Exciton stabilizing high triplet energy n-type hosts for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2021.109297

An exciton stabilizing electron transport type (n-type) host material was developed for application in long lifetime blue phosphorescent organic light-emitting diodes (PhOLEDs). A conjugation breaking diphenylsilyl backbone structure combined with a diphenyltriazine unit was a basic molecular platform to obtain high triplet energy and electron transport property. Two hosts with and without carbazole were synthesized as the bipolar and unipolar n-type hosts derived from diphenylsilyl backbone and diphenyltriazine unit. The blue PhOLEDs using the two hosts showed high external quantum efficiency above 20% and blue color coordinate of (0.14, 0.17). Particularly, the bipolar n-type host with the carbazole unit extended the device lifetime by more than 1.4 times compared with the unipolar host and by more than three times relative to a conventional diphenyltriazine based host without the diphenylsilyl core because of the exciton stabilizing function of the host. This work proved the potential of the diphenyltriazine and carbazole merged diphenylsilyl skeleton as a chemical platform of the n-type hosts.

Full text of DOI:10.1016/j.dyepig.2021.109297

Dyes and Pigments 190 (2021) 109297
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Exciton stabilizing high triplet energy n-type hosts for blue phosphorescent
organic light-emitting diodes
Ju Hui Yun1, Junseop Lim1, Won Jae Chung, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
An exciton stabilizing electron transport type (n-type) host material was developed for application in long
lifetime blue phosphorescent organic light-emitting diodes (PhOLEDs). A conjugation breaking diphenylsilyl
backbone structure combined with a diphenyltriazine unit was a basic molecular platform to obtain high triplet
energy and electron transport property. Two hosts with and without carbazole were synthesized as the bipolar
and unipolar n-type hosts derived from diphenylsilyl backbone and diphenyltriazine unit. The blue PhOLEDs
using the two hosts showed high external quantum efficiency above 20% and blue color coordinate of (0.14,
0.17). Particularly, the bipolar n-type host with the carbazole unit extended the device lifetime by more than 1.4
times compared with the unipolar host and by more than three times relative to a conventional diphenyltriazine
based host without the diphenylsilyl core because of the exciton stabilizing function of the host. This work
proved the potential of the diphenyltriazine and carbazole merged diphenylsilyl skeleton as a chemical platform
of the n-type hosts.
Blue PHOLEDs
Device lifetime
Triplet energy
Electron transport type host
1. Introduction
selection of the hole transport type (p-type) and electron transport type
(n-type) hosts of the mixed hosts [19,20].
Blue phosphorescent organic light-emitting diodes (PhOLEDs) have
been studied because of potential to obtain high external quantum ef-
ficiency (EQE) over 20% by full utilization of excitons in the emitting
layer [1–9]. Both host and phosphors were critical to the device per-
formances of the blue PhOLEDs and the host materials affected the EQE,
driving voltage, and device lifetime. Therefore, the host materials for
blue PhOLEDs should have high triplet energy to prevent back energy
transfer from phosphors to host, good hole and electron transport
properties, and good stability against carriers during device operation.
Single bipolar hosts can satisfy the several host requirements, but the
mixed host system is favored to meet the requirements and to achieve
high efficiency and long device lifetime, simultaneously. The main
causes of short device lifetime of blue PhOLEDs, triplet-triplet annihi-
lation (TTA) and triplet-polaron annihilation (TPA) [10–18], can be
suppressed in the mixed host by managing the triplet excitons and po-
larons. Triplet exciton density triggering the degradation mechanisms of
blue PhOLEDs can be controlled by broad recombination zone assisted
by bipolar charge transport properties of the mixed host, and polaron
leakage and polaron density can be well manipulated by proper
A lot of p-type and n-type hosts have been reported in the literatures
and representative n-type hosts have generally been diphenylphosphine
oxide, diphenylsulfone, pyridine, pyrimidine and diphenyltriazine de-
rivatives [21–29]. However, the diphenylphosphine oxide and sulfone
derivatives are unstable due to small bond dissociation energy, which
made them unsuitable for long lifetime blue PhOLEDs [30]. In the case
of the diphenyltriazine derivatives, it was difficult to obtain high triplet
energy in solid state because of planar backbone structure, which made
it difficult to develop the high triplet energy n-type hosts compared to
the p-type hosts. Moreover, the design of the n-type hosts which are
stable against polarons and excitons formed during the electrical oper-
ation of the mixed host device is also challenging. Recently, several
diphenylsilyl group based host materials were exemplified as the high
triplet energy hosts, but the device lifetime is still short [31,32].
In this work, we developed new n-type host materials with high
triplet energy for high EQE and long device lifetime in the blue PhO-
LEDs. Diphenylsilane was used as a backbone structure and diphenyl-
traizine unit was merged for electron transporting property. Two n-type
hosts,
bis(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)diphenylsilane
* Corresponding author.
E-mail address: leej17@skku.edu (J.Y. Lee).
1
Ju Hui Yun and Jun Seop Lim contributed equally.
https://doi.org/10.1016/j.dyepig.2021.109297
Received 4 November 2020; Received in revised form 25 February 2021; Accepted 8 March 2021
Available online 16 March 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

J.H. Yun et al.
Dyes and Pigments 190 (2021) 109297
(2TrzSi) and bis(3-(4-(9H-carbazol-9-yl)-6-phenyl-1,3,5-triazin-2-yl)
phenyl)diphenylsilane (2CzTrzSi), were synthesized and evaluated as
the n-type hosts in blue PhOLEDs. It was demonstrated that the blue
PhOLEDs built on the 2CzTrzSi based mixed host showed high EQE
above 20% and extended device lifetime by more than 1.4 times
compared with the 2TrzSi device and more than 3.7 times relative to
that of the control device with a planar diphenyltriazine derived host.
The exciton stabilizing function of the 2CzTrzSi host was responsible for
the improved device lifetime.
2. Results and discussion
Two n-type hosts, 2TrzSi and 2CzTrzSi, were synthesized based on
tetraphenylsilane backbone as a conjugation breaking group and
diphenyltriazine as a strong electron transport group. The molecular
structure of two hosts was targeted to have tetrahedral geometry unlike
other aromatic linked diphenyltriazine derived materials with planar
structure in order to obtain high triplet energy in solid state by sup-
pressing intermolecular stacking. The diphenyltriazine functional
groups were connected to the meta position of the phenyl unit to deliver
the 2TrzSi host. The 2CzTrzSi host had extra carbazole moieties to
control the singlet energy for exciton stabilization.
The synthetic processes of 2TrzSi and 2CzTrzSi are depicted in
Scheme 1. A triazine-carbazole intermediate 3 was produced through a
lithiation reaction of carbazole followed by reaction with 2,4-dichloro-
6-phenyl-1,3,5-triazine. The brominated tetraphenylsilane intermedi-
ate 1 prepared according to reported work [33,34] was transformed into
a boronic ester form and coupled with 2-chloro-4,6-diphenyl-1,3,
5-triazine and intermediate 3 by Suzuki-Miyaura coupling reaction to
provide the 2TrzSi and 2CzTrzSi hosts. All reaction steps were pro-
ceeded in good yields over 50% and the chemical structure of the final
hosts was confirmed by mass spectrometer and 1H/13C nuclear magnetic
resonance (NMR) spectrometer (Figure S1 in supporting information).
Photoluminescence (PL) analysis of the two compounds was carried
out to investigate the possibility of them as n-type hosts for blue PhO-
LEDs. Fluorescent and phosphorescent emission spectra in Fig. 1 were
collected to estimate the singlet and triplet energies of the hosts. Room
temperature fluorescence spectra of 2TrzSi and 2CzTrzSi in neat film
provided singlet energies of 3.51 and 3.14 eV, while the low temperature
(77K) phosphorescence spectra in frozen tetrahydrofuran solution
delivered triplet energies of 2.97 and 2.98 eV, respectively. The singlet
energy of 2CzTrzSi was smaller than that of 2TrzSi due to weak charge
transfer (CT) character between carbazole and diphenyltriazine. How-
ever, the triplet energy was similar in the two hosts because the addi-
Fig. 1. UV–Vis absorption and PL spectra of (a) 2TrzSi and (b) 2CzTrzSi.
UV–Vis absorption and phosphorescence were measured using dilute tetrahy-
drofuran solution. Fluorescence was obtained from neat film. Phosphorescence
was measured at 77 K after delay time of 1 ms.
2CzTrzSi compared to the local emission accompanying vibrational
peaks in the 2TrzSi. The ultraviolet–visible (UV–Vis) absorption spectra
of 2TrzSi and 2CzTrzSi also reflected the CT properties of the 2CzTrzSi
host. The UV–Vis absorption of the two materials below 300 nm was
tional carbazole unit did not extend the degree of π-conjugation. The
extra carbazole in the 2CzTrzSi played a role of stabilizing the singlet
energy while preserving the high triplet energy of the 2TrzSi. It also
induced broad CT emission spectrum in the fluorescence spectra of
originated from strong π-π* absorption of the aromatic structure sepa-
rated by the diphenysilane group. In the case of 2CzTrzSi, an extra weak
absorption at 300–350 nm was observed by CT absorption of 2CzTrzSi
Scheme 1. Synthetic scheme of 2TrzSi and 2CzTrzSi.
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J.H. Yun et al.
Dyes and Pigments 190 (2021) 109297
The lowest absorption energies of 2TrzSi and 2CzTrzSi corresponding to
HOMO-LUMO gap were 4.12 and 3.65 eV, respectively.
between the LUMO and optical HOMO-LUMO gap from UV–Vis ab-
sorption spectra, while that of 2CzTrzSi was obtained from onset value
of oxidation potential (Figure S2) as the 2CzTrzSi is a CT type material.
The HOMO/LUMO of 2TrzSi and 2CzTrzSi were ꢀ 7.36/-3.24 and
ꢀ 6.40/-3.32 eV, respectively. The LUMO levels were similar because the
same triazine unit was employed as the electron deficient unit, but the
HOMO level of 2CzTrzSi was shallower than 2TrzSi due to the carbazole
moiety. The experimental energy level analysis results were well
correlated with a molecular orbital simulation result by B3LYP 6-31G
(Becke, 3-parameter, Lee–Yang–Parr 6-31G) basis set in Fig. 5. The
HOMO and LUMO frontier molecular orbitals of 2TrzSi were spread over
the diphenyltriazine unit through the tetraphenylsilane backbone
structure. However, the HOMO of 2CzTrzSi was predominantly local-
ized on the carbazole. The HOMO/LUMO energy levels from the mo-
lecular orbital calculation were ꢀ 6.60/-1.83 and ꢀ 5.76/-1.95 eV in the
2TrzSi and 2CzTrzSi, respectively, which followed the trend of the
experimentally validated energy levels. The singlet and triplet energies
were also kept on the experimentally determined emission energies. All
calculated data are summarized in Table 1.
The solid state PL emission of the hosts was further studied to figure
out the complex formation with p-type host in the mixed host because
the 2TrzSi and 2CzTrzSi hosts were developed as n-type hosts. The
fluorescence spectra of the mixed film of p-type and n-type hosts were
compared with those of neat films of each host to verify the exciplex
formation. Carbazole based conventional host, 3,3-di(9H-carbazol-9-yl)
biphenyl (mCBP), was used as the p-type host. The emission spectra of
the mixed host films (Fig. 2), mCBP:2TrzSi and mCBP:2CzTrzSi, re-
flected the PL emission of each host without indistinctive red-shift,
indicating no exciplex formation. Two host materials can be justified
as exciplex free type mixed host. In the case of 2TrzSi, donor-acceptor
interaction between carbazole of mCBP and triazine of 2TrzSi caused
the slight red shift of the emission spectrum in the mixed film. Whereas,
the slight blue shift of the emission spectrum of 2CzTrzSi is due to
suppressed intermolecular interaction between 2CzTrzSi molecules by
the mCBP host.
Transient PL (TRPL) of 2CzTrzSi in Fig. 3 was measured to assess the
thermally activated delayed fluorescence (TADF) characteristic of
2CzTrzSi because the singlet-triplet energy gap was small. A clear
Thermal properties like glass transition temperature (Tg) and ther-
mal decomposition temperature (Td) are depicted in Figure S3. The Tgs
were 118 and 153 ◦C, the Tds were 442 and 526 ◦C in 2TrzSi and
2CzTrzSi, respectively. The Tg and Td of 2CzTrzSi were higher than those
of 2TrzSi due to the large molecular weight and intermolecular inter-
action induced CT character. The two host materials showed high Td for
good thermal stability in device fabrication process and high Tg for
stable device operation.
delayed fluorescence with an excited state lifetime of 15.5
μs was
observed in the TRPL decay data, suggesting that the 2CzTrzSi is a TADF
type host for the mixed host.
The lowest unoccupied molecular orbital (LUMO) was calculated
from onset value of reduction potential by cyclic voltammetry (CV) in
Fig. 4. The HOMO of 2TrzSi was determined from the difference
Based on the material characteristics, the 2TrzSi and 2CzTrzSi were
used as the n-type hosts in blue PhOLEDs with mCBP as a p-type host.
The mixed host was composed of n-type: p-type host (50:50) and Ir(cb)3
[35] was utilized as a blue triplet emitter at 20% doping concentration.
The device energy diagram and molecular structure of fac-tris(5-(tert--
butyl)-1,3-diphenyl-2,3-dihydro-1H-imidazo[4,5-b]pyrazine)iridium (Ir
(cb)3) are represented at Fig. 6. A planar type triazine based DBFTrz [28]
reported in our previous paper was a reference n-type host. The current
density (J) and luminance (L) plots against voltage (V) are presented in
Fig. 7(a). The J of the mCBP:2TrzSi and mCBP:2CzTrzSi was similar, but
high J was observed in the mCBP:DBFTrz device. This was well corre-
lated with electron transport properties of the hosts. The single carrier
device data in Figure S4 confirmed the electron transport property and
the electron current density of hosts were in the order of DBFTrz >
2TrzSi ≈ 2CzTrzSi. The high electron current density of DBFTrz is
related with the planar molecular structure by hydrogen bonding be-
tween diphenyltriaizne and dibenzofuran. The 2TrzSi and 2CzTrzSi
showed low electron current density because the diphenylsilyl backbone
structure favors tetrahedral geometry. The tetrahedral structure hinders
molecular orbital overlap between molecules, retarding the electron
transport of the hosts. The optimized geometry of 2TrzSi and 2CzTrzSi is
given in the Figure S5. The turn-on voltage of three mixed host devices
were under 2.7 V and driving voltage at 10/100/1000 cdm-2 of the
mCBP:2TrzSi and mCBP:2CzTrzSi devices were 3.1/3.6/4.5 and
3.1/3.6/4.7 V. The turn-on and driving voltages of blue PhOLED devices
were low by transporting carriers separately without energy barrier by
mixing the mCBP and n-type hosts.
The maximum EQEs of the mCBP:2TrzSi and mCBP:2CzTrzSi devices
were 21.4 and 20.7%, respectively. The EQEs at 100/500/1,000 cd/mꢀ 2
were 21.3/21.2/20.5% in mCBP:2TrzSi and 20.7/20.4/19.8% in
mCBP:2CzTrzSi devices. The mCBP:2TrzSi device maintained the high
EQE over 20% even at high luminance and the EQE of the
mCBP:2CzTrzSi device was still high though it was slightly declined
under 20%, indicating the efficiency drop at high luminance was
improved. The mCBP:DBFTrz device showed poor maximum EQE of
9.7% because the energy transfer from host to guest was inefficient due
to the low triplet energy of DBFTrz (2.94 eV) in film state. The EQE plots
are described in Fig. 7(b) and device performances are summarized in
Table 2.
Fig. 2. Film PL spectra of (a) mCBP:2TrzSi and (b) mCBP:2CzTrzSi in com-
parison with those of mCBP, 2TrzSi and 2CzTrzSi.
3

J.H. Yun et al.
Dyes and Pigments 190 (2021) 109297
Fig. 3. Transient photoluminescence data of (a) prompt decay and (b) delayed decay of 2CzTrzSi.
Fig. 4. Reduction CV curves of 2TrzSi and 2CzTrzSi in dichloro-
methane solution.
Fig. 5. Molecular simulation results of 2TrzSi and 2CzTrzSi using B3LYP 6-31G
(Becke, 3-parameter, Lee–Yang–Parr 6-31G) basis set.
The electroluminescence (EL) spectra in Fig. 7(c) represented Ir(cb)3
phosphor emission without host emission. The EL peak wavelength of
the mCBP:2TrzSi and mCBP:2CzTrzSi devices was 466 nm, which agreed
with pure Ir(cb)3 EL emission with color coordinate of (0.14, 0.17) and
PL spectrum with peak wavelength of 463 nm (Figure S6). In the case of
the mCBP:DBFTrz device, the EL wavelength was slightly red-shifted to
468 nm and y color coordinate was also shifted with color coordinate of
(0.14, 0.19) owing to the endothermic energy transfer.
Table 1
Measured material properties of 2Trzsi and 2CzTrzSi.
ES
ET
Eg
HOMO
LUMO
(eV)
Tg/Td
(◦C)
(eV)
(eV)
(eV)a
(eV)
2TrzSi
3.51
2.97
2.98
4.12
3.65
ꢀ 7.36b
ꢀ 6.40d
ꢀ 3.24
ꢀ 3.32
118/442
153/527
The device operational lifetime confirmed at a constant current
density operating condition is in Fig. 7(d). The device lifetime at 50% of
initial luminance (LT50) was 923 h in the mCBP:2TrzSi device and 1270
h in the mCBP:2CzTrzSi device, whereas that of mCBP:DBFTrz device
was only 347 h. The device lifetime of the mCBP:2CzTrzSi device was
extended by more than 1.4 times and 3.7 times compared to that of the
mCBP:2TrzSi device and mCBP:DBFTrz device, respectively. The longer
device lifetime of the mCBP:2CzTrzSi and mCBP:2TrzSi devices than
that of the mCBP:DBFTrz device is due to the prevention of the reverse
energy transfer by high solid state triplet energy of the 2CzTrzSi and
2TrzSi hosts. The improved device lifetime of the mCBP:2CzTrzSi device
2CzTrzSi
3.14c
a
Eg was absorption energy calculated from onset value of UV–Vis absorption
spectrum.
b
c
HOMO of 2TrzSi was calculated from difference between LUMO and Eg.
ES of 2CzTrzSi was obtained from onset value of fluorescence spectrum.
HOMO of 2CzTrzSi was calculated from oxidation potential of CV.
d
is due to the exciton stabilization effect of the 2CzTrzSi host through low
singlet energy by TADF character. Moreover, the short triplet exciton
lifetime of 15.5
μs by reverse intersystem crossing mechanism also
contributed to the improved device lifetime of the 2CzTrzSi device.
4

J.H. Yun et al.
Dyes and Pigments 190 (2021) 109297
Fig. 6. (a) Device energy diagram and (b) molecular structure of Ir(cb)3.
Fig. 7. (a) Current density-voltage-luminance plots, (b) EQE-luminance plots, (c) EL spectra and (d) device operation lifetime of the Ir(cb)3 doped blue PhOLEDs.
Additionally, the mixed host devices worked better than the single host
devices of mCBP, 2TrzSi, 2CzTrzSi as presented in Figure S7. In the
electrical operation process of the mixed host devices, the hole carriers
are transported through the p-type host dominantly, but they can be
leaked to the n-type host under high electric field and the excitons are
generated in the n-type host. The low singlet energy of 2CzTrzSi de-
creases the generated exciton energy and can stabilize the host against
excitons. This can be proven by tracing the degree of PL intensity decline
under UV exposure in Fig. 8. The reduction of PL intensity according to
UV irradiation time was observed, which demonstrated that the degree
of PL degradation was small in the 2CzTrzSi host compared to the 2TrzSi
host. The result indicated that 2CzTrzSi was stable against UV light
induced PL degradation and the improved exciton stability of 2CzTrzSi
assisted achieving longer device lifetime in the 2CzTrzSi device than the
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J.H. Yun et al.
Dyes and Pigments 190 (2021) 109297
anthracen-2-yl)-phenyl]-1-phenyl-1H-benzoimidazole. The emitting
layer (EML) was a mixed host of mCBP and electron transport type host
materials doped with 20% of Ir(cb)3 triplet emitter. The host materials
were mixed at a ratio of 50:50.
Table 2
Device data of blue PhOLEDs.
Von
EQE (%)
CIE
Lifetime (h,
(V)a
LT50
)
100
1000
Max
cdm-2
cdm-2
CRediT authorship contribution statement
mCBP:2TrzSi
mCBP:2CzTrzSi
mCBP:DBFTrz
2.61
2.65
4.14
21.3
20.7
9.31
20.5
19.8
8.06
21.4
20.7
9.66
(0.14,
0.17)
(0.14,
0.17)
(0.14,
0.19)
923
Ju Hui Yun: Investigation, Writing – original draft. Junseop Lim:
Investigation, Writing – original draft. Won Jae Chung: Investigation.
Jun Yeob Lee: Supervision, Writing – review & editing.
1270
347
a
Von is turn-on voltage at 1 cdm-2
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
This work was supported by National Research Foundation of Korea
(2016M3A7B4909243).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109297.
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