter. Although the
64
energy difference between the triplet of the phosphorescent sensitizer and the
singlet of the fluorescent emitter was small, we fabricated a fluorescent OLED
with an EQE of 15.3% by efficient triplet harvesting via sensitization.
Combined with the recent development of deep blue phosphorescent OLEDs,
this work demonstrates the potential for realizing highly efficient blue
fluorescent OLEDs using phosphorescent sensitizers.
65
4.2 Experimental
Preparation of solutions and organic film samples: Organic materials
were diluted in methylene chloride solvent to prepare solutions, and the molar
concentration was 10-5 M. To fabricate organic film samples, piranha solution
was prepared by mixing hydrogen peroxide (H2O2) and sulfuric acid (H2SO4)
with a ratio of 1:3. Then, fused silica substrates were cleaned by soaking in
piranha solution about 2 hrs, followed by rinsing with deionized water. After
cleaning, organic layers were deposited on the fused silica substrates under a
pressure lower than 5 × 10–7 Torr. The total deposition rate of organic materials
for each process were 1 Å s-1.
Measurement of absorption spectra: The absorption spectra of
organic materials in solution or films were measured using a spectrophotometer
(Cary 5000; Agilent). Extinction coefficient of the materials were calculated by
dividing absorbance into molar concentration and the width of quartz cuvette
used in the measurement.
Photoluminescence (PL) characteristics of the solutions and organic
films: The PL spectra of the solutions and host materials were obtained using
a spectrofluorometer (Photon Technology International). A pulsed xenon flash
was used for the excitation source. The excitation wavelengths were selected
using a monochromator (250 nm for diphenylphosphineoxide-4-
(triphenylsilyl)phenyl (TSPO1), 290 nm for 3,3-di(9H-carbazol-9-yl)biphenyl
66
(mCBP), 325 nm for phosphorescent sensitizer, and 410 nm for fluorescent
emitter).
The PL spectra of the doped films were obtained using a fiber-optics
spectrometer (Maya2000; Ocean Optics). A continuous-wave He-Cd laser (325
nm) was used as the excitation source for PL measurements. PL spectra and
angle-dependent PL of films were measured using a fiber-optics spectrometer
(Maya2000; Ocean Optics) as the optical detection system. P-polarized light
emission was to calculate the horizontal dipole. The PL quantum yields
(PLQYs) of the films were measured using an integrating sphere and a
photomultiplier tube. Details of the experimental procedures for PLQY are
described elsewhere.6,102
The transient PL was measured using a pulsed nitrogen laser (337 nm;
Usho Optical Systems) as the excitation light source and a streak camera
(C10627; Hamamatsu Photonics) as the optical detection system.
Fabrication of OLEDs: For the fabrication of the OLEDs, 70-nm-thick
ITO-coated glass substrates rinsed with deionized water were dipped in acetone
and isopropylalcohol and boiled in isopropylalcohol, followed by the exposure
of UV-ozone flux for 10 min. OLEDs were fabricated by the deposition of
organic layers and aluminum electrode on precleaned 70-nm-thick indium-tin
oxide (ITO)-coated glass substrates. All of the layers were deposited by thermal
evaporation under a pressure of 5 × 10–7 Torr, and the deposition rate was 1 Å
s-1 for the organic layers, and 4 Å s-1 for the aluminum electrode.
67
Device characteristics of OLEDs: Current density, luminance, and
electroluminescence (EL) spectra were measured using a programmable source
meter (Keithley 2400) and a spectrometer (Spectrascan PR650; Photo
Research). The EQEs, current efficiencies, and power efficiencies of the
OLEDs were calculated using current density-voltage-luminance (J–V–L)
characteristics and EL spectra with the assumption of Lambertian emission
patterns. The operational lifetimes of the OLEDs were measured until the
luminance decreases to 50% of the initial luminance (LT50) under constant
current using a lifetime measurement system (M6000T; Polaronix).
Calculation of bond dissociation energy: Density functional theory
calculation was used to optimize geometry for neutral, radical and ionic states
of molecules. B3LYP functional and 6-31G** basis set were used in the
geometry optimization. The bond dissociation energy and geometry
optimization of molecules were calculated by using Jaguar in Schrӧdinger
Materials Science Suite.103,104
68
4.3 Result and discussion
Figure 4.1a shows the molecular structures of the compounds used in
this study. mCBP and TSPO1 were combined into a mixed cohost with a molar
ratio of 1:1. 2,5,8,11-Tetra-tert-butylperylene (TBPe) was used as a
conventional fluorescent emitter and bis[2′,6′-difluoro-4-(trimethylsilyl)-2,3′-
bipyridine]iridium(III)(4-methylpyridinepicolinate) ((dfpysipy)2Ir(mpic)) as a
phosphorescent sensitizer.105
Figure 4.1b compares the absorption and PL spectra of
(dfpysipy)2Ir(mpic) and TBPe in methylene chloride with the PL spectra of
mCBP and TSPO1 films. The PL spectra of mCBP and TSPO1 overlapped with
the absorption of (dfpysipy)2Ir(mpic) and TBPe, and the absorption spectrum
of TBPe also overlapped with the PL spectrum of (dfpysipy)2Ir(mpic). The
Fӧrster radius from (dfpysipy)2Ir(mpic) to TBPe was calculated to be 3.17 nm,
indicating that Fӧrster ET from the sensitizer to the fluorescent emitter can be
efficient even at low doping concentrations. The triplet energies of mCBP (2.90
eV)106 and TSPO163 (3.36 eV) are higher than that of (dfpysipy)2Ir(mpic) (2.76
eV; estimated from the peak wavelength of PL emission), so that triplet exciton
quenching from the phosphorescent dye to the host triplets is expected to be
small. Additionally, it is possible to prevent the singlet excited state of TBPe
from quenching to the triplet of (dfpysipy)2Ir(mpic), since the triplet energy of
(dfpysipy)2Ir(mpic) is higher than the singlet energy of TBPe. The PL spectra
69
Figure 4.1 (a) Molecular structures of the host (mCBP, TSPO1), iridium (Ir)
complex sensitizer [(dfpysipy)2Ir(mpic)], and fluorescent emitter (TBPe) (b)
Absorption and PL spectra of (dfpysipy)2Ir(mpic) and TBPe in methylene
chloride with the PL spectra of mCBP and TSPO1 films. (c) PL spectra of films
in the mCBP:TSPO1 mixed host doped with 1 wt% TBPe (black square), 1 and
5 wt% (dfpysipy)2Ir(mpic) (red circle and blue triangle), and 1 and 5 wt%
(dfpysipy)2Ir(mpic) with 1 wt% TBPe (dark cyan down triangle and magenta
diamond).
70
of the films codoped with TBPe (1 wt%) and (dfpysipy)2Ir(mpic) in the
mCBP:TSPO1 mixed host showed almost the same spectra as TBPe-doped
films, indicating that the PL emissions of the codoped film originated from the
fluorescence of TBPe. (Figure 4.1c) In the PL spectra of the films, a slight host
emission was also observed at a wavelength shorter than about 420 nm. The
PLQY of doped TBPe (1 wt%) in the mCBP:TSPO1 mixed-host was 73 ± 2
and 60 ± 2%, respectively, when 1 and 5 wt% of (dfpysipy)2Ir(mpic) was
additionally doped.
To understand the emission process, we measured the transient PL of the
films with and without the phosphorescent sensitizer at the excitation
wavelength of 337 nm where the absorption by mCBP and (dfpysipy)2Ir(mpic)
is dominant. Figure 4.2a shows the transient PL of the films integrated from
435 to 620 nm. The doping concentration of TBPe and (dfpysipy)2Ir(mpic) was
1 and 5 wt%, respectively. TBPe doped in the mCBP:TSPO1 mixed host
without the (dfpysipy)2Ir(mpic) showed monoexponential decay with a lifetime
of 5.65 ns (inset of Figure 4.2a). The transient PL of (dfpysipy)2Ir(mpic) doped
in the cohost without TBPe also showed monoexponential decay with an
exciton lifetime of 2.75 μs, characteristic of phosphorescent emission. When
the phosphorescent and fluorescent dyes were codoped, however, the transient
decay curve showed multiexponential decay combined with prompt emission
in the range of nanoseconds and delayed emission with a lifetime of 1.21 μs.
71
Figure 4.2 (a) Transient PL of three films in mCBP:TSPO1 mixed host doped
with 5 wt% (dfpysipy)2Ir(mpic) (red), and 5 wt% (dfpysipy)2Ir(mpic) + 1 wt%
TBPe (blue). The inset shows the transient PL of 1 wt% TBPe in mCBP:TSPO1
mixed film. (b) Time-resolved PL spectra of the three films.
72
Figure 4.3 Absorption spectra of mCBP and TSPO1
73
Figure 4.4 Transient PL of the mCBP:TSPO1:1 wt% TBPe film co-doped with
(dfpysipy)2Ir(mpic) integrated from 435 to 620 nm.
74
The decay rate of the delayed emission was faster than the phosphorescent
emission. Notably, this emission originated from TBPe, as manifested in the
time-resolved PL spectra (Figure 4.2b). The prompt and delayed emission
spectra of the film doped with (dfpysipy)2Ir(mpic) and TBPe matched the
emission spectrum of the TBPe-doped film in terms of the positions and
intensities of the emission peaks. The prompt and delayed emission from TPBe
emission in the codoped film indicates that various excitonic processes were
operating within the system; direct ET from the host molecules (mostly mCBP,
considering the excitation wavelength and absorption spectra of the host
molecules displayed in Figure 4.3) to TBPe, direct ET from (dfpysipy)2Ir(mpic)
to TBPe, and cascade ET from the host to (dfpysipy)2Ir(mpic) to TBPe. The
increase of the delayed portion in the transient PL with an increase in the
concentration of (dfpysipy)2Ir(mpic) resulted from the increased cascade ET
from the host to the TBPe via (dfpysipy)2Ir(mpic), or direct ET from
(dfpysipy)2Ir(mpic) to TBPe. (Figure 4.4). Taken together, these results imply
that the spin-mixing process in the sensitizing system can be efficient with very
little efficiency loss in OLEDs.
The fabrication of blue fluorescent OLEDs based on our sensitizing
system proved that triplet harvesting by spin-mixing in the sensitizer was
indeed efficient. Figure 4.5a illustrates the device structures of the fluorescent
OLEDs: ITO (70 nm)/mCBP:6 wt% Re2O7 (40 nm)/mCBP (10 nm)/emitting
layer (EML) (30 nm)/TSPO1 (55 nm)/Rb2CO3 (1 nm)/Al (100 nm). mCBP and
TSPO1 were used as the hole transporting layer (HTL) and electron
75
transporting layer (ETL), respectively, and as the mixed cohost to improve
charge balance in the EML. p-doped mCBP and Rb2CO3 layers were used for
efficient charge injection from the electrodes. Three different fluorescent
OLEDs with different concentration of the (dfpysipy)2Ir(mpic) sensitizer (0, 1,
and 5 wt%) in the EML were fabricated. Figure 4.5b shows the J–V–L curves
of the devices. The devices showed the same turn-on voltage of 3.6 V, but the
current density was reduced as the concentration of the sensitizer was increased.
This indicates that charge trapping to the sensitizer must contribute the reduced
current density in the device. The EL spectra of the devices are shown in Figure
4.5c. The emission peaks of the EL spectra were the same, and the CIE
coordinates of the devices were slightly changed from (0.13, 0.19) to (0.14,
0.19) when the sensitizer was codoped regardless of the concentration of
sensitizer. The EQEs of the devices are compared in Figure 4.5d. Even though
the PLQY of the doped TBPe was reduced as the concentration of sensitizer
was increased, the EL efficiency of device was increased. This demonstrates
that charge trapping to the sensitizer resulted in efficient triplet harvesting via
sensitization in the devices. The maximum EQEs of the fluorescent OLEDs was
increased from 7.0% to 15.3% when 5 wt% of (dfpysipy)2Ir(mpic) was co-
doped, which correspond to the maximum current efficiency and power
efficiency of 18.0 cd A-1 and 15.7 lm W-1 for the fluorescent OLED with the
sensitizer, respectively. (Figure 4.6)
76
Figure 4.5 (a) Schematic diagram of fluorescent organic light-emitting diodes
(OLEDs) (b) Current density–voltage–luminance (J–V–L) characteristics, (c)
Electroluminescence (EL) spectra at 1,000 cd m-2, and (d) External quantum
efficiencies of OLEDs as a function of luminance.
77
Figure 4.6 (a) Current efficiency, and (b) Power efficiency of OLEDs according
to luminance.
78
Figure 4.7 Angle-dependent PL intensity of mCBP:TSPO1:1 wt% TBPe film
measured at 463 nm and the fitting with the horizontal emitting dipole
orientation of 76%.
79
According to the classical dipole model, with a PLQY of 60 ± 2% and a
horizontal dipole ratio of 76 ± 1% for TBPe (Figure 4.7),6 69% of the total
excitons are converted to light in the device with the sensitizer under the
assumption of an exciton formation efficiency of 100% without any electrical
loss. The singlet ratio in the sensitized device can be higher if electrical loss
without forming excitons is considered because the device structure cannot
confine the electrons in EML due to the low electron barrier between the EML
and HTL. The fluorescent OLED with 5 wt% of sensitizer also showed higher
EQE (5.6%) at 1,000 cd m-2 and lower efficiency roll-off compared to the
fluorescent OLED without the sensitizer (1.7%; Table 4.1) The efficiency roll-
off characteristics of the fluorescent OLEDs at high current density might
originate from the electrical loss caused by field-dependent mobility
differences or charge trapping.107 If we can use a deep blue exciplex cohost
having balanced hole and electron mobilities, and a lower triplet energy level
than those of the constituent materials for efficient spin mixing, the fluorescent
OLEDs would have further increased efficiency and yielded better roll-off
characteristics.
We also measured operational lifetimes of three devices to compare the
device stability. (Figure 4.8) The LT50 of the fluorescent OLED with 5 wt% of
the sensitizer at the initial luminance of 400 cd m-2 was 0.89 hr, which was
about 10 times longer than that of the fluorescent OLED without the sensitizer.
However, the LT50 of three devices in this study are still short compared to
80
Table 4.1 Device characteristics of the fluorescent OLEDs
Device
EQE [%] Current efficiency (cd A-1) Power efficiency (lm W-1)
Max. 1,000 cd m-2 Max. 1,000 cd m-2 Max. 1,000 cd m-2
TBPe 7.0 1.7 8.2 2.3 7.2 0.7
TBPe+1 wt% (dfpysipy)2Ir(mpic) 12.4 4.0 15.0 5.5 13.1 1.8
TBPe+5 wt% (dfpysipy)2Ir(mpic) 15.3 5.6 18.0 7.7 15.7 2.4
81
Figure 4.8 Device lifetime of OLEDs at an initial luminance of 400 cd m-2
82
Table 4.2 Bond dissociation energies of host materials
Materials Bond dissociation energy (eV)
Cation Anion Neutral
mCBP 4.48 1.81 3.64
TSPO1 1.59 1.89 3.63
83
those in industry. TSPO1 used as EML and ETL in the devices must contribute
to the short operational lifetime due to low bond dissociation energy of 1.89 eV
in the anion state. (Table 4.2). Further studies about the development of stable
host materials should be accompanied to realize improved device stability with
high efficiency.
It is also beneficial for phosphorescent dye-sensitized fluorescent
OLEDs that the difference in emission wavelength between the phosphorescent
sensitizer and fluorescent emitter used in this study was only 10 nm, with
emission energies of 2.76 eV for phosphorescent emission and 2.71 eV for
fluorescent emission. This demonstrates that sensitization is possible with blue-
emitting phosphorescent dyes, with no requirement for a UV-emitting
phosphorescent dyes, contrary to common belief. This work, together with
recent reports of high-efficiency deep-blue OLEDs, suggests a bright future for
phosphorescent dye-sensitized blue OLEDs with high efficiencies and long
operational lifetimes.
84
4.4 Conclusion
In conclusion, an efficient blue fluorescent OLED was developed using
a deep-blue-emitting phosphorescent sensitizer and a blue-emitting
conventional fluorescent emitter. The emission energy difference between the
phosphorescent and fluorescent emission was only 0.05 eV, but the emission
from the fluorescence dopant was dominant in the device. The EQE of the
sensitized OLED was 15.3%, which is 2.2 times that of the fluorescent OLED
without the sensitizer (7.0%). These results clearly demonstrate that
sensitization by a phosphorescent dye to harvest triplets by a fluorescent emitter
does not require a large energy difference between the phosphorescent
sensitizer and fluorescent receiver. Using the recently developed deep-blue
phosphorescent dyes as sensitizers in the phosphorescent dye-sensitized
OLEDs developed in this work is expected to yield fluorescent OLEDs with
efficiencies comparable to those of phosphorescent OLEDs but with greatly
improved operational lifetimes comparable to those of fluorescent OLEDs.
85
Summary and Conclusion
OLEDs have been commercialized in mobile display as well as large-
area or curved displays since they can be applied to various substrates and have
various advantages such as high color gamut and light weight. In addition,
several companies manufacture lighting products using OLED because they
can be fabricated as a surface or flexible light source, and have high color
temperature and color rendering index. For several decades, the development
of various emitters such as phosphorescence and TADF materials has enabled
the production of highly efficient OLEDs and contributed to the
commercialization of OLEDs. However, in order for OLEDs to be
commercialized more widely in the future, researches should be performed to
increase both the efficiency and device stability of OLEDs. For this propose,
this thesis represents a system which can efficiently harvest triplet excited states
while using conventional fluorescent dye for highly efficient fluorescent
OLEDs with long operational lifetime, and blue fluorescent OLEDs based on
phosphorescent sensitizer. Based on these researches, efficient fluorescent
OLEDs with improved device stability and various colors including blue have
been reported.
In Chapter 2, we developed the novel method to utilize both induced
heavy atom effect and sensitization for efficient fluorescent OLEDs. The use of
exciplex cohost and phosphorescent codopant successfully increased the
fraction of singlet excitons in fluorescent dye (65.9%), resulting in a red
86
fluorescent OLED with EQE of 23.7%. Quantitative analysis showed that
fluorescent OLED with EQE of 29.7% could be obtained if all the singlet
excitons in the fluorescent dye showed radiative decay.
In Chapter 3, we successfully demonstrated fluorescent OLEDs with
high efficiency and long operational lifetime. By using fluorescent emitter with
high PLQY, the fluorescent OLEDs based on exciplex cohost and
phosphorescent sensitizer showed EQE over 25.0%, which is comparable to
phosphorescent OLEDs. In addition, the fluorescent OLEDs also showed
higher stability (T50 over 99 hr) compared to fluorescent OLEDs based on
exciplex cohost and phosphorescent OLEDs, which is attributed to the reduced
triplet density in OLEDs and reduced lifetime of exciplex and phosphorescent
materials.
According to the previous results, we expected that using a combination
of an exciplex cohost and a phosphorescent sensitizer in the fluorescent OLEDs
could solve both the efficiency and lifetime issues of blue OLEDs. For the
purpose, we reported the triplet harvesting using a phosphorescent sensitizer in
blue fluorescent OLEDs in Chapter 4. We confirmed that triplet harvesting by
sensitization occurred even with the use of deep-blue phosphorescent material
with a small energy difference of 0.05 eV compared to the fluorescent emitter.
As a result, the fluorescent OLEDs based on phosphorescent sensitizer showed
EQE of 15.3%.
87
We believe that the development of fluorescent OLEDs using exciplex
cohost and phosphorescent sensitizer has provided a direction for achieving
OLEDs with high efficiency and long operational lifetime. Since we
demonstrate that the phosphorescent sensitization can be applied even in blue
fluorescent OLEDs, the development of stable host, deep-blue phosphorescent
materials, and a deep-blue exciplex with high triplet energy and efficient RISC
will lead to the development of highly efficient blue OLEDs with long
operational lifetime, which has been the goal of many researchers working on
OLEDs.
88
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초 록
다양한 전자 장비의 개발로 디스플레이 산업이 확장되는
가운데, 유기발광소자는 높은 색 순도 등의 다양한 장점으로 액정
디스플레이와 같은 기존 디스플레이를 대체하고 있다. 또한,
유기발광소자는 조명 분야로까지 확대되어 사용되고 있다. 하지만,
유기발광소자가 미래 기술에서도 광범위하게 상용화 되기 위해서는
효율과 소자 안정성 문제를 모두 해결해야 할 필요가 있다.
유기발광소자에서 효율을 나타내는 지표인 외부양자효율은
외부발광효율과 내부양자효율로 구성된다. 유기발광소자 및 유기
반도체 분야에 종사하는 많은 연구자들은 광추출 구조, 수평 배향을
가지는 발광체, 저 굴절률 호스트 및 전하전달 물질의 개발 등을
통해 외부발광효율을 증가시켜왔다. 내부양자효율을 증가시키기
위한 방안으로 광발광 효율이 높은 발광체 물질과 양극성 호스트
물질 개발, 그리고 엑시플렉스 기반의 소자와 같은 전하균형이 맞는
소자 구조 개발 등이 보고되었다.
한편, 내부양자효율을 중가 시키기 위해서 75% 의 삼중항을
빛으로 활용하는 것이 중요하다. 많은 연구자들은 스핀혼합 과정을
활용하는 인광 도펀트, 열 활성 지연형광 발광체를 개발하여
100
유기발광소자의 효율을 개선시켜왔다. 하지만, 발광체 내에서의
스핀 혼합과정에 따른 느린 감쇄 속도로 인해 소자의 구동 안정성
측면에서 문제가 되고 있다. 특히, 청색 영역에서 수명 문제로 인해
낮은 외부양자효율을 보임에도 형광 유기발광소자가 주로 상용화
되어 사용되고 있다.
이러한 문제를 해결하고자 기존 형광 염료를 사용하면서
삼중항을 수확하는 연구가 진행되어왔다. 하지만, 현재까지 개발된
기존 형광 염료 기반의 소자들은 적은 삼중항의 수확 때문에 인광
유기발광소자나 열 활성 지연형광 유기발광소자와 비교하여 낮은
효율을 보이고 있다. 따라서 유기발광소자의 효율과 수명 문제를
모두 해결할 수 있는 새로운 시스템에 대한 개발이 필요한
상황이다.
본 논문은 고효율, 장수명의 유기발광소자를 제작하기 위해
안정한 기존 형광 염료를 사용하면서 효율적인 삼중항을 수확하는
방법에 대한 연구 주제를 다루고 있다.
제 2 장은 감광 효과와 유도된 중금속 효과를 함께 활용하여
기존 형광 염료의 삼중항을 효율적으로 활용하는 시스템에 대한
내용을 담고 있다. 인광 도펀트의 스핀 혼합과정과 유도된 중금속
효과에 의한 엑시플렉스의 증가된 역항간 교차에 의해 형광
101
도펀트의 일중항 비율을 증가시키고자 엑시플렉스 호스트와 인광
도펀트를 함께 사용하였다. 새롭게 설계한 시스템을 활용하여,
23.7% 의 외부양자효율을 갖는 형광 유기발광소자를 구현하였고,
이는 기존 형광 염료 기반의 적색 유기발광소자 기준으로 세계
최고의 효율이었다. 또한 광학 계산과 실험 결과를 이용하여 기존
형광 염료의 일중항이 모두 발광할 경우 최대 외부양자효율 30%
에 가까운 형광 유기발광소자를 얻을 수 있다는 것을 밝혔다.
이러한 내용을 근간으로, 제 3 장에서는 엑시플렉스 호스트와
인광 감광제를 사용하여 고효율, 장수명의 형광 유기발광소자를
달성한 결과를 담고 있다. 광 발광 효율이 높은 형광 도펀트를
이용하여 25.0% 이상의 외부양자효율을 가지는 형광
유기발광소자를 보고하였다. 에너지 전달 과정은 스핀 혼합
과정으로 긴 감쇄 과정을 보이는 엑시플렉스와 인광 물질의 감쇄
시간을 감소시켰다. 그 결과, 엑시플렉스와 인광 감광제를 포함하는
형광 유기발광 소자의 구동 수명은 99 시간 이상으로, 인광
유기발광소자, 엑시플렉스 기반 형광 유기발광소자의 구동수명보다
향상된 값을 보였다.
제 2, 3 장의 연구 결과를 기반으로 유기발광소자에서 핵심
문제인 청색 유기발광소자의 효율, 수명문제를 개선하기 위해
엑시플렉스 호스트와 인광 감광제 조합을 청색 형광 유기발광
102
소자에도 적용시키고자 하였다. 하지만, 지금까지 청색 영역에서
인광 도펀트에 의한 감광효과는 원활한 에너지 전달을 위한 자외선
영역의 물질 개발 필요 때문에 어려운 영역으로 여겨져 왔다. 제 4
장에서는 인광물질을 감광제로 활용한 청색 형광 유기발광소자에
대한 결과를 포함하고 있다. 광 물리적 특성에 따라, 우리는 청색
형광 발광체와 적은 에너지 차이를 보이는 진청색 인광 감광제를
사용하더라도 감광제에서 스핀 혼합과정에 이은 형광 도펀트로의
에너지 전달 과정이 발생할 수 있음을 알 수 있었다. 인광 감광제를
이용한 효율적인 삼중항 수확을 통해 최대 외부양자효율 15.3% 인
청색 형광 유기발광소자를 보고하였다.
주요어: 형광 유기발광소자, 엑시플렉스, 인광 감광제, 기존 형광
발광체, 삼중항 수확, 에너지 전달, 구동 수명
학번: 2013 – 23037