High-performance sky-blue phosphorescent organic light-emitting diodes employing wide-bandgap bipolar host materials with thermally activated delayed fluorescence characteristics

Article abstract of DOI:10.1016/j.orgel.2020.105660

Up to date, several kinds of thermally activated delayed fluorescence (TADF) host molecules have been rapidly developed to fulfill the increasing demand of high-efficiency phosphorescent organic light-emitting diodes (PHOLEDs). Herein, three wide-bandgap TADF materials BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz are newly designed and synthesized as host materials for sky-blue PHOLEDs. By modulation of carbazole-units in the molecular skeleton, we find that the optimized molecules can realize both high triplet energies and balanced carrier transport properties with obvious TADF properties. The sky-blue PHOLEDs with ultralow phosphor doping ratio employing BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as host materials demonstrate really high performance with alleviated roll-offs and achieve impressive current efficiencies (CEs) of 46.5 cd A^?1, 50.0 cd A^?1, and 34.5 cd A^?1, power efficiencies (PEs) of 40.6 lm W^?1, 46.8 lm W^?1, and 34.8 lm W^?1, and external quantum efficiencies (EQEs) of 22.2%, 24%, and 15.8%, respectively. More encouragingly, even at an applied luminance of 1000 cd m^?2, the EQEs could maintain high levels of 21.0%, 21.5% and 12.8%, respectively, which represents the potential utility value of the new family of these molecules in blue PHOLEDs. Our molecular design strategy provides a practical way to construct promising wide-bandgap TADF host materials with an excellent combination of TADF property, high triplet energy, and balanced carrier-transporting ability for high-performance devices.

Full text of DOI:10.1016/j.orgel.2020.105660

Journal Pre-proof
High-performance sky-blue phosphorescent organic light-emitting diodes employing
wide-bandgap bipolar host materials with thermally activated delayed fluorescence
characteristics
Yuan-Lan Zhang, Quan Ran, Qiang Wang, Qi-Sheng Tian, Fan-Cheng Kong, Jian
Fan, Liang-Sheng Liao
PII:
S1566-1199(20)30046-X
DOI:
https://doi.org/10.1016/j.orgel.2020.105660
ORGELE 105660
Reference:
To appear in: Organic Electronics
Received Date: 9 November 2019
Revised Date: 28 December 2019
Accepted Date: 20 January 2020
Please cite this article as: Y.-L. Zhang, Q. Ran, Q. Wang, Q.-S. Tian, F.-C. Kong, J. Fan, L.-S. Liao,
High-performance sky-blue phosphorescent organic light-emitting diodes employing wide-bandgap
bipolar host materials with thermally activated delayed fluorescence characteristics, Organic Electronics
(2020), doi: https://doi.org/10.1016/j.orgel.2020.105660.
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High-Performance Sky-Blue Phosphorescent Organic Light-
Emitting Diodes Employing Wide-Bandgap Bipolar Host Materials
with Thermally Activated Delayed Fluorescence Characteristics
Yuan-Lan Zhang,1,† Quan Ran,1,† Qiang Wang,1 Qi-Sheng Tian,1 Fan-Cheng Kong,1 Jian
Fan,1,2,* and Liang-Sheng Liao1,2,*
1Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-
Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R.
China.
2Institute of Organic Optoelectronics, Jiangsu Industrial Technology Research Institute (JITRI),
Wujiang, Suzhou, Jiangsu 215211, P. R. China.
E-mail: lsliao@suda.edu.cn; jianfan@suda.edu.cn
†These two authors contributed equally to this work.
Abstract: Up to date, several kinds of thermally activated delayed fluorescence (TADF) host
molecules have been rapidly developed to fulfill the increasing demand of high-efficiency
phosphorescent organic light-emitting diodes (PHOLEDs). Herein, three wide-bandgap TADF
materials BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz are newly designed and synthesized as
host materials for sky-blue PHOLEDs. By modulation of carbazole-units in the molecular
skeleton, we find that the optimized molecules can realize both high triplet energies and balanced
carrier transport properties with obvious TADF properties. The sky-blue PHOLEDs with

ultralow phosphor doping ratio employing BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as host
materials demonstrate really high performance with alleviated roll-offs and achieve impressive
current efficiencies (CEs) of 46.5 cd A-1, 50.0 cd A-1, and 34.5 cd A-1, power efficiencies (PEs)
of 40.6 lm W-1, 46.8 lm W-1, and 34.8 lm W-1, and external quantum efficiencies (EQEs) of
22.2%, 24%, and 15.8%, respectively. More encouragingly, even at an applied luminance of
1000 cd m-2, the EQEs could maintain high levels of 21.0%, 21.5% and 12.8%, respectively,
which represents the potential utility value of the new family of these molecules in blue
PHOLEDs. Our molecular design strategy provides a practical way to construct promising wide-
bandgap TADF host materials with an excellent combination of TADF property, high triplet
energy, and balanced carrier-transporting ability for high-performance devices.
Keywords: thermally activated delayed fluorescence, bipolar host molecule, blue organic light-
emitting diodes
1 Introduction
Blue emission takes a significantly important role in the three primary colors (RGB) based
organic light-emitting diodes (OLEDs).[1-5] A great challenge still exists for researchers to
achieve high-efficiency blue OLEDs owing to many strict requirements and corresponding
difficulties.[3, 6-8] Nowadays, phosphorescent dyes have drawn tremendous attention worldwide
due to their excellent exciton-harvesting ability in electroluminescence performance, realizing
approximately 100% internal quantum efficiency (IQE).[9-11] Therefore, blue phosphorescent
organic light-emitting diodes (PHOLEDs) outstand unparalleled merits in blue emission field of
organic optoelectronic devices.[12-20] However, in spite of promising performance, the required
high doping ratio of noble-metal based phosphorescent dyes in device fabrication is still

problematic considering the resource rarity.[21, 22] Fortunately, host materials with thermally
activated delayed fluorescence (TADF) characteristics have become fashionable to relieve this
stressed condition due to their effective reverse intersystem crossing (RISC) process and efficient
resonance energy transfer to the dopants.[23-29] In general, phosphorescent dyes are normally
dispersed in host molecular matrix to suppress the concentration quenching.[30, 31] An ideal
host for phosphorescent dyes should fulfill the following demands[32, 33]: (1) a high triplet
energy (ET) level sufficient for impeding the reserve energy transfer; (2) high thermal stability;
(3) reasonable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) to match frontier molecular orbitals of its adjacent charge transport molecules;
(4) balanced hole-/electron- transporting properties to pursue efficient generation process of
excitons. As we can see, first of all, a high triplet excited state for confining excitons on the
phosphorescent molecules is essential to host materials.[34] It is a general way to increase the ET
by decreasing the π-conjugation length of molecules. However, the reduction in the conjugation
length always takes a heavy toll on the charge transport properties.[35] Therefore, high ET and
high charge transport property are contradictory. Particularly, for blue emitters with a relatively
high ET, this strict requirement becomes a challenge for molecular design and limits the diversity
of host molecules prepared for blue OLEDs.[36, 37]
Carbazole is a well-known electron donor with high ET (3.0 eV) and many carbazole
derivative wide-bandgap host materials have been developed.[38-45] The triplet energy level is
very sensitive to the extension of the molecular π-conjugation. For instance, in contrast to the
origin of carbazole, a conventional host material CBP has a sharp decrease in the ET to be 2.5
eV, which is too low to afford the host for blue dyes. By reducing the π-conjugation of the
carbazole moieties and the biphenyl units, two isomers m-CBP and o-CBP could again result in

higher ETs of 2.84 and 3.00 eV due to the limited conjugation length.[46] Therefore, tunable ET
energy could be realized by exactly controlling the π-conjugation length of carbazole moieties
and the π-conjugation length need to be strictly limited for a wide-bandgap host molecule with
high ET. On the other hand, considering carrier transport property of a host molecule, a sufficient
π-conjugation system of carbazole-units is another requirement that should be fulfilled in the
molecular design. Otherwise, it may induce unbalanced hole-/electron- transporting property and
thus a deficient process of exciton generation.
Herein, we report three wide-bandgap TADF host materials utilizing a benzene ring to
combine carbazole derivative electron donors with high ET and CN substituted benzene acceptor
with electron withdrawing character to give new bipolar host materials of 4'-(3,6-diphenyl-9H-
carbazol-9-yl)-[1,1'-biphenyl]-2-carbonitrile
(BPCN-Cz2Ph),
4'-(9'-phenyl-9H,9'H-[3,3'-
bicarbazol]-9-yl)-[1,1'-biphenyl]-2-carbonitrile (BPCN-2Cz), and 4'-(9,9''-diphenyl-9H,9'H,9''H-
[3,3':6',3''-tercarbazol]-9'-yl)-[1,1'-biphenyl]-2-carbonitrile (BPCN-3Cz). By modulation of the
carbazole units in the molecular skeleton frame, tunable bandgap and bipolarity, appropriate
highest occupied molecular orbital (HOMO) energy level, small singlet-triplet splitting (∆EST),
high triplet energy, and balanced hole-/electron- transporting ability could be achieved. These
three molecules designed on the same platform show impressively high performance when they
are used as the host for sky-blue PHOLEDs with ultralow iridium(III) bis(4,6-
(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) doping ratio (5 wt%). The devices hosted
by BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz respectively achieved current efficiencies (CEs)
of 46.5 cd A-1, 50.0 cd A-1 and 34.5 cd A-1, power efficiencies (PEs) of 40.6 lm W-1, 46.8 lm W-1
and 34.8 lm W-1, and external quantum efficiencies (EQEs) of 22.2%, 24% and 15.8%. More
encouragingly, even at an applicable luminance of 1000 cd m-2, the EQEs could maintain high

levels of 21.0%, 21.5% and 12.8%, respectively, which represents the potential utility value of
this new family of host molecules in blue PHOLEDs. By optimizing the carbazole derivative
donor segments in the molecular skeleton, we could obtain ideal TADF host materials featured
with both high ETs and balanced charge transport properties. This structural optimization method
provides a practical way to build promising host materials for blue PHOLEDs and evidently
confirm that an excellent combination of TADF property, high triplet energy, and balanced
carrier transport capacity allow bipolar host materials to deliver a high performance.
2 Results and discussion
2.1 Synthesis and characterization
The synthetic processes of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz are described in
Scheme 1. The di/tri-carbazole precursors 9-phenyl-9H,9'H-3,3'-bicarbazole (2) and 9,9''-
diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole (3) were prepared according to the literature.[47-49]
The final products can be obtained via Buchwald-Hartwig cross coupling reaction between
related carbazole derivatives (1, 2, 3) and 4'-chloro-[1,1'-biphenyl]-2-carbonitrile. The complexes
1
were fully identified by H/13C NMR, MALDI-TOF mass spectrometry and elemental analysis
(Fig. S8-S15).
2.2 Theoretical calculations
To primarily get insight in the electronic structure of BPCN-Cz2Ph, BPCN-2Cz and BPCN-
3Cz, density function theory (DFT) calculations were carried out to simulate the frontier orbitals
of HOMO/LUMO distributions of these three molecules as shown in Fig. 1. We can see that the
electron clouds of HOMOs were mainly spread on the donor moiety- 3,6-diphenyl-9H-carbazole
(Cz2Ph), 9-phenyl-9H,9'H-3,3'-bicarbazole (2Cz) and 9,9''-diphenyl-9H,9'H,9''H-3,3':6',3''-
tercarbazole (3Cz) as well as central benzene motif for BPCN-Cz2Ph, BPCN-2Cz and BPCN-

3Cz, respectively, while LUMOs of these three compounds were found to be mostly located on
the the acceptor moiety- the group of biphenyl carbonitrile (BPCN). The highly separated
distributions of HOMO and LUMO would lead to a small ∆EST and an effective hole/electron
transport route in these bipolar host materials, benefitting the upconversion process of RISC and
the formation of excitons, respcetively. To investigate the characteristics of the lowest excited
states (S1 and T1) of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz, we further used natural
transition orbitals (NTO) analysis to study their excited states. As shown in Fig. S1, BPCN-
Cz2Ph, BPCN-2Cz and BPCN-3Cz show very similar hole and electron distribution behavior in
their S1 states, the holes are mainly located on the Cz2Ph, 2Cz and 3Cz units, and electrons are
mainly distributed on BPCN group. This typical S1 state distribution of these three compounds
suggests the characteristics of a charge transfer (CT) state, revealing the reason of their feature-
less broad PL spectra. As depicted in Fig. S2, the T1 holes are mostly located on the BPCN
group and partially extended to the donor moieties. In addition, the T1 electrons of these three
compounds exhibit nearly the same distributions. This observation results in the hybrid CT and
locally excited (LE) state of T1 for BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz.
2.3 Photophysical properties
To study the photophysical behaviors of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz, UV-Vis
absorption and photoluminescence (PL) measurement were carried out. Firstly, at room
temperature (RT), the absorption and fluorescence emission characteristics of BPCN-Cz2Ph,
BPCN-2Cz and BPCN-3Cz were evaluated in toluene solution, and then low-temperature (77K)
phosphorescent spectra were investigated in toluene glass matrix (Fig. 2a). Their experimental
data are summarized in Table 1. BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz show similar
absorption feature with a strong absorption band around 296 nm for BPCN-Cz2Ph and 305 nm

for both BPCN-2Cz and BPCN-3Cz, which are deemed to be the intrinsic π-π* transitions,
besides, the relatively weak absorption band in the range of 325–350 nm could be attributed to
the transition of intramolecular charge-transfer (ICT). The resemble characteristics of absorption
spectra can be explained by their similar molecular backbone used in BPCN-Cz2Ph, BPCN-2Cz
and BPCN-3Cz. According to the absorption edges, the optical bandgaps (Egs) can be estimated
to be 3.35 eV for BPCN-Cz2Ph, 3.36 eV for BPCN-2Cz and 3.30 eV for BPCN-3Cz. On
account of the strong electron-donating property of carbazole, it is easily understood that BPCN-
3Cz showed a bathochromic-shift in its absorption spectra compared with those of BPCN-
Cz2Ph and BPCN-2Cz. Thus, BPCN-3Cz would have the smallest bandgap among the three
compounds. In the PL spectra, the feature-less broad emission peaked at 404 nm, 423 nm and
429 nm, corresponding to the singlet energies (ESs) of 3.07 eV, 2.93 eV, and 2.89 eV, were
observed for BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz, respectively. In the phosphorescent
spectra (Phos), from the first vibrational peaks, the ETs of BPCN-Cz2Ph, BPCN-2Cz and
BPCN-3Cz could be respectively determined to be 2.76 eV, 2.71 eV, and 2.72 eV. Due to the
high enough ET values, all the three molecules are inferred appropriate host materials for blue
PHOLEDs. In the interest of gaining more insight into the ICT characteristics of these three
molecules, the solvent-polarity-dependent PL was tested as shown in Fig. 2b. For BPCN-
Cz2Ph, BPCN-2Cz and BPCN-3Cz, with the increasing polarity of solvent from low-polarity
toluene to high-polarity DMF, obvious red-shift in the PL spectra of all the three molecules were
observed, indicating extensive ICT characteristics in their excited states. To confirm if BPCN-
Cz2Ph, BPCN-2Cz, and BPCN-3Cz possess TADF character in such small ∆ESTs (0.31 eV,
0.22 eV, and 0.17 eV), the transient PL decay characteristics of BPCN-Cz2Ph/BPCN-
2Cz/BPCN-3Cz neat films were investigated (Fig. 2c). All the neat films exhibit dual decay

characteristics with a short lifetime (τP) of 6.4/12.1/9.0 ns and a relatively long lifetime (τD) of
0.46/0.34/0.34 µs, corresponding to the prompt and delayed components of a typical TADF
material. We also investigated their prompt and delayed fluorescence spectra using time-resolved
PL spectra test (Fig. S3). Besides, photoluminescence quantum efficiency (ηPL) of their neat
films were measured (78% for BPCN-Cz2Ph, 68% for BPCN-2Cz, 73% for BPCN-3Cz). In
addtion, to verify the triplet exciton confinement ability of these current TADF hosts, we also
performed the transient PL decay measurement of their doped films co-deposited with a well-
known sky-blue triplet emitter of Firpic (Fig. 2d). Although all the co-deposited films didn’t
exhibit a mono-exponential decay curve, but its second exponential decay component is far
fewer than the first exponential decay part, which indicates the triplet energy transfer from FIrpic
to these host materials was successfully suppressed. The detailed data are summarized in Table
S1. Among these doped films, ηPL of 91% was achieved for BPCN-2Cz in contrast with BPCN-
2Cz (85%) and BPCN-3Cz(52%), which could be due to the more efficient host-guest energy
transfer and good exciton-confinement on guest molecules. In addition, typical emission spectra
of FIrpic were obtained (Fig. S4), indicating the direct and full radiation from T1 of FIrpic. Last,
we compared the absorption spectrum of FIrpic and PL spectra of BPCN-Cz2Ph, BPCN-2Cz,
and BPCN-3Cz neat films (Fig. S5).
2.4 Thermal properties
High thermal stability is prerequisite for organic molecules as the host material in OLED
fabrication and operation. To study the thermal stability of BPCN-Cz2Ph, BPCN-2Cz and
BPCN-3Cz, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC)
were performed (Fig. 3). All these molecules exhibited high thermal properties with the
decomposition temperature (Td, corresponding to 5 wt% loss) over 400
and glass transition

temperature (Tg) over 100 . Their good thermal stability could be due to the rigid molecular
skeleton frame of these three molecules. Particularly, carefully comparing the molecular
configuration of these three molecules, BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz show a
relative relationship with Td (408 , 458 and 478 ) and Tg (101 , 127 and 172 ). The
synchronously increasing Td and Tg along with the number of carbazole-units was observed,
which can be explained by the increasing rigid carbazole-units in molecular skeleton frame. The
excellent thermal stability suggests that BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz could
produce morphologically stable thin films during the thermal evaporation.
2.5 Electrochemical properties
The electrochemical properties of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz were studied
via the cyclic voltammetry (CV) technique using ferrocene as the internal reference in degassed
DCM (Fig. S6). From threshold of the electrochemical oxidation potential vs Fc+/Fc, HOMO
energy levels of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz were determined to be -5.49 eV, -
5.25 eV, and -5.21eV, respectively. In addition, based on the empirical equation and the optical
energy gap, LUMO energy levels of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz could be
respectively estimated to be -2.14 eV, -1.89 eV, and -1.91 eV. The HOMO values of BPCN-
Cz2Ph, BPCN-2Cz and BPCN-3Cz show a tendency to increase on account of the increasing
electron-donating capability of the moiety- Cz2Ph, 2Cz and 3Cz. Since all these products
contained the same electron-withdrawing moiety, it easily explained the tendency of reducing
bandgap for the three compounds. The HOMOs and LUMOs of these host materials need to
suitably match with frontier molecular orbitals of the neighboring layers to prefer the electron-
hole pair recombination in the emitting layer (EML).
2.6 Electroluminescence (EL) properties

To evaluate the potential of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz as real host character,
sky-blue PHOLEDs were fabricated with 5 wt% of FIrpic as a dopant via co-evaporation with
BPCN-Cz2Ph, BPCN-2Cz, or BPCN-3Cz. It should be noted that the optimized doping
concentration of 5 wt% FIrpic in these three host materials is much lower than the conventional
doping values in the range of 15-20 wt%, which might be benefitted from the TADF
characteristics of host molecules. The device configuration is depicted as following: indium tin
oxide (ITO)/dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) (10
nm)/1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC) (40 nm)/tris(4-(9H-carbazol-
9-yl)phenyl)amine (TCTA) (10 nm)/Host: FIrpic (5 wt%, 20 nm)/1,3,5-tri[(3-pyridyl)-phen-3-
yl]benzene (TmPyPB) (55 nm)/8-hydroxyquinolinolato lithium (Liq) (2 nm)/Al (120 nm). Fig.
4a shows the energy level diagrams, and the chemical structures of the molecules applied in the
OLEDs are given in Fig. 4e. Here, HAT-CN and Liq were respectively used as hole-injecting
layer (HIL) and electron-injecting layer (EIL). TAPC acted as hole-transporting layer (HTL). A
10-nm-thick TCTA was inserted between HTL and EML as electron-blocking layer (EBL).
TmPyPB took roles of both electron-transporting layer (ETL) and hole-blocking layer (HBL) in
the meantime. Fig. 4c depicts the current density-voltage-luminescence (J-V-L) curves of the
PHOLEDs employing these three host materials. And the devices employing BPCN-Cz2Ph,
BPCN-2Cz and BPCN-3Cz show relative low turn-on voltages at 100 cd m-2 of 3.60 V, 3.36 V,
and 3.17 V, respectively. The slight difference might be due to the different HOMO/LUMO
levels and charge transport ability of the three host molecules. To fully verify the bipolar
property of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz, we have further fabricated hole-only
devices (HODs) and electron-only devices (EODs) (Fig. 5). Based on space-charge-limited
current (SCLC) model, the hole/electron mobilities of BPCN-Cz2Ph, BPCN-2Cz, and BPCN-

3Cz are calculated to be 7.77×10-6/1.57×10-7, 9.94×10-6/3.65×10-7, and 1.48×10-5/3.23×10-6 cm2
v-1 s-1, respectively. Fig. 4b,4d give CE-J-PE, EQE-J characteristics of BPCN-Cz2Ph, BPCN-
2Cz and BPCN-3Cz based sky-blue PHOLEDs. The device performances are summarized in
Table 2. At 100 cd m-2, the BPCN-2Cz based OLEDs achieved 50.0 cd A-1, 46.8 lm W-1 and
24.0% of CE, PE and EQE, respectively, which are higher than those of BPCN-Cz2Ph (46.5 cd
A-1, 40.6 lm W-1 and 22.2%) and BPCN-3Cz (34.5 cd A-1, 34.8 lm W-1 and 15.8%) based
devices. To our knowledge, the superior device efficiency based on BPCN-2Cz are among the
best values of FIrpic-based sky-blue PHOLEDs.[16, 50-63] In fact, BPCN-Cz2Ph, BPCN-2Cz
and BPCN-3Cz all show triplet energies high enough to be host materials for FIrpic (2.62 eV).
An efficient host-guest energy transfer from BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz to
FIrpic and a splendid triplet exciton confinement on the FIrpic molecules should be achieved.
Thus, the slight difference in device performance should be due to the difference in electron-hole
balance injection/transport in the EML, which is depended on the bipolar property of the host
material. A nice host molecule not only possesses a high ET to confine the excitons on dopant but
also has a balanced carrier transport character to favor the recombination process of excitons in
EML. In addition, at a high driving voltage in PHOLEDs, efficiency roll-off is a typical
phenomenon and always be related to the quenching effect of triplet-triplet annihilation (TTA)
and the triplet-polaron annihilation (TPA). In FIrpic-based sky-blue PHOLEDs, this efficiency
roll-off could be alleviated by utilizing our new family of TADF molecules as the host materials.
With the TADF nature of these three host materials, the up-conversion of triplet excitons to
singlets will reduce the concentration of triplets, thus resulting in reduced TTA and TPA at high
driving current density and smaller efficiency roll-off. Impressively, even at 1000 cd m-2, the
performances of BPCN-2Cz and BPCN-CZ2Ph based devices could maintain high levels of

44.8 cd A-1, 34.5 lm W-1, 21.5% and 44.2 cd A-1, 32.6 lm W-1, 21.0%, respectively. While for
BPCN-3Cz, due to the unbalanced carrier transport ability at high driving voltage, BPCN-3Cz
based devices showed a slightly high EQE roll-off of 19.0% from 100 cd m-2 to 1000 cd m-2 in
contrast to BPCN-2Cz (10.4%) and BPCN-CZ2Ph (5.4%). The results indicate the critical
importance of a well-balanced charge transport property for a host material. The typical FIrpic
emission were observed in the electroluminescence (EL) spectra of BPCN-Cz2Ph, BPCN-2Cz
and BPCN-3Cz based devices as shown in the inset of Fig. 4d, which means a well triplet
energy confinement on the FIrpic molecules could be achieved by using BPCN-Cz2Ph, BPCN-
2Cz and BPCN-3Cz as the host materials. Moreover, due to the increasing long-range Förster
energy transfer from TADF host to guest, low doping ratio of dopants is needed. To verify this
superiority of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz, we have fabricated sky-blue devices
with higher doping concentration of 10 wt% and 15 wt% (Fig. S7). As we expected, the
maximum EQEs of the BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz based devices with 10 wt%
and 15 wt% doping concentrations were 19.1%/22.6%/15.6% and 18.8%/22.0%/15.3%,
respectively. These results indicate that high performance could be achieved even at a low
doping concentration like 5 wt% when using BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as
host.
3 Conclusions
In summary, a unique molecular design of TADF host materials has been demonstrated by
combining carbazole electron-donors and CN substituted benzene electron-acceptor with a
benzene ring. Excellent performance and reduced efficiency roll-off could be achieved in FIrpic-
based PHOLEDs. Considering an ideal TADF host material should simultaneously fulfil the two

requirements: (1) a sufficiently high ET, (2) a well-balanced carrier transport ability, we
modulated the carbazole units in the novel molecular skeleton frame to study the influence of the
increase in the number of the donor moieties on the molecular physical property. Although the
family of these molecules could realize high ET as host materials for FIrpic, slight difference in
device performance was observed, which might be due to the difference in carrier transport
ability of host materials. Based on these present findings, we believe our method of molecular
design provides a practical way to construct promising host molecules and reveal device
performance enhancement for blue PHOLEDs.
4 Experimental section
4.1 Synthesis of 4'-chloro-[1,1'-biphenyl]-2-carbonitrile
2-iodobenzonitrile (5 g, 21.8 mmol), (4-chlorophenyl)boronic acid (4.08 g, 26.16 mmol),
K2CO3 (12.03 g, 87.2 mmol), and Pd(PPh3)4 (254 mg, 0.22 mmol) were dissolved in a 200 mL
mixture of 1,4-dioxane and water (10/1, v/v) under argon atmosphere. The mixture was stirred
for 24 h at 90 and cooled down to room temperature. Afterwards, the resulting mixture was
mixed with 200 mL water and then dichloromethane was used to extract the desired product,
which would be collected and evaporated under reduced pressure. After evaporation, the organic
phase was further purified via column chromatography using petroleum ether and
1
dichloromethane (3/1, v/v) as the eluent solvents to give a white solid (4.57 g, 98%). H NMR
(400 MHz, CDCl3) δ 7.77 (d, J = 7.6 Hz, 1H), 7.66 (t, J = 7.7 Hz, 1H), 7.48 (tt, J = 4.6 Hz, 6H).
4.2 Synthesis of 3,6-diphenyl-9H-carbazole (1)
3,6-dibromo-9H-carbazole (2 g, 6.15 mmol), phenylboronic acid (1.80 g, 14.76 mmol), K2CO3
(3.4 g, 24.6 mmol), and Pd(PPh3)4 (208 mg, 0.18 mmol) were dissolved in a 100 mL mixture of
1,4-dioxane and water (10/1, v/v) under argon atmosphere. The mixture was stirred for 24 h at 90

and cooled down to room temperature. Afterwards, the resulting mixture was mixed with 200
mL water and then dichloromethane was used to extract the desired product, which would be
collected and evaporated under reduced pressure. After evaporation, the organic phase was
further purified via column chromatography using petroleum ether and dichloromethane (2/1,
1
v/v) as the eluent solvents to afford 3,6-diphenyl-9H-carbazole a white solid (1.69 g, 86%). H
NMR (400 MHz, CDCl3) δ 8.35 (s, 2H), 8.09 (s, 1H), 7.73 (t, J = 6.8 Hz, 5H), 7.69 (s, 1H), 7.49
(t, J = 7.2 Hz, 6H), 7.36 (t, J = 7.3 Hz, 2H).
4.3
Synthesis
of
4'-(3,6-diphenyl-9H-carbazol-9-yl)-[1,1'-biphenyl]-2-
carbonitrile (BPCN-Cz2Ph)
A mixture of 1 (1 g, 3.13mmol), 4'-chloro-[1,1'-biphenyl]-2-carbonitrile (805 mg, 3.76 mmol),
Pd2(dba)3 (147 mg, 0.16 mmol), s-Phos (193 mg, 0.47 mmol) and t-BuONa (1.2 g, 12.52 mmol)
was put in 100 mL toluene. The mixture was stirred and heated for 24 h at 110 under argon
and cooled down to the room temperature. Then, dichloromethane was used to extract the
resulting product. The combined organic extracts were dried over Na2SO4 and evaporated under
reduced pressure. After evaporation, the product was purified via column chromatography on
silica gel while using petroleum ether and dichloromethane (1/1, v/v) as the eluent solvents to
1
afford BPCN-Cz2Ph a white solid (1.24 g, 80%). H NMR (600 MHz, DMSO) δ 8.77 (s, 2H),
8.04 (d, J = 7.8 Hz, 1H), 7.93 (d, J = 7.8 Hz, 2H), 7.90 – 7.85 (m, 3H), 7.83 (d, J = 8.1 Hz, 5H),
7.80 (d, J = 11.6 Hz, 2H), 7.66 (t, J = 7.6 Hz, 1H), 7.57 (d, J = 8.6 Hz, 2H), 7.51 (t, J = 7.3 Hz,
13
4H), 7.37 (t, J = 7.3 Hz, 2H). C NMR (151 MHz, CDCl3) δ 144.41, 141.79, 140.49, 138.15,
137.06, 133.97, 133.91, 133.04, 130.40, 130.10, 128.80, 127.94, 127.32, 126.93, 126.66, 125.78,
124.22, 118.90, 118.68, 111.22, 110.26. MALDI-TOF-MS: m/z: calcd for C37H24N2: 496.61,

found: 496.64. Anal. calcd for C37H24N2 (%): C 89.49, H 4.87, N 5.64; found: C 89.28, H 4.96, N
5.58.
4.4 Synthesis of 4'-(9'-phenyl-9H,9'H-[3,3'-bicarbazol]-9-yl)-[1,1'-biphenyl]-2-
carbonitrile (BPCN-2Cz)
The work-up procedures of BPCN-2Cz were similar to those used for BPCN-Cz2Ph to obtain
a white solid (1.22 g, 85%). 1H NMR (600 MHz, DMSO) δ 8.71 (d, J = 7.4 Hz, 2H), 8.41 (dd, J
= 7.7 Hz, 2H), 8.04 (d, J = 7.8 Hz, 1H), 7.95 – 7.84 (m, 7H), 7.79 (d, J = 7.7 Hz, 1H), 7.71 (t, J =
7.6 Hz, 2H), 7.66 (dd, J = 8.0 Hz, 3H), 7.60 – 7.54 (m, 2H), 7.50 (q, J = 8.3 Hz, 3H), 7.46 (d, J =
7.5 Hz, 1H), 7.42 (d, J = 8.2 Hz, 1H), 7.35 (dt, J = 7.2 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ
144.48, 141.33, 141.05, 140.03, 139.71, 138.33, 137.74, 136.92, 134.65, 134.23, 133.97, 133.03,
130.35, 130.11, 129.90, 127.89, 127.43, 127.05, 127.02, 126.20, 126.04, 125.94, 125.82, 124.19,
123.96, 123.77, 123.54, 120.47, 120.43, 120.30, 119.98, 118.92, 118.89, 118.70, 111.22, 110.11,
110.03, 109.98, 109.88. MALDI-TOF-MS: m/z: calcd for C43H27N3: 585.71, found: 585.23.
Anal. calcd for C43H27N3 (%): C 88.18, H 4.65, N 7.17; found: C 87.98, H 4.78, N 7.13.
4.5 Synthesis of 4'-(9,9''-diphenyl-9H,9'H,9''H-[3,3':6',3''-tercarbazol]-9'-yl)-
[1,1'-biphenyl]-2-carbonitrile (BPCN-3Cz)
The work-up procedures of BPCN-3Cz were similar to those used for BPCN-Cz2Ph to obtain
a white solid (1.02 g, 80%). 1H NMR (600 MHz, CDCl3) δ 8.58 (s, 2H), 8.51 (s, 2H), 8.27 (d, J =
7.6 Hz, 2H), 7.87 (t, J = 6.9 Hz, 3H), 7.84 (t, J = 7.9 Hz, 6H), 7.75 (t, J = 7.5 Hz, 1H), 7.66 (d, J
= 21.0 Hz, 11H), 7.54 (d, J = 8.0 Hz, 3H), 7.50 (s, 2H), 7.45 (q, J = 8.2 Hz, 4H), 7.33 (t, J = 7.0
Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 144.69, 141.61, 140.44, 140.32, 138.70, 138.04, 137.12,
134.97, 134.44, 134.02, 132.98, 130.47, 130.20, 129.94, 127.94, 127.51, 127.23, 127.11, 126.16,
126.10, 125.93, 124.63, 124.18, 123.78, 120.50, 120.08, 119.11, 118.98, 118.61, 111.58, 110.32,

110.11, 109.96. MALDI-TOF-MS: m/z: calcd for C61H38N4: 827.00, found: 826.31. Anal. calcd
for C61H38N4 (%): C 88.59, H 4.63, N 6.77; found: C 88.06, H 4.77 N 6.52.
Notes
The authors declare no competing financial interest.
Acknowledgements
The authors are funded by the National Natural Science Foundation of China (51821002,
61575136, and 21871199) and the National Key R&D Program of China (2016YFB0400700).
“111” Project of The State Administration of Foreign Experts Affairs of China, Priority
Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and
Collaborative Innovation Centre of Suzhou Nano Science and Technology (Nano-CIC) also
provide the financial support to this work.
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Scheme 1. The synthetic route of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz.

Fig. 1. Theoretically calculated spatial distributions and energy levels of the HOMO and LUMO
of BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz as well as their values of Eg.

Fig. 2. (a) The absorption spectra (UV-Vis), photoluminescence spectra (PL) in toluene solution
at room temperature (RT) and phosphorescent spectra (Phos) in toluene glass matrix at 77 K of
BPCN-Cz2Ph, BPCN-2Cz and BPCN-3Cz. (b) The PL spectra of BPCN-Cz2Ph, BPCN-2Cz,
and BPCN-3Cz in different solvents. (c, d) The transient PL decay curves of BPCN-Cz2Ph,
BPCN-2Cz, and BPCN-3Cz neat films and 5 wt%-FIrpic-doped films.