Simultaneously enhancement of quantum efficiency and color purity by molecular design in star-shaped solution-processed blue emitters

Article abstract of DOI:10.1016/j.orgel.2016.06.005

A series of fluorene-free bipolar star-shaped molecules, Sn-Cz-OXD (n = 1-5), with increasing conjugated length in branches were synthesized as high efficient blue emitters for OLEDs. With the extension of conjugated branches, the solid PL quantum efficiency and external quantum efficiency of Sn-Cz-OXD significantly increased with longer spacer, while the emission spectrum of these materials exhibited a blue-shift with enhanced color purity due to the unique molecular design. All materials maintained exceptionally high thermal stability after prolonged heat treatment at 150 °C in air. The photophysical, electrochemical, thermal properties of these emitters were studied in relation to the molecular structure. Nondoped device based on S4-Cz-OXD with structure ITO/PEDOT:PSS/EML/TPBI/LiF/Al emitted stable pure blue light with CIE coordinates of (0.157, 0.146). It exhibited high current efficiency and external quantum efficiency of 4.96 cd A^-1 and 4.20%, respectively. These values are among the best results for solution-processed non-doped blue device based on fluorene-free materials, indicating its potential for commercial applications.

Full text of DOI:10.1016/j.orgel.2016.06.005

Organic Electronics 37 (2016) 14e23
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Simultaneously enhancement of quantum efficiency and color purity
by molecular design in star-shaped solution-processed blue emitters
Xuehan He a, Lei Chen b, Yongbiao Zhao c, Hui Chen a, Siu Choon Ng d, Xizu Wang e,
,
*
Xiaowei Sun c, Xiao ‘Matthew’ Hu a
a School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
b Department of Materials Science and Engineering, National University of Singapore, Singapore
c School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore
d Singapore Institute of Technology, Singapore
e Institute of Materials Research and Engineering (IMRE), Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of fluorene-free bipolar star-shaped molecules, Sn-Cz-OXD (n ¼ 1e5), with increasing conju-
gated length in branches were synthesized as high efficient blue emitters for OLEDs. With the extension
of conjugated branches, the solid PL quantum efficiency and external quantum efficiency of Sn-Cz-OXD
significantly increased with longer spacer, while the emission spectrum of these materials exhibited a
blue-shift with enhanced color purity due to the unique molecular design. All materials maintained
exceptionally high thermal stability after prolonged heat treatment at 150 ꢀC in air. The photophysical,
electrochemical, thermal properties of these emitters were studied in relation to the molecular structure.
Nondoped device based on S4-Cz-OXD with structure ITO/PEDOT:PSS/EML/TPBI/LiF/Al emitted stable
pure blue light with CIE coordinates of (0.157, 0.146). It exhibited high current efficiency and external
quantum efficiency of 4.96 cd Aꢁ1 and 4.20%, respectively. These values are among the best results for
solution-processed non-doped blue device based on fluorene-free materials, indicating its potential for
commercial applications.
Received 21 March 2016
Received in revised form
2 June 2016
Accepted 3 June 2016
Keywords:
Organic semiconductor
Star-shaped bipolar molecule
Conjugation spacer
Light emitting diode
Blue emitter
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
combine the advantages of both small molecules and polymers
[5e8]. Compared with polymers, which are actually a mixture of
Solution-processed organic light-emitting diodes (OLEDs) have
received tremendous attention in both academic research and po-
tential commercial applications as low cost, flexible, large area full-
color displays [1,2]. To realize full-visible-spectra display, blue
emitters play an very important role because they not only supply
blue emission as one of the three-primary colors, but also generate
other colors by energy cascade to lower energy fluorescent or
phosphorescent dopants [3]. However, efficient blue emitters are
difficult to achieve due to their intrinsic wide band gap, which also
causes the difficulty in injection of electron. As a result, blue OLEDs
usually show inferior device performance compared with that of
green and red OLEDs [4].
different molecules, star-shaped oligomers have precise structures
and are easy to purify and reproduce. On the other hand, they
possess excellent solubility in organic solvents, which pave the way
for solution-processing and fabrication of device. Besides, star-
shaped molecules usually have high photoluminescent (PL) quan-
tum efficiency than its linear counterparts [9] because their star-
branched structure could suppress the intermolecular interaction,
prevent aggregation and close packing, and reduce concentration
quenching. Besides, star-shaped structure also facilitates the for-
mation of amorphous film.
So far, most reported star-shaped blue emitters were oligo-
fluorenes [8e12]. Yang et al. designed a series of starburst fluorene
Recently, star-shaped blue emitters have attracted much
research interest in the development of OLEDs since they can
oligomers with a planar triphenylamine core to achieve a
maximum CE of 3.83 cd Aꢁ1 and a maximum EQE of 4.19% [11]. Zou
et al. synthesized a series of starburst oligofluorenes using phenyl
as the core with a maximum EQE of 6.8% and CE of 5.4 cd Aꢁ1 [8].
Subsequently, they fine-tuned the turn-on voltage of 3.6 V by
* Corresponding author.
E-mail address: ASXHU@ntu.edu.sg (X. ‘Matthew’ Hu).
http://dx.doi.org/10.1016/j.orgel.2016.06.005
1566-1199/© 2016 Elsevier B.V. All rights reserved.

X. He et al. / Organic Electronics 37 (2016) 14e23
15
introducing electron donor diphenylamine end-caps to raise the
HOMO level although this may reduce the spectral stability [12].
Improved device performance was also reported in star-like
branched polyfluorene [13,14].
Although these star-shaped oligofluorenes were found to have
good device efficiency, their practical application is hindered due to
some remaining issues. Fluorene unit intrinsically has unstable EL
spectrum so that an additional green emission could be observed
during long time operation or thermal treatment in air due to the
generation of ketonic defects [15e18]. Besides, fluorene is a strong
p-type hole conductors, which might lead to an unbalanced charge
injection and transportation. In order to develop better blue
emitting material, recently we designed a fluorene-free star-shaped
Among these molecules, S1-Cz-OXD has been studied earlier
[19]. It was found to have superior high solid PL efficiency and
excellent thermal stability due to its molecular design. And the
performance of its non-doped device was comparable with other
solution processed blue OLEDs. Based on this molecule, S2-Cz-OXD
and S3-Cz-OXD with increased arm length were designed to
further suppress the molecular aggregation in order to achieve
higher quantum efficiency in solid state. Considering the potential
poor solubility of S3-Cz-OXD due to its rigid construction, S4-Cz-
OXD with alkoxy chains in the arms were also designed to improve
the solubility, and facilitate the solution processing in device
fabrication.
The synthesis route is shown in Scheme 1. The synthesis of the
core (15) and S1-Cz-OXD was described in an earlier paper [19]. 1,4-
Hydroquinone (compound 1) was reacted with bromohexane in
DMF at 80 ꢀC in the presence of K2CO3 to give compound 2 with 51%
yield. Then compound 3 was obtained by reaction of compound 2
with bromine at a yield of 82% [27]. Subsequent treatment of the
D-p-A blue emitter S1-Cz-OXD with excellent thermal stability. It
showed much higher solid PL quantum efficiency compared with a
linear counterpart of it and had great potential to be a good blue
emitting material for OLEDs. This indicates to a new direction to
develop star-shaped blue emitters with high stability and effi-
ciency. D-
p
-A design was also introduced in emitter S1-Cz-OXD,
above bromides
3
with n-BuLi and 2-isopropoxy-4,4,5,5-
endowing it with bipolar properties, to improve the charge trans-
portation balance in the device [19].
tetramethyl-1,3,2-dioxaborolane gave the corresponding boronic
esters 4 [28]. Then compound 4 underwent a Suzuki reaction to
produce compound 5 in a yield of 60%. An Ullmann reaction [29]
was used to attach the phenyl bridge onto the nitrogen atom of
3,6-Di-tert-butyl-9H-carbazole (6) to give the N-phenylcarbazole
derivatives 7 and 9 with 61% and 64% yields. The corresponding
boronic esters 8 or 10 were again obtained by the similar procedure
to that used for the synthesis of compound 4. Boronic esters 8 react
with bromides 3 and 5 through Suzuki reaction to produce bro-
mides 11 and 13 with yields of 45% and 42%, which produce boronic
ester 12 and 14 by the same esterification for the synthesis of
compound 4. A classic Suzuki reaction was used to coupling the
bromides core (15) and different boronic esters 8, 10, 12 and 14, and
to give the corresponding target molecules S2-Cz-OXD, S3-Cz-OXD,
S4-Cz-OXD and S5-Cz-OXD, respectively. The identity and purity of
these compounds were confirmed by 1H NMR, 13C NMR spectra, MS
(MALDI-TOF), and elemental analysis, as presented in the experi-
mental part.
S1-Cz-OXD was tested to have good solubility. Its solubility was
found to be around 25 mg mLꢁ1 in CHCl3 and 18 mg mLꢁ1 in mixed
solution of toluene: chlorobenzene (1:1). The solubility of S2-Cz-
OXD in CHCl3 reduced to 15 mg mLꢁ1 and 12 mg mLꢁ1. And, for S3-
Cz-OXD, the solubility fell to less than 3 mg mLꢁ1 and less than
1 mg mLꢁ1 in CHCl3 as expected. S4-Cz-OXD and S5-Cz-OXD, with
the solubilization function of six alkoxy chains, showed increased
solubility of 20 mg mLꢁ1 and 15 mg mLꢁ1 in CHCl3 as well as
10 mg mLꢁ1 and 8 mg mLꢁ1 in mixed toluene: chlorobenzene (1:1),
respectively.
To further optimization, extending of the conjugated branches
of the star-shaped emitters is employed to enhance the solid PL
quantum efficiency (Ff), since it would suppress the intermolecular
interaction more effectively, as observed in many star-shaped oli-
gofluorenes molecules [8,12,20]. Additionally, this was supposed to
improve the color purity of the emission blue light. For the unipolar
oligofluorene emitters, more fluorene repeat units would reduce
the energy bandgap and cause red shift of the emission spectra due
to extent
p-conjugation [21e24]. For example, starburst oligo-
fluorenes with a phenyl core exhibited red-shift of 43 nm when the
amount of fluorene unit in branches increased from 1 to 4 [8].
However, in D-p-A molecules, when the conjugated spacer be-
tween donor and acceptor increased, there would be less overlap of
electron orbit between donor and acceptor moiety. This would
reduce the charge transfer ability from donor to acceptor and
leading to blue-shift in PL spectrum [25]. Thus the undesired red-
shift could be prevented.
In this paper, we present the strategy toward novel star-shaped
blue emitters Sn-Cz-OXD (n ¼ 2e5) with a tris(1,3,4-oxadiazole)
phenylene as core, 3,6-di-tert-butyl-9H-carbazolas as terminal
groups, and benzene units as the spacers. We also report on their
high solid PL quantum efficiency, high thermal and electrochemical
stability. Moreover, S4-Cz-OXD showed best performance and was
selected as emitting material for the solution-processed non-doped
OLED device. Compared with the control device based on S1-Cz-
OXD (459 nm, 3.13%, 3.64 cd Aꢁ1, 1.58 lm Wꢁ1), S4-Cz-OXD based
non-doped device exhibited a blue-shift emission at 452 nm,
enhanced external quantum efficiency (EQE) of 4.20%, current ef-
ficiency (CE) of 4.96 cd Aꢁ1 and power efficiency (PE) of
2.2. Properties and characterizations
2.08 lm Wꢁ1
.
2.2.1. Photophysical properties
Fig. 1a and b show the UVevisible absorption and PL spectra of
the four molecules in diluted toluene solution. Their peak absorp-
2. Results and discussions
tion and emission wavelengths as well as
Фf are listed in Table 1. In
2.1. Molecular design and synthesis
dilute solution of toluene, the main absorption peaks are located at
369 nm, 356 nm, 348 nm, 350 nm and 349 nm for S1-Cz-OXD, S2-
Cz-OXD, S3-Cz-OXD, S4-Cz-OXD and S5-Cz-OXD. And the peaks of
their PL emission wavelengths (excited at 350 nm) are 422 nm,
420 nm, 418 nm, 428 nm and 425 nm, respectively. Both values
showed slight blue-shift from S1-Cz-OXD to S3-Cz-OXD with the
Chemical structures of target molecules are shown in Scheme 2.
These four star-shaped molecules were designed based on a radial
donor-p-acceptor architecture, whereby the molecules can possess
a bipolar characteristic favorable for more balanced charge injec-
tion/transport as well as higher carrier recombination efficiency
[26]. The electron-rich carbazole and electron-deficient oxadiazole
units were selected to serve as donor/acceptor units because of
their excellent charge transport properties and good chemical
stability.
increase of arm length. This is because for D-p-A type molecules,
the light absorption and emission are determined by the energy of
intramolecular charge transfer from donor unit to acceptor unit. In
this case, with the increasing arm length, the distance between
donor and acceptor unit was also increased, this will weaken the

16
X. He et al. / Organic Electronics 37 (2016) 14e23
C6H13
C6H13
O
C6H13
C6H13
O
O
O
OH
O
O
O
O
Br
Br
B
B
Br
Br
I
Br
HO
O
O
O
O
C6H13
C6H13
C6H13
C6H13
4
3
5
2
1
O
NH
N
Br
N
B
O
I
Br
7
8
6
O
O
B
N
6
Br
N
I
Br
10
9
C6H13
O
C6H13
O
O
O
N
Br
N
B
8
3
O
O
C6H13
C6H13
12
11
C6H13
O
C6H13
O
O
O
N
Br
N
B
8
5
O
O
C6H13
C6H13
13
14
Br
S1-Cz-OXD
S2-Cz-OXD
6
8
N
N
O
COCl
H
O
N
N
N
Br
Br
10
12
14
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
N
N N
ClOC
COCl
N
N
O
15 (core)
Br
Scheme 1. Synthesis route: i) 1-bromohexane, K2CO3, DMF, heated at 80 ꢀC for 30 h; ii) CHCl3, bromine, stir at room temperature for 20 h; iii) ꢁ78 ꢀC, THF, n-BuLi, 2-Isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane, stir for 24 h; iv) K2CO3, toluene, Aliqud 336, Pd(PPh3)4, stir in dark, heated at 100 ꢀC for 20 h; v) KOH, CuCl, 1,10-phenanthroline, toluene,
reflux for 24 h; vi) anhydrous pyridine, reflux for 24 h; vii) 3,6-Di-tert-butyl-9H-carbazole, CuI, K2CO3, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) and 18-crown-
6, heat at 170 ꢀC for 48 h.

X. He et al. / Organic Electronics 37 (2016) 14e23
17
N
O
O
C
D
N
C H
O
O
C
H
O
O
H
CC6H13
S5-Cz-OXD
Scheme 2. Chemical structure of target molecules.
intramolecular charge transfer process, leading to a wider molec-
ular energy bandgap and a slight blue-shift. S4-Cz-OXD and S3-Cz-
OXD are of the same arm length, the red-shift in both absorption
and emission spectra of S4-Cz-OXD could be attributed to the
electron donating ability of the alkoxy chains, which could reduce
the energy bandgap. Besides, it also enhanced the polarity of S4-Cz-
OXD, leading to a bathochromic shift in polar toluene solvent. S5-
Cz-OXD again showed blue-shift in both spectra compared with
S4-Cz-OXD due to the extended conjugation. The corresponding PL
quantum efficiency in dilute toluene solution of these emitters are
calculated to be 93%, 91%, 87%, 79% and 75% using 9,10-
diphenylanthracene as reference. This is because in solution state,
bigger molecules tend to lose energy by molecular vibration and
single bond rotation, especially the flexible alkoxy chains.
It is important to study the condensed state optical properties.
Both neat and doped films (PMMA) [10,30] of target materials are
spin-casted on quartz substrates from solution followed by
annealing treatment. The resultant spectra of them are shown in
Fig. 1, c and d. Compare with the dilute solution, the absorption
peak of the films of Sn-Cz-OXD (n ¼ 1e5) showed slightly red-shift
to 372 nm, 358 nm, 349 nm, 352 nm and 351 nm due to the close
packing in solid state. And their corresponding emission peaks (at
excitation of 355 nm) exhibited obviously red-shifts to 455 nm,
450 nm, 446 nm, 447 nm and 445 nm, respectively. The blue-shift
from S1-Cz-OXD to S3-Cz-OXD was due to the same reason.
Without the bathochromic shift, S4-Cz-OXD only showed a very
slight red-shift compared with S3-Cz-OXD, indicating that the
alkoxy chains with electron donating ability can reduce the mo-
lecular energy bandgap. Their corresponding Ff values in doped
PMMA films are calculated to be 72%, 76%, 79%, 81% and 84%, and
are 54%, 58%, 61%, 62% and 65% in neat films, using 9,10-
diphenylanthracene as reference. These values of Ff are quite
high for a solid neat film, implying the star-shaped structure could
reduce the fluorescence quenching by effectively suppressing the
intermolecular interactions in solid state and is benefit for non-
doped emitters with high quantum efficiencies. From S1-Cz-OXD to
S5-Cz-OXD, Ff is increasing because of the intermolecular in-
teractions could be more effectively suppressed by longer branches
as well as the bulky alkoxy chains.
We reported earlier that S1-Cz-OXD exhibited excellent spec-
trum thermal stability compared with those fluorene-based blue
emitters [15e18]. The PL of fluorene-based polymer thin film
exhibited obvious change in spectra after only 4 h heating at 150 ꢀC
[31]. For comparison, thin films of the target molecules in this work
were annealed at a similar condition. As shown in Fig. 2, the shape
of the PL spectra of S2-Cz-OXD and S3-Cz-OXD keeps unchanged

18
X. He et al. / Organic Electronics 37 (2016) 14e23
(b) 1,0
1,0
0,8
0,6
0,4
0,2
0,0
(a)
S1-Cz-OXD
S2-Cz-OXD
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
S1-Cz-OXD
S2-Cz-OXD
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
0,8
0,6
0,4
0,2
0,0
300
350
400
450
500
550
600
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
(c)
1,0
(d) 1,0
0,8
S1-Cz-OXD
S2-Cz-OXD
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
S1-Cz-OXD
S2-Cz-OXD
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
0,8
0,6
0,4
0,2
0,0
0,6
0,4
0,2
0,0
300
350
400
450
500
550
600
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 1. (a) Absorption and (b) PL spectra of Sn-Cz-OXD in dilute toluene solution; (c) Absorption and (d) PL spectra of Sn-Cz-OXD in doped PMMA films.
Table 1
Photophysical, electrochemical and thermal properties of target molecules.
Ffb (%)
E
(V)c
EHOMO (eV)
ELUMO (eV)
Eg (eV)
Tg (ꢀC)
Td (ꢀC)
red
PL
max
Abs
max
oxd
l
a (nm)
E
c (V)
onset
a
Compound
l
(nm)
onset
S1-Cz-OXD
S2-Cz-OXD
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
369/372
356/358
348/349
350/352
349/351
422/455
420/450
418/446
428/447
425/445
93/72/54
91/76/58
87/79/61
79/81/62
75/84/65
1.26
1.27
e
ꢁ1.51
ꢁ1.53
e
ꢁ5.66
ꢁ5.67
e
ꢁ2.89
ꢁ2.87
e
2.77
2.80
e
137
141
145
137, 151
140, 160
447
453
470
413
421
1.17
1.19
ꢁ1.55
ꢁ1.56
ꢁ5.57
ꢁ5.59
ꢁ2.85
ꢁ2.84
2.72
2.75
a
Values measured in dilute solution/neat films.
b
c
Values of Ff in state of solution/doped PMMA films/neat films.
Using 0.1 M (C4H9)4NClO4 in CH2Cl2 as supporting electrolyte, Ag/AgCl as reference electrode.
after thermal annealing for 220 h in ambient condition, favorable
for the overall stability of the electroluminescent devices. Specif-
ically, the insert window in Fig. 2 shows the relative PL intensity of
S2-Cz-OXD changing with heating treatment. The PL intensity still
maintained at a high level of 95% of pristine film after 3 h, and 39%
after 220 h at 150 ꢀC. The results are much better than fluorene
based materials, whose PL relative intensity dropped to less than
40% of the original value after only 4 h annealing at 160 ꢀC under
similar conditions [32].
The PL spectrums of S4-Cz-OXD and S5-Cz-OXD were also quite
stable. Only a very slight increase in the tail of the spectrums of S4-
Cz-OXD and S5-Cz-OXD was observed after being annealed for
about 100 h. However, the spectrum stability of S4-Cz-OXD and S5-
Cz-OXD are still much better than those fluorene-based materials,
which will show an additional green emission just about several
hours after thermal annealing at the same condition [31].
2.2.2. Electrochemical and thermal properties
Cyclic voltammetry measurement has been carried out to
investigate the electrochemical properties of the blue emitters.
Since S3-Cz-OXD has poor solubility in methylene chloride, we only
measured other four emitters and their results are shown in Fig. 3.
All these materials exhibited reversible redox behavior under both
positive and negative drive bias, which can be attributed to the t-
BCz donor units and the OXD acceptor core, respectively, indicating
their distinct bipolar properties. Their highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels are calculated from the onset oxidation and reduction
potential according to the formula EHOMO
¼
ꢁe(Eoxd
onset þ 4.4)(eV) and ELUMO ¼ ꢁe(Ered onset þ 4.4)(eV). Data and
calculated values are summarized in Table 1.
The calculated HOMO energy levels are ꢁ5.66 eV, ꢁ5.67 eV,
ꢁ5.57 eV and ꢁ5.59 eV for S1-Cz-OXD, S2-Cz-OXD, S4-Cz-OXD and

X. He et al. / Organic Electronics 37 (2016) 14e23
19
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
1 hour
24 hours
70 hours
120 hours
170 hours
220 hours
0,4
0,2
0,0
1 hour
S3-Cz-OXD
24 hours
70 hours
120 hours
170 hours
220 hours
S2-Cz-OXD
400
450
500
550
600
650
700
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
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 hour
1 hour
24 hours
48 hours
72 hours
96 hours
24 hours
48 hours
72 hours
96 hours
120 hours
120 hours
S5-Cz-OXD
S4-Cz-OXD
400
450
500
550
600
650
700
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 2. PL spectra of Sn-Cz-OXD (n ¼ 2, 3, 4, 5) during long period heating treatment at 150 ꢀC in ambient air. Inserted spectra: Relative PL intensities of S2-Cz-OXD at different
heating time interval.
The corresponding calculated energy bandgaps are 2.77 eV, 2.80 eV,
2.78 eV and 2.79 eV. These findings are consistent with the pho-
tophysical measurement results.
The thermal properties of Sn-Cz-OXD (n ¼ 1e5) were explored
by thermogravimetric analysis (TGA) and differential scanning
S1-Cz-OXD
S2-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
calorimetry (DSC) measurement as shown in Fig. 4. The results are
also listed in Table 1. The glass transition temperature (Tg) is 137 ꢀC,
141 ꢀC and 145 ꢀC for Sn-Cz-OXD (n ¼ 1e3). S4-Cz-OXD and S5-Cz-
OXD showed two Tg, at 137 ꢀC and 151 ꢀC, 140 ꢀC and 160 ꢀC,
respectively.
The lower value of Tg at 137 ꢀC and 140 ꢀC might correspond to
the alkoxy chains which are easier to undergo rearrangement,
while the higher Tg at 151 ꢀC and 160 ꢀC might attribute to the rigid
framework of the molecules. The decomposition temperatures (Td,
5% weight loss) for these five emitters are 447 ꢀC, 453 ꢀC, 470 ꢀC,
413 ꢀC and 421 ꢀC, respectively. Td increased with molecular size
from S1-Cz-OXD to S3-Cz-OXD, and dropped to a lower value when
the alkoxy chains were introduced. The high Td and Tg temperatures
further guarantee the thermal stability during practical application.
Surface morphology of neat films of Sn-Cz-OXD (n ¼ 1e5) are
recorded by using AFM technique and the resultant images of both
non-annealed and annealed films are shown in Fig. 5. The RMS
roughness is 0.48 nm, 0.70 nm, 0.82 nm, 0.54 nm and 0.67 nm for
films of Sn-Cz-OXD (n ¼ 1e5) without annealing treatment and is
0.41 nm, 0.66 nm, 0.80 nm, 0.52 nm and 0.65 nm for those after
annealed. For both non-annealed and annealed films, the rough-
ness is obviously increasing from S1-Cz-OXD to S3-Cz-OXD and
from S4-Cz-OXD to S5-Cz-OXD. It is mainly due to the poorer
-2,0
-1,5
-1,0
-0,5
0,5
1,0
1,5
2,0
Voltage (V) vs Ag/AgCl
Fig. 3. Cyclic voltammograms of S1-Cz-OXD, S2-Cz-OXD, S4-Cz-OXD and S5-Cz-OXD.
S5-Cz-OXD. The values for S1-Cz-OXD and S2-Cz-OXD are quite
similar, indicating the electron-donating ability; while for S4-Cz-
OXD and S5-Cz-OXD, this value increase to ꢁ5.60 and ꢁ5.59 with
two oxidized peaks, which can be ascribed to the electron-donating
alkoxy chains. The calculated LUMO energy levelsare ꢁ2.89 eV,
ꢁ2.87 eV, ꢁ2.79 eV and ꢁ2.80 eV, respectively, indicating weaker
electron-withdrawing ability for tris(1,3,4-oxadiazole)phenylene
core with the increased distance between donor and acceptor unit.

20
X. He et al. / Organic Electronics 37 (2016) 14e23
1,22
(a)
(b)
1,0
S1-Cz-OXD
S2-Cz-OXD
S3-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
0,7
0,8
0,4
0,0
-0,4
-0,8
-1,2
-1,6
8
0,9
4
0,8
0
0,6
4
S1-C
z
z
-OXD
-OXD
-OXD
-OXD
-OXD
0,5
0,4
0,3
0,2
S2-C
S3-C
S4-C
S5-C
zz
z
zz
z
8
z
2
z
6
50
100
150
200
250
300
0
100
20
0
300
400
500
600
700
800
Temperature (oC)
Temperature (oC)
Fig. 4. (a) DSC traces and (b) TGA thermograms of Sn-Cz-OXD (n ¼ 1e5).
Fig. 5. AFM images of neat films of Sn-Cz-OXD (n ¼ 1e5).
solubility with extension of the conjugated branch and leading to
less smooth film by spin-coating. S4-Cz-OXD exhibits much lower
roughness than S3-Cz-OXD due to the introduction of alkoxy
chains, which could improve the solubility and film forming ability.
The same trend can also be observed in AFM images, where the
fluctuation range of height is more and more apparent from S1-Cz-
OXD to S3-Cz-OXD and from S4-Cz-OXD to S5-Cz-OXD. Moreover,
the slightly decreased RMS roughness for the annealed films
compared with those non-annealed ones indicates annealing
treatment could improve the surface quality of these films.
diagram of the devices and the chemical structure of TPBI were
shown in Scheme 3.
The electroluminescent (EL) spectra of the devices at different
applied voltages could be observed in Fig. 6. Pure blue emission
peaking at 459 nm and 452 nm were obtained for device based on
S1-Cz-OXD and S4-Cz-OXD. They have nearly identical PL spectra of
2.2.3. Electroluminescent properties
In order to further investigate the luminescence properties of
these emitters, S1-Cz-OXD and S4-Cz-OXD were selected as the
emitting material in nondoped solution processed OLEDs because
they have good solubility and could form nice amorphous films by
spin-coating. The non-doped devices based on emitters S1-Cz-OXD
and S4-Cz-OXD with configurations of ITO/PEDOT:PSS/EML/TPBI/
LiF/Al were fabricated. PEDOT:PSS is Poly(3,4-ethylenedioxy-thio-
phene) Polystyrene sulfonate, serving as the hole transporting
layer. TPBI is 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene,
serving as both electron transporting and hole blocking layer. LiF is
used as electron injecting material in the device. The energy
Scheme 3. Energy level diagram of the OLED devices and molecular structure of TPBI.

X. He et al. / Organic Electronics 37 (2016) 14e23
21
1,0
1,0
0
7V
8V
9V
10V
7V
8V
9V
10V
S4-Cz-OXD
S1-Cz-OXD
0,8
0,88
0,6
0,66
0,4
0,2
0,0
0,4
4
0,22
0,00
350 400 450 500 550 600 650 700 750 800
350 400 450 500 550 600 650 700 750 800
Wavelengthh (nm)
Wavelength (nm)
Fig. 6. Normalized EL spectra of devices at different applied voltages.
the films with only a little red-shift. The CIE coordinates of the
devices are demonstrated to be (0.167, 0.168), (0.157, 0.146), as
shown in Table 2. Moreover, no significant change in the EL spectra
was observed at various driving voltages, indicating an excellent
voltage-independent and stable EL spectrum.
3. Conclusion
We developed a series of fluorene-free star-shaped bipolar blue
emitters, Sn-Cz-OXD (n ¼ 1e5), with extended conjugated length.
With longer arms, the interactions between molecules could be
suppressed more effectively to achieve a high solid PL quantum
efficiency. Besides, the extension of conjugated length would
reduce the charge transfer ability between Donor and Acceptor,
leading to a blue-shift of the emission spectrum with better color
purity. Moreover, these materials show excellent thermal stability
due to the fluorene-free design. Among these emitters, S4-Cz-OXD
with high quantum efficiency, high solubility and improved blue
color purity in emission spectrum was selected as emitting material
for non-doped blue device. Efficient and stable blue electrolumi-
nescence based on S4-Cz-OXD was realized with a current effi-
ciency of 4.96 cd Aꢁ1 and an EQE of 4.20%. It is among the best
results for solution-processed non-doped blue device based on
fluorene-free materials, indicating its great potential for commer-
cial applications.
Although there is large hole injection barrier from PEDOT:PSS
layer (ꢁ5.2 eV) to the HOMO level of emitting layer (ꢁ5.66 eV and
ꢁ5.57 eV), both devices show low turn-on voltages at 4.1 V and
4.0 V. The reason might be the introduction of both carbazole and
oxadiazole units into endowed our materials with enhanced HOMO
level and lower LUMO level, which matched the work function of
PEDOT:PPS layer and LUMO level of TPBI layer better. These
reduced the injection barriers to achieve a low turn-on voltage.
Fig. 7 illustrates the detailed results of the non-doped device based
on the blue emitters. The electroluminescent performances were
outlined in Table 2. The devices of S4-Cz-OXD show high EL per-
formances. It can reach a maximum luminance of 7804 cd mꢁ2, a
maximum CE of 4.96 cd Aꢁ1, an EQE of 4.20%, and a PE of
2.08 lm Wꢁ1. The control device of S1-Cz-OXD exhibits poorer EL
performance, with a maximum brightness of 5672 cd mꢁ2, a CE of
3.64 cd Aꢁ1, an EQE of 3.13% and a PE of 1.58 lm Wꢁ1
.
4. Experimental section
Besides, devices based on emitters S2-Cz-OXD and S5-Cz-OXD
with identical configurations were also fabricated for comparison
study and the corresponding performance of them was also sum-
marized in Table 2. Fabrication of device based on S3-Cz-OXD was
failure due to its poor solubility. Device based on S2-Cz-OXD
exhibited higher PL efficiencies and maximum brightness than that
of S1-Cz-OXD because of the high quantum efficiency and good
surface morphology of S2-Cz-OXD. Device based on S5-Cz-OXD
showed higher maximum brightness values but less efficiency
compared with that of S4-Cz-OXD. This was because S5-Cz-OXD
has higher value of PL quantum efficiency but worse surface
morphology.
4.1. Measurement and instruments
1H and 13C nuclear magnetic resonance (NMR) spectra were
collected on a Bruker DRX spectrometer operating at 400 MHz
using deuterated chloroform as solvent with tetramethylsilane
(TMS) as reference. Elemental analyses were carried out on Ele-
mentarVario EL. Molecular weight was determined using Matrix
assisted laser desorption ionization/Time-of-flight (MALDI-TOF,
AXIMA Performance). Thermal analyses were carried out using
differential scanning calorimetry (DSC, Q10 TA Instrument) and
thermogravimetric analyses (TGA 2950, TA Instrument) at a heating
rate of 10 ꢀCminꢁ1. UVevisible absorption spectra and emission
spectra were recorded on UVeVis Spectrophotometer (Shimadzu
Table 2
Electroluminescence characteristics of the devices.
Max L (cd mꢁ2
)
EQEb (%)
CEb (cd Aꢁ1
)
PEb (lm Wꢁ1
)
lmax (nm)
CIE (x, y)
a
Emitter
Von (V)
S1-Cz-OXD
S2-Cz-OXD
S4-Cz-OXD
S5-Cz-OXD
4.1
4.1
4.0
4.0
5672
6538
7804
8213
3.13/3.1/3.0
3.42/3.4/3.3
4.20/4.2/4.1
3.93/3.8/3.9
3.64/3.6/3.6
3.86/3.8/3.8
4.96/4.9/4.9
4.66/4.6/4.6
1.58/1.5/1.4
1.65/1.6/1.5
2.08/2.0/1.9
1.99/1.9/1.8
459
455
452
449
(0.166, 0.169)
(0.159, 0.157)
(0.157, 0.146)
(0.150, 0.141)
a
Turn-on voltage at a brightness of 1 cd mꢁ2
.
b
Maximum value, estimated values at 500 cd mꢁ2 and 1000 cd mꢁ2
.

22
X. He et al. / Organic Electronics 37 (2016) 14e23
(a) 1400
(b) 5,0
10000
,
,
S1-Cz-OXD
S4-Cz-OXD
S1-Cz-OXD
S4-Cz-OXD
1200
4,5
1000
4,0
1000
800
600
400
200
0
3,5
100
3,0
10
1
2,5
2,0
1,5
1,0
0,1
0
2
4
6
8
10
12
0
50
100
150
200
250
300
350
400
Current Density (mA cm-2)
Voltage (V)
2,5
6,0
(d)
(c)
S1-Cz-OXD
S4-Cz-OXD
S1-Cz-OXD
S4-Cz-OXD
5,5
5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
2,0
1,5
1,0
0,5
0
50
100
150
200
250
300
350
400
0
50
100
150
200
250
300
350
400
Current Density (mA cm-2)
Current Density (mA cm-2)
Fig. 7. EL performance: (a) JeV and LeV characteristics; (b) EQEeJ characteristics; (c) CEeJ characteristics and (d) PEeJ characteristics of devices based on S1-Cz-OXD and S4-Cz-
OXD.
UV-2501 PC) and Shimadzu RF 5301 Spectrofuorophotometer,
respectively. Fluorescence quantum yields (Ff) of the samples both
in toluene solutions and as films (neat films and doped films in
PMMA matrix) were measured by using 9,10-diphenylanthracene
and isopropanol, each for 20 min in an ultrasonic bath. After
cleaning the ITO substrates were blow-dried with nitrogen gas and
undergo O2 plasma cleaning for 10 min, PEDOT:PSS (4083) (30 nm)
was then spin-coated on the ITO substrate at 4000 rpm for 60 s.
After annealing for 15 min at 120 ꢀC, the emitting layer was spin
coated on PEDOT:PSS layer from a 12 mg mLꢁ1 solution in a mixed
solvent (1:1 v/v) of toluene: chlorobenzene for 60 s at 3000 rpm,
and annealed at 120 ꢀC for 30 min (70 nm). Then TPBI (40 nm), LiF
(0.5 nm) and Al (120 nm) were successively evaporated onto the
emitting layer in a multi-source vacuum evaporator at a base
pressure of 4 ꢂ 10ꢁ4 Pa. The evaporation rates and thickness were
monitored with several quartz crystals located above the crucibles.
Calibration of the layer thickness was performed using a surface
profilometer. Each patterned device has a 2 mm ꢂ 2 mm active
area. Except for PEDOT deposition, all other fabrication processes
were carried out in a glove box with less than 1 ppm oxygen and
moisture. J-V and L-V data were collected using a source meter
(Yokogawa GS610) and a luminance meter (Konica Minolta LS-110)
(
Ff ¼ 0.92 in toluene and Ff ¼ 0.83 dispersed in PMMA films with a
concentration lower than 1 ꢂ 10ꢁ3 M) as standards. The concen-
tration of their dilute toluene solution was around 10ꢁ6 M with
absorbance lower than 0.05, while the thickness of films was
around 70 nm. Values are calculated according to equation:
Fn
¼
FsInAshn2/AnIshs2. where Fn is the fluorescence quantum yield
of the sample, Fs is the fluorescence quantum yield of the standard
reference, In and Is are the integrated emission intensities of the
sample and the standard, respectively, An and As are the absorbance
of the sample and the standard at the excitation wavelength,
respectively, and hn and hs are the refractive indexes of the corre-
sponding solutions. Cyclic voltammetry was carried out on a
CHI660A electrochemical workstation at a scan rate of 100 mV sꢁ1
in a nitrogen-saturated solution of 0.1 M Tetrabutylammonium
perchlorate ((C4H9)4NClO4) in anhydrous methylene chloride
(CH2Cl2), using a glassy carbon electrode as working electrode, a Pt
wire as counter electrode, and an Ag/AgCl electrode as reference
using
a customized Labview program. Electroluminance (EL)
spectra were recorded with a spectrometer (Photo Research
PR705). The AC voltage used to drive the device for EL spectrum
measurement was generated by the same sourcemeter operated in
a pulse mode (rectangular shape, 50% duty cycle, 50 Hz). Many
batches of devices (for each emitter) were fabricated and opti-
mized. The results of the best ones were shown in this text.
electrode. The energy band gap was calculated from Eg ¼ ELUMO
-
E
HOMO (eV). Surface topography of films is studied by Atomic Force
Microscopy (AFM) Bruker DI 3100 in tapping mode.
4.2. Device fabrication and characterizations
Acknowledgements
Patterned ITO-coated glass (sheet resistance 10
U
per square)
was cleaned sequentially with detergent, de-ionized water, acetone
Xuehan He and Lei Chen acknowledge the financial support

X. He et al. / Organic Electronics 37 (2016) 14e23
23
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Supplementary data related to this article can be found at doi:
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