Zig-Zag Type Molecular Design Strategy of N-Type Hosts for Sky-Blue Thermally-Activated Delayed Fluorescence Organic Light-Emitting Diodes

Article abstract of DOI:10.1002/chem.201904477

N-type hosts for long lifetime in sky-blue thermally-activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) were investigated by synthesizing four hosts with zig-zag-type backbone structure for high triplet energy. The four hosts had

Full text of DOI:10.1002/chem.201904477

A Journal of
Accepted Article
Title: Zig-zag type molecular design strategy of n-type hosts for sky-
blue thermally activated delayed fluorescence organic light-
emitting diodes
Authors: Chang Yoon Yang, Kyung Hyung Lee, and Jun Yeob Lee
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201904477
Link to VoR: http://dx.doi.org/10.1002/chem.201904477
Supported by

Chemistry - A European Journal
10.1002/chem.201904477
Zig-zag type molecular design strategy of n-type hosts for sky-blue
thermally activated delayed fluorescence organic light-emitting diodes
Chang Yoon Yang, Kyung Hyung Lee, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2
066, Seobu-ro, Jangan-gu, Seobu-ro, Suwon, Gyeonggi, 16419, Korea
E-mail : leej17@skku.edu
Abstract
N-type hosts for long lifetime in sky-blue thermally activated delayed fluorescence (TADF)
organic light-emitting diodes (OLEDs) were investigated by synthesizing four hosts with zig-
zag type backbone structure for high triplet energy. The four hosts had two CN units at different
positions of the zig-zag type backbone structure and two dibenzofuran units through either 2
or 4 position of dibenzofuran. The position of the CN unit was controlled as meta and para
positions in the zig-zag type backbone to study the relationship between material parameters
and lifetime of the TADF OLEDs. It was revealed that the meta orientation of the CN units in
the backbone was advantageous to extend device lifetime of the sky-blue TADF OLEDs.
Keywords: sky-blue devicelifetimeTADFOLED
Introduction
Blue organic light emitting diodes (OLEDs) are currently experiencing difficulty in achieving deep blue
color coordinates, high efficiency and long device lifetimes at the same time. Among these issues, the
lifetime is considered as the most serious problem of blue OLEDs.1-5 In particular, the lifetime of high
efficiency blue OLEDs such as phosphorescent OLEDs and thermally activated delayed fluorescence
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10.1002/chem.201904477
(
TADF) OLEDs is too short to meet the specification of the commercial products. Therefore, intensive
efforts to overcome the lifetime hurdle of the highly efficient OLEDs are essential.6-15
Until now, host-emitter system has been widely used in the emitting layer of the phosphorescent and
TADF OLEDs to improve the OLED performances by reducing exciton quenching.16-18 The host
materials generally play a vital role in forming the excitons through carrier balance. In the luminescent
process, the holes and electrons are predominantly transported by the host in the emitting layer to
generate excitons dominantly in the host and deliver the exciton energy to the emitters. For this purpose,
the emitters are uniformly dispersed in the host to inhibit triplet exciton quenching. Thus, the exciton
formation is dominated by the host and various kinds of hosts have been used. Among the several host
types, bipolar host and mixed host have been popular because of balanced carrier transport and wide
recombination zone for high external quantum efficiency (EQE) and long lifetime.19-30 In particular, the
mixed host has been proven to be effective to upgrade the device performances. However, in the case
of deep blue OLEDs, it has been challenging to develop the mixed hosts because of difficulty of the
design of high triplet energy electron transport (n-type) host for phosphors or TADF emitters. Only
several n-type hosts were demonstrated to perform as the n-type host of the mixed host for blue
phosphorescent or TADF OLEDs. Therefore, it is strongly required to explore high triplet energy n-
type host for mixed host.
Herein, we describe the synthesis and application of four n-type hosts with high triplet energy for mixed
host of sky-blue TADF OLEDs. We demonstrate systematic studies clarifying the effect of molecular
design strategy on the EQE and device lifetime of sky-blue TADF OLEDs using four hosts with zig-
zag type backbone structure substituted with two CN units and dibenzofuran moieties. The position of
the CN unit in the zig-zag type backbone was controlled as meta and para positions to adjust the LUMO
energy level and electron transport properties, and the substitution position of dibenzofuran connected
to the backbone structure was managed for additional support for good electron transport properties. It
was revealed that the electron transport properties dominated by the CN substitution position are well
correlated with the device lifetime of the sky-blue TADF OLEDs.
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Results and discussion
Molecular design, synthesis and electronic calculation
The four hosts, 6,6'-bis(dibenzo[b,d]furan-4-yl)-[1,1'-biphenyl]-3,3'-dicarbonitrile (mCN4DBF), 2,2'-
bis(dibenzo[b,d]furan-4-yl)-[1,1'-biphenyl]-4,4'-dicarbonitrile(pCN4DBF), 6,6'-bis(dibenzo[b,d]furan-
2-yl)-[1,1'-biphenyl]-3,3'-dicarbonitrile(mCN2DBF)
and
2,2'-bis(dibenzo[b,d]furan-2-yl)-[1,1'-
biphenyl]-4,4'-dicarbonitrile(pCN2DBF) were designed as the n-type hosts having the same CN
moieties and dibenzofuran for electron transport. Only the position of the CN unit in the backbone
structure and the interconnecting position of dibenzofuran was different. The four hosts were intended
to have high triplet energy by the zig-zag type backbone structure inducing distortion of the backbone
structure and good electron transport ability by the CN substituted biphenyl backbone structure. High
triplet energy can be obtained through large dihedral angle by connecting dibenzofuran to the biphenyl
backbone structure at ortho position of the molecule. Electron transport can be plausible through
dibenzofuran and CN modified biphenyl backbone with electron accepting ability.31-35 It is expected
that the management of the substitution position and interconnect position may control the carrier
transport properties and energy levels of the hosts.
The molecular design for electron transport properties was confirmed by picturing the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) based on the frontier
molecular orbital calculation using the B3LYP 6-31G* basis set of Gaussian software. Figure 1
displays the HOMO and LUMO dispersion of mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF. In
the case of mCN4DBF and mCN2DBF with the same meta positioned CN modified biphenyl backbone
structure, their LUMO was located on the backbone structure and partially on the dibenzofuran.
Whereas, the LUMO dispersion of pCN4DBF and pCN2DBF with the para positioned CN modified
biphenyl backbone structure was localized on the CN modified biphenyl backbone structure.
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Figure 1. Calculated HOMO and LUMO distributions of mCN4DBF, pCN4DBF, mCN2DBF and
pCN2DBF through DFT calculation.
3
-Bromo-4-chlorobenzonitrile and 4-bromo-3-chlorobenzonitrile were used as starting materials and
these materials were converted into ester forms using bis(pinacolato)diboron. Then, they were coupled
each other by Suzuki coupling process using K CO base and tetrakis(triphenylphosphine)palladium(0).
Final synthetic method of coupling two CN modified biphenyl backbones with two kinds of
dibenzofuran intermediates by Suzuki coupling using Cs CO base was shared in the synthesis of the
2
3
2
3
four hosts. The reaction was carried out in a 2 neck round-bottomed flask under a reflux condition. The
final products all had insolubility problem in any solvents. They were precipitated out of the reaction
vessel. Then, they just were purified through washing process and recrystallization. Production yields
of mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF were 32, 38, 25 and 30% in the final step of wet
purification, respectively. The wet-purified final products were completely purified to have a purity
level higher than 99% by high performance liquid chromatography analysis using a train sublimation
1
13
apparatus. The four hosts were identified by chemical analysis methods of H and C nuclear magnetic
resonance and mass measurement. Scheme 1 describes the brief synthetic procedure of the four hosts.
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Scheme 1. Synthetic route to mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF
Photophysical and electrochemical analysis
The four hosts were photophysically characterized through ultraviolet-visible (UV-vis) and
photoluminescence (PL) analysis. Figure 2 represents the UV-vis absorption and fluorescence spectra
of solid films at room temperature and phosphorescence spectra at 77 K to obtain singlet and triplet
energies, respectively. In the UV–vis absorption spectra, these four hosts showed strong absorption by
the π−π* transition of the zig-zag type backbone structure which consisted of two CN modified biphenyl
units and dibenzofuran. UV–vis absorption edges were similar in the four compounds.
The singlet energy/triplet energy values of the mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF
hosts were 3.73/2.84, 3.49/2.87, and 3.60/2.87, 3.50/2.86, respectively. The singlet energy of pCN4DBF
and pCN2DBF was lower than that of mCN4DBF and mCN2DBF, while the triplet energy of the four
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hosts was similar. The low singlet energy of the pCN4DBF and pCN2DBF relative to that of mCN4DBF
and mCN2DBF can be interpreted as the effect of HOMO and LUMO separation by the para oriented
CN units as depicted in the HOMO and LUMO pictures of the hosts. The triplet energy of the hosts was
high because the aromatic moieties were largely distorted in zig-zag fashion by the steric hindrance.
Furthermore, the high triplet energy of the hosts proposes that the four hosts would function as the host
of sky-blue TADF emitters without any reverse energy transfer.
The HOMO and LUMO energy levels of the hosts were also measured to check the effect of molecular
structure on the energy levels. Electrochemical methods employing cyclic voltammetry estimated the
HOMO/LUMO levels of the mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF as -6.67/-2.97, -6.80/-
3.13, and -6.69/-2.95, -6.85/-3.14 respectively (Figure S1 in supporting information). The HOMO and
LUMO energy levels were dominated by the CN location in the backbone structure. The para oriented
CN units deepened the HOMO and LUMO levels by the isolated HOMO and LUMO distribution. The
dissimilarity of the energy levels indicates that the four hosts have differences in terms of carrier
injection, carrier accumulation and carrier trapping by energy barrier in the devices. These results are
expected to directly affect the efficiency and lifetime of the device.
Table 1. Photophysical properties of mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF.
Singlet energy
eV)
Triplet energy
(eV)
HOMO
(eV)
LUMO
(eV)
HOMO-LUMO gap
(
(eV)
mCN4DBF
pCN4DBF
mCN2DBF
pCN2DBF
3.73
3.49
3.60
3.50
2.84
-6.67
-2.97
3.70
2.87
2.87
2.86
-6.80
-6.69
-6.85
-3.13
-2.95
-3.14
3.67
3.74
3.71
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Figure 2. UV-vis absorption (black line), solid PL spectra in polystyrene (red line), low temperature PL
spectra (blue line) of (a) mCN4DBF (b) pCN4DBF (c) mCN2DBF and (d) pCN2DBF
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Device application
In order to check the electron transport ability of the n-type host materials, electron only devices were
fabricated. The electron transport feature and electron stability of the four n-type hosts were studied.
Electron only device data of mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF are presented in
Figure 3. The electron current density of the electron only devices proposed that mCN4DBF and
mCN2DBF are better than pCN4DBF and pCN2DBF in terms of electron transport properties. As the
electron current density reflects only electron transport character without reflecting energy barrier for
electron injection, it can be proposed that electrons are well transported in the mCN4DBFand
mCN2DBF materials. The mCN4DBF and mCN2DBF had the same backbone structure in which the
CN units are positioned at meta to the phenyl unit. This result is due to the fact that the LUMO was
largely dispersed over the CN modified backbone and dibenzofuran. The widespread LUMO increases
the overlap integral for electron transport, facilitates electron hopping between molecules in the
mCN4DBF and mCN2DBF. On the other hand, the LUMO is spread only in the CN modified backbone
structure in the pCN4DBF and pCN2DBF, which hindered the electron transport.
Figure 3. The change of the current density of the electron only devices of the mCN4DBF, pCN4DBF,
mCN2DBF and pCN2DBF
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The mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF were applied as the hosts of the sky-blue
TADF OLED to understand the relationship between the molecular structure of hosts and device
performances. The TADF emitter was 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzCN) which is
known as a sky-blue TADF dopant for device study of TADF OLEDs. The device structure of the sky-
blue TADF OLED is in Figure 4. The chemical structures of materials used in OLEDs are in Figure
S3.
Figure 4. Device structure of sky-blue TADF OLEDs with mCN4DBF, pCN4DBF, mCN2DBF and
pCN2DBF hosts.
Figure 5 (a) represents current density (J) and luminance (L) data of the sky-blue TADF OLEDs. The
order of J and L of the sky-blue TADF OLEDs was mCN4DBF > mCN2DBF > pCN4DBF > pCN2DBF.
This result was well matched with the electron current density result of the electron only device. The
low J and L of the pCN4DBF and pCN2DBF device are caused by low electron current density induced
by poor electron transport properties of the two hosts. External quantum efficiency (EQE) of the TADF
devices according to luminance is shown in Figure 5 (b) and Table 2. Maximum EQEs of the
mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF devices were 17.3, 14.8, 12.9 and 12.2,
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respectively. The order of device EQE was mCN4DBF > pCN4DBF > mCN2DBF = pCN2DBF.
Correlation of the molecular structure with the EQE of the devices demonstrated that the hosts with the
dibenzofuran substituted through 4 position were better than those with the dibenzofuran substituted
through 2 position and the hosts with the CN unit at meta position performed better than those with the
CN unit at para position. As the determining factor of the EQE is the carrier balance and PL quantum
yield, the EQE trend in the four hosts can be interpreted as the effect of the electron density in the
emitting layer reflected in the current density and PL quantum yield of the 5CzCN doped emitting layer
films. The 4 position substituted dibenzofuran derived hosts showed higher current density than the 2-
position substituted dibenzofuran derived hosts, which balanced carriers in the electron trap dominant
5CzCN doped emitting layer. Likewise, the hosts with the CN unit at meta position of biphenyl had
higher current density than the corresponding hosts with the CN unit at para position of biphenyl, which
explains the EQE trend in the four hosts. Therefore, the EQE of the pCN4DBF device was higher than
that of the mCN2DBF device by reduced electron trapping caused by the deep LUMO level of the
pCN4DBF host. The PL quantum yield of the emitting layer also affected the EQE of the devices. The
PL quantum yields of the 5CzCN doped mixed films of mCN4DBF, pCN4DBF, mCN2DBF and
pCN2DBF were 70.9, 79.7, 48.6, 60.4%, respectively. The EQE of the mCN4DBF device was higher
than of the mCBP reference device as shown in Figure S5.
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Figure 5. (a) Current density-voltage-luminance curves of four hosts (b) Quantum efficiency-luminance
plots of four hosts
Table 2. Device results of mixed host devices with mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF
mCN4DBF
14.2
pCN4DBF
12.2
mCN2DBF
10.7
pCN2DBF
10.8
5
00 cd/m2
Max
a)
EQE (%)
17.3
14.8
12.9
12.2
CIE b)
(0.17,0.35)
3
(0.17,0.35)
2.4
(0.17, 0.35)
2.8
(0.17, 0.35)
1.7
c)
LT 90 (h)
d)
LT 75 (h)
19.1
14.1
19.9
9.9
a)
External quantum efficiency, b) color coordinates, c) device lifetime of 90% luminance decay at initial luminance of 500
cd/m2, d) device lifetime of 75% luminance decay at initial luminance of 500 cd/m2
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The device lifetime of the sky-blue TADF OLEDs with the four hosts is shown in Figure 6. The order
of device lifetime was mCN4DBF = mCN2DBF > pCN4DBF > pCN2DBF at a constant current driving
mode. The order of lifetime was the same as the electron current tendency. The hosts with the meta
positioned CN in the biphenyl backbone structure generally showed longer lifetime than those with the
para positioned CN in the backbone structure. The main reason for the difference in the device lifetime
between the four host materials was the electron transport properties. Good electron transport properties
are favored in the 5CzCN devices because electron trapping by the 5CzCN dopant localizes the emission
36
zone near the electron transport layer. Localized emission zone accelerates the degradation process of
the TADF OLEDs and shortens the device lifetime. Therefore, the mCN4DBF and mCN2DBF devices
extended the device lifetime of the 5CzCN devices relative to the pCN4DBF and pCN2DBF devices.
The lifetime of the mCN4DBF and mCN2DBF devices was even longer than that of the mCBP device
(
Figure S5) and it was similar to that of the devices with triazine derived n-type hosts as reported in
our previous work.37
Figure 6. Lifetime data of 5CzCN doped mCN4DBF, pCN4DBF, mCN2DBF, and pCN2DBF devices
To understand the reason for the improved lifetime in mixed host devices, the stability of the host under
negative polarons was simulated by the stability test of the electron only device (EOD) as shown in
Figure 7. They were stressed under constant current driving condition to monitor the change of the
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driving voltage of the devices. The four hosts exhibited voltage change during the electrical operation.
The voltage changes of the mCN4DBF, mCN2DBF pCN4DBF and pCN2DBF hosts were of no big
difference between them. This result indicates that they are stable under negative polaron stress. As the
four hosts showed good electron stability under negative polaron stress, the lifetime difference in the
four hosts can be clearly explained by the electron transport properties of the hosts. Therefore, the host
design stabilizing the host under electron stress and with good electron transport ability is necessary to
improve the device lifetime of the TADF OLEDs.
Figure 7. The electron stability test of mCN4DBF, pCN4DBF, mCN2DBF and pCN2DBF
Conclusions
In conclusion, four hosts which had good electron transport properties and electron stability were
synthesized for sky-blue TADF device application by adopting zig-zag type backbone structure
modified with CN and dibenzofuran units. The substitution position of CN unit in the biphenyl
backbone structure was critical to the EQE and device lifetime, and the substitution of the CN unit at
meta position of the biphenyl backbone structure was effective for electron transport and improved
device performances. Therefore, the zig-zag type host design modified with CN and dibenzofuran units
can be effectively used to extend the device lifetime of the sky-blue TADF OLEDs.
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