Design of hole transport type host for stable operation in blue organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2020.105724

Molecular design of the hole transport type host for mixed host was investigated to improve the lifetime of the phosphorescent organic light-emitting diodes. A negative polaron stabilizing hole transport type host design was employed and the effect of the negative polaron stabilizing unit was investigated. Dibenzofuran or benzonitrile was introduced as the negative polaron stabilizing unit in the bicarbazole backbone structure of the hole transport type host. Two host materials were synthesized and the comparison of them proposed that the negative polaron stabilizing unit is a key to the lifetime of the phosphorescent organic light-emitting diodes. The dibenzofuran and benzonitrile embedded bicarbazole hosts performed better than the mCBP host. The dibenzofuran and benzonitrile modified bicarbazole hosts demonstrated high external quantum efficiency of 18.6 and 19.1%, respectively and lifetime extension by 30% compared with the conventional host without the negative polaron stabilizing unit.

Full text of DOI:10.1016/j.orgel.2020.105724

Organic Electronics 82 (2020) 105724
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
Design of hole transport type host for stable operation in blue organic
light-emitting diodes
Cho long Kim, Won Jae Chung, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 16419, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Molecular design of the hole transport type host for mixed host was investigated to improve the lifetime of the
phosphorescent organic light-emitting diodes. A negative polaron stabilizing hole transport type host design was
employed and the effect of the negative polaron stabilizing unit was investigated. Dibenzofuran or benzonitrile
was introduced as the negative polaron stabilizing unit in the bicarbazole backbone structure of the hole
transport type host. Two host materials were synthesized and the comparison of them proposed that the negative
polaron stabilizing unit is a key to the lifetime of the phosphorescent organic light-emitting diodes. The
dibenzofuran and benzonitrile embedded bicarbazole hosts performed better than the mCBP host. The diben-
zofuran and benzonitrile modified bicarbazole hosts demonstrated high external quantum efficiency of 18.6 and
19.1%, respectively and lifetime extension by 30% compared with the conventional host without the negative
polaron stabilizing unit.
Hole type host
Mixed host
Lifetime
Negative polaron
1. Introduction
over 20% [18]. Whereas, the development of the p-type host for blue
phosphors has not been popular in spite of the importance of the p-type
Phosphorescent organic light-emitting diodes (PhOLEDs) employing
phosphorescent emitters are very popular because of high efficiency
close to the theoretical maximum quantum efficiency of OLEDs [1–4].
The PhOLEDs already reached the upper limit of the maximum quantum
efficiency. However, the lifetime of the blue PhOLEDs is still far below
that of the red and green PhOLEDs, which is a big hurdle for the com-
mercial use of the blue PhOLEDs [5–8].
host although several p-type hosts have been documented [19–21]. The
n-type hosts can be damaged by positive polarons during light emission
process because holes are injected into the host. The electrical burden of
the n-type host can be relieved by mixing the p-type host with the n-type
host, which can stabilize the devices. As the common direction in the
p-type host development was to design strong p-type host, not many
strategies were tried. Therefore, further concept and strategy of the hole
transport type host for the blue PhOLEDs is strongly required.
The short lifetime issues of the blue PhOLEDs can be regarded as the
combined effect of the organic materials, carrier transport, carrier
recombination and device structure. One of the key factors from the
material aspect is the host material. In general, a mixed host consisted of
hole transport type (p-type) host and electron transport type (n-type)
host has been widely used due to high efficiency and long lifetime in the
blue PhOLEDs [9–14]. In particular, the n-type host was a main focus
because electrons are mostly trapped by the blue phosphor and high
triplet energy n-type hosts are not common. Several n-type type hosts
already demonstrated good performances in the blue PhOLEDs [15–18].
For example, triphenylsilyl modified triazine type hosts showed high
external quantum efficiency (EQE) about 20% and moderate lifetime
[16]. A carbazole and triphenylsilyl co-embedded triazine type host was
also effective to extend the device lifetime while achieving high EQE
Herein, we describe the study of the hole transport type host derived
from bicarbazole backbone structure and negative polaron stabilizing
unit. The bicarbazole was 9-phenyl-9H,90H-3,40-bicarbazole which has
linkage between 3 position and 4 position of carbazole. The negative
polaron stabilizing units were dibenzofuran and benzonitrile. Two hosts,
9’-(dibenzo[b,d]furan-2-yl)-9-phenyl-9H,90H-3,40-bicarbazole
(DBFBCz) and 2-(9-phenyl-9H,90H-[3,40-bicarbazol]-90-yl)benzonitrile
(BNBCz), were synthesized to trace the role and influence of the negative
polaron stabilizing units. It was demonstrated that the negative polaron
stabilizing unit is crucial to the device lifetime of the blue PhOLEDs. The
dibenzofuran and benzonitrile embedded bicarbazole host performed
better than the mCBP host. The DBFBCz and BNBCz hosts demonstrated
high EQE of 18.6 and 19.1%, respectively and lifetime extension by 30%
* Corresponding author.
E-mail address: leej17@skku.edu (J.Y. Lee).
https://doi.org/10.1016/j.orgel.2020.105724
Received 15 January 2020; Received in revised form 16 March 2020; Accepted 16 March 2020
Available online 21 March 2020
1566-1199/© 2020 Elsevier B.V. All rights reserved.

C. Kim et al.
Organic Electronics 82 (2020) 105724
Scheme 1. Synthetic scheme of DBFBCz and BNBCz.
Fig. 1. Calculated HOMO and LUMO distributions of DBFBCz and BNBCz.
relative to the negative polaron stabilizing unit free host.
2. Results and discussion
forecast before synthesis to judge the appropriateness of the host design
for blue device application. The frontier molecular orbital (FMO)
simulation results of the DBFBCz and BNBCz are displayed in Fig. 1. The
FMO calculation results were similar in the two hosts in that the HOMO
was in the bicarbazole backbone structure and the LUMO was found in
the negative polaron stabilizing unit. The LUMO localization in the
dibenzofuran or benzonitrile unit suggests that the negative polaron
related properties would be governed by the negative polaron stabilizing
units. The calculated highest occupied molecular orbital (HOMO)/
lowest unoccupied molecular orbital (LUMO) values of the DBFBCz and
BNBCz hosts were À 5.11/-1.17 eV and À 5.21/-1.60 eV, respectively.
The LUMO levels of the two hosts indicate that the BNBCz is a strongly
electron accepting hole transport type host and the DBFBCz is a weakly
electron accepting hole transport type host.
2.1. Synthesis and orbital calculation
The backbone structure of the host was 9-phenyl-9H,90H-3,40-bicar-
bazole which has linkage between two carbazoles through the 3 and 4
position of the two carbazoles to increase the triplet energy of the host.
As the well-known 9-phenyl-9H,90H-3,30-bicarbazole is not suitable for
blue phosphors due to low triplet energy, the 9-phenyl-9H,90H-3,40-
bicarbazole backbone structure was employed. As the negative polaron
stabilizing unit, weakly electron accepting dibenzofuran and strongly
electron accepting benzonitrile units were introduced to correlate the
electron accepting units of the hosts with the device performances. Two
hosts, DBFBCz and BNBCz, shared the same backbone structure and had
different negative polaron stabilizing units. The synthetic process is
explained in Scheme 1 and details of the synthesis are described in
Experimental section.
The two hosts were prepared by the similar synthetic pathways
coupling the negative polaron stabilizing unit to the 9-phenyl-9H,90H-
3,40-bicarbazole backbone structure produced by Suzuki coupling re-
action between (9-phenyl-9H-carbazol-3-yl)boronic acid and 4-chloro-
9H-carbazole. The synthetic yield of the bicarbazole backbone struc-
ture was 60%, while that of the DBFBCz and BNBCz from the bicarbazole
The photophysical properties and energy levels of the hosts were
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Organic Electronics 82 (2020) 105724
Fig. 2. UV–vis and PL spectra of DBFBCz and BNBCz.
Fig. 4. (a) DSC (Differential Scanning Calorimetry) and (b) TGA (Thermogra-
vimetric analysis) data of DBFBCz and BNBCz at a heating rate of 10 �C/min in a
nitrogen atmosphere.
eV. The linkage via the 3 and 4 positions of the bicarbazole core allowed
the high triplet energy in the two hosts.
The electrochemical characterization data of the two hosts are shown
in Fig. 3. Both oxidation and reduction of the hosts were detected during
cyclic voltage scan. The conversion of the oxidation and reduction po-
tentials of the hosts into the energy levels using the ferrocene standard
material provided the HOMO/LUMO levels of À 5.98/-2.59 eV and
À 6.06/-2.64 eV for the DBFBCz and BNBCz hosts, respectively. The
strongly electron accepting nature of the benzonitrile slightly deepened
the HOMO and LUMO levels of the BNBCz host.
Fig. 3. Cyclic voltammetry (CV) data of DBFBCz and BNBCz.
intermediate was 40 and 55%, respectively. Multi-step purification
processes of column chromatography, recrystallization, and sublimation
were engaged to afford the final hosts for device lifetime study. The
identification of the final compounds was conducted using 1H and 13
C
nuclear magnetic resonance (NMR) spectrometer, mass spectrometer
and elemental analysis.
The thermal characterization of the hosts was performed to evaluate
the potential of the hosts in terms of electrical operation stability and
thermal evaporation process stability. Glass transition temperature (Tg)
and thermal decomposition temperature (Td) at 5% weight loss indi-
rectly guide the thermal stability of the hosts. The thermal character-
ization data in Fig. 4(a) and (b) enabled the determination of the Tg/Td
2.2. Material characterization
The basic properties of the hosts were analyzed by photophysical,
electrochemical, and thermal characterizations. The photophysical
analysis results of the hosts are presented in Fig. 2. Absorption, fluo-
rescence and phosphorescence of the hosts were characterized using
tetrahydrofuran solution (1.0 � 10À 5 M) of the hosts. The absorption
behavior in the ultraviolet–visible (UV–vis) region was similar in the
two hosts because the main UV–vis absorption is from the bicarbazole
backbone structure which is shared in the two hosts. The fluorescence of
the hosts collected at room temperature was slightly different in that the
fluorescence spectrum of the BNBCz was relatively broad and red-shifted
because of charge transfer (CT) character by the strongly electron
accepting benzonitrile unit. Bathochromic shift of the fluorescence
spectrum by 13 nm resulted from the CT character. Whereas, the
phosphorescence spectra of the two hosts gathered at 77 K after delay
time of 1 ms agreed each other and provided high triplet energy of 2.90
�
as 143/442 and 138/385 C in the DBFBCz and BNBCz hosts, respec-
tively. The high Tg and Td of the two hosts confirmed the thermal sta-
bility of them in the operation and fabrication processes. All material
characterization data explained above proved that the DBFBCz and
BNBCz hosts would perform as the hosts for the blue phosphors.
2.3. Device analysis
Device evaluation of the two hosts was carried out by doping fac-tris
(3-(1-(2,6-diisopropylphenyl)-1H-imidazole-2-yl)benzonitrile)iridium
(CNIm) phosphor in the mixed hosts. The CNIm was reported as a blue
phosphor realizing high EQE and long device lifetime in our previous
work and was also used in this work for EQE and lifetime study of the
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Organic Electronics 82 (2020) 105724
Fig. 5. Energy level diagram of the blue PhOLEDs (a), device structure of hole only device (b) and electron only device (c), and chemical structure of the mate-
rials (d).
4

C. Kim et al.
Organic Electronics 82 (2020) 105724
Fig. 6. (a) The current density and luminance according to driving voltage and (b) the external quantum efficiency (EQE) according to luminance of the PhOLEDs.
Fig. 9. Lifetime data of CNIm doped DBFBCz, BNBCz, and mCBP devices.
Fig. 7. Hole current density of non-doped DBFBCz and BNBCz hole
only devices.
the BNBCz device might be due to the strongly electron accepting
character of the benzonitrile unit. The driving voltages of the DBFBCz
and BNBCz devices at 1000 cd/m2 were 6.0 and 6.3 V, respectively.
The EQE of the blue PhOLEDs is presented according to the lumi-
nance of the devices. The EQE of the BNBCz device was slightly higher
than that of the DBFBCz device possibly due to balanced carrier density
triggered by the low hole carrier density of the BNBCz hole only device
considering the same triplet energy of the two hosts. As shown Fig. 8, the
same EL spectra presenting pure CNIm emission in the two devices
support the exclusion of the energy transfer as the factor for the
disagreement of the EQE. The maximum EQEs of the DBFBCz and BNBCz
devices were 18.6 and 19.1%, respectively. The CIE color coordinates of
the DBFBCz and BNBCz devices were (0.15,0.25) and (0.15,0.26),
respectively.
Fig. 8. EL spectra of DBFBCz, BNBCz, and mCBP device at 1000 cd/m2.
hosts [22]. As the DBFBCz and BNBCz were developed as the p-type
hosts in the mixed host, they were mixed with a 2-phenyl-4,6-bis(3-(tri-
phenylsilyl)phenyl)-1,3,5-triazine (mSiTrz) n-type host reported in our
previous work [16]. The device structure is schematized in Fig. 5
including the energy levels and chemical structure of the organic ma-
terials. The device evaluation data of the DBFBCz and BNBCz hosts are
presented in Fig. 6. In the current density and luminance characteriza-
tion data according to driving voltage, the current density and lumi-
nance of the DBFBCz device were higher than those of the BNBCz device.
Hole only devices were fabricated to confirm the current density
discrepancy in the two hosts, which showed that the better hole trans-
port properties of the DBFBCz host are responsible for the high current
density of the DBFBCz device (Fig. 7). The low hole current density of
Fig. 10. The change of driving voltage according to time in electron only de-
vices of DBFBCz, BNBCz and mCBP.
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C. Kim et al.
Organic Electronics 82 (2020) 105724
The device lifetime of the DBFBCz and BNBCz devices was evaluated
References
at a constant current driving condition. A conventional p-type host
without the negative polaron stabilizing unit, mCBP, was a standard
material to validate the host design strategy. The device lifetime data are
presented in Fig. 9. The device lifetime of the DBFBCz and BNBCz de-
vices was longer than that of the mCBP control device. The lifetime
values of the DBFBCz, BNBCz, and mCBP devices up to 80% of initial
luminance were 8.8, 9.3, and 6.8 h, respectively. In order to understand
the origin of the lifetime extension by the DBFBCz and BNBCz hosts,
device analysis was carried out by tracing the negative polaron stability
of the hosts. The stability test of the hosts against the polarons was
performed by stressing the electron only devices of the hosts. The
negative polaron stability of the DBFBCz and BNBCz hosts was
compared with that of the mCBP host. The data of polaron stability test
are presented in Fig. 10. In the negative polaron stability test using
electron only devices, the change of driving voltage according to time in
DBFBCz and BNBCz devices was smaller than that of the mCBP device.
This indicates that DBFBCz and BNBCz have better negative polaron
stability than mCBP. The benzonitrile and dibenzofuran units made the
hosts stable under negative polarons because they tend to accept elec-
trons and their electron deficiency can stabilize injected electrons.
Therefore, the DBFBCz and BNBCz hosts showed enhanced negative
polaron stability and extended the device lifetime.
[1] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, Nearly 100% internal
phosphorescence efficiency in an organic light-emitting device, J. Appl. Phys. 90
(2001) 5048–5051.
[2] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Very high-
efficiency green organic light-emitting devices based on electrophosphorescence,
Appl. Phys. Lett. 75 (1999) 4–6.
[3] M.A. Baldo, D. O’brien, Y. You, A. Shoustikov, S. Sibley, M. Thompson, S.
R. Forrest, Highly efficient phosphorescent emission from organic
electroluminescent devices, Nature 395 (1998) 151–154.
[4] B. Minaev, G. Baryshnikov, H. Agren, Principles of phosphorescent organic light
emitting devices, Phys. Chem. Chem. Phys. 16 (2014) 1719–1758.
[5] Y.J. Cho, K.S. Yook, J.Y. Lee, A universal host material for high external quantum
efficiency close to 25% and long lifetime in green fluorescent and phosphorescent
OLEDs, Adv. Mater. 26 (2014) 4050–4055.
[6] B. Liu, J. Zhao, C. Luo, F. Lu, S. Tao, Q. Tong, A novel bipolar phenanthroimidazole
derivative host material for highly efficient green and orange-red phosphorescent
OLEDs with low efficiency roll-off at high brightness, J. Mater. Chem. C 4 (2016)
2003–2010.
[7] H. Sasabe, J. Kido, Recent progress in phosphorescent organic light-emitting
devices, Eur. J. Org Chem. (2013) 7653–7663, 2013.
[8] H.J. Jang, J.Y. Lee, J. Kwak, D. Lee, J.-H. Park, B. Lee, Y.Y. Noh, Progress of display
performances:AR, VR, QLED, OLED, and TFT, J. Infect. Dis. 20 (2019) 1–8.
[9] N. Chopra, J.S. Swensen, E. Polikarpov, L. Cosimbescu, F. So, A.B. Padmaperuma,
High efficiency and low roll-off blue phosphorescent organic light-emitting devices
using mixed host architecture, Appl. Phys. Lett. 97 (2010), 03304.
[10] V. Jankus, C.J. Chiang, F. Dias, A.P. Monkman, Deep blue exciplex organic light-
emitting diodes with enhanced efficiency; P-type or E-type triplet conversion to
singlet excitons? Adv. Mater. 25 (2013) 1455–1459.
[11] B.S. Kim, J.Y. Lee, Engineering of mixed host for high external quantum efficiency
above 25% in green thermally activated delayed fluorescence device, Adv. Funct.
Mater. 24 (2014) 3970–3977.
3. Conclusions
[12] J. Lee, J.-I. Lee, J.Y. Lee, H.Y. Chu, Stable efficiency roll-off in blue phosphorescent
organic light-emitting diodes by host layer engineering, Org. Electron. 10 (2009)
1529–1533.
The negative polaron stabilizing hole transport type hosts of DBFBCz
and BNBCz were designed and synthesized as high triplet energy host
with a triplet energy above 2.90 eV. In blue PhOLEDs, the DBFBCz and
BNBCz showed high EQE of 18.6 and 19.1%, respectively, and extended
device lifetime compared with the mCBP. The improved negative
polaron stability of the hosts by the dibenzofuran and benzonitrile units
was responsible for the improved device lifetime. This result revealed
that negative polaron stability of hole transport type hosts play an
important role in improving the driving lifetime.
[13] J. Lee, J.-I. Lee, J.Y. Lee, H.Y. Chu, Improved performance of blue phosphorescent
organic light-emitting diodes with a mixed host system, Appl. Phys. Lett. 95
(2009), 253304.
[14] J.-H. Lee, C.-I. Wu, S.-W. Liu, C.-A. Huang, Y. Chang, Mixed host organic light-
emitting devices with low driving voltage and long lifetime, Appl. Phys. Lett. 86
(2005), 103506.
[15] H.-F. Chen, T.-C. Wang, S.-W. Lin, W.-Y. Hung, H.-C. Dai, H.-C. Chiu, K.-T. Wong,
M.-H. Ho, T.-Y. Cho, C.-W. Chen, C.-C. Lee, Peripheral modification of 1,3,5-
triazine based electron-transporting host materials for sky blue, green, yellow, red,
and white electrophosphorescent devices, J. Mater. Chem. 22 (2012)
15620–15627.
Declaration of competing interest
[16] K.H. Choi, K.H. Lee, J.Y. Lee, T. Kim, Simultaneous achievement of high efficiency
and long lifetime in deep blue phosphorescent organic light-emitting diodes, Adv.
Opt. Mater. 7 (2019), 1901374.
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.
[17] H. Fukagawa, N. Yokoyama, S. Irisa, S. Tokito, Pyridoindole derivative as electron
transporting host material for efficient deep-blue phosphorescent organic light-
emitting diodes, Adv. Mater. 22 (2010) 4775–4778.
[18] M. Jung, K.H. Lee, J.Y. Lee, T. Kim, A bipolar host based high triplet energy
electroplex for an over 10 000 h lifetime in pure blue phosphorescent organic light-
emitting diodes, Mater. Horiz. 7 (2020) 559–565.
Acknowledgement
[19] H. Sasabe, N. Toyota, H. Nakanishi, T. Ishizaka, Y.J. Pu, J. Kido, 3,3’-Bicarbazole-
based host materials for high-efficiency blue phosphorescent OLEDs with
extremely low driving voltage, Adv. Mater. 24 (2012) 3212–3217.
[20] M.H. Tsai, Y.H. Hong, C.H. Chang, H.C. Su, C.C. Wu, A. Matoliukstyte,
J. Simokaitiene, S. Grigalevicius, J.V. Grazulevicius, C.P. Hsu, 3-(9-Carbazolyl)
carbazoles and 3,6-Di(9-carbazolyl)carbazoles as effective host materials for
efficient blue organic electrophosphorescence, Adv. Mater. 19 (2007) 862–866.
[21] S.G. Yoo, S.K. Jeon, S.-H. Hwang, J.Y. Lee, A strong hole transport type host
material for high quantum efficiency blue phosphorescent organic light-emitting
diodes, J. Ind. Eng. Chem. 32 (2015) 72–76.
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.orgel.2020.105724.
[22] Y. Kwon, S.H. Han, S. Yu, J.Y. Lee, K.M. Lee, Functionalized phenylimidazole-
based facial-homoleptic iridium(III) complexes and their excellent performance in
blue phosphorescent organic light-emitting diodesJ, Mater. Chem. C 6 (2018)
4565–4572.
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