Design Approach of Lifetime Extending Thermally Activated Delayed Fluorescence Sensitizers for Highly Efficient Fluorescence Devices

Article abstract of DOI:10.1002/chem.202004042

In this work, a design approach of three thermally activated delayed fluorescence (TADF) emitters to extend the device lifetime of the TADF sensitized fluorescent devices was studied. Three TADF materials, 5-{4,6-bis[4-(tert-butyl)phenyl]-1,3,5-triazin-2-yl}-2-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazol-5-yl)benzonitrile (tTCNTruX), 4-[3-cyano-4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazol-5-yl)phenyl]-2,6-diphenylpyrimidine-5-carbonitrile (PCNTruX) and 4-(4-{10,15-bis[4-(tert-butyl)phenyl]-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazol-5-yl}-3-cyanophenyl)-2,6-diphenylpyrimidine-5-carbonitrile (PCNtTruX), were synthesized as sensitizers for TADF-sensitized fluorescent organic light-emitting diodes. The two tTCNTruX and PCNtTruX TADF emitters were designed to have Dexter energy transfer with blocking groups either in the donor or acceptor unit of the donor–acceptor-type TADF sensitizer. The TADF materials showed small singlet–triplet energy splitting and a high reverse intersystem crossing (RISC) rate for effective sensitization of the fluorescent emission of the fluorescent emitter. tTCNTruX- and PCNtTruX-sensitized fluorescent devices showed maximum external quantum efficiencies (EQEs) of 17.7 % and 11.5 % in the yellow and red devices, respectively, which were higher than those of TADF-sensitized devices with the corresponding TADF sensitizer without a blocking group. Moreover, the device lifetime was also extended by employing the tTCNTruX and PCNtTruX sensitizers. This work demonstrated that the tTCNTruX and PCNtTruX sensitizers are effective to improve the maximum EQE and device lifetime of TADF-sensitized fluorescent devices.

Full text of DOI:10.1002/chem.202004042

Accepted Article
Accepted Article
Title: Design approach of lifetime extending thermally activated
delayed fluorescence sensitizers for highly efficient fluorescence
devices
Authors: Sung Joon Yoon, Ji Han Kim, Won Jae Chung, and Jun Yeob
Lee
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004042
Link to VoR: https://doi.org/10.1002/chem.202004042
0
1/2020

Chemistry - A European Journal
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Table-of-content
Efficient TADF sensitizer: Three TADF materials having fast reverse intersystem crossing rate
were developed for TADF sensitizer. Among these, two TADF materials include tert-butyl
blocking group to manage Dexter energy transfer. The TADF sensitized fluorescent devices
with blocking groups showed high maximum external quantum efficiencies. In addition, the
device operational lifetime was extended by adding blocking groups.
1
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Chemistry - A European Journal
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Design approach of lifetime extending thermally activated delayed fluorescence
sensitizers for highly efficient fluorescence devices
Sung Joon Yoon, Ji Han Kim, Won Jae Chung, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu,
Suwon-si, Gyeonggi-do, 440-746, Republic of Korea
*
Corresponding author
E-mail: leej17@skku.edu
Abstract
In this work, a design approach of three thermally activated delayed fluorescence (TADF)
emitters to extend the device lifetime of the TADF sensitized fluorescent devices was studied.
Three TADF materials, 5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-(10,15-diphenyl-
1
4
0,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)benzonitrile (tTCNTruX), 4-(3-cyano-
-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl)-2,6-
diphenylpyrimidine-5-carbonitrile (PCNTruX) and 4-(4-(10,15-bis(4-(tert-butyl)phenyl)-
0,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)-3-cyanophenyl)-2,6-
1
diphenylpyrimidine-5-carbonitrile (PCNtTruX), were synthesized as the sensitizers of the
TADF sensitized fluorescent organic light-emitting diodes. The two tTCNTruX and
PCNtTruX TADF emitters were designed to have Dexter energy transfer managing blocking
groups either in the donor or acceptor unit of the donor-acceptor type TADF sensitizer. The
TADF materials showed small singlet-triplet energy splitting and high reverse intersystem
crossing (RISC) rate for effective sensitization of the fluorescent emission of the fluorescent
emitter. The tTCNTruX and PCNtTruX sensitized fluorescent devices showed maximum
external quantum efficiencies (EQEs) of 17.7% and 11.5% in the yellow and red devices,
2
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respectively, which were higher than those of the TADF sensitized devices with the
corresponding TADF sensitizer without the blocking group. Moreover, the device lifetime was
also extended by employing the tTCNTruX and PCNtTruX sensitizers. This work
demonstrated that the tTCNTruX and PCNtTruX sensitizers are effective to upgrade the
maximum EQE and device lifetime of the TADF sensitized fluorescent devices.
Key words : Organic light-emitting diodes, TADF, TADF sensitizer, blocking group
Introduction
Fluorescence emitters have been the first generation light-emitting materials of organic light-
emitting diodes (OLEDs) showing long lifetime and high color purity. However, fluorescence
emitters can achieve theoretical internal quantum efficiency (IQE) of only 25% according to
the spin statistics, which a big hurdle of them. To tackle the low efficiency of the fluorescence
emitters, thermally activated delayed fluorescence (TADF) emitters enabling theoretical IQE
of 100% were developed recently [1-4]. The basic principle of TADF is the triplet exciton up-
conversion from triplet excited state to singlet excited state through the reverse intersystem
crossing (RISC) due to small singlet-triplet energy splitting (∆EST). To get the small ∆EST, the
orbital overlap between the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) should be minimized. Therefore, the TADF materials
are generally designed to have electron acceptor and electron donor moieties to separate the
HOMO and LUMO [1]. However, this design strategy involves structure relaxation that results
in increased Stokes shift and broad full width at half maximum (FWHM) harmful to the color
purity [5, 6]. Moreover, the TADF devices suffer from short device lifetime compared to the
fluorescence devices.
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In 2014, Adachi group developed the so-called hyperfluorescence (TADF sensitized
fluorescence) system combining TADF sensitizer and fluorescence emitter to overcome the
disadvantages of the TADF and fluorescence [7]. In the hyperfluorescence system, the TADF
sensitizer harvests triplet excitons to singlet excitons by RISC process and the up-converted
singlet excitons transfer emission energy to the fluorescence emitter by Förster resonance
energy transfer (FRET) process. The hyperfluorescence device can have advantage over the
TADF device for operation lifetime because of accelerated RISC process reducing the triplet
exciton lifetime in the emission process [8]. Additionally, the color purity is improved since
the emission comes from the fluorescence materials. However, the triplet exciton energy of the
TADF sensitizer is also transferred to the fluorescence emitter by Dexter energy transfer (DET)
which is critical to the device performances. Therefore, fast RISC of the TADF sensitizer and
suppression of DET from the TADF sensitizer to the fluorescence emitter are needed to show
high efficiency. To hinder the DET process, interrupting orbital overlap by increasing the
distance between the TADF sensitizer and fluorescence emitter is efficient [9, 10]. In particular,
the addition of blocking groups to the TADF sensitizer can reduce the intermolecular
interaction and inhibit the DET process.
In this work, we developed 5-(4,6-bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-2-(10,15-
diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)benzonitrile (tTCNTruX), 4-
(3-cyano-4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl)-
2
1
,6-diphenylpyrimidine-5-carbonitrile (PCNTruX) and 4-(4-(10,15-bis(4-(tert-butyl)phenyl)-
0,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)-3-cyanophenyl)-2,6-
diphenylpyrimidine-5-carbonitrile (PCNtTruX) as TADF sensitizers in yellow and red
hyperfluorescence devices. The tTCNTruX was compared with 5-(4,6-diphenyl-1,3,5-triazin-
2
-yl)-2-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)benzonitrile
4
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(TCNTruX) in the yellow hyperfluorescence OLEDs doped with 2,8-di-tert-butyl-5,11-bis(4-
tert-butylphenyl)-6,12-diphenyltetracene (TBRb) [11]. Whereas, the PCNTruX and
PCNtTruX were used as TADF sensitizers in the red hyperfluorescence OLEDs doped with
5
,10,15,20-tetraphenylbisbenz[5,6]indeno[1,2,3-cd:1’,2’,3’-lm]perylene
(DBP).
The
tTCNTruX and PCNtTruX sensitized hyperfluorescence devices showed maximum external
quantum efficiencies (EQEs) of 17.7% and 11.5% in the yellow and red devices, respectively,
which were higher than those of the TCNTruX and PCNTruX sensitized devices. Additionally,
the device lifetime of the hyperfluorescence devices was extended by using the TADF
sensitizers with the blocking groups.
Results and discussion
The main design concept of the TADF sensitizers is to add tert-butyl units to the donor or
acceptor part of the TADF molecules to suppress the DET process and enhance device
operational lifetime in the yellow and red hyperfluorescence system. The basic donor moiety
of the molecular design was 5,10-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole
(TruX) which causes a large torsion angle by connection with a phenyl linker. In previous work,
3
-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (TCN) and 4-(3-cyanophenyl)-2,6-
diphenylpyrimidine-5-carbonitrile (PCN) were integrated with the TruX unit. The TCN unit
was modified with tert-butyl blocking groups, offering the tTCNTruX TADF material. The
acceptor of the red TADF sensitizer was the PCN moiety which has stronger electron deficiency
than the TCN moiety because of additional CN group. Instead of the tert-butyl modification of
the acceptor unit, the TruX donor unit was protected with the tert-butyl blocking group to
synthesis PCNtTruX. Two TADF materials, tTCNTruX and PCNtTruX, were designed to
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identify the influence of blocking groups on the energy transfer, efficiency and device lifetime
of the hyperfluorescence system. The synthesis of the TADF materials is shown in Scheme 1
and the scheme for the intermediate materials is presented in the supporting Scheme S1. The
details of the reaction conditions and reagents are presented in experimental part of the
supporting information. The final products purified by column chromatography and
sublimation were confirmed through nuclear magnetic resonance (NMR) and gas
chromatography high resolution mass spectrometry (GC-HRMS).
Scheme 1. Synthesis scheme of tTCNTruX, PCNTruX and PCNtTruX, and chemical
structure of TCNTruX.
Through Gaussian 16 based on the B3LYP 6-31G basis set, the orbital distribution and
optimized geometry of the three TADF materials in the ground state were determined by
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electronic calculations. Figure 1 shows the orbital distribution of the HOMO/LUMO and the
calculated photophysical properties of the tTCNTruX, PCNTruX and PCNtTruX. The
optimized structure of the TADF materials in ground state is shown in Figure S1. As a result of
the simulation, the dihedral angles between the acceptor and donor moieties were 72.5°, 76.9°
and 65.6° in the tTCNTruX, PCNTruX and PCNtTruX owing to the steric hindrance of the
TruX unit. Due to the twisted structure between the donor and acceptor units, the HOMO and
LUMO distributions showed almost separated molecular orbitals. The HOMO was commonly
localized on the TruX electron donating unit, and the LUMO was dispersed over the TCN and
PCN electron withdrawing unit. The tert-butyl groups had little effect on the HOMO and
LUMO orbital pictures. The calculated HOMO/LUMO levels of the tTCNTruX, PCNTruX
and PCNtTruX were -4.92/-2.12, -4.85/-2.42 and -4.87/2.41 eV, respectively. The LUMO level
was estimated to be deeper in the PCNTruX and PCNtTruX due to the strongly electron
deficient PCN moiety by the pyrimidine and an additional CN unit. The calculated
singlet/triplet energies of tTCNTruX, PCNTruX and PCNtTruX were 2.37/2.33 eV,
2
.06/2.05 eV and 2.11/2.10 eV, respectively. The ΔEST values were less than 0.04 eV, which
was small enough to trigger RISC process. The tert-butyl addition had little effect on the energy
level and emission energy of the TADF materials. The calculated oscillator strength was higher
in the tTCNTruX than the PCNtTruX, indicating that tTCNTruX may show higher PLQY
value than PCNtTruX.
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Figure 1. Calculated HOMO and LUMO distributions, and energy levels of tTCNTruX,
PCNTruX and PCNtTruX.
The ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) measurements were
used to determine the photophysical properties of the tTCNTruX, PCNTruX and PCNtTruX
in dilute tetrahydrofuran (THF) solution. Figure 2 shows UV-Vis absorption and PL spectra of
the three TADF materials. In the UV-Vis absorption spectra, the three materials showed similar
spectra because their backbone structure was analogous. The only difference was the slight red
shift of the absorption spectra in the PCN derived materials. The broad absorption below 500
nm region demonstrates the intramolecular charge transfer (ICT) transition from the TruX
donor to the acceptor unit. The molar extinction coefficient (ε) of the tTCNTruX material was
3
-1
-1
3
-1
-1
relatively larger (6.6 × 10 M cm ) than that of the others (4.5 × 10 M cm ), indicating the
strong absorption by large HOMO/LUMO overlap in tTCNTruX. The onset values of the UV-
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Vis absorption were 476 nm, 482 nm and 484 nm in the tTCNTruX, PCNTruX and
PCNtTruX, respectively. The singlet and triplet energy values of the TADF materials were
measured by fluorescent and phosphorescent PL measurements in dilute THF solution at 77 K
without and with 2.0 ms delay time. The singlet/triplet energy values of the tTCNTruX,
PCNTruX and PCNtTruX were 2.68/2.65, 2.66/2.67 and 2.64/2.68 eV, respectively. The peak
wavelengths of the fluorescence emission of the tTCNTruX, PCNTruX and PCNtTruX were
5
32, 544 and 547 nm, respectively. Two materials, PCNTruX and PCNtTruX, showed long
emission wavelength because of the strong PCN acceptor moiety. The measured ΔEST values
were small enough to show TADF characteristics. The negative ΔEST of the PCNTruX and
PCNtTruX might be due to the local triplet emission of the emitters.
Cyclic voltammetry (CV) was used to estimate the HOMO and LUMO energy levels of the
TADF materials. The CV data of three TADF materials are shown in Figure S2. The
HOMO/LUMO energy levels of tTCNTruX, PCNTruX and PCNtTruX were -5.78/-3.53, -
5
.83/-3.64 and -5.83/-3.63 eV, respectively, which agreed with the tendency of the molecular
simulation. As the PCNTruX and PCNtTruX contain the strongly electron deficient PCN unit,
the LUMO energy level was deep.
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1
0
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Figure 2. UV-Vis absorption spectra (a) and fluorescence/phosphorescence emission spectra
of the (b) tTCNTruX (c) PCNTruX and (d) PCNtTruX.
The absolute PL quantum yields (PLQYs) of tTCNTruX and PCNtTrux were measured to
identify PL efficiency of the TADF materials. The TCN derived tTCNTruX were analyzed in
the 9-(3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-3-yl)-9H-carbazole (CzTrz) host at
2
0 wt% doping concentration, and the PCN derived PCNtTruX were investigated in the
2,12′-(6-phenyl-1,3,5-triazine-2,4-diyl)bis(11-phenyl-11,12-dihydroindolo[2,3-a]carbazole)
1
(PBICT)/4-(3-(triphenylen-2-yl)phenyl)dibenzo[b,d]thiophene (DBTTP1) mixed host (70/30)
at 10 wt% doping concentration [12]. The PLQY of the TCNTruX and PCNTruX was also
measured for comparison. The PLQY value of TCNTruX/tTCNTruX under nitrogen was
measured to be 47.9/65.2%, and that of PCNTruX/PCNtTruX was 47.1/45.0%. The
TCNTruX and PCNTruX showed similar PLQY values, whereas the tTCNTruX exhibited
higher PLQY than PCNtTruX. The tert-butyl modification increased the PLQY of the
tTCNTruX. The reason might be the difference of CT character by the tert-butyl modification.
The tert-butyl groups on the acceptor moiety in tTCNTruX enhance the HOMO/LUMO
overlap by decreasing the CT character through the weakened acceptor character. The
enhancement of HOMO/LOMO overlap leads to relatively high PLQY value. Whereas, the
PCNtTruX which has tert-butyl groups on the donor moiety tends to have opposite effect [13].
Additionally, the tert-butyl group of tTCNTruX can prevent intermolecular interaction
between planar acceptors for suppressed exciton quenching, enhancing the PLQY.
The transient PL (TRPL) was used to measure prompt fluorescence decay time (τ
p
) and
delayed fluorescence decay time (τ ) to study photophysical dynamics of the emitters. The
d
1
1
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TRPL decays of the TCN derivatives and PCN derivatives were almost the same, indicating
that tert-butyl groups did not affect the decay time. The τ values were 75.8/76.3 ns in
TCNTruX/tTCNTruX, and 102.4/126.5 ns in PCNTruX/PCNtTruX, respectively. The τ
p
d
values were 1.08/1.28 μs in TCNTruX/tTCNTruX, and 1.06/1.00 μs in
PCNTruX/PCNtTruX. The prompt and delayed fluorescence curves of TRPL are shown in
Figure 3. The intersystem crossing (ISC), RISC, radiative (k S) and non-radiative (knrT) rate
r
constants of the TADF emitters were calculated by PLQY and TRPL measurement values using
the method reported in the literature [14]. Table 1 shows the summary of the PLQY, TRPL data
5
-1
and rate constants of the four TADF materials. The RISC rate constants (×10 s ) of the four
5
-1
TADF materials were calculated to be over 13.6, and non-radiative rate constants (×10 s )
were less than 6.94 indicating that the four TADF materials are suitable for the TADF sensitizer
in the hyperfluorescence devices.
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Figure 3. Transient PL graphs of (a) Prompt and (b) Delay fluorescence of tTCNTruX,
PCNTruX, PCNtTruX and reference material TCNTruX.
Table 1. The PLQY, transient PL data and calculated rate constants of tTCNTruX, PCNTruX,
PCNtTruX and reference material TCNTruX.
PLQY
S
T
τ
p
τ
d
k
ISC
6
k
RISC
k
r
k
nr
(
a)
(b)
Ф
Ф
F
Ф
TADF
-1
5
-1
6
-1
5 -1
[
ns]
[μs]
[×10 s ] [×10 s ] [×10 s ] [×10 s ]
[
%]
[%]
21.2
29.3
[%]
TCNTruX
tTCNTruX
47.9
65.2
26.7
35.9
75.8
76.3
1.08
1.28
10.4
9.27
14.8
13.6
2.80
3.84
6.14
3.86
1
3
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PCNTruX
PCNtTruX
47.1
45.0
21.4
21.0
25.7
24.0
102.4
126.5
1.06
1.00
7.67
6.24
14.4
14.4
2.09
1.66
6.35
6.94
(
a) Absolute PL quantum yield measured under an oxygen atmosphere (b) Absolute PL quantum yield of the
TADF component
In addition, the TBRb and DBP fluorescence dopants were additional doped at 0.5 wt%
doping concentration to the TADF materials to verify the photophysical dynamics in
hyperfluorescence system by PLQY and TRPL measurements. The TRPL graphs of the four
TADF emitters with the fluorescence dopants are shown in Figure 4. The delayed fluorescence
decay time of the TADF materials was shortened by the doping of the fluorescent emitters. The
fast FRET process from the TADF sensitizer to the fluorescent dopant accelerates the RISC
process of the TADF materials, shortening the delayed decay time. The PLQY values of
TCNTruX:TBRb/tTCNTruX:TBRb and PCNTruX:DBP/PCNtTruX:DBP were 72.8/82.8%
and 76.4/75.7%, respectively. The PLQY was increased by the doping of the fluorescence
dopants because the emission energy of the non-radiative excitons were transferred to the
fluorescent dopant.
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Figure 4. Transient PL graphs of the (a) TCNTruX/tTCNTruX without or with TBRb and (b)
PCNTruX/PCNtTruX without or with DBP.
To verify the performance of the TADF materials as the TADF emitters and sensitizers, the
electroluminescence (EL) devices were fabricated with the TADF materials and fluorescent
emitters. The common structure of the EL devices was indium tin oxide (ITO) (50 nm)/ N1,N1-
(biphenyl-4,4-diyl)bis(N1-phenyl-N4,N4-di-m-tolylbenzene-1,4-diamine) (DNTPD) (60 nm)/
N,N,N′,N′-tetra[(1,1′-biphenyl)-4-yl]-(1,1′-biphenyl)-4,4′-diamine (BPBPA) (20 nm)/ 9,9-
dimethyl-10-(9-phenyl-9H-carbazol3-yl)-9,10-dihydroacridine (PCZAC) (10 nm)/ emitting
layer (30 nm)/ 2,8-bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]furan (DBFTrz) (5 nm)/ 2-
1
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[
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 20 wt% tTCNTruX or TCNTruX doped emitting
(
layer with and without the 0.5 wt% doped TBRb yellow fluorescence dopant was used in the
yellow devices. The CzTrz host material was commonly applied in the yellow TADF and
hyperfluorescence device. Similarly, the 10 wt% doped PCNTruX or PCNtTruX TADF
materials were used as the emitting layer in the TADF device and that of 20 wt% TADF
materials and 0.5 wt% DBP red fluorescence dopant was employed in the red devices. The
PBICT and DBTTP1 mixed host (70:30) was commonly used in the red TADF and
hyperfluorescence devices. Figure 5 shows the energy level diagram of the devices.
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Figure 5. The energy level diagram of the TCNTruX, tTCNTruX, PCNTruX and
PCNtTruX devices.
The current density (J)-voltage (V)-luminance (L) data, EQE-L plots and EL spectra of the
TADF devices and hyperfluorescence devices are shown in Figure 6 and Figure 7. The J values
of the TCNTruX and tTCNTruX devices were analogous, indicating that the tert-butyl
blocking group has little effect on the carrier transport properties of the emitters considering
the similar HOMO and LUMO levels of the two TADF materials. The PCNTruX and
PCNtTruX devices followed the same trend. The maximum EQEs of the TCNTruX and
1
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tTCNTruX devices were 15.3 and 18.4 % at 20 wt% doping concentration with efficiency
-2
drop of only 0.6 and 0.5% at 1,000 cd m , respectively. The EQE was rather degraded in the
PCNTruX (10.2%) and PCNtTruX (9.3%) devices by the non-radiative loss according to the
energy gap law. The maximum EQEs of the four TADF materials followed the trend of the
-2
PLQY values. The EQE reduction at 1,000 cd m was only 2.2%. Generally, the TADF
5
-1
materials have RISC rate of about 10 s , but the four TADF materials achieved 10 times higher
RISC rate than other TADF materials. Accordingly, the up-conversion was facilitated, which
suppressed the EQE roll-off of the TADF devices.
The ELpeak wavelengths of the TCNTruX, tTCNTruX, PCNTruX and PCNtTruX devices
were 554, 548, 563 and 565 nm, respectively. The PCNTruX and PCNtTruX devices emitted
relatively at long wavelengths as shown in the PL spectra of the TADF materials. The short
wavelength shift of the EL spectrum of tTCNTruX relative to that of TCNTruX is due to the
slightly weakened acceptor strength by the electron donating character of the tert-butyl unit.
The Commission International de l’Eclairage (CIE) coordinates of the TCNTruX, tTCNTruX,
PCNTruX and PCNtTruX devices were (0.42, 0.55), (0.40, 0.56), (0.46, 0.52) and (0.47, 0.52),
respectively.
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Figure 6. (a) The J-V-L data, (b) EQE-L plots and (c) EL spectrum of the TCNTruX and
tTCNTruX TADF devices and sensitized TBRb devices.
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Figure 7. (a) The J-V-L data, (b) EQE-L plots and (c) EL spectrum of the PCNTruX and
PCNtTruX TADF devices and sensitized DBP devices.
In the hyperfluorescence devices, the maximum EQEs of the TCNTruX and tTCNTruX
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devices with the TBRb yellow dopant were 15.7 and 17.7%, while those of the PCNTruX and
PCNtTruX devices with the DBP red dopant were 10.4 and 11.5%, respectively. As a result,
the hyperfluorescence devices with the tert-butyl modified TADF materials showed improved
EQE. In the TCN derivatives, the EQE of the hyperfluorescence devices followed the EQE
tendency of the TADF devices. Whereas, in the red hyperfluorescence devices, the PCNtTruX
device showed higher EQE than the PCNTruX device in spite of low EQE in the TADF device.
This result can be interpreted based on the energy transfer process from the TADF materials
to the fluorescent dopant. In general, both FRET and DET contribute to the energy transfer
process in the hyperfluorescence devices. The FRET process is desirable for high EQE, but the
DET process has to be avoided because it is an exciton loss mechanism. Therefore, the DET
has to be managed by molecular engineering. In the PCNtTruX material, the tert-butyl was
added as the bulky blocking group and it can control the DET from it to the DBP dopant. The
suppression of the DET process can up-convert more triplet excitons into singlet excitons,
which can increase the EQE of the hyperfluorescence devices. To prove the DET suppressing
role of the tert-butyl blocking groups, PLQY dependence of the PCNTruX and PCNtTruX
emitting layer was measured by changing the doping concentration of the TADF compound
and DBP. Table S1 and Table S2 show the PLQY values of hyperfluorescence emitting layer.
When the tert-butyl group was absent, the PLQY was reduced by increasing the TADF and
DBP doping concentration because of DET. However, the PLQY remains almost constant
irrespective of the TADF and DBP doping concentration because the DET is suppressed. In the
case of the tTCNTruX, the tert-butyl unit suppressed quenching mechanisms of the TADF
emission and improve radiative transition probability as shown in the PLQY, which increased
the EQE of the hyperfluorescence devices. Therefore, the device performances of the
hyperfluorescence device agreed with the trend of those of the TADF device.
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The EL spectra of the hyperfluorescence devices represent that the EL emission is originated
partially from the TADF materials and partially from the fluorescence dopant. The CIE
coordinates of the TCNTruX and tTCNTruX sensitized TBRb devices were (0.43, 0.54) and
(0.43, 0.55), respectively, and that of the PCNTruX and PCNtTruX sensitized DBP devices
was (0.63, 0.37). Table 2 and Table 3 show the EL performances of the TADF and
hyperfluorescence devices.
Table 2. EL performance of the TCNTruX and tTCNTruX TADF devices and
hyperfluorescence devices.
Device
lifetime(e)
[h]
External Quantum
efficiency [%]
Power efficiency
[lm/W]
Current efficiency
[cd/A]
(
b)
(c)
V
on
CIE(d)
(x, y)
V
d
Device(a)
[
V]
[V]
4.4
4.1
4.5
4.1
2
2
2
Max
15.3
Max
26.5
Max
48.1
1
000cd/m
1000cd/m
1000cd/m
TCNTruX
(
(
(
(
0.42,
6
6
6
5
.0
.0
.3
.9
15.2
25.1
47.8
393
734
392
593
0
.55)
(
20%)
tTCNTruX
20%)
0.40,
18.3
15.5
17.5
18.4
15.7
17.7
30.5
25.0
30.5
32.5
26.8
32.5
58.3
50.2
57.6
58.5
50.7
58.1
0
.56)
(
TCNTruX:TBRb
20%:0.5%)
0.43,
0
.54)
(
tTCNTruX:TBRb
20%:0.5%)
0.43,
0
.55)
(
-
2
(
a) ITO/DNTPD/BPBPA/PCZAC/emitter:CzTrz/DBFTrz/ZADN/LiF/Al (b) V
d
: driving voltage at 1000 cd m (c) Von: turn
-
2
-2
-2
on voltage (driving voltage at 1 cd m ) (d) At 1000 cd m (e) LT90 at 1000 cd m
Table 3. EL performance of the PCNTruX and PCNtTruX TADF devices and
hyperfluorescence devices.
External Quantum
efficiency [%]
Power efficiency
[lm/W]
Current efficiency
[cd/A]
Device
lifetime(e)
[h]
(
b)
(c)
V
on
CIE(d)
(x, y)
V
d
Device(a)
[
V]
[V]
2
2
2
Max
Max
Max
1
000cd/m
1000cd/m
1000cd/m
2
2
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10.1002/chem.202004042
PCNTruX
(
(
(
(
0.46,
5
5
6
6
.5
.8
.6
.7
3.3
3.5
3.5
3.5
10.1
9.1
10.2
9.3
16.8
14.0
4.3
22.1
19.1
10.3
13.1
29.4
25.7
9.0
29.7
26.1
13.1
14.6
2360
1685
599
0
.52)
(
10%)
PCNtTruX
10%)
0.47,
0
.52)
(
PCNTruX:DBP
20%:0.5%)
0.63,
6.4
10.4
11.5
0
.37)
(
PCNtTruX:DBP
20%:0.5%)
0.63,
.37)
6.7
4.6
9.7
919
0
(
(
(
a) ITO/DNTPD/BPBPA/PCZAC/emitter:PBICT:DBTTP1/DBFTrz/ZADN/LiF/Al (b) V
d
: driving voltage at 1000 cd m-2
-
2
-2
-2
c) Von: turn on voltage (driving voltage at 1 cd m ) (d) At 1000 cd m (e) LT90 at 1000 cd m
The device operational lifetime of the TADF and hyperfluorescence devices was measured at
-2
1
,000 cd m at a constant current operation mode. Figure 8 shows the device lifetime curves
of the TADF and hyperfluorescence devices of TCNTruX, tTCNTruX, PCNTruX and
PCNtTruX. In the case of TCN derivatives, tTCNTruX achieved long device lifetime
compared to TCNTruX in both TADF and hyperfluorescence devices. The TCNTruX and
tTCNTruX sensitized TBRb devices showed the device lifetime of 392 and 593 h up to 90%
-2
of the initial luminance (LT90) at 1,000 cd m , respectively. Compare with 4CzIPN-Me:TBRb
hyperfluorescence device in previous work, the device lifetime was extended by more than 2
times [15]. However, in the case of the PCN derivatives, the PCNtTruX with the tert-butyl
blocking groups achieved shorter device lifetime in the TADF device, but longer lifetime in the
hyperfluorescence device than the PCNTruX device. The opposite trend of the device lifetime
in the TADF and hyperfluorescence devices might be due to the DET control in the
hyperfluorescence devices. As the DET is an exciton loss mechanism, the DET process would
accelerate the efficiency loss of the hyperfluorescence device corresponding to the lifetime
degradation. Therefore, the DET management is helpful to the device lifetime of the
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hyperfluorescence device, increasing the device lifetime of the PCNtTruX device. The
PCNTruX and PCNtTruX sensitized DBP devices showed the lifetime of 599 and 919 h
-2
(
LT90) at 1,000 cd m , respectively. This shorter device lifetime of the PCNtTruX TADF
device may be due to long delayed fluorescence lifetime and low EQE of the PCNtTruX emitter
relative to that of PCNTruX in spite of DET suppressing function of the tert-butyl units. The
relatively short device lifetime of the red hyperfluorescence device compared to the TADF
device is caused by red shift of the emission spectrum. As the device lifetime was measured at
the same luminance, the red hyperfluorescence device shows shorter device lifetime than the
yellow TADF device.
Figure 8. Device operational lifetime curves of (a) TCNTruX, tTCNTruX and (b) PCNTruX,
-2
PCNtTruX TADF and hyperfluorescence devices at an initial luminance 1000 cd m .
Conclusions
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In this work, three TADF materials were synthesized to study the effect of tert-butyl blocking
groups on the EQE and device lifetime of the hyperfluorescence system. The tert-butyl unit
managed the DET process from the TADF sensitizer to the fluorescenct dopant, increasing the
EQE and device operational lifetime in the yellow and red hyperfluorescence devices.
Especially, tTCNTruX and PCNtTruX assisted hyperfluorescence devices demonstrated long
device operational lifetime in the yellow and red hyperfluorescence devices, respectively.
Therefore, the design of adding the tert-butyl blocking groups to the TADF sensitizer is
effective to achieve high EQE and long device operational lifetime.
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