New naphthyridine-based bipolar host materials for thermally activated delayed fluorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2019.04.003

A series of bipolar host materials, namely, o-, m-, p-NPCz and o-, m-, p-NPDa, composing of electron-transporting naphthyridine (NP) and phenylene bridge with ortho-, meta-, and para-substituted hole-transporting carbazole (Cz)/diphenylamine (Da) were synthesized and characterized. By adjusting the linkage topology, the physical properties are subtly tuned. The characteristics of devices employing these new bipolar hosts with green thermally activated delayed fluorescence (TADF) emitter 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) employed as emitter were investigated under the same device structure. Among these hosts, carbazole-based NPCz can perform efficient energy transfer from the host to the dopant and better exciton confinement in the emitting layer due to higher triplet energy and deeper HOMO/LUMO level than those of diphenylamine-based NPDa. The device with o-NPCz as bipolar host exhibits the best device performance with external quantum efficiency of 18.4% and low efficiency roll-off. However, o-, m-, p-NPDa with lower E_T and shallower HOMO levels than 4CzIPN exhibit inferior host to dopant energy transfer. Instead, the exciplex formation between host and 4CzIPN, which was verified by TRPL, led the resulting EL spectra of the NPDa-based devices broad with yellow to orange emissions.

Full text of DOI:10.1016/j.orgel.2019.04.003

Accepted Manuscript
New naphthyridine-based bipolar host materials for thermally activated delayed
fluorescent organic light-emitting diodes
Tzu-Chin Yeh, Jhen-De Lee, Lu-Yu Chen, Tanmay Chatterjee, Wen-Yi Hung, Ken-
Tsung Wong
PII:
S1566-1199(19)30168-5
DOI:
https://doi.org/10.1016/j.orgel.2019.04.003
ORGELE 5197
Reference:
To appear in: Organic Electronics
Received Date: 15 January 2019
Revised Date: 12 March 2019
Accepted Date: 2 April 2019
Please cite this article as: T.-C. Yeh, J.-D. Lee, L.-Y. Chen, T. Chatterjee, W.-Y. Hung, K.-T. Wong, New
naphthyridine-based bipolar host materials for thermally activated delayed fluorescent organic light-
emitting diodes, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.04.003.
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TOC graphic
Bipolar TADF Host
ortho > meta > para
20
16
12
8
4CzIPN
4
0
100
101
102
103
104
Luminance (cd/m2)

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New Naphthyridine-based Bipolar Host Materials for Thermally Activated
Delayed Fluorescent Organic Light-Emitting Diodes
Tzu-Chin Yeha, Jhen-De Leeb, Lu-Yu Chenb, Tanmay Chatterjeea, Wen-Yi Hung*b,c and
Ken-Tsung Wong*a,d
a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
b Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202,
Taiwan
c
Center of Excellence for Ocean Engineering, National Taiwan Ocean University,
Keelung 202, Taiwan
d Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan
Corresponding authors:
W.-Y. Hung (wenhung@mail.ntou.edu.tw),
K.-T. Wong (kenwong@ntu.edu.tw)
Abstract
A series of bipolar host materials, namely, o-, m-, p-NPCz and o-, m-, p-NPDa,
composing of electron-transporting naphthyridine (NP) and phenylene bridge with
ortho-, meta-, and para-substituted hole-transporting carbazole (Cz)/diphenylamine
(Da) were synthesized and characterized. By adjusting the linkage topology, the
physical properties are subtly tuned. The characteristics of devices employing these
new bipolar hosts with green thermally activated delayed fluorescence (TADF)
emitter 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) employed as
emitter were investigated under the same device structure. Among these hosts,
carbazole-based NPCz can perform efficient energy transfer from the host to the
dopant and better exciton confinement in the emitting layer due to higher triplet
energy and deeper HOMO/LUMO level than those of diphenylamine-based NPDa.

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The device with o-NPCz as bipolar host exhibits the best device performance with
external quantum efficiency of 18.4% and low efficiency roll-off. However, o-, m-,
p-NPDa with lower ET and shallower HOMO levels than 4CzIPN exhibit inferior host to
dopant energy transfer. Instead, the exciplex formation between host and 4CzIPN,
which was verified by TRPL, led the resulting EL spectra of the NPDa-based devices
broad with yellow to orange emissions.
Keywords: TADF, bipolar host, naphthyridine, carbazole, diphenylamine, exciplex

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1. Introduction
High-performance organic light-emitting diodes (OLEDs) have been a subject of
extensive research both in academia and in industry for the last two decades. The
utilization of thermally activated delayed fluorescence (TADF) in OLEDs for
harnessing singlet as well as triplet excitons was for the first time brought about by
Adachi et al. [1,2]. Organic materials which possess TADF characteristics are highly
promising owing to their small singlet-triplet energy difference (ΔEST), which
facilitates efficient reverse intersystem crossing (RISC) [3]. In principle, TADF OLEDs
can lead to a maximum internal quantum efficiency of 100% [4,5], similar to
phosphorescent OLED (PhOLED) employing Ir-based emitters. Analogous to PhOLEDs,
the emitters in TADF-based OLEDs are generally homogeneously dispersed into a
suitable host matrix so as to minimize de-excitation pathways, such as, triplet–triplet
annihilation and/or concentration quenching, and thus, augment the efficacy of the
device [6,7]. Thus, the astute choice of the host material is equally important to that
of the emitter system. As host materials often take up the majority part (60–99%) in
the emitting layer (EML), they are especially responsible for the power efficiency and
turn-on voltage, thereby demonstrating their influence on the overall device
performance [8]. In a typical TADF-based OLED, holes and electrons injected from the
two opposing electrodes recombine in the EML to form excitons; whereby the EML
comprise the wide-energy-gap host doped with a TADF emitter. This allows for
efficient energy transfer from the host to the dopant and confinement of both singlet
and triplet excitons within the TADF guest. The hosts for TADF-based OLEDs must
generally fulfill certain critical requirements, such as, (a) the host should possess
higher singlet and triplet (ET) energies than the emitter for efficient energy transfer
and thus confine the triplet excitons on the dopant, (b) the host should have suitably
aligned energy levels for better charge injection, (c) the host material should have

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bipolar properties (containing both hole-transporting and electron-transporting
moieties) for better charge transport, which in turn, can aid in realizing much higher
efficiency and smaller efficiency roll-offs [9-12]. Though three types of host materials
(depending on charge transport properties) are employed in such OLEDs, viz.,
hole-transporting (p-type) [13], electron-transporting (n-type) [14,15], and bipolar
(containing both p- and n-type functional moieties) [16], in which the bipolar hosts
are found to be most successful [ 17 , 18 ]. However, the selection of the
electron-transporting n-type units always remains the most essential but difficult
part during the molecular design of bipolar hosts, since most of the n-type molecules
have electron mobilities lower than the hole mobilities of p-type groups [19,20].
However, obtaining host materials with all the above properties are still a challenge.
Thus, the host materials should be carefully designed so that the ET, frontier orbital
(HOMO/LUMO) levels and other parameters are well balanced with easy charge
injection and transportation, rendering an efficient energy transfer to the dopant
feasible and consequently an improved device performance.
Thus, it is also evident that host materials play a vital role in the photophysics of
TADF emitters [21-23]. As the emitter is dispersed into the host matrix, therefore the
host acts as a solid state solvent and thus, the various excited states (for example,
local excited and charge transfer states etc.) are affected differently based on the
polarity of the host via dipole-dipole interactions. As a result, analogous to the
solution state, a polar host stabilizes the charge-transfer singlet excited state (1CT)
more than the charge-transfer triplet state (3CT) [24] as well as local excited triplet
state (3LE), and thus, the ΔEST value is also affected. However, the molecular kinetics
are relatively restricted to a large extent in the solid phase compared to the liquid
state which has greater entropy. In a recent article, Penfold et al. has revealed the
influence of solid state solvation and its effect on TADF emitters based on simulation

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studies [25]. Furthermore, the article reported by Monkman and coworkers clearly
demonstrates the effect of host polarity on the RISC efficiency as well as the role of
the host matrix to fine tune the ΔEST value [26]. In addition, Adachi and coworkers
demonstrated that by controlling molecular orientation of the host, efficiency roll-off
could be suppressed in a TADF-based OLED [27]. The simulation and photo-physical
studies revealed that the efficiency roll-off was influenced by the hole mobility in the
emission layer rather than by the optical characteristic of the devices or by the
photo-physical characteristics of the TADF emitter. It was found that the host
molecules oriented themselves randomly at elevated temperatures during device
fabrication, which in turn reduced the hole mobility. Consequently, the
recombination zone was shifted away from the interface of emission and electron
transporting layers and thus, probably suppressed the efficiency roll-off. In addition,
Adachi
et
al.,
demonstrated
a
green
OLED
using
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as the TADF emitter
doped in a CBP host matrix which exhibited a maximum external quantum efficiency
(EQE) of 19.3%.[2]
Earlier reports suggest that bipolar hosts can provide optimized charge balance
and activate TADF emission of the emitter and result in greater than 20% quantum
efficiency. For example, Lee et al., reported green TADF OLEDs with
dicarbazole/dicarbonitrile hybridized bipolar host systems, DCzDCN [ 28 ] and
mCzB-2CN [29] doped with 4CzIPN as emitter yielding high EQEs (> 25%) . Another
interesting bipolar host 3CzPFP composing of carbazole as donor and
3-(3-(carbazole-9-yl)phenyl) pyrido[3′,2′:4,5]furo[2,3-b]pyridine as acceptor was also
reported by Lee group to achieve EQE over 30% with only 1% of 4CzIPN as emitter
[17]. Another feasible approach to achieve balance charge transport is using
physically blended mixture that can form exciplex as co-host system. For example,

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Lee and co-workers developed a 4CzIPN-based OLED with a mixed host system
composing of N,N′-dicarbazolyl-3,5-benzene (mCP) as donor and 1,3,5-tri[(3-pyridyl)-
phen-3-yl]benzene (BmPyPB) exhibits a maximum EQE of 28.6% [30]. Kim and
co-workers also developed mCP: bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine
(B3PYMPM) as the exciplex-forming co-host system which can further improve the
efficiency of 4CzIPN-based OLED to 29.6% [31]. These results clearly reveal the
potential of using host materials with bipolar character for TADF-based OLEDs.
In this study, we report the design, synthesis and properties of a series of
bipolar host materials (o-, m-, p-NPCz and o-, m-, p-NPDa) with different linking
topologies of the electron-withdrawing naphthyridine as acceptor (A) [32] and the
widely used electron-donating carbazole or diphenylamine as donor (D) via a
phenylene bridge. By adjusting the electro-donating ability of two different kind of
donors and the ortho-, meta-, or para-linking manner of the functional donor units,
the physical parameters are subtly tuned. The transient photoluminescence (TRPL)
decay and device performance of the hosts doped with green TADF emitter 4CzIPN
are fully investigated. According to the literature [19], 4CzIPN emitter possesses a
singlet/triplet energy of 2.45/2.40 eV and the energy levels (HOMO/LUMO) of
−5.80/−3.40 eV. Interesꢀngly, the emission of o-, m-, p-NPCz: 4CzIPN films exhibited
similar spectral profile to that of 4CzIPN, whereas that of the o-, m-, p-NPDa: 4CzIPN
films were broad and featureless. This phenomenon was attributed to
diphenylamine-based hosts, o-, m-, p-NPDa, with lower triplet energy (ET= 2.22~2.28
eV) than 4CzIPN, which indicate inefficient host to dopant energy transfer. In addition,
shallower HOMO/LUMO levels (-5.55~-5.48/-2.65~2.62 eV) of o-, m-, p-NPDa than
4CzIPN, result in the formation of intermolecular charge transfer (CT) complex [33].
The photoluminescence quantum yields (PLQY) of o-, m-, p-NPCz co-deposited films
are found to be much higher than o-, m-, p-NPDa co-deposited films. Subsequently,

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the OLEDs using o-, m-, p-NPCz and o-, m-, p-NPDa as hosts were fabricated with the
configuration: ITO/Tris-PCz:4% ReO3 (60 nm)/Tris-PCz (15 nm)/Host:10 wt% 4CzIPN
(25 nm)/3N-T2T (50 nm)/Liq/Al. Among these six host materials, the highest EQE
(18.4%) with current efficiency (CE) of 60.8 cd/A and power efficiency (PE) of 53.8
lm/W was achieved by the ortho-interconnection of carbazole-naphthyridine, namely
o-NPCz, which has sufficient ET of 2.56 eV, appropriate HOMO/LUMO energy levels
(-5.82/-2.7 eV), bipolar charge transport, and effective at reducing the
non-radioactive decay of 4CzIPN co-deposited thin film.

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2. Results and Discussions
2.1 Synthesis and Photo-physical Properties
Synthetic procedures and chemical structures of the host materials are
presented in Scheme 1. The intermediates o-, m-, p-NPBr, naphthyridine with
Br-substituted phenylene bridge, were synthesized by the condensation of
2-amino-3-pyridinecarboxaldehyde with appropriate bromoacetophenones. The o-,
m-, p-NPBr were then connected to the electron-donating carbazole or
diphenylamine moieties via palladium-catalyzed Buchwald-Hartwig amination. The
materials were obtained in good yields (70~90%) after purification by column
chromatography and subsequent recrystallization. The chemical structures of all
1
intermediates and host materials were confirmed by mass, H NMR and 13C NMR
spectra, and elemental analysis. Detailed synthetic procedures are summarized in the
Supporting information. All these compounds were further purified through vacuum
sublimation and used for their analyses of physical properties and application as host
materials of TADF-based OLEDs.
The room temperature UV-vis absorption and photoluminescence (PL) spectra
in neat films of these materials are depicted in Figure 1 and the data are summarized
in Table 1. The absorption band (λabs) of o-, m-, p-NPCz and o-, m-, p-NPDa were
observed at 296, 296, 287, 304, 314, and 299 nm, respectively, which are assigned to
the π-π* absorption of the phenyl-substituted naphthyridine. The relatively weaker
absorption of 300-350 nm was assigned to the carbazole to naphthyridine charge
transfer (CT) absorption overlapped with n-π* transition of carbazole in o-, m-,
p-NPCz. Obviously, the CT absorption spectra of o-, m-, p-NPDa are rather red-shifted
as compared to those of carbazole-substituted counterparts o-, m-, p-NPCz due to
the stronger electron-donating character of diphenylamino group. It is worthy to
note that the p-substituted cases, p-NPCz and p-NPDa, exhibit evident red-shifted

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absorption peak centered at 355 and 398 nm, respectively, owing to stronger D-A
interaction. The photoluminescence maximum (λPL) of o-, m-, p-NPCz in neat film is
centered at 436, 456, and 464 nm, respectively, which is blue-shifted as compared to
those diphenylamino-substituted counterparts due to the weaker electron-donating
ability of carbazole group. The PL λPL of o-, m-, p-NPDa is located at 517, 508, and 514
nm, respectively. The ET of o-, m-, p-NPCz and o-, m-, p-NPDa was obtained from the
phosphorescence spectra recorded in neat films (10K). The o-, m-, p-NPCz exhibit
higher ET of 2.56-2.44 eV than ET of o-, m-, p-NPDa (2.28-2.22 eV). The higher ET of o-,
m-, p-NPCz indicates their better potential for serving as effective host for the green
TADF emitter 4CzIPN (ET =2.4 eV).
2.2 Electrochemical and Thermal Properties
To obtain electrochemical properties, cyclic voltammetry technique was used to
probe the redox behavior of o-, m-, p-NPCz and o-, m-, p-NPDa as well as their
electrochemical stability in solutions. The cyclic voltammograms are presented in
Figure 2 and the corresponding data are summarized in Table 1. The oxidation scans
of o-, m-, p-NPCz and o-, m-, p-NPDa host materials were measured in
dichloromethane solution (10-3 M) with 0.1
M
tetrabutylammoniuum
hexafluorophosphate (TBAPF6) as supporting electrolyte and the reduction scans
were performed in degassed N,N-dimethylformamide solution (10-3 M) with 0.1 M
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The irreversible
oxidation potentials of o-, m-, p-NPCz are were observed with the oxidation onset at
1.25, 1.27 and 1.26 V, respectively, which can be assigned to the typical oxidation of
unprotected carbazole. The reversible reduction of the naphthyridine moiety in o-,
m-, p-NPCz with half-way potential at 1.55, 1.52 and 1.58 V, respectively. Similarly to
the cases of o-, m-, p-NPCz and o-, m-NPDa also showed irreversible oxidation

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behavior with oxidation onset observed at 0.90 and 0.96 V respectively, which is
significantly lower as compared to those of carbazole-substituted analogues,
indicating the better electron-donating feature of diphenylamino group. Surprisingly,
the oxidation of p-NPDa is quasi-reversible with peak centered at 1.04 V. The
reversible reduction potential of o-, m-, p-NPDa is -1.61, -1.63 and -1.60 V,
respectively, which is slightly lower as compared to the corresponding
carbazole-substituted analogues. The results indicate that the electronic coupling
between diphenylamino group and naphthyridine is stronger in the cases of o-, m-,
p-NPDa, resulting in the reduced electron-accepting character of naphthyridine
moiety, which is consistent with the stronger CT character observed in electronic
absorption spectra.
For a practical application consideration, the HOMO levels of compounds o-, m-,
p-NPCz and o-, m-, p-NPDa were determined by a photo-electron spectroscopy (AC-2)
(Figure S1). The LUMO levels of compounds were then calculated by subtracting the
HOMO level from the band gaps (ΔEg) determined from the absorption edge. The
HOMO/LUMO energy levels of o-, m-, p-NPCz are -5.82/-2.70, -5.75/-2.78, and
-5.75/-2.75 eV, respectively, which may have better capability to confine the excitons
within the 4CzIPN (-5.8/-3.4 eV) and thus suppress unwanted exciton-polaron
quenching. In contrast, the HOMO energy levels (-5.55~-5.48 eV) of o-, m-, p-NPDa
are relatively shallower than 4CzIPN, which might lead to the possibility of interfacial
charge transfer between the 4CzIPN and o-, m-, p-NPDa host materials, leading to the
low photoluminescence quantum yield which will be discussed later. This
phenomena has also been reported by Lee’s group [33]. In general, similar to the
literature, the optical energy gap (Eg) and triplet energy (ET) are shifted to longer
wavelengths followed by the ortho-, meta-, para-interconnections [29,34].
The glass transition temperature (Tg) and thermal decomposition temperature

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(Td, corresponding to a 5% weight loss) of the compounds o-, m-, p-NPCz and o-, m-,
p-NPDa were obtained from differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) measurements, respectively. As indicated in Table 1,
o-, m-, p-NPCz exhibit higher morphological and thermal stabilities in terms of their
Tg (76–81 °C) and Td (281–338 °C) as compared to those of o-, m-, p-NPDa [Tg (54–60
°C) and Td (244–302 °C)].
2.3 Charge Carrier Mobility
The charge-carrier transport properties of o-, m-, p-NPCz and o-, m-, p-NPDa
were evaluated by measuring the carrier mobilities of their vacuum-deposited film
using photoinduced time-of-flight (TOF) technique. The representative TOF transient
photocurrent signals for holes and electrons were illustrated in Figure S2 and S3
(Supporting Information). Figure 3(a) depicts the obtained carrier mobilities for o-,
m-, p-NPCz, shown as a function of the square root of the electric field. The hole
mobilities varied within the range from 2 × 10–7 to 4 × 10–6 cm2 V–1s–1 for fields
varying from 3.3 × 105 to 8.3 × 105 V cm–1 and the electron mobilities fell within the
range from 2 × 10–7 to 10–5 cm2 V–1s–1 for fields varying from 2.9 × 105 to 8.3 × 105 V
cm–1. The field-dependence of the carrier mobilities followed the nearly universal
Poole–Frenkel relationship µ ∝ exp(βE1/2) typically observed for disordered organic
systems, where β is the Poole–Frenkel factor [35]. Apparently, the connecting
topology has crucial influence on the charge transporting behavior. The observed
hole mobilities have the trend of para> ortho> meta, whereas the electron mobilities
have the trend of ortho> meta> para in NPCz case. Figure 3(b) shows the field
dependence of the hole mobilities of o-, m-, p-NPDa. The observed hole mobilities in
the range of 10-5 cm2 V–1s–1 s order has the trend of para> meta> ortho in NPDa case.
Unfortunately, the transient photocurrent profiles are too dispersive to enable the

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clear determination of electron mobility.
2.4 Transient PL characteristics
In Figure 4, the photoluminescence decay transients and spectra of 10 wt%
4CzIPN: o-, m-, p-NPCz co-deposited films were conducted to ensure that the origin
of observed delayed fluorescence of 4CzIPN is from TADF. o-, m-, p-NPCz due to its
high triplet energy (ET = 2.56-2.44 eV) and suitable HOMO/LUMO levels, allowing an
efficient confinement of excitons within 4CzIPN. The transient PL decay
characteristics of these co-deposited films show two-component decays, that is, a
fast decay with a lifetime of 24.6-33.2 ns and a slow decay with a lifetime of 1.8-2.3
μs, which can be assigned to fluorescence and triplet exciton up-conversion via TADF,
respectively. In addition, the PL spectra of 10 wt% 4CzIPN: o-, m-, p-NPCz
co-deposited films showed
a peak centered at ca. 535 nm and the
photoluminescence quantum yield (PLQY) were measured to be 50-73% (Figure S4).
In contrast, the transient PL decay of 10 wt% 4CzIPN: o-, m-, p-NPDa
co-deposited films were best fitted by three single exponential decay components as
Figure 5 shown. The two slow decay components display one is TADF of 4CzIPN
emitter and another is the formation of intermolecular CT complex (exciplex)
between 4CzIPN and o-, m-, p-NPDa. The two overlapped and red-shift emission
bands observed in the co-deposited films can be explained by the residual emission
(4CzIPN) and exciplex formation, respectively, which show very low PLQYs (10-20%).
In the case of 10 wt% 4CzIPN: p-NPDa co-deposited film, the exciplex emission peak
(592 nm) is in good agreement with the energy difference (2.08 eV) between the
HOMO of p-NPDa (-5.48 eV) and the LUMO of 4CzIPN (-3.4 eV). This result is
consistent with our speculation that o-, m-, p-NPDa with lower ET and shallower
HOMO/LUMO levels than 4CzIPN, which indicate inefficient host to dopant energy

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transfer and exciplex formation. In addition, the TRPL and spectra of o-, m-,
p-NPDa:3N-T2T (by 1:1 weight ratio) films were measured in order to examine the
possibility of exciplex formations between hosts (o-, m-, p-NPDa) and ETL (3N-T2T).
As indicated in Figure S5, the spectra only showed the PL of o-, m-, p-NPDa and the
decay lifetimes are in nano-second region. The result indicates no exciplex formed in
the interface of host:ETL.
2.5 Electroluminescent properties
Using 10 wt% 4CzIPN doped in o-, m-, p-NPCz and o-, m-, p-NPDa films as the
emitting layers (EML), multilayer OLEDs were fabricated with a configuration of ITO/
4 wt% ReO3:Tris-PCz (60 nm)/ Tris-PCz (15 nm)/ EML (25 nm)/ 3N-T2T (30 nm)/ Liq (1
nm)/Al,
where
3N-T2T
[
36
,
37
]
and
Liq
represent
2,4,6-tris(3-(pyridin-3-yl)phenyl)-1,3,5-triazine and 8-hydroxyquinolinolatolithium.
Scheme 2 shows the device structure and the schematic diagram of the energy levels
of materials used in the device. To lower the hole injection barrier from ITO to
Tris-PCz, we used rhenium oxides (ReO3) as a dopant material in Tris-PCz to produce
ohmic contact [38]. Tris-PCz with a high ET (2.76 eV) and hole mobility can efficiently
confine the triplet energy of EML, is selected as the HTL [34]. 3N-T2T (ET: 2.82 eV;
HOMO/LUMO: -6.77/-2.7 eV) is selected as the electron transport layer (ETL) to block
the excitons within the EML.
Figure 6 depicts the electroluminescent (EL) characteristics of the o-, m-, p-NPCz
and o-, m-, p-NPDa hosted devices, and the pertinent data are summarized in Table 2.
Devices exhibited a lower turn-on voltage (Von) of 2.0 V, excepting o-NPCz-based
device (Von of 2.4 V, high-energy barrier for hole injection from Tris-PCz to o-NPCz).
The difference in the J–V performance is presumably because of the different hosts
that influence the charge carrier transporting behavior of the EML. o-, m-,

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p-NPCz-based devices display higher device efficiency than those of o-, m-, p-NPDa,
which are consistent with previous discussion.
Among these six bipolar hosts, o-NPCz exhibits a higher ET of 2.56 eV that can
perform effective confinement of the 4CzIPN excitons. Therefore, o-NPCz-hosted
device revealed the best device performance with a maximum luminance (Lmax) of
53880 cd/m2 at 11.0 V and the CIE coordinate of (0.36, 0.58). The maximum external
quantum (EQE), current (CE), and power (PE) efficiencies were 18.4%, 60.8 cd/A, and
53.8 lm/W, respectively. The m-NPCz and p-NPCz hosted devices exhibited relatively
lower EQEs (15.6% and 15.5%). The best performances of o-NPCz hosted device can
be rationalized by the higher PLQY of 4CzIPN:o-NPCz film (0.73) than that of
4CzIPN:m-NPCz (0.56) and 4CzIPN:o-NPCz (0.56) films (Figure S4).
Quantum efficiency-luminance curves of o-, m-, p-NPDa hosted devices are
shown in Figure 6(d), which were sharply dropped, and the EQEs were only 5.3-3.1%
compared with those of o-, m-, p-NPCz -based devices. There are two possible
reasons to explain such low EL performance of o-, m-, p-NPDa-hosted devices. One is
lower ETs of o-, m-, p-NPDa than 4CzIPN, which cannot undergo efficient energy
transfer to 4CzIPN dopant. Another is the shallower HOMO levels of o-, m-, p-NPDa
than 4CzIPN that leads to the exciplex formation between o-, m-, p-NPDa and 4CzIPN.
The EL spectra of the devices were broad and showed yellow to orange emissions
(Figure 6f).
.
3. Conclusion
In summary, a series of novel naphthyridine-based bipolar hosts, o-, m-, p-NPCz
and o-, m-, p-NPDa, were designed and synthesized by incorporating naphthyridine
unit and carbazole/diphenylamine via the ortho-, meta-, and para-position of

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phenylene bridge. The optical, electrochemical properties, transient
photoluminescence decay and device performances of the newly synthesized host
materials were fully investigated. The pertinent compounds were used as host
materials for the green TADF emitter, 4CzIPN. The photoluminescence quantum
yields of o-, m-, p-NPCz co-deposited films are found to be much higher than o-, m-,
p-NPDa co-deposited films. Subsequently, the green TADF-OLEDs using o-, m-, p-NPCz
and o-, m-, p-NPDa as bipolar hosts were fabricated with the configuration: ITO/
Tris-PCz:4% ReO3 (60 nm)/ Tris- PCz (15 nm)/ Host: 10% 4CzIPN (25 nm)/ 3N-T2T (50
nm)/ Liq/ Al. Amongst these six host materials, the highest EQE (18.4%) with CE (60.8
cd/A) and PE (53.80 lm/W) is displayed by the ortho-interconnection of
donor-acceptor, namely o-NPCz, which has sufficiently high triplet energy,
appropriate HOMO/LUMO energy levels, and effectively reducing the
non-radioactive decay of 4CzIPN co-deposited thin film. For the stronger
electron-donating feature of dinphenylamino groups, leads molecules o-, m-, p-NPDa
to have higher HOMO energy levels as well as reduced optical energy gaps and triplet
energies, giving less effective confinement of emissive excitons within 4CzIPN, which
was verified by the observed lower PLQYs in doped films and lower EQEs from the
exciplex in the devices.
Acknowledgements
The authors gratefully acknowledge financial supports from Universal Display
Cooperation and Ministry of Science and Technology of Taiwan. (MOST
106-2112-M-019-002-MY3, 107-2113-M-002-019-MY3).

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List of Figures
Scheme 1. Molecular structures and synthesis of new bipolar host molecules o-, m-,
p-NPCz and o-, m-, p-NPDa.
Figure 1. Room-temperature normalized UV-Visible absorption and PL emission
spectra of o-, m-, p-NPCz (a,b,c) and o-, m-, p-NPDa (d,e,f) in neat films;
phosphorescence spectra (red lines) in neat films at 10 K.
Figure 2. Cyclic voltammograms of (a) o-, m-, p-NPCz and (b) o-, m-, p-NPDa.
Figure 3. Hole and electron mobilities of (a) o-, m-, p-NPCz as well as hole mobilities
of (b) o-, m-, p-NPDa with different linking positions.
Figure 4. Room-temperature transient PL decay as well as normalized UV-Vis
absorption and PL emission for the 4CzIPN-doped film in (a)/(d) o-NPCz, (b)/(e)
m-NPCz and (c)/(f) p-NPCz.
Figure 5. Room-temperature transient PL decay as well as normalized UV-Vis
absorption and PL emission for the 4CzIPN-doped film in (a)/(d) o-NPDa, (b)/(e)
m-NPDa and (c)/(f) p-NPDa.
Scheme 2. Molecular structures used in this study and an energy level diagram of the
device.
Figure 6. (a)/(b) Current density-voltage-luminance (J–V–L) characteristics, (c)/(d)
EQE and PE as a function of luminance, (e)/(f) EL spectra of o-, m-, p-NPCz and o-, m-,
p-NPDa doped with 10 wt% 4CzIPN.

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Table 1. Photophysical and Electrochemical Properties of o-, m-, p-NPCz, and o-, m-,
p-NPDa.
OX
REV
c
d
d
λabs
λPL
(nm)
aE1/2
aE1/2
HOMO b
LUMO b
ΔEg
ET
Tg
(oC)
Td
Host
(nm)
(V)
(V)
(eV)
(eV)
(eV)
(eV)
(oC)
o-NPCz
296,326
436
456
1.25
1.27
-1.55
-1.52
-5.82
-5.75
-2.70
-2.78
3.12
2.97
2.56 80
2.46 76
281
m-NPCz 296,331
p-NPCz 287,355
296
338
464
1.26
-1.58
-5.75
-2.75
3.0
2.44 81
o-NPDa 304,382
m-NPDa 314,389
p-NPDa 299,398
517
508
514
0.90
0.96
1.04
-1.61
-1.63
-1.60
-5.55
-5.54
-5.48
-2.63
-2.62
-2.65
2.92
2.92
2.83
2.28 56
2.24 54
2.22 60
244
296
302
a
REV
E1/2OX oxidation onset potential and E1/2 reduction half-wave potential are determined from cyclic
voltammetry; The E1/2OX of p-NPDa are calculated from its oxidation half-wave potential. Determined
b
through AC2 measurements; LUMO = HOMO + Eg; the value of Eg was calculated from the absorption
c
d
onset of the solid film. Triplet energy ET was calculated from the peak of phosphorescence spectra. Tg
was measured by DSC technique; Td was determined by TGA analysis.

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Table 2 EL performance of devices incorporating various host.
Von
Lmax
Imax
EQE max CE max PE max
at 1000 nit
[%, V]
CIE
[x,y]
Host
[V] [cd/m2] [mA/cm2]
[%]
[cd/A] [lm/W]
53880
(11.0V)
o-NPCz 2.4
m-NPCz 2.0
p-NPCz 2.0
o-NPDa 2.0
m-NPDa 2.0
p-NPDa 2.0
2096
2458
1026
1280
2595
2451
18.4
60.8
51.6
51.2
13.7
12.3
5.6
53.8
46.4
50.5
17.0
14.9
7.9
17.7, 4.4
15.1, 4.4
14.5, 4.8
3.7, 3.8
1.9, 4.6
0.6, 5.6
0.36, 0.58
0.33,0.59
0.34,0.59
0.42, 0.53
0.43, 0.52
0.5, 0.47
74280
(11.4V)
15.6
15.5
5.3
51700
(10.4V)
7690
(8.6V)
5720
(9.6V)
5.1%
2670
(9.4V)
3.1
a Turn-on voltage at which emission became detectable. b The values of driving
voltage and EQE of device at 1000 cd/m2 are depicted in parentheses.

ACCEPTED MANUSCRIPT
Figure 1
(a)
(b)
(c)
m-NPCz
Abs
o-NPCz
Abs
p-NPCz
Abs
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
PL
Phos
PL
Phos
PL
Phos
300
400
500
600
700
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
(d)
(e)
(f)
m-NPDa
Abs
p-NPDa
Abs
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
o-NPDa
Abs
PL
Phos
PL
Phos
PL
Phos
300
400
500
600
700
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)

ACCEPTED MANUSCRIPT
Figure 2
(a)
(b)
o-NPCz
m-NPCz
p-NPCz
o-NPDa
m-NPDa
p-NPDa
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
Potential vs. Ag/AgCl (V)
Potential vs. Ag/AgCl (V)

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Figure 3
1E-4
1E-5
1E-6
(b)
(a)
hole / electron mobilities
hole mobilities
o-NPDa
/
/
/
o-NPCz
m-NPCz
p-NPCz
1E-5
m-NPDa
p-NPDa
.
.
1E-6
1E-7
500
600
700
800
900
700 800 900 1000 1100 1200
E1/2 (V/cm)1/2
E1/2 (V/cm)1/2

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Figure 4
105
104
103
102
105
(a)
105
104
103
102
(b)
(c)
m-NPCz:10% 4CzIPN
o-NPCz:10% 4CzIPN
ex285_em535
Fitting
p-NPCz:10% 4CzIPN
ex285_em535
Fitting
ex285_em535
Fitting
104
103
I(t)=ΣAiexp(-t/τi)
I(t)=ΣAiexp(-t/τi)
I(t)=ΣAiexp(-t/τi)
A1=0.02; τ1=1.81 (
µs)
A1=0.04; τ1=2.33 (
µs)
A1=0.03; τ1=2.28 (
µs)
A2=0.98; τ2=24.6 (ns)
A2=0.96; τ2=33.2 (ns)
A2=0.97; τ2=30.3 (ns)
102
0
2
4
6
0
2
4
6
0
2
4
6
Time (
µ
s)
Time (µs)
Time (
µ
s)
(d)
(e)
(e)
535
532
535
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
m-NPCz:10% 4CzIPN
p-NPCz:10% 4CzIPN
o-NPCz:10% 4CzIPN
Abs
PL
Abs
PL
Abs
PL
0.8
0.6
0.4
0.2
0.0
200
400
600
200
400
600
200
400
600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)

ACCEPTED MANUSCRIPT
Figure 5
104
103
102
101
104
104
103
102
101
(b)
(a)
(c)
m-NPCz:10% 4CzIPN
o-NPDa:10% 4CzIPN
ex285_em545
Fitting
p-NPCz:10% 4CzIPN
ex285_em590
Fitting
ex285_em580
Fitting
103
102
101
I(t)=ΣAiexp(-t/τi)
I(t)=ΣAiexp(-t/τi)
I(t)=ΣAiexp(-t/τi)
A1=0.03; τ1=1.77 (µs)
A2=0.27; τ2=0.29 (µs)
A3=0.70; τ3=32.9 (ns)
A1=0.03; τ1=1.6 (µs)
A2=0.19; τ2=0.23 (µs)
A2=0.78; τ2=25.8 (ns)
A1=0.04; τ1=1.11 (µs)
A1=0.31; τ1=0.25 (µs)
A2=0.65; τ2=29.9 (ns)
0
2
4
6
8
10
(e)
0
2
4
6
8
10
(e)
0
2
4
6
8
10
Time (µs)
Time (µs)
Time (µs)
(d)
544
592
578
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
m-NPDa:10% 4CzIPN
p-NPDa:10% 4CzIPN
o-NPCz:10% 4CzIPN
Abs
PL
Abs
PL
Abs
PL
200
400
600
800
200
400
600
800
200
400
600
800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)

104
2500
2500
2000
1500
1000
500
(b)
(a)
ACCEPTED MANUSCRIPT
Figure 6
104
102
100
10-2
2000
102
Host
Host
o-NPCz
m-NPCz
p-NPCz
1500
1000
500
0
o-NPDa
m-NPDa
p-NPDa
100
10-2
0
10
0
2
4
6
8
10
12
0
2
4
6
8
Voltage(V)
Voltage(V)
(c)
(d)
20
16
12
8
6
4
2
0
80
60
40
20
0
40
30
20
10
0
4
0
100
101
102
103
104
100
101
102
103
104
Luminance (cd/m2)
Luminance (cd/m2)
(e)
(f)
CIE(x,y)
1.0
1.0
o-NPCz (0.36,0.58)
m-NPCz (0.33,0.59)
p-NPCz (0.34,0.59)
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
CIE(x,y)
o-NPDa (0.42,0.53)
m-NPDa(0.43,0.52)
p-NPDa (0.50,0.47)
400
500
600
700
800
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)

ACCEPTED MANUSCRIPT
Scheme 1