Effect of increasing electron donor units for high-efficiency blue thermally activated delayed fluorescence

Article abstract of DOI:10.1016/j.dyepig.2017.01.053

To enhance the efficiency of organic light-emitting diodes (OLEDs), the application of triplet excitons must be maximized. Two donor–acceptor-type molecules bearing bis-carbazolyl and tris-carbazolyl groups at the ortho position of a phenyl-attached triazine ring were synthesized. These molecules showed very small singlet–triplet energy gaps (ΔE_ST) of 0.06 and 0.03?eV, respectively, and transient photoluminescence (PL) characteristics, which indicate that they are thermally activated delayed fluorescence (TADF) materials. A high external quantum efficiency (EQE) of 16.5% with deep blue color coordinates of (0.15, 0.22) was observed using a TrisCz–Trz TADF emitter. This interaction between the electron donor and electron acceptor substituents plays a key role in achieving a very small ΔE_ST and good intramolecular charge transfer (ICT) activities with high photoluminescence (PL).

Full text of DOI:10.1016/j.dyepig.2017.01.053

Accepted Manuscript
Effect of increasing electron donor units for high-efficiency blue thermally activated
delayed fluorescence
Jae-Ryung Cha, Chil-Won Lee, Myoung-Seon Gong
PII:
S0143-7208(16)31482-6
10.1016/j.dyepig.2017.01.053
DYPI 5753
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 December 2016
Revised Date: 20 January 2017
Accepted Date: 20 January 2017
Please cite this article as: Cha J-R, Lee C-W, Gong M-S, Effect of increasing electron donor units for
high-efficiency blue thermally activated delayed fluorescence, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2017.01.053.
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ACCEPTED MANUSCRIPT
Graphical Abstract

ACCEPTED MANUSCRIPT
Effect of increasing electron donor units for high-efficiency blue
thermally activated delayed fluorescence
a
b
a,
Jae-Ryung Cha , Chil-Won Lee , and Myoung-Seon Gong *
Department of Nanobiomedical Science and BK21 PLUS NBM Global Research
Center, Dankook University, Cheonan 31116, Republic of Korea
a
b
Department of Chemistry, Dankook University, Cheonan 31116, Republic of Korea
*
Corresponding author. Tel.: +82 41 5501476; fax: +82 41 5503431.
E-mail: msgong@dankook.ac.kr (M. S. Gong)
ABSTRACT
To enhance the efficiency of organic light-emitting diodes (OLEDs), the application
of triplet excitons must be maximized. Two donor–acceptor-type molecules bearing bis-
carbazolyl and tris-carbazolyl groups at the ortho position of a phenyl-attached triazine
ring were synthesized. These molecules showed very small singlet–triplet energy gaps
(
ꢀEST) of 0.06 and 0.03 eV, respectively, and transient photoluminescence (PL)
characteristics, which indicate that they are thermally activated delayed fluorescence
TADF) materials. A high external quantum efficiency (EQE) of 16.5% with deep blue
(
color coordinates of (0.15, 0.22) was observed using a TrisCz–Trz TADF emitter. This
interaction between the electron donor and electron acceptor substituents plays a key
role in achieving a very small ꢀEST and good intramolecular charge transfer (ICT)
activities with high photoluminescence (PL).
Keywords: TADF; OLED; Sky blue; Carbazole; Triazine; Blue dopant
1

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1
. Introduction
To date, the development of organic light-emitting diodes (OLEDs) has mainly
focused on enhancing the operational lifetime and power consumption of devices to
improve their overall efficiency. Commercialized OLED panels use phosphorescent
materials with an internal quantum efficiency of 100% as their green and red light
emitters. However, blue phosphorescent materials have a very short lifetime because of
their relatively low molecular stability and high operation voltage in blue OLED
devices. Therefore, a new methodology is required to achieve a high electroluminescent
efficiency for blue OLED devices.
Recently, highly efficient fluorescent materials with high external quantum
efficiencies have received increased attention owing to the use of thermally activated
delayed fluorescence (TADF) emitters [1-9]. By combining the molecular structures of
electron donors and electron acceptors, the energy difference between the singlet and
triplet excited states (ꢀEST) can be minimized to develop highly efficient TADF
materials [9-13]. TADF phenomena are processes which apply thermal energy
(approximately 28 meV) at room temperature to materials having an extremely small
energy difference between the singlet and triplet states. These processes induce a singlet
state via the reverse intersystem crossing (RISC), where a triplet state passes to a more
excited singlet state, displaying delayed fluorescence as the singlet state transitions to
the ground state. TADF emitters which use singlet and triplet excitons can be exploited
to enhance the external quantum efficiency and thus overcome the problems associated
with traditional fluorescent materials.
2

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To develop efficient TADF materials, the RISC and intramolecular charge transfer
ICT) need to occur efficiently. To enhance the RISC process, a smaller gap between
(
the S
the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) overlap, the S energy level increases as a result. Therefore, the
1 1
and T energy levels is desirable. However, when the electronic distributions of
1
electron density overlap of the HOMO and LUMO levels needs to be minimized. To
prevent electron density overlap, twisted molecular structures are preferred [14]. There
is a trade-off between the RISC efficiency and the ICT efficiency. Therefore, the
overlap of the HOMO and LUMO distributions needs to be adequately adjusted.
Carbazole derivatives with a stable structure have been used as electron donor moieties
of TADF emitters [15-23]. Because carbazole consists only of aromatic moieties; thus,
it has a high bond dissociation energy. Triazine derivatives have also proven to be stable
because of their aromatic character. Therefore, TADF emitters have been developed
using carbazole (electron donor moiety) and triazine (electron acceptor moiety) [23-30].
However, the TADF materials reported to date have a severe efficiency roll-off at a high
luminance owing to triplet–triplet annihilation and the triplet–polaron quenching effect
[
31].
In this work, we designed and synthesized donor–acceptor-type molecules [9,9'-((6-
phenyl-1,3,5-triazine-2,4-diyl)bis(2,1-phenylene))bis(9H-carbazole) (BisCz–Trz) and
,4,6-tris(2-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TrisCz–Trz)] for application as
2
blue TADF emitters in OLEDs. In these molecules, two and three carbazole electron
donor units are introduced at the ortho position of the phenyl ring of the triazine
electron acceptor. These molecules exhibit a very small ꢀEST (0.03–0.06 eV) with very
high electroluminescence efficiencies. Optimized blue TADF devices using TrisCz-Trz
3

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show a maximum quantum efficiency of 16.5% and chromaticity coordinates (0.15,
2
0
.22) in the deep blue range. Moreover, the roll-off at 100–1000 cd/m is less than 10%
(
16.5% → 15.6%; 5.45% efficiency drop compared to the maximum efficiency),
meaning that the quantum efficiency drop phenomena have been greatly improved in
the high luminescence range compared to existing blue TADF materials.
4

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2
. Experimental
2
.1. Materials and Measurements.
Zinc chloride, 9H-carbazole, 1,2-dibromobenzene, copper iodide (Aldrich Chem. Co.),
,1'-bis(diphenylphosphino)ferrocene-palladium(II) dichloride dichloromethane
complex, tetrakis(triphenylphosphine)palladium(0), 2,4-dichloro-6-phenyl-1,3,5-triazine,
,4,6-trichloro-1,3,5-triazine, and 4,4,4',4',5,5,5',5'-octamethyl-2,2'-bi(1,3,2-
1
2
dioxaborolane) (P&H Tech. Co. Korea) were used without further purification.
Potassium carbonate, potassium acetate, N,N-dimethylacetamide, 1,4-dioxane,
tetrahydrofuran, dichloromethane, n-hexane, and nitromethane (DukSan Sci. Co. Korea)
were used as received.
1
13
The H and C nuclear magnetic resonance (NMR) spectra were recorded using a
Varian Ascend 500 (500 MHz) spectrometer. The photoluminescence (PL) spectra were
recorded using
a
fluorescence spectrophotometer (Jasco FP-6500), and
photoluminescence quantum yield (PLQY) was measured by absolute PL quantum yield
spectrometer (Hamamatsu, C11347-11). Ultraviolet–visible (UV-vis) spectra were
obtained by means of a UV-vis spectrophotometer (Shimadzu UV-1601PC). Elemental
analyses (EAs) were performed using a Thermo Fisher Scientific Flash 2000 elemental
analyser, and low- and high-resolution mass spectra were recorded using a JEOL JMS-
600W spectrometer in the electron ionization (EI) and fast atom bombardment (FAB)
modes. Differential scanning calorimetry (DSC) measurements were performed using a
Shimadzu DSC-60 calorimeter in nitrogen at a heating rate of 10 °C/min, and the
thermogravimetric analysis (TGA) measurements were performed using a Shimadzu
TGA-50 thermogravimetric analyser at a heating rate of 10 °C /min.
2.2. Preparation of 9-(2-bromophenyl)-9H-carbazole
5

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A mixture of 9H-carbazole (5.5 g, 32.89 mmol), 1,2-dibromobenzene (11.64 g, 49.34
mmol), copper iodide (3.13 g, 16.45 mmol), and potassium carbonate (9.09 g, 65.79
mmol) in anhydrous N,N-dimethylacetamide (120 mL) was degassed with nitrogen for
30 min while stirring. The reaction mixture was then refluxed in nitrogen for 24 h. The
crude mixture was filtered, and the residue was washed with dichloromethane (3 ꢀ 50
mL). The combined filtrates were evaporated until they were dry The residue was
purified by column chromatography (silica gel, 50% n-hexane in dichloromethane) to
give 9-(2-bromophenyl)-9H-carbazole as a white solid.
1
H NMR (500 MHz, CDCl
H, J = 18 Hz), 7.43-7.52 (m, 2H), 7.84 (d, 1H, J = 12 Hz), 8.14 (d, 2H, J = 12 Hz). C
NMR (CDCl ): δ 110.0, 120.0, 120.3, 123.2, 123.8, 125.9, 128.8, 130.1, 131.1, 134.2,
3
): δ 7.06 (d, 2H, J = 12 Hz), 7.28 (t, 2H, J = 18 Hz), 7.39 (t,
13
3
3
136.7, 140.8.
2.3. Preparation of 9-(2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-
carbazole
A mixture of 9-(2-bromophenyl)-9H-carbazole (10 g, 31.04 mmol), 4,4,4',4',5,5,5',5'-
octamethyl-2,2'-bi(1,3,2-dioxaborolane) (10.25 g, 40.35 mmol), 1,1'-
bis(diphenylphosphino)ferrocene-palladium(II) dichloride dichloromethane complex
0.76 g, 0.93 mmol), potassium acetate (9.14 g, 93.11 mmol), and anhydrous 1,4-
(
dioxane (180 ml) was degassed with nitrogen for 1 h while stirring. The reaction
mixture was then maintained in nitrogen at 80 °C for 24 h. The mixture was diluted with
dichloromethane and washed with distilled water (100 mL) three times. The organic
4
layer was dried over anhydrous MgSO and evaporated in vacuo to give the crude
product, which was purified by column chromatography using dichloromethane/n-
6

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hexane and gave a white powder.
1
H NMR (500 MHz, CDCl
3
): δ 0.79 (s, 12H), 7.18 (d, 2H, J = 12 Hz), 7.25 (t, 2H, J =
1
1
8 Hz), 7.37 (t, 2H, J = 18 Hz), 7.53 (t, 2H, J = 18 Hz), 7.69 (t, 1H, J = 16 Hz), 7.96 (d,
13
3
H, J = 12 Hz), 8.14 (d, 2H, J = 12 Hz). C NMR (CDCl ): δ 24.4, 83.4, 109.7, 119.1,
119.9, 123.1, 125.5, 127.6, 128.6, 132.1, 136.3, 142.1, 142.4.
2.4. Preparation of 9,9'-((6-Phenyl-1,3,5-triazine-2,4-diyl)bis(2,1-phenylene))bis(9H-
carbazole) (BisCz-Trz)
-(2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole (11.04 g,
7.87 mmol), 2,4-dichloro-6-phenyl-1,3,5-triazine (3 g, 13.27 mmol), and
9
2
tetrakis(triphenylphosphine)palladium(0) (1.53 g, 1.33 mmol) were dissolved in a 2 M
potassium carbonate solution (40 mL) and tetrahydrofuran (100 mL). The reaction
mixture was stirred for 24 h at 80 °C. The mixture was diluted with dichloromethane
and washed with distilled water (100 mL) three times. The organic layer was dried over
4
anhydrous MgSO and evaporated in vacuo to give the crude product, which was
purified by column chromatography using dichloromethane/n-hexane and gave a pale
yellow powder.
o
o
o
1
BisCz-Trz: Yield 71%. T
NMR (500 MHz, CDCl ): δ 6.79 (d, 2H, J = 8 Hz), 6.895 (t, 2H, J = 15 Hz), 6.965 (d,
H, J = 8 Hz), 7.163–7.217 (m, 5H), 7.263 (t, 4H, J = 15 Hz), 7.478 (t, 2H, J = 15 Hz),
g m d
: 100 C, T : 263 C, T : 374 C (5% weight loss). H
3
4
7
13
.563 (d, 2H, J = 8 Hz), 7.659–7.708 (m, 4H), 7.986 (d, 4H, J = 8Hz). C NMR
): δ 109.4, 119.4, 120.1, 123.3, 125.9, 127.8, 128.4, 128.6, 130.4, 131.9, 132.4,
(CDCl
3
+
+
1
29 5
32.6, 135.3, 136.4, 141.5. Mass (EI ) m/z 639 [(M+H) ]. Anal Calcd for C45H N : C,
84.48%; H, 4.57%; N, 10.95%. Found: C, 84.48%; H, 4.57%; N, 10.86%. UV-vis: λmax
7

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=
339 (THF) nm. Fluorescence: λmax (emission) = 457 (solid state) nm. Φfl = 0.756.
2.5. Preparation of 2,4,6-Tris(2-(9H-carbazol-9-yl)phenyl)-1,3,5-triazine (TrisCz-Trz)
9-(2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole (15.36 g,
53.68 mmol), 2,4,6-trichloro-6-1,3,5-triazine (3 g, 16.27 mmol), and
tetrakis(triphenylphosphine)palladium(0) (2.82 g, 2.44 mmol) were dissolved in a 2 M
potassium carbonate solution (40 mL) and tetrahydrofuran (100 mL). The reaction
o
mixture was stirred for 24 h at 80 C. The mixture was diluted with dichloromethane
and washed with distilled water (100 mL) three times. The organic layer was dried over
4
anhydrous MgSO and evaporated in vacuo to give the crude product, which was
purified by column chromatography using dichloromethane/n-hexane and gave a pale
yellow powder.
o
o
o
1
TrisCz-Trz: Yield 37%. T
NMR (500 MHz, CDCl ): δ 6.238 (d, 3H, J = 8 Hz), 6.558 (d, 6H, J = 8 Hz), 6.968 (t,
H, J = 15 Hz), 7.216ꢁ7.261 (m, 12H), 7.341 (d, 3H, J = 8 Hz), 7.485 (t, 3H, J = 15 Hz),
g m d
: 126 C, T : 323 C, T : 395 C (5% weight loss). H
3
3
1
3
8
.100 (d, 6H, J = 7 Hz). C NMR (CDCl
32.2, 133.6, 136.0, 141.3, 170.2. Mass (EI ) m/z 803 [(M+H) ]. Anal Calcd for
C, 85.05%; H, 4.51%; N, 10.44%. Found: C, 85.07%; H, 4.49%; N, 10.45%.
UV-vis: λmax = 339 (THF) nm. Fluorescence: λmax (emission) = 457 (solid state) nm. Φfl
0.708.
3
): δ 109.4, 119.2, 123.2, 125.9, 128.2, 130.0,
+
+
1
57 36 6
C H N
=
2.6. Device fabrication and measurements
All devices were basically prepared using a vacuum thermal evaporation process.
,4'-cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC) was chosen as a hole
4
transport layer; 1,3-bis(N-carbazolyl)benzene (mCP) and diphenylphosphine oxide-4-
8

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(triphenylsilyl)phenyl (TSPO1) were applied as the exciton blocking and hole blocking
layers; 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) was used as the electron
transport layer; and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
PEDOT:PSS) and N,N'-diphenyl-N,N'-bis-[4-phenyl-m-tolyl-amino]-phenyl]-biphenyl-
(
4,4'-diamine (DNTPD) were used as the hole injection layer. The reason for using
DNTPD instead of PEDOT-PSS is to adjust the charge injection and charge transfer for
charge balance. Device structures for devices I, II, and III are as follows and are shown
in Fig. 1.
<Fig. 1>
Device I: ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/mCP:BisCz-Trz (25
nm, 1, 3, and 5% doping)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200 nm).
Device II: ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/mCP:TrisCz-Trz
(25 nm, 1, 3, 7 and 10% doping)/TSPO1 (5 nm) )/TPBi (30 nm)/LiF (1 nm)/Al (200
nm).
Device III: ITO/DNTPD (50 nm)/TAPC (20 nm)/mCP (10 nm)/mCP:TrisCz-Trz (25nm,
7% doping)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200 nm).
The electrical evaluation of the devices was performed with a Keithley 2400
sourcemeter, and the optical evaluation was carried out with CS 2000 spectroradiometer.
9

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3
. Results and discussion
3.1. Synthesis and characterization
The two TADF emitters developed in this work have two and three carbazole units as
the electron donor moieties, and the triazine electron acceptor moiety is linked to the
two- and three-carbazole moieties through a phenyl spacer at the ortho position. Scheme
1
presents the synthesis of the two blue TADF materials. BisCz-Trz and TrisCz-Trz
were prepared by the Suzuki coupling reaction between the boronic ester form of alkyl-
substituted 9-phenylcarbazole, 2,4-dichloro-6-phenyl-1,3,5-triazine, and 2,4,6-trichloro-
6-1,3,5-triazine. BisCz-Trz and TrisCz-Trz purified by column chromatography and
vacuum sublimation were used for material characterization and device fabrication.
Fig. 2 presents the molecular orbitals of the TADF emitter calculated by using the
B3LYP 6-31G* basis set of the Gaussian 09 program. The HOMO was mainly located
at the carbazole unit of the TADF emitters, and the LUMO was dispersed over the
phenyl-spacerꢁtriazine unit regardless of the type of electron donor moiety. The large
torsion angles separating the HOMO and LUMO of BisCz-Trz and TrisCz-Trz were 66°
and 67°, respectively. The ortho linkage can distort the carbazole from the phenyl plane
by steric hindrance, and the degree of HOMOꢁLUMO overlap at the TADF emitter can
be decreased by the large torsion angle between the electron donor and electron
acceptor moiety.
<Fig. 2>
The Calculated ꢀEST of BisCz-Trz and TrisCz-Trz was 0.06 and 0.04 eV,
respectively, and was expected to cause an RISC. Moreover, the HOMO and LUMO are
not completely separated but adequately separated. It is expected that the fluorescence
1
0

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quantum efficiency, because the partially overlapping HOMO and LUMO in the phenyl
spacer improves the ICT characteristics.
3
.2. Thermal properties
The thermal stability of BisCz-Trz and TrisCz-Trz was studied by using thermograms
from DSC and TGA. The glass transition temperature T was determined from the
g
inflection point of the endothermic curves: 100 °C for BisCz-Trz and 126 °C for
TrisCz-Trz. The value of T was 26 °C higher for TrisCz-Trz compared to BisCz-Trz
g
because the former has a symmetric structure and high molecular weight. The melting
points of BisCz-Trz and TrisCz-Trz were 263 and 323 °C, respectively. From the TGA
measurements, we determined the 5% weight loss temperature to be 374 °C for BisCz-
Trz and 395 °C for TrisCz-Trz (ESI Fig. S5-S6). BisCz-Trz and TrisCz-Trz have high
thermal stability, which is expected to show favorable properties for device stability and
lifetime.
3.3. Optical properties
The light absorption and emission of the blue TADF materials were analysed by using
UV-vis and PL spectroscopy. Fig. 3 presents the UV-vis absorption and PL spectra of
BisCz-Trz and TrisCz-Trz. The 290ꢁ300 nm range is the π–π* transition; the 300ꢁ350
nm range is the n–π* transition of carbazole; and the 350ꢁ450 nm range shows the ICT
absorption characteristics between electron accepter unit (triazine moiety) and electron
donor unit (carbazole moiety). This type of absorption occurs more strongly in TrizCz-
Trz where the electron donor unit is increased. The measurement of the singlet energy
of BisCz-Trz and TrisCz-Trz was determined in the maximum emission wavelength of
1
1

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PL in polystyrene matrix, which was measured the same at 2.71 eV (PLmax 457 nm.)
The triplet energies of BisCz-Trz and TrisCz-Trz were 2.65 and 2.68 eV,
respectively, on the basis of the phosphorescent emission peak in a frozen
tetrahydrofuran solution measured at 77 K after a delay time of 100 µs was applied. The
singletꢁtriplet energy gaps (ꢀEST) of BisCz-Trz and TrisCz-Trz were 0.06 and 0.03 eV,
respectively. BisCz-Trz and TrisCz-Trz emitters designed on the basis of the ortho-
phenyl linkage platform exhibited a very small ꢂEST less than 0.1 eV via the distortion
of the molecular geometry. The singlet and triplet energies of the TADF emitters were
reduced by modifying the number of carbazole moieties as the electron donor group.
<Fig. 3>
The decay curves of the delayed fluorescence emissions of BisCz-Trz and TrisCz-
Trz were investigated by transient PL measurements of BisCz-Trz and TrisCz-Trz
dispersed in mCP. Fig. 4 shows the transient PL decay curves of BisCz-Trz and TrisCz-
Trz. The lifetimes of the delayed emission calculated from the decay curves were 5.4 µs
for BisCz-Trz and 5.0 µs for TrisCz-Trz. This indicates that the delayed fluorescence
process contributes to the light emissions of BisCz-Trz and TrisCz-Trz. The reason for
the relatively short decay time is that the effective RISC occurs as a result of the small
singletꢁtriplet energy difference; therefore, the concentration of the triplet-state excitons
is rapidly exhausted.
<Fig. 4>
3
.4. Electrochemical properties
The ionization potential (IP) and electron affinity (EA) of the blue TADF emitters
were measured by cyclic voltammetry (CV) to predict the HOMO and LUMO, as
1
2

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shown Fig. 5. The IP/EA of BisCz-Trz and TrisCz-Trz were -6.10/-3.35 eV and -6.11/-
3.33 eV, respectively. The IP/EA gaps were 2.75 and 2.78 eV for BisCz-Trz and
TrisCz-Trz, respectively. The HOMO/LUMO levels were similar in the BisCz-Trz and
TrisCz-Trz emitters. These results show similar LUMO levels, despite the increased
carbazole unit because the interaction between the carbazole and triazine units also
minimizes the influence on triazine. Table 1 presents the basic thermal and
photophysical properties of BisCz-Trz and TrisCz-Trz.
<
Fig. 5> <Table. 1>
3. 5. Device properties
The two TADF emitters were evaluated as blue emitters by doping in the mCP host
material. Fig. 6 shows the current-densityꢁvoltage and luminanceꢁvoltage curves of
devices I, II, and III. The decrease in the current density of device II for the increase
from 1% doping to 3% or 7% doping is due to the charge (electron) trapping effect of
the TADF materials in the LUMO level. The decrease in the current density when the
dopant concentration is changed from 7% to 10% is due to the increase in charge
transfer between dopants.
<Fig. 6>
The quantum efficiencies of devices I, II, and III were calculated from the current
density and luminance are plotted versus the luminance in Fig. 7. The increase in the
number of electron donor units enhanced the external quantum efficiency (EQE). The
maximum EQEs of devices I (3%), II (7%), and III (7%) were 12.2%, 14.9%, and
16.5%, respectively. A dramatic improvement in the EQE was realized by increasing the
number of carbazole units attached to the ortho-phenyl linkage. The EQE decreased
slowly with increasing luminance and the EQE of device III remained 16.5% at 100
1
3

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2
2
cd/m and 15.6% at 1000 cd/m , corresponding to 100% and 94.5% of the maximum
EQE, respectively.
The triplet energy difference between mCP of host material and BisCz-Trz or TrisCz-
Trz was less than 0.3 eV. Triplet excitons formed in the light emitting layer mainly
remained stayed in dopants, thus the dopant-dopant TTA was caused because of a long
lifetime relative to the singlet excitons. Generally, TTA can be explained by the Dexter
or Förster transfer mechanism, when two triplet excitons are in close proximity.
Through this energy transfer process, one is formed in a high energy state (T
n n
or S ) and
the other is formed in the bottom energy state (S ) [32]. If the concentration of the
o
dopant is high or if the dopant is aggregated, the triplet excitons can cause TTA through
Dexter energy transfer in principle. An exchange of electrons occurred through overlap
o n n
of Dexter energy transfer molecular orbital, and the S state and T or S state are
formed through electron transfer of HOMOꢀHOMO and LUMOꢀLUMO.
In the molecular structure of TrisCz-Trz, the acceptor unit of triazine is surrounded
by a donor unit in the carbazole. Therefore, HOMO and LUMO is located outside and
inside of the molecular, so that the overlap of LOMO between two triplet excitons
become limited. As a result, in order to generate TTA by the Dexter mechanism, both
HOMO and LUMO electrons must be able to move. However, in the case of TrisCz-Trz,
the transfer of LUMO electrons is limited and the TTA phenomenon is expected to be
reduced. Due to these molecular structural properties, the roll-off properties of TTA in
the device are improved compared to other TADF materials.
Table 2 presents the characteristics of devices I, II, and III at different doping
concentrations.
<
Fig. 7> <Table. 2>
1
4

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Fig. 8 presents the electroluminescence (EL) spectra of BisCz-Trz and TrisCz-Trz
devices characterized with a spectroradiometer. The maximum emission peaks of
devices I, II, and III were 465, 467, and 468 nm, respectively, and very similar
characteristics were obtained. The full width at half maximum (FWHM) values of
devices I, II, and III were 63, 62, and 64 nm, respectively. The color coordinates of
devices I, II, and III were (0.15, 0.20), (0.15, 0.21) and (0.15, 0.22) respectively.
Although the number of carbazole units increased, devices I, II, and III had similar color
coordinates and maximum emission wavelengths with a narrow emission spectrum.
<Fig. 8>
4
. Conclusion
In conclusion, new blue TADF emitters, BisCz-Trz and TrisCz-Trz, were designed and
synthesized. These molecules demonstrated very small ꢂEST (0.03ꢁ0.06 eV) and
improved ICT characteristics by increasing the number of electron donor units and
ortho-linking electron donor and electron acceptor moieties. The blue TADF device
using TrisCz-Trz showed a high quantum efficiency and deep blue emission in the high
2
luminescence range (100ꢁ1000 cd/m ). Therefore, the ortho-linked design approach can
be useful for TADF material design to obtain a small ꢀEST and good ICT transition. A
molecular design strategy which includes more electron donor units and distributes the
HOMO uniformly would allow for the development of high-EQE TADF devices.
Acknowledgements
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education
(2013R1A1A2011560) the MOTIE (Ministry of Trade, Industry & Energy (project
number 10051350)) and KDRC (Korea Display Research Consortium) support program
for the development of future devices technology for display industry.
1
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Table captions
Table 1. Basic thermal and photophysical properties of BisCz-Trz and TrisCz-Trz.
Table 2. Blue TADF OLEDs characteristics using BisCz-Trz and TrisCs-Trz at different
doping concentrations.
1

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Table 1. Basic thermal and photophysical properties of BisCz-Trz and TrisCz-Trz.
Materials
a)
BisCz-Trz
TrisCz-Trz
Highes
Properties
t
Purity
HPLC
DSC
(℃)
%
9
9.9
00
263
39
99.9
126
323
339
occupie
d
Thermal
Analysis
T
g
1
Tm
molecul
ar
UV
PL(Solid)
LT PL
Max(nm)
3
Optical
S
1
orbital
2
.71 (457 nm) 2.71 (457 nm)
a)
Lowes
Analysis
T1
%
2.65
5.6
2.68
70.8
t
unoccup
ied
PLQY
7
a
Electrical
Analysis
HOMO
-
6.10
3.35
-6.11
-3.33
CV(eV)
b
LUMO
-
molecul
ar orbital.
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Table 2. Blue TADF OLEDs characteristics using BisCz-Trz and TrisCs-Trz at
different doping concentrations.
-2
-2
Maximum
500 cd m (1000 cd m )
a)
b)
c)
Devices
QE
PE
CE
QE
PE
CE Voltage
Color
-1
-1
-1
-1
[
%]
[lm W ]
[cd A ]
[%]
[lm W ]
[cd A ]
[V]
coordinate
BisCz-Trz 1%
BisCz-Trz 3%
BisCz-Trz 5%
TrisCz-Trz 1%
TrisCz-Trz 3%
4.2
2.7
6.5
6.4
7.3
8.2
9.6
8.5
6.3
3.2
1.7
4.9
9.1
0.18, 0.21
0.15, 0.20
0.15, 0.21
0.15, 0.19
0.15, 0.21
0.15, 0.21
0.15, 0.23
12.2
10.9
8.8
18.5
16.7
12.8
17.0
23.0
18.3
11.5 (8.9)
8.4 (6.8)
7.0
5.7 (4.2)
4.2 (3.2)
4.3
17.4 (13.5)
12.9 (10.4)
10.2
9.6 (8.9)
9.7 (6.8)
7.5
11.1
14.9
8.2 (6.9)
14.7 (13.4)
10.6 (9.2)
4.3 (3.3)
8.4 (7.1)
6.8 (5.3)
12.6 (10.5)
22.8 (20.8)
17.2 (15.0)
9.2 (10.0)
8.5 (9.3)
8.0 (8.9)
TrisCz-Trz 7%
TrisCz-Trz
1
1
1.3
6.5
1
0%
TrisCz-Trz 7%
DNTPD)
12.3
b)
23.9
14.8 (15.6)
c)
8.1 (9.3)
21.3 (22.5)
8.3 (7.7)
0.15, 0.22
(
a)
QE, quantum efficiency; PE, power efficiency; CE, current effieciency.
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Scheme and Figure captions
Scheme 1. Synthetic scheme of BisCz-Trz and TrisCz-Trz.
Fig. 1. Device structure of the blue thermally activated delayed fluorescent device using
BisCz-Trz and TrisCz-Trz.
Fig. 2. Simulated HOMO and LUMO distribution of BisCz-Trz and TrisCz-Trz.
Fig. 3. UV-vis solid PL and low temperature PL spectra of BisCz-Trz and TrisCz-Trz.
Fig. 4. Photoluminescence decay curves of BisCz-Trz and TrisCz-Trz.
Fig. 5. Electrochemical oxidation and reduction curves of BisCz-Trz and TrisCz-Trz.
Fig. 6. Current density-voltage and luminance-voltage curves of blue TADF OLEDs.
Fig. 7. Quantum efficiency-luminance curves of blue TADF OLEDs.
Fig. 8. EL spectra of blue TADF OLEDs.
1

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Scheme 1. Synthetic scheme of BisCz-Trz and TrisCz-Trz.
2

ACCEPTED MANUSCRIPT
Fig. 1. Device structure of the blue thermally activated delayed fluorescent device using
BisCz-Trz and TrisCz-Trz.
3

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Fig. 2. Simulated HOMO and LUMO distribution of BisCz-Trz and TrisCz-Trz.
4

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Fig. 3. UV-vis solid PL and low temperature PL spectra of BisCz-Trz and TrisCz-Trz.
5