The effect of frontier orbital distribution of the core structure on the photophysics and device performances of thermally activated delayed fluorescence emitters

Article abstract of DOI:10.1039/c9tc01577a

The effect of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of the core structure on the thermally activated delayed fluorescence (TADF) behavior of the TADF emitters was investigated. Dibenzofuran was used as the core structure of the TADF materials, and its 2 and 3 positions were substituted with a donor and an acceptor. Two TADF emitters with the donor and acceptor positions exchanged with each other were synthesized and suggested that the substitution of the donor at the LUMO dominant position and the acceptor at the HOMO dominant position is beneficial to improve the efficiency of the TADF OLEDs. It was described that the substitution positions of the donor and acceptor to the core structure should be managed to increase the quantum efficiency of the TADF devices by enlarged orbital overlap.

Full text of DOI:10.1039/c9tc01577a

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Effect of frontier orbital distribution of the core structure onDtOhI: 1e0.1039/C9TC01577A
photophysics and device performances of thermally activated delayed
fluorescent emitters
Mina Jung1, Kyung Hyung Lee1, Wan Pyo Hong2*, Jun Yeob Lee1*
1 School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea
2 LG Chem, Ltd, LG science Park
30, Magokjungang 10-ro, Gangseo-gu, Seoul, 07796, Korea
*E-mail: leej17@skku.edu, whongw@lgchem.com
Abstract
The effect of the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) distribution of the core structure on the thermally activated
delayed fluorescent (TADF) behavior of the TADF emitters was investigated. Dibenzofuran
was used as the core structure of the TADF materials, and the 2 and 3 positions of it were
substituted with a donor and an acceptor. Two TADF emitters with the donor and acceptor
position exchanged each other were synthesized and they suggested that the substitution of
donor at LUMO dominant position and acceptor at HOMO dominant position is beneficial to
improve the efficiency of the TADF OLEDs. It was described that the substitution position of
the donor and acceptor to the core structure should be managed to increase quantum
efficiency of the TADF devices by enlarged orbital overlap.

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DOI: 10.1039/C9TC01577A
Keywords: TADFfluorescent emitterquantum efficiencymolecular orbitaldibenzofuran

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Introduction
Thermally activated delayed fluorescent (TADF) type dopants are very popular as high
efficiency organic emitters because the maximum external quantum efficiency (EQE) of them
can be close to the theoretical EQE of organic light-emitting diodes (OLEDs). Many works
already reported the maximum theoretical EQE in TADF OLEDs by molecular engineering
of the TADF dopants.1-3
In many TADF dopants, basic platform of the molecular structure was donor-acceptor or
donor-acceptor-donor.4-9 Main approach to control the TADF features based on the basic
chemical platform was to manage the donor and acceptor moieties because the type of donor
and acceptor was critical to the TADF parameters. In addition to the donor and acceptor
moieties, the organization of the donor and acceptor was also important to achieve the high
EQE in the TADF emitters. For example, the positions of the donor and acceptor in the
common phenyl core such as ortho, meta, and para played a decisive role in determining the
EQE, efficiency roll-off and lifetime of the TADF OLEDs. It was reported that the ortho or
para position substitution is advantageous to achieve good device performances in the TADF
emitters.10-14 In particular, the ortho substitution approach was useful in the design of the
TADF emitters for small singlet-triplet energy splitting (EST) and high reverse intersystem
crossing (RISC) rate.
Until now, the phenyl unit has been popular as the core structure of the TADF dopants,15-28
but other aromatics have a potential as the core structure because the role of the core is to
manage the orbital distribution and emission energy. Therefore, other aromatics can also be
considered as the core structure of the TADF design.29-34 In previous work, we reported that
dibenzofuran can work as the core structure of the TADF emitters for high external quantum
efficiency and narrow emission due to rigidity of the molecular structure.35,36 However,

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detailed study about the effect of the substitution position of the donor and acceptor has not
been carried out. Compared to the common phenyl core with electronically equivalent carbon
atoms at all positions, the dibenzofuran has four different carbons with different electronic
orbital distribution. Therefore, the study about the correlation of substitution position of the
donor and acceptor with the photophysical and device properties of the dibenzofuran cored
TADF emitters would be important.
Herein, we describe the design of the TADF emitters built on the chemical platform of donor-
dibenzofuran core-acceptor. Instead of the common phenyl core, the dibenzofuran core was
introduced and the effect of the donor and acceptor position on the TADF performances of
the TADF emitters was investigated. Two TADF emitters, 9-(3-(4,6-diphenyl-1,3,5-triazin-2-
yl)dibenzo[b,d]furan-2-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (2Cz3Trz) and 9-(2-(4,6-diphenyl-
1,3,5-triazin-2-yl)dibenzo[b,d]furan-3-yl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (3Cz2Trz), were
designed and synthesized as the dibenzofuran core based TADF dopants with different
substitution positions of the donor and acceptor. It was described that 2 position substitution
of donor and 3 position substitution of acceptor (2Cz3Trz) are preferred for high EQE in the
TADF dopants because of large overlap of the frontier orbital for radiative transition. From
this work, it was demonstrated that the management of the substitution position in the
dibenzofuran core based TADF emitters is critical to the TADF performances.
Results and discussion
Theoretical calculation
Dibenzofuran is a well-known aromatic unit with 4 different substitution positions. As
reported in our previous work, functional groups can be introduced at 4 different positions of
dibenzofuran.35,36 Therefore, the dibenzofuran unit can work as the core structure in the
design of donor-acceptor type TADF dopant in place of a common phenyl core. Compared

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with the phenyl unit which has six substitution positions equivalent each other, the
dibenzofuran unit has four different substitution positions and can diversify the design of the
TADF emitters. In the case of the TADF emitters, at least one donor and one acceptor have to
be substituted to the dibenzofuran core to realize TADF property. In this work, a molecular
design adopting the donor and acceptor at 2 and 3 positions of dibenzofuran was devised to
develop the dibenzofuran core based TADF emitters.
The underlying background utilizing the 2 and 3 positions of dibenzofuran is that the donor
and acceptor adjacent each other would induce the distortion of the donor from the
dibenzofuran core and strong charge transfer (CT) character for small EST. The donor and
acceptor substitution positions of 2Cz3Trz were 2 and 3, respectively, and they were
exchanged in the 3Cz2Trz because the 2 and 3 positions of dibenzofuran are not equivalent in
terms of orbital distribution. The frontier orbital calculation results of dibenzofuran based on
B3LYP 6-31G* basis set are shown in Figure 1. The 2 position of dibenzofuran has high
highest occupied molecular orbital (HOMO) density but is a node for the lowest unoccupied
molecular orbital (LUMO) density, while the 3 position has both HOMO and LUMO
distribution. Therefore, the position of the donor and acceptor may affect the molecular
orbital of the TADF emitters and TADF properties.
LUMO
HOMO
Figure 1. Simulated HOMO and LUMO distribution of the dibenzofuran core calculated
using basis set of B3LYP 6-31G*.

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The frontier molecular orbital calculation results of the 2Cz3Trz and 3Cz2Trz dopants that
calculated using basis set of B3LYP 6-31G* are presented in Figure 2. Clear change of the
HOMO and LUMO distribution was observed in the TADF dopants by exchanging the
substitution position of the donor and acceptor. The HOMO distribution of the two TADF
emitters was quite similar in that the HOMO is mostly localized on the bicarbazole donor
with weak extension to the dibenzofuran core. However, the LUMO distribution was
dissimilar in the two TADF dopants. The LUMO of the 3Cz2Trz emitter was completely
isolated in the diphenyltriazine acceptor with no LUMO extension to the dibenzofuran core.
Whereas, the LUMO of the 2Cz3Trz emitter was extensively dispersed over the
diphenyltriazine acceptor and dibenzofuran core. Therefore, the HOMO and LUMO overlap
was relatively large in the 2Cz3Trz dopant, resulting in relatively large oscillator strength in
the 2Cz3Trz (0.089) compared to 3Cz2Trz (0.0045). This indicates that the 2Cz3Trz has a
potential as a better emitter than the 3Cz2Trz by efficient radiative transition. The reason for
this can be found in the LUMO distribution of the dibenzofuran core and geometrical
structure. As described in the molecular orbital distribution of the dibenzofuran core, the 2
position is the node for LUMO. Therefore, the substitution of the diphenyltriazine acceptor to
the 2 position cannot extend the LUMO further, isolating the LUMO only on the
diphenyltriazine acceptor. Moreover, the triazine unit is connected to the two phenyl units of
dibenzofuran in a slightly bent manner, which hinders the extension of the LUMO to the
dibenzofuran core. The dihedral angles of the diphenyltriazine unit from the dibenzofuran
plane were 34.7o and 36.9o in the 2Cz3Trz and 3Cz2Trz emitters. The experimental
HOMO/LUMO values of 2Cz3Trz and 3Cz2Trz emitters were -5.78/-3.32 and -5.77/-3.20 eV,
respectively (Figure S2 in supporting information). The HOMO-LUMO gap was diminished
in the 2Cz3Trz emitter by the extended conjugation.

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2Cz3Trz
3Cz2Trz
LUMO
HOMO
67.5 ˚
34.7 ˚
72.7 ˚
Optimized
Geometry
36.9 ˚
Figure 2. Simulated HOMO and LUMO distribution of the 2Cz3Trz and 3Cz2Trz emitters
based on the optimized geometry of the molecules.
Natural transition orbitals (NTO) calculation of the 2Cz3Trz and 3Cz2Trz was also carried
out using time-dependent density functional theory with IP-tuned LC-RS parameter
calculation method with w value as 0.1500.[37] The calculation results are in Figure S1. In the
two emitters, the origin of the lowest excited state was found to charge transfer (CT) state by

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the orbital separation. The calculated S1/T1 energy were 2.88/2.80 and 2.99/2.90 eV for
2Cz3Trz and 3Cz2Trz emitters.
Synthesis
The synthesis of the 2Cz3Trz and 3Cz2Trz was carried out using F and Br functionalized
dibenzofuran intermediates prepared by ring closing reaction. The starting materials of
2Cz3Trz and 3CzTrz were dibromodifluorobenzene and bromodifluorobenzene, respectively,
and they were coupled with 2-methoxyphenylboronic acid to prepare a precursor of the F and
Br functionalized dibenzofuran. Demethylation followed by ring closing reaction provided
the dibenzofuran intermediates with the F and Br functional groups. The F unit was
substituted with a 3,3-bicarbazole donor moiety by Cs2CO3 mediated reaction and the Br unit
was replaced with diphenyltriazine unit by Suzuki coupling reaction after boration of the Br
unit. Structural confirmation of the 2Cz3Trz and 3Cz2Trz emitters was performed using 1H
and 13C nuclear magnetic resonance spectrometer and mass analysis. All synthetic procedures
are schematized in Scheme 1. More details can be found in supporting information.

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P1-4
P1-5
P1-3
2Cz3Trz
P1-1
P2-5
P1-2
P2-6
P2-4
3Cz2Trz
P2-3
P2-1
P2-2
Scheme 1. Synthetic process of the 2Cz3Trz and 3Cz2Trz emitters.
Photophysical analysis
The photophysical analysis of the two TADF emitters was performed to examine the effect of
discrepancy of the molecular orbital distribution on the light absorption and emission
behaviors. Figure 3 shows the ultraviolet-visible (UV-vis) absorption and photoluminescence
(PL) emission spectra of the 2Cz3Trz and 3Cz2Trz emitters. Tetrahydrofuran solution (1.0 
10-5 M) of the emitters was used for the UV-vis absorption measurement because toluene has
optical absorption below 280 nm. Toluene solution (1.0  10-5 M) was utilized for the PL
measurement at room temperature and low temperature (77 K). The two TADF emitters
exhibited strong UV-vis absorption by the backbone structure corresponding to the −* and
n-* transition from dibenzofuran and bicarbazole donor. Additionally, they showed very
weak charge transfer (CT) absorption above 350 nm by the donor-acceptor structure of the
emitters. The CT absorption was relatively strong in the 3Cz2Trz due to strong CT character

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by the isolated LUMO. This can also be confirmed by the relatively large bathochromic shift
of the PL emission in the 3Cz2Trz (58 nm) compared to that in the 2Cz3Trz (54 nm) by
changing solvent from nonpolar toluene to polar tetrahydrofuran. Oscillation strength were
calculated from the UV-Vis absorption. The 2Cz3Trz(f=0.026, 0.022) emitter showed higher
oscillator strength than the 3Cz2Trz(f=0.017, 0.023) emitter. [38-39] (Figure 4)
(a)
(b)
UV-vis absorption
Fluorescence
Phosphorescence
Oscillation
UV-vis absorption
Fluorescence
Phosphorescence
Oscillation
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
strength
strength
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 3. UV-vis absorption, fluorescence and phosphorescence spectra of the
tetrahydrofuran solution of the 2Cz3Trz (a) and 3Cz2Trz (b).
(a)
(b)
Methylene chloride
Tetrahydrofuran
Toluene
Methylene chloride
Tetrahydrofuran
Toluene
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Figure 4. PL spectra of the (a) 2Cz3Trz and (b) 3Cz2Trz in toluene, tetrahydrofuran and
methylene chloride.

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The emitters were analyzed at 77 K without and with delayed time of 100 s to record
fluorescence and phosphorescence. The fluorescence and phosphorescence spectra of the
2Cz3Trz emitter were observed at a longer wavelength than those of the 3Cz2Trz emitter.
The singlet energy/triplet energy values of the 2Cz3Trz and 3Cz2Trz were 2.92/2.86 and
3.06/3.01 eV, respectively. The low emission energy of the 2Cz3Trz can be correlated with
the degree of conjugation of the emitters. The diphenyltriazine unit at 3 position of
dibenzofuran extends the degree of conjugation due to linear alignment of the triazine unit
relative to two phenyl units of dibenzofuran as reflected in the LUMO extension to the
dibenzofuran core and reduces the emission energy of the 2Cz3Trz. Whereas, the triazine unit
at 2 position in the 3Cz2Trz limits the extension of the conjugated structure and increases the
emission energy. The EST values of the 2Cz3Trz and 3Cz2Trz were 0.06 and 0.05 eV,
respectively. The EST was slightly small in the 3Cz2Trz emitter because of the large HOMO
and LUMO separation.
The effect of the EST is noticed in the PL decay of the two TADF emitters. The delayed
fluorescence decay of the 2Cz3Trz and 3Cz2Trz emitters is presented in Figure 5 according
to temperature. The temperature dependent PL decay was different in the two emitters. The
2Cz3Trz emitter showed little delayed fluorescence at 100 K, but the delayed fluorescence
was clearly detected at 200 and 300 K. Whereas, the 3Cz2Trz emitter showed delayed
fluorescence even at 100 K because of small EST. The delayed fluorescence lifetimes at 300
k of the 2Cz3Trz and 3Cz2Trz emitters were 4.80 and 2.84 s, respectively. Short delayed
fluorescence lifetime was obtained in the 3Cz2Trz emitter as predicted from the small EST
value.

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(a)
(b)
1
1
IRF
IRF
100 K
200 K
300 K
0.01
100 K
200 K
300 K
0.1
0.1
0.01
1E-3
1E-4
1E-5
1E-3
1E-4
0
40
80
120
160
200
0
40
80
120
160
200
Second (us)
Second (us)
Figure 5. Transient PL decay curves of the 2Cz3Trz (a) and 3Cz2Trz (b) emitters doped in
CzTrz host according to measurement temperature.
Absolute PL quantum yield of the two emitters was also measured using an integrating sphere.
The PL quantum yields of the 2Cz3Trz and 3Cz2Trz were 74.2 and 69.7% under nitrogen,
respectively, indicating that 2Cz3Trz is superior to 3Cz2Trz in terms of radiative transition.
This result is in agreement with the oscillator strength of the two emitters in the molecular
simulation. High oscillator strength of the 2Cz3Trz emitter is responsible for the PL quantum
yield. The extension of the HOMO and LUMO to the dibenzofuran core overlapped the
HOMO and LUMO in the 2Cz3Trz emitter, which increased the PL quantum yield of the
2Cz3Trz emitter. Moreover, the contribution of the TADF component was significant in the
two TADF emitters. The PL quantum yield and decay parameters of the 2Cz3Trz and
3Cz2Trz are summarized in Table 1 along with other photophysical parameters. The reverse
intersystem crossing rate was high in the 3Cz2Trz relative to that of the 2Cz3Trz by the short
delayed fluorescence lifetime.
Table 1. Summary of photophysical analysis results of the 2Cz3Trz and 3Cz2Trz emitters.

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UV-
vis
(nm)a
Singlet Triplet
τd
(s)e
ΔEST
(eV)
PLQY
(%)d
τp
(ns)e
KISC
(s-1)f
KRISC
(s-1)f
sf
energy
(eV)b
energy
(eV)c
Krsf
Knr
0.24 
107
2.46 
105
9.92 
106
34.5 
106
2Cz3Trz 364.5
3Cz2Trz 366.7
2.92
3.06
2.86
3.01
0.06
0.05
74.2
69.7
63.2
28.4
4.80
2.84
1.01 
107
4.93 
105
17.5 
106
7.60 
106
(a) measured by UV-Vis absorption edge. (b) Singlet energy was calculated from onset
wavelength of fluorescent emission. (c) Triplet energy was calculated from onset wavelength
of phosphorescent emission. (d) PLQY measurement was performed on 5wt. % doped film in
CzTrz under nitrogen. (e) Prompt and delayed fluorescence lifetime from transient PL. (f)
rate constants of emitters were calculated using the equation in the reference.[40]
Thermal properties
Basic thermal properties of the emitters were analyzed by differential scanning calorimeter
and thermogravimetric analyzer to gather glass transition temperature and thermal
decomposition temperature. The analysis data are described in Figure S3 in supporting
information. The glass transition temperatures are 166 oC and 167 oC for 2Cz3Trz and
3Cz2Trz. The thermal decomposition temperatures of the 2Cz3Trz and 3Cz2Trz at 5%
weight loss were 516.1 and 501.4 oC, respectively. The two emitters exhibited similar thermal
properties because they are isomers with the same molecular weight.
Device performances
The photophysical properties of the 2Cz3Trz and 3Cz2Trz emitters were appropriate for use
of them as the TADF emitters for TADF OLEDs. Therefore, TADF devices of 2Cz3Trz and
3Cz2Trz were developed by dispersing them in the CzTrz host at a doping concentration of
10%. The device performances at 10% doping concentration were described because the
device performances of 2Cz3Trz and 3Cz2Trz were optimized near 10% doping

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concentration. The device results at other doping concentrations are shown in Figure S4 and
Table S1 in supporting information.
The current density (J) and luminance (L) data of the TADF OLEDs are presented in Figure
6 according to driving voltage of the devices. The J and L of the two devices were analogous
because the same host was used in the device structure. Although the TADF dopant was
different, the energy level of the dopant was similar as shown in the energy level diagram in
Figure 7, which did not make any difference in the J and L of the devices.
60
50
40
30
20
10
0
100000
10000
1000
100
2Cz3Trz
3Cz2Trz
2Cz3Trz
3Cz2Trz
10
1
0.1
4
6
8
10
12
14
Voltage (V)
Figure 6. J and L data of the 2Cz3Trz and 3Cz2Trz devices according to voltage.
2.1
2.36
2.4
3.07
3.18
2.9
3.20
3.36
3.36
5.1
5.2
5.5
5.71
5.77
5.78
6.08
6.1
6.64

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Figure 7. Energy level diagram of the 2Cz3Trz and 3Cz2Trz devices.
However, the EQE of the TADF devices was high in the 2Cz3Trz device as can be inferred
from the high PL quantum yield (Figure 8). The higher PL quantum yield of the 2Cz3Trz
emitter than that of the 3Cz2Trz increased the EQE due to high probability of radiative
transition assisted by the HOMO and LUMO overlap caused by the acceptor substitution to
the position with the LUMO distribution. The maximum EQEs of the 2Cz3Trz and 3Cz2Trz
devices were 17.9 and 15.0%, respectively. The J values at the 50% of the maximum EQE
were 67.4 and 72.8 mA/cm2 in the 2Cz3Trz and 3Cz2Trz, suggesting that 3Cz2Trz is
advantageous to improve efficiency roll-off of the devices. The noticeable difference of the
efficiency roll-off of the two devices can be correlated with the reverse intersystem crossing
(RISC) process of the emitters. Fast RISC process would suppress exciton quenching
processes triggered by long-living triplet excitons at high J, whereas slow RICS process
would initiate them. Therefore, the 3Cz2Trz device with the large RISC rate constant (Table
1) showed improved efficiency roll-off behavior in the devices.
The device stability data of the 2Cz3Trz and 3Cz2Trz at 1,000 cd/m2 are shown in supporting
information (Figure S5). The device lifetime of the 2Cz3Trz device was longer than that of
the 3Cz2Trz device due to the red-shifted EL emission spectrum.

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20
16
12
8
2Cz3Trz
3Cz2Trz
4
0
0.1
1
10
100
1000
Current density (mA/cm2)
Figure 8. EQE of the 2Cz3Trz and 3Cz2Trz devices according to J.
Figure 9 shows the electroluminescence (EL) spectra of the TADF devices. A noticeable red
shift of the EL spectrum was noted in the 2Cz3Trz device relative to that of the 3Cz2Trz
device. The peak wavelength of the EL spectra was shifted from 512 nm in the 3Cz2Trz
device to 521 nm in the 2Cz3Trz device. The relative position of the EL spectra matched with
that of the PL spectra. The color coordinates of the 2Cz3Trz and 3Cz2Trz were (0.30, 0.56)
and (0.26, 0.50), respectively. Both x and y coordinates were increased in the 2Cz3Trz device
by the red-shift of the EL spectra. Table 2 summarized device performances of the TADF
OLEDs.
2Cz3Trz
3Cz2Trz
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)

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Figure 9. EL spectra of the 2Cz3Trz and 3Cz2Trz devices at 1,000 cd/m2.
Table 2. Summarized device data of the 2Cz3Trz and 3Cz2Trz devices.
Power
efficiency
(lm/W)
Current
efficiency
(cd/A)
EQEmax
(%)
ELmax
(nm)
CIE (x,y)
2Cz3Trz]
3Cz2Trz
17.9
15.0
28.3
21.8
55.8
42.9
(0.30,0.56)
(0.26,0.50)
521
512
Conclusions
The synthesis and comparison of the two dibenzofuran cored TADF emitters with different
donor and acceptor positions, 2Cz3Trz and 3Cz2Trz, revealed that the substitution of the
acceptor at the node position of LUMO in dibenzofuran is disadvantageous to increase the PL
quantum yield of the emitters, but is advantageous to reduce the delayed fluorescence lifetime
of the TADF emitters. The importance of the substitution position based on the frontier
molecular orbital of the core structure was recognized by collecting photophysical and device
data of the two TADF emitters. Therefore, the selection of the substitution position
considering the frontier molecular orbital needs to be carefully managed to achieve high EQE
and proper color coordinate.
Experimental
Synthetic details are in supporting information.
Device fabrication and measure

Journal of Materials Chemistry C
Page 18 of 21
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DOI: 10.1039/C9TC01577A
The OLED structure is indium tin oxide (ITO, 50 nm)/N,N′-diphenyl-N,N′-bis-[4-(phenyl-m-tolyl-
amino)-phenyl]-biphenyl-4,4′-diamine (DNTPD, 60 nm)/N,N,N′N′-tetra[(1,10-biphenyl)-4-yl]-(1,10-
biphenyl)-4,4′-diamine
(BPBPA,
20
nm)/9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-
dihydroacridine (PCZAC, 10 nm)/(3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-(1,1′-biphenyl)-3-yl)-9-
carbazole (CzTrz):TADF dopant (30 nm, 10wt. % doping)/CzTrz (5 nm)/2-[4-(9,10-Di-naphthalen-2-
yl-anthracene-2-yl)-phenyl]-1-phenyl-1H-benzimidazole (ZADN, 30 nm)/LiF (1.5 nm)/Al (200 nm).
The emitting layer comprises a CzTrz film doped with 10 wt. % TADF molecules. All of devices were
fabricated under a high vacuum condition (<1.0×10−7 Torr). J–V–L measurements were carried out
using an electrical and photometric measurement system equipped with a CS 2000 (Konica Minolta
Inc.) spectroradiometer and a Keithley 2400 sourcemeter (Keithley Instruments, Inc) for J–V
measurement.
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