New carbazole-substituted anthracene derivatives based non-doped blue light-emitting devices with high brightness and efficiency

Article abstract of DOI:10.1016/j.dyepig.2013.06.025

A series of anthracene and 2-tert-butyl-substituted anthracene derivatives featuring 9-ethyl-9H-carbazol-3-yl or 4-(9-ethyl-9H-carbazol-3-yl)phenyl groups were synthesized for use as light-emitting layer in sky-blue organic light emitting devices (OLEDs).

Full text of DOI:10.1016/j.dyepig.2013.06.025

Dyes and Pigments 99 (2013) 577e587
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New carbazole-substituted anthracene derivatives based non-doped
blue light-emitting devices with high brightness and efficiency
,
Yu-Chen Chang a, Shih-Chieh Yeh b, Ying-Hsiao Chen a, Chin-Ti Chen c, Rong-Ho Lee a
**
,
*
,
Ru-Jong Jeng b
a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
b Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan
c Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of anthracene and 2-tert-butylesubstituted anthracene derivatives featuring 9-ethyl-9H-car-
bazol-3-yl or 4-(9-ethyl-9H-carbazol-3-yl)phenyl groups were synthesized for use as light-emitting layer
in sky-blue organic light emitting devices (OLEDs). Efficient conjugation between the carbazole and
anthracene unit resulted in strong blue emissions and high quantum efficiencies of anthracene de-
rivatives in solution. Electroluminescence (EL) properties of non-doped OLEDs incorporated with these
anthracene derivatives, along with an electron-transporting layer (ETL), tri-(8-hydroxyquinoline)
aluminum (Alq3) or 1,3,5-tris(N-phenyl benzimidizol-2-yl)benzene (TPBI) were investigated. The OLEDs
incorporated with TPBI provided superior EL performance than did those incorporated with Alq3. A hole-
blocking layer 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) was inserted at the interface of light
emitting layer and ETL to enhance the EL performance for the OLED with Alq3 as ETL. High brightness
(37423 cd/m2), and great current efficiency (5.89 cd/A) were observed for the 4-(9-ethyl-9H-carbazol-3-
yl)phenyl-substituted anthracene derivatives based OLEDs.
Received 18 April 2013
Received in revised form
13 June 2013
Accepted 17 June 2013
Available online 27 June 2013
Keywords:
Organic light-emitting diodes
Carbazole group
Anthracene derivative
Electroluminescence properties
Light-emitting layer
Photoluminescence
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Several small-molecule emitters, including pyrene [13,14], oxa-
diazole [15,16], carbazole [17,18], fluorene [19,20], and anthracene
Organic light-emitting diodes (OLEDs) have attracted much
attention because of their excellent performance and great poten-
tial in flat-panel displays [1e6]. In full-color displays, pure blue
emission is very difficult to achieve because a larger energy band
gap is required as compared to green or red light-emitting mate-
rials. In order to fabricate the OLED with excellent performance, the
dopantehost system is often adopted. However, electrolumines-
cence (EL) properties, including color purity, are extremely sensi-
tive to the dopant concentration, and the thickness of light-
emitting layer [7e10]. In addition, precise control of the dopant
concentration using co-evaporation methods is not an easy task.
The development of high performance blue-light-emitting mate-
rials for nondoped OLEDs is one of the effective approaches to
circumvent such fabrication problems [9,11,12].
derivatives [21e36], were reported for applications in nondoped
type blue OLEDs. Several anthracene derivatives attached with
electron-donating or -withdrawing moieties at C9 and C10 posi-
tions, exhibiting hole-transporting/light-emitting or electron-
transporting/light-emitting properties were reported [21e29].
A direct substitution of either strong electron-withdrawing or
electron-donating groups at C9 and C10 positions changes the
electron density on the anthracene ring and consequently enhances
the conjugation length and photoluminescence (PL) performance
of the anthracene derivative [23]. Particularly high brightness and
large current efficiency were observed for the triphenylamine
moieties attached anthracene derivative based OLEDs [24,25].
Moreover, the attachment of bulky side-groups to the anthracene
core can prevent pep* stacking interactions of the molecules in the
solid state and reduce self-quenching effects [27,28]. Various
electron-donating and -accepting groups were attached to tert-
butyl substituted anthracene derivatives for use as blue emitters in
OLEDs [21,27e32]. In addition, a series of OLEDs based on the
anthracene derivatives attached with N-phenylcarbazole, triphe-
nylphosphine, tetraphenylethylene, or oxadiazole exhibiting deep
* Corresponding author. Tel.: þ886 4 22854308; fax: þ886 4 22854734.
** Corresponding author. Tel.: þ886 2 33665884; fax: þ886 2 33665237.
E-mail addresses: rhl@dragon.nchu.edu.tw (R.-H. Lee), rujong@ntu.edu.tw(R.-J. Jeng).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.025

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Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
blue emission were reported in literature [30,32e35]. However, the
brightness and current efficiency of these OLEDs are not among the
high numbers. In fact, it is very difficult for the anthracene deriv-
ative based non-doped blue OLEDs to achieve all of the benchmark
features at once (high brightness, great current efficiency, deep
blue emission, and excellent operational stability) [21e36]. Apart
from that, the conjugated side-group effect on the EL performance
of anthracene derivatives has not been thoroughly discussed.
Therefore, it is important to further discuss the chemical structure
effect of the anthracene derivatives on the photo-physical proper-
ties, electrochemical behavior, and EL performance for developing
practical blue emitters.
In general, the incorporation of a carbazole moiety into a mo-
lecular scaffold can significantly improve the glassy-state durability
and thermal stability of an organic compound [17,35]. Moreover, the
3, 6, and 9 positions of the carbazole moiety can be readily func-
tionalized, allowing the fine-tuning of the electro-optical properties
of the molecules [37e44]. Recently, we synthesized a series of N-
arylated carbazole moiety attached anthracene derivatives for use
as blue emitters in nondoped-type OLEDs [45]. Chemical structures
of the N-arylated carbazole moiety attached anthracene derivatives
are shown in Fig. S1. However, the EL properties of the anthracene
derivative based OLEDs were only moderate, which is attributed to
the relatively poor conjugation of the anthracene derivatives
attached with the N-arylated carbazole moieties. The feature of poor
conjugation affected the photophysical properties and electro-
chemical behaviors of the blue emitter negatively, and subsequently
resulted in poor EL performance for OLEDs. With that in mind, a
more efficient conjugation would be obtained for the attachment of
C3-position of carbazolyl groups onto the C9 and C10 positions of
anthracene unit. In this study, we designed and synthesized a series
of anthracene and 2-tert-butylesubstituted anthracene derivatives
featuring carbazole moieties as side groupsd9,10-bis(9-ethyl-9H-
carbazol-3-yl)anthracene (Cz3An), 2-tert-butyl-9,10-bis(9-ethyl-
9H-carbazol-3-yl)anthracene (Cz3Ant), 9,10-bis[4-(9-ethyl-9H-
carbazol-3-yl)phenyl]anthracene (Cz3PhAn) and 2-tert-butyl-9,10-
presence of benzophenone. The syntheses of intermediates (CzBr,
CzSn, and CzPhBr) and carbazole-substituted anthracene de-
rivatives (Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt) are presented in
Schemes 1 and 2.
2.2. Synthesis
2.2.1. 3-Bromo-9-ethyl-9H-carbazole (CzBr)
N-bromosuccinimide (1.77 g, 10 mmol) in 10 mL DMF was
added dropwise to a DMF (25 mL) solution containing 9-ethyl-
9H-carbazole (1.95 g, 10 mmol) at 0 ꢀC in a nitrogen atmosphere.
The reaction mixture was reacted for 8 h, then quenched with
water, and extracted with ethyl acetate (EA). The organic layer
was dried over anhydrous magnesium sulfate followed by solvent
evaporation in a rotary evaporator. A solid crude product was re-
crystallized in ethanol to afford a white solid product. The white
solid product yield was 80.0% (2.19 g). 1H NMR (400 MHz, CDCl3)
d
(ppm): 1.41 (t, J ¼ 7.2 Hz, 3H), 4.26 (q, J ¼ 7.2 Hz, 2H), 7.24 (d,
J ¼ 8.7 Hz, 1H), 7.30 (t, J ¼ 7.3 Hz, 1H), 7.41 (d, J ¼ 8.26 Hz, 1H),
7.50 (d, 1H), 7.57(dd, J ¼ 8.6, 1.8 Hz, 1H), 8.08 (d, J ¼ 7.3 Hz, 1H),
8.27 (s, 1H). 13C NMR (125 MHz, CDCl3)
d (ppm): 140.3, 138.6,
128.3, 126.4, 124.7, 123.2, 121.9, 120.7, 119.3, 111.6, 109.9, 108.8,
37.6, 13.8.
2.2.2. 9-Ethyl-3-(tributylstannyl)-9H-carbazole (CzSn)
A solution of CzBr (2.73 g, 10 mmol) in dry THF (50 mL) was
stirred at ꢁ78 ꢀC under N2 for 10 min and then n-BuLi (2.5 M in
hexane, 4.0 mL) was added dropwise. The mixture was maintained
at ꢁ78 ꢀC with continued stirring for a further 1 h, at which point
the tributylchlorostannane (3.26 g, 10 mmol) was added dropwise.
After warming to room temperature and stirring for 24 h, methanol
was added to quench the reaction. The solution was partitioned
between EA and water; the organic phase was collected, dried
(MgSO4), filtered, and evaporated to dryness. The residue was pu-
rified chromatographically (SiO2; EA/hexanes) to provide a brown
solid (3.41 g, 71.0%). 1H NMR (400 MHz, CDCl3)
1.00(s, 9H), 1.16e1.69 (t, 3H), 4.40(q, J ¼ 6.8 Hz, 2H), 7.27e7.63(m,
5H), 8.09e8.27(m, 2H). 13C NMR (125 MHz, CDCl3)
(ppm):
d (ppm): 0.85e
bis[4-(9-ethyl-9H-carbazol-3-yl)phenyl]anthracene
(Cz3PhAnt)
for use as light-emitting layers in blue OLEDs. EL properties of non-
doped OLEDs incorporating these anthracene derivatives, with tri-
(8-hydroxyquinoline)aluminum (Alq3) or 1,3,5-tris(N-phenyl ben-
zimidizol-2-yl)benzene (TPBI) as electron transporting layer (ETL)
were investigated. In order to improve the EL performance of OLEDs,
d
140.1, 139.5, 133.2, 128.3, 126.3, 125.5, 120.3, 118.7, 37.3, 29.2, 27.4,
13.7, 9.7, 8.7.
2.2.3. 3-(4-Bromo-phenyl)-9-ethyl-9H-carbazole (CzPhBr)
a
hole-blocking layer (HBL), 2,9-dimethyl-4,7-diphenyl-1,10-
A solution of CzSn (4.84 g, 10 mmol), 1,4-dibromobenzene (2.3 g,
10 mmol), Pd(OAc)2 (0.01 g, 0.045 mmol) in dry toluene (100 mL)
was stirred and reacted at 120 ꢀC under N2 for 48 h. After cooling to
room temperature, the solution was evaporated to dryness. The
phenanthroline (BCP) was inserted at the interface of light emit-
ting layer and ETL for these anthracene derivatives based
nondoped-type blue emitting OLEDs. In addition, we analyzed the
influence of the attached carbazole moieties on the thermal sta-
bility, electro-chemical properties, photo-physical behavior, and EL
performances of the anthracene derivativeebased devices.
2. Experimental section
2.1. Materials
All reactions and manipulations were performed under a N2
atmosphere using standard Schlenk techniques. All chromato-
graphic separations were performed using SiO2. Anthraquinone
and 2-tert-butylanthraquinone were obtained from SHOWA.
Compounds 9-ethyl-carbazole, 1,4-dibromobenzene, tributyl-
chlorostannane, palladium(II) acetate (Pd(OAc)2), and n-butyl
lithium (n-BuLi), N, N-dimethylformamide (DMF), and toluene
were obtained from Aldrich and used as received. N-bromosucci-
nimide (NBS), potassium iodide (KI), and sodium hypophosphite
monohydrate (NaHPO2) were purchased from Acros. Tetrahydro-
furan (THF) was purified through distillation from Na in the
Scheme 1. Synthetic routes of compounds CzBr, CzSn, and CzPhBr.

Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
579
Scheme 2. Synthetic routes of Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt.
cold water was then added to the mixture and finally extracted
2.2.5. 2-tert-Butyl-9,10-bis(9-ethyl-9H-carbazol-3-yl)anthracene
(Cz3Ant)
with EA. The organic phase was collected, dried (MgSO4), filtered,
and evaporated to dryness. The crude product was purified chro-
matographically (SiO2; EA/hexanes) to provide a white solid (2.50 g,
A solution of CzBr (2.73 g, 10 mmol) in dry THF (500 mL) was
stirred at ꢁ78 ꢀC under N2 for 10 min and then n-BuLi (2.5 M in
hexane, 4.8 mL) was added dropwise. A solution of 2-tert-buty-
lanthraquinone (1.31 g, 5 mmol) in THF (20 mL) was then added
dropwise to the mixture at ꢁ78 ꢀC. The resulting mixture was
warmed to room temperature and stirred for 24 h. After the reac-
tion had finished, methanol was added to the mixture, which was
then partitioned between DCM and water. The combined organic
fractions were dried (MgSO4) and the volatiles evaporated under
reduced pressure. The residue was added to a mixture of KI (2.99 g,
18 mmol), NaHPO2 (2.96 g, 34 mmol) and AcOH (50 mL) and then
the mixture was heated under reflux for 24 h. After the reaction had
finished, the mixture was poured into water and the precipitate
filtered off and washed with plenty of water. The yellowish crude
product was purified chromatographically (SiO2; hexane/CH2Cl2) to
give a yellow solid (3.22 g, 52.0%). 1H NMR(400 MHz, CDCl3)
72.0%). 1H NMR(400 MHz, CDCl3)
d
(ppm): 1.47(t, J ¼ 7.2 Hz, 3H),
4.41(q, J ¼ 7.2 Hz, 2H), 7.26(t, J ¼ 7.4 Hz, 1H), 7.45-7.51(m, 3H),
7.59(s, 4H), 7.67(dd, J ¼ 8.4,1.8 Hz,1H), 8.16(d, J ¼ 7.7 Hz,1H), 8.28(s,
1H). 13C NMR (125 MHz, CDCl3)
d (ppm): 141.0, 140.3, 139.5, 131.7,
130.9, 128.8, 125.9, 124.8, 123.4, 122.9, 120.4, 119.0, 118.7, 108.7,
108.6, 37.6, 13.8.
2.2.4. 9,10-Bis(9-ethyl-9H-carbazol-3-yl)anthracene (Cz3An)
A solution of CzBr (2.73 g, 10 mmol) in dry THF (500 mL) was
stirred at ꢁ78 ꢀC under N2 for 10 min and then n-BuLi (2.5 M in
hexane, 4.8 mL) was added dropwise. A solution of anthraqui-
none (1.04 g, 5 mmol) in THF (20 mL) was then added dropwise to
the mixture at ꢁ78 ꢀC. The resulting mixture was warmed to
room temperature and stirred for 24 h. After the reaction had
finished, methanol was added to the mixture, which was then
partitioned between dichloromethane (DCM) and water. The
combined organic fractions were dried (MgSO4) and the volatiles
evaporated under reduced pressure. The residue was added to a
mixture of KI (2.99 g, 18 mmol), NaHPO2 (2.96 g, 34 mmol) and
AcOH (50 mL) and then the mixture was heated under reflux for
24 h. After the reaction had finished, the mixture was poured into
water, and the precipitate was filtered off and washed with plenty
of water. The yellowish crude product was purified chromato-
graphically (SiO2; hexane/CH2Cl2) to give a yellow solid (2.93 g,
d
(ppm): 1.19 (s, 9H), 1.54e1.58 (t, 6H), 4.50e4.52 (q, J ¼ 2.6 Hz, 4H),
7.25e7.28 (m, 4H), 7.38e7.41 (dd, J ¼ 9.2, 1.6 Hz, 1H), 7.52e7.50 (t,
J ¼ 3.8 Hz, 4H), 7.56e7.64 (m, 4H), 7.73e7.78 (m, 4H), 8.04e8.09 (q,
J ¼ 6.7 Hz, 2H), 8.23e8.21 (d, J ¼ 8.0 Hz, 2H). 13C NMR (125 MHz,
CDCl3)
d (ppm): 147.2, 140.6, 139.6, 137.7, 137.5, 131.1, 130.8, 130.6,
129.8, 129.3, 127.5, 127.3, 126.1, 126.0, 124.8, 124.5, 123.4, 123.2,
123.1,121.8,120.8,119.2,108.8,108.5, 38.0, 35.1, 31.0,14.2. HRMS (m/
z): Calcd for C46H40N2: 620.32, Found: 620.32 (Mþ). Anal. Calcd for
C46H40N2: C, 88.99; H, 6.49; N, 4.51, Found: C, 89.27; H, 6.53;
N, 4.22.
52.0%). 1H NMR (400 MHz, CDCl3)
d
(ppm):1.56e1.59 (t, J ¼ 7.2 Hz,
6H), 4.49e4.52 (q, J ¼ 8.2 Hz, 4H), 7.27e7.30 (q, J ¼ 3.46 Hz, 6H),
7.50e7.51 (m, 4H), 7.58e7.65 (m, 4H), 7.78e7.81 (q, J ¼ 3.33 Hz,
4H), 8.05e8.09 (m, J ¼ 6.4 Hz, 2H), 8.23 (s, 2H). 13C NMR
2.2.6. 9,10-Bis[4-(9-ethyl-9H-carbazol-3-yl)phenyl]anthracene
(Cz3PhAn)
Cz3PhAn was obtained as a yellow solid (3.58 g, 50.0%) after
(125 MHz, CDCl3)
d (ppm): 131.1, 129.9, 129.4, 128.9, 127.7, 126.2,
using a procedure similar to that described above for Cz3An. 1H
125.0, 123.6, 120.9, 119.3, 108.9, 108.6, 38.1, 14.2. HRMS (m/z):
Calcd for C42H32N2: 564.26, Found: 564.26 (Mþ). Anal. Calcd for
NMR(400 MHz, CDCl3) d (ppm): 1.49 (t, 6H), 4.43e4.45 (q,
J ¼ 7.2 Hz, 4H), 7.25e7.29 (m, 2H), 7.37e7.39 (dd, J ¼ 6.8, 3.2 Hz, 4H),
C
42H32N2: C, 89.33; H, 5.71; N, 4.96, Found: C, 89.20; H, 5.90;
7.46e7.61 (m, 10H), 7.85e7.97 (m, 10H), 8.19e8.21 (d, J ¼ 7.6 Hz,
N, 4.66.
2H), 8.50 (s, 2H). 13C NMR (125 MHz, CDCl3)
d (ppm): 141.6, 140.9,

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Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
140.0,137.4,132.4,132.1,130.5,128.3,127.9,127.5,127.4,127.3,126.2,
125.5, 125.4, 125.3, 124.0, 123.9, 120.8, 119.4, 109.1, 109.0, 38.0, 14.1.
HRMS (m/z): Calcd for C54H40N2: 716.32, Found: 716.32 (Mþ). Anal.
Calcd for C54H40N2: C, 90.47; H, 5.62; N, 3.91, Found: C, 90.28; H,
5.24; N, 3.75.
anthracene derivative (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (BCP, 10 nm)/Alq3 (30 nm)/LiF (0.5 nm)/Al
(150 nm), and (type D) ITO/NPB (40 nm)/anthracene derivative
(30 nm)/BCP (10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Al (150 nm), where
NPB, the anthracene derivative, BCP, and Alq3 (or TPBI) were the
hole-transporting layer (HTL), blue light-emitting layer, hole-
blocking layer (HBL), and electron-transporting layer (ETL),
respectively. The cathode deposition rate was determined using a
quartz thickness monitor (STM-100/MF, Sycon). The thin film
thickness was determined using a surface texture analysis system
(3030ST, Dektak). UVeVis spectra of the organic thin films were
measured using a HewlettePackard 8453 spectrometer equipped
with a photodiode array detector. PL and EL spectra were recorded
using a Hitachi F-4500 fluorescence spectrophotometer. Currente
voltage characteristics were measured using a programmable
electrometer equipped with current and voltage sources (Keithley
2400). Luminance was measured with a BM-9 luminance meter
(Topcon).
2.2.7. 2-tert-Butyl-9,10-bis[4-(9-ethyl-9H-carbazol-3-yl)phenyl]
anthracene (Cz3PhAnt)
Cz3PhAnt was obtained as a yellow solid (3.63 g, 47.0%) after
using a procedure similar to that described above for Cz3Ant. 1H
NMR(400 MHz, CDCl3)
d (ppm): 1.29 (s, 9H), 1.48e1.52 (m, 6H),
4.44e4.45 (q, J ¼ 7.2 Hz, 4H), 7.26e7.29 (t, J ¼ 7.4 Hz, 2H), 7.34e3.36
(dd, J ¼ 6.8, 3.6 Hz, 2H), 7.44e7.61 (m, 11H), 7.76e7.99 (m, 10H),
8.19e8.22 (q, J ¼ 4.3 Hz, 2H), 8.53e8.50 (d, J ¼ 11.6 Hz, 2H). 13C NMR
(125 MHz, CDCl3)
d (ppm): 147.5, 141.4, 141.1, 140.7, 139.8, 137.5,
137.0, 136.7, 132.3, 132.1132.0, 130.5, 130.2, 123.0, 128.9, 127.4, 127.3,
127.0, 126.1, 125.4, 125.1, 124.9, 123.8, 123.4, 121.5, 120.8, 119.2,
109.0, 108.9, 37.9, 35.2, 31.0, 14.1. HRMS (m/z): Calcd for C58H48N2:
772.38, Found: 772.38 (Mþ). Anal. Calcd for C58H48N2: C, 90.12; H,
6.26; N, 3.62, Found: C, 90.12; H, 6.60; N, 3.90.
3. Results and discussion
2.3. Measurements
3.1. Synthesis and characterization
1H and 13C NMR spectra were recorded using a Varian Gemini
NMR spectrometer operated at 400 and 125 MHz, respectively.
Elemental analysis was performed using an elemental analyzer
(Elementar Vario EL III). High-resolution mass spectra were recor-
ded using a Finnigan/Thermo Quest MAT mass spectrometer. Glass
transition temperatures (Tgs) were measured under N2 using a
differential scanning calorimeter (TechMax Instruments DSC 6220)
operated at a heating rate of 10 ꢀC minꢁ1. Thermogravimetric
analysis (TGA) was performed under a N2 atmosphere using a
thermogravimetric analyzer (VersaTherm thermogravimetric
analyzer) operated at a heating rate of 10 ꢀC minꢁ1. UVeVis ab-
sorption and PL spectra were recorded using a Shimadzu UV-1240
spectrophotometer and an Acton Research Spectra Pro-150 spec-
trometer, respectively. We measured the fluorescence quantum
yields (Ffl) of the four anthracene derivatives in dilute CHCl3 so-
lution by comparing their emissions with that of a standard solu-
tion of DPA in cyclohexane (Ffl ¼ 0.90) at room temperature [29]. In
addition, solid state Ffl values of vacuum-deposited thin films were
determined by the integrating-sphere method described by de
Mello et al. [46]. Cyclic voltammetry (CV) was conducted using a
CHI model611D apparatus and a three-electrode cell, in which an
indium tin oxide (ITO) sheet, a Pt wire, and silver/silver nitrate (Ag/
Agþ) were used as the working electrode, counter electrode, and
reference electrode, respectively. All electrochemical experiments
were performed using deoxygenated MeCN solutions containing
0.1 M tetrabutylammonium perchlorate (TBAP) as the electrolyte.
Schemes 1 and 2 illustrate the routes for the syntheses of the
anthracene derivatives containing carbazole moieties. The
carbazole-substituted anthracene derivatives Cz3An, Cz3Ant
Cz3PhAn, and Cz3PhAnt were synthesized through reactions of
anthraquinone or 2-tert-butylanthraquinone with various carba-
zole derivatives. The chemical structures of these compounds were
confirmed by 1H and 13C NMR spectroscopy, elemental analysis,
and high-resolution mass spectrometry. The relative intensities of
the signals in the various spectra were in agreement with the
proposed structures of these anthracene derivatives. The opera-
tional stability of an OLED is directly related to the thermal stability
of the light-emitting layer. Thus, high Tg and thermal degradation
temperature (Td) are desirable for a light-emitting material to be
utilized in OLEDs. Fig. 1 presents DSC thermograms of the
carbazole-substituted anthracene derivatives; Table 1 summarizes
their values of Tg, crystallization temperature (Tc), melting tem-
perature (Tm), and Td. DSC results indicate that no transition was
observed at temperature below 350 ꢀC in the second-heating scan
for Cz3An and Cz3Ant. The melting peak was observed at 422 ꢀC for
Cz3PhAn. The tight molecular stacking resulted in the high Tm value
2.4. EL device fabrication and electro-optical characterization
Fabrication of OLEDs was conducted through high-vacuum
thermal evaporation of the organic materials onto ITO-coated
glass (sheet resistance: 15
U/square; Applied Film Corp). Glass
substrates with patterned ITO electrodes were washed well and
then cleaned through O2 plasma treatment. Furthermore, a LiF layer
was thermally deposited onto the organic material layer, followed
by Al metal deposition as the top layer in a high-vacuum chamber.
OLED configurations in this study were (type A) ITO/4,40-bis[N-(1-
naphthyl)-N-phenylamino]biphenyl (NPB, 30 nm)/anthracene de-
rivative (30 nm)/tri-(8-hydroxyquinoline)aluminum (Alq3, 30 nm)/
LiF (0.5 nm)/Al (150 nm), (type B) ITO/NPB (30 nm)/anthracene
derivative (30 nm)/1,3,5-tris(N-phenyl benzimidizol-2-yl)benzene
(TPBI, 30 nm)/LiF (0.5 nm)/Al (150 nm), (type C) ITO/NPB (30 nm)/
Fig. 1. DSC thermograms of Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt.

Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
581
g
Table 1
Thermal properties, UVeVis absorption, and PL emission and quantum efficiency of the carbazole-substituted anthracence derivatives.
abs
max
abs
onset
PL
max
,e
,e
Compound
Tg (ꢀC)a
Tc (ꢀC)a
Tm (ꢀC)a
Td (ꢀC)b
l
(nm)d
l
(nm)e
l
(nm)d
FWHM of PL (nm)d
Ff (％)f,
Cz3An
Cz3Ant
Cz3PhAn
Cz3PhAnt
n.a.c
n.a.
n.a.
169
n.a.
n.a.
n.a.
238
n.a.
n.a.
422
344
448
401
495
486
397, 378, 355
397, 378, 354
397, 378, 359
397, 378, 358
443
444
445
446
450/456
453/455
440/458
444/457
61/52
60/44
62/53
61/51
71/24
83/27
72/25
85/28
a
Determined by DSC at heating rate and cooling rate of 10 ꢀC/min and 50 ꢀC/min under nitrogen.
Determined by TGA at heating rate of 10 ꢀC/min under nitrogen. (Td: at 5% loss weight.)
n.a.: not available.
Measured in dilute DCM solution.
Measured in solid film.
b
c
d
e
f
PL quantum efficiency (Ffl) of the compounds, measured in CHCl3 solution using 9,10-diphenylanthracene as a standard.
PL quantum efficiency (Ffl) of the compounds, measured as solid film using 9,10-diphenylanthracene as a standard.
g
of Cz3PhAn. The glass transition (169 ꢀC), and re-crystallization
(238 ꢀC) and melting (344 ꢀC) peaks were clearly observed in the
heating-scan for the anthracene derivative Cz3PhAnt. As compared
to Cz3PhAn, the incorporation of the tert-butyl groups on the C2
position of anthracene unit led to the lower values of Tg and Tm of
Cz3PhAnt. Nevertheless, the presence of the sterically bulky
carbazole side groups meant that these carbazole-substituted
anthracene derivatives exhibited higher value of Tg than those of
the well-known blue-emitting materials DPVBi (64 ꢀC) [16] and
MADN (120 ꢀC) [47]. Consequently, these carbazole-substituted
anthracene derivatives were capable of forming the morphologi-
cally stable and uniform amorphous films required to improve the
efficiencies and lifetimes of OLEDs. In addition, the values of Td of
Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt were 448, 401, 495, and
486 ꢀC, respectively. The values of Td of Cz3PhAn and Cz3PhAnt are
much higher than those of Cz3An and Cz3Ant, which is attributed
to the presence of more aromatic rings in the molecules of Cz3PhAn
and Cz3PhAnt. Moreover, higher thermal stability was observed for
the Cz3PhAn and Cz3An relative to those of the Cz3PhAnt and
Cz3Ant. This is due to the close molecular stacking of Cz3PhAn and
Cz3An in the absence of the tert-butyl groups on the C2 position of
anthracene units. It is also important to note that these carbazole-
substituted anthracene derivatives show higher thermal stability
than that of the 9,10-diphenylanthracene (DPA, Td ¼ 241 ꢀC).
wavelengths corresponding to the carbazole-centered transitions
were slightly red-shifted for the three anthracene derivatives in
thin film state, presumably because of the interactions and aggre-
gation of the molecules in thin film state.
Upon excitation at 350 nm, these anthracene derivatives exhibi-
ted deep-blue emissions with PL maxima at ca. 440e453 nm in
dilute DCM solution. The maximum emission wavelength of the PL
PL
ð
l
Þ and the full width at half-maximum (FWHM) of the PL were
max
dependent on the chemical structure of the carbazole-substituted
anthracene derivative. The difference of
PL
l
was not significant
max
3.2. Photophysical properties
Fig. 2 presents UVeVis absorption and PL spectra of the
carbazole-substituted anthracene derivatives in dilute DCM solu-
tions and as solid films. The UVeVis absorption spectra of the
emitters in solution were measured at a concentration of 10ꢁ5 M;
the thickness of the emitter layer for the UVeVis absorption mea-
surements was approximately 100 nm. Table 1 summarizes the
maximal absorption and PL emission wavelengths of the carbazole-
substituted anthracene derivatives. The maximal absorption
wavelengths of the anthracene derivatives appeared in the range
from 280 to 320 nm due to carbazole-centered transitions [48],
along with moderately weak absorptions in the ranged from 350 to
425 nm (characteristic vibronic patterns attributed to the
transition of anthracene [49]). In solution, the maximal absorption
wavelengths (corresponding to the * transition of anthracene)
pep*
pep
of Cz3PhAn and Cz3PhAnt were slightly red-shifted as compared
with those of the Cz3An and Cz3Ant. Moreover, as thin film state,
the onset absorption wavelength of Cz3PhAn and Cz3PhAnt were
also slightly red-shifted when compared with those of the Cz3An
and Cz3Ant. This implies that the conjugation of the carbazole-
substituted anthracene derivatives was somewhat enhanced via
the incorporation of a phenyl ring between the carbazole and
anthracene units. In addition, relative to the absorption behavior of
the anthracene derivatives in solution, the maximal absorption
Fig. 2. UVeVis absorption and PL spectra of Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt
((a) UVeVis absorption spectra and (b) PL spectra).

582
Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
for Cz3An and Cz3Ant in solution. Moreover, a slightly red-shift of
PL
max
l
was observed for Cz3An and Cz3Ant in the thin film state. As
compared to the Cz3An and Cz3Ant, the Cz3PhAn and Cz3PhAnt
exhibited blue-shifts of approximately 10 nm when in solution. The
free rotation of the phenyl ring interrupted the conjugation between
the carbazole and anthracene units for Cz3PhAn and Cz3PhAnt. As a
PL
max
result, a blue shifting of
l
was observed for Cz3PhAn and
Cz3PhAnt. Relative to the PL spectra of the samples in the dilute
solutions, however, the emission spectra of Cz3PhAn and Cz3PhAnt
in the thin film state exhibited red-shifts of 12e16 nm. The differ-
ences in the PL spectra of the compounds in solution and as thin film
suggested the presence of intermolecular interactions or aggregation
PL
max
in thin film states. In the thin film state, the values of
l
of the
Cz3PhAn and Cz3PhAnt were slightly red-shifted as compared with
those of Cz3An and Cz3Ant. This implies that the conjugation lengths
of these anthracene derivatives were slightly enhanced by the
incorporation of a phenyl ring between the carbazole and anthra-
cene groups. On the other hand, the FWHM values of the PL emission
band were almost the same for these anthracene derivatives in the
solution state, while the FWHM values varied slightly in the thin film
state for these anthracene derivatives with different chemical
structures. With the incorporation of the tert-butyl group at the C2
position of the anthracene unit, the FWHM of Cz3Ant was smaller
than that of Cz3An. The steric hindrance of the tert-butyl group on
the peripheral anthracene group reduced the degree of intermolec-
Fig. 3. CV spectra of Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt.
(LUMO) of the carbazole-substituted anthracene derivatives. Fig. 4
presents the energy band diagram of the carbazole-substituted
anthracene derivatives along with BCP, TPBI and Alq3. The elec-
trochemical properties of the anthracene derivatives are summa-
ular pep stacking of Cz3Ant, leading to the smaller FWHM value of
on
rized in Table 2. The oxidation potentials ðE Þ of Cz3An, Cz3Ant,
ox
the PL emission band. Furthermore, the FWHM value of Cz3PhAn
was slightly larger than that of Cz3An. The incorporation of a phenyl
ring between the carbazole and anthracene units enhanced the de-
Cz3PhAn, and Cz3PhAnt were 0.76, 0.78, 0.80, and 0.84 V, respec-
tively. From these values, the HOMO levels of the anthracene de-
rivatives were calculated according to the following equation:
gree of intermolecular
pep stacking of Cz3PhAn and Cz3PhAnt,
leading to the larger FWHM value of the PL emission band. Apart
ꢀ
ꢁ
on
HOMO ¼ ꢁe E ꢁ Eooxn; ferrocene þ 4:8 ðeVÞ
PL
from that, the
l
values of anthracene derivatives attached with
ox
max
the carbazolyl groups at C3-positions were slightly larger than those
the anthracene derivatives attached with N-arylated carbazole
where 4.80 eV is the energy level of ferrocene below the
PL
vacuum level and the E of ferrocene/ferroceneþ is 0.08 V in 0.1 M
on
moieties. The
l
values of the anthracene derivatives attached
max
ox
with N-arylated carbazole moieties are summarized in Table S1
(Supporting Information).
Bu4NClO4eMeCN solution. The HOMO levels were obtained
as ꢁ5.48, ꢁ5.50, ꢁ5.52 and ꢁ5.56 eV for Cz3An, Cz3Ant, Cz3PhAn,
and Cz3PhAnt, respectively. The HOMO energy levels of Cz3PhAn
and Cz3PhAnt were slightly lower than those of the Cz3An and
Cz3Ant. The incorporation of a phenyl ring between the carbazole
and anthracene units would dilute the density of the electron-rich
carbazole moiety. As a result, lower HOMO energy levels were
observed for Cz3PhAn and Cz3PhAnt. In addition, the HOMO en-
ergy levels of carbazole-substituted anthracene derivatives are
slightly higher than those of 9,10-di(2-naphthyl)anthracene
(ADN, ꢁ5.8 eV) [22,26]. The incorporation of electron-rich
The blue emitters Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt in
solution exhibited high quantum yields of 0.71, 0.83, 0.72, and 0.85,
respectively. Moreover, the values of Ffl of Cz3An, Cz3Ant, Cz3PhAn,
and Cz3PhAnt in thin film state were 0.24, 0.27, 0.25, and 0.28,
respectively. Relative to the Ffl values of anthracene derivatives in
dilute solution, lower Ffl values in thin film state were attributed to
the intermolecular
pep stacking and PL quenching of these
anthracene derivatives. The values of Ffl of Cz3Ant and Cz3PhAnt
were larger than that of Cz3An and Cz3PhAn. This is because the
incorporation of the tert-butyl group on the anthracene unit would
possibly reduce the degree of intermolecular
pep stacking of
Cz3Ant and Cz3PhAnt, leading to the lower PL emission quenching
of carbazole-substituted anthracene derivatives. In addition, the
conjugation lengths of the anthracene derivatives were slightly
enhanced via inserting a phenyl ring between the carbazole and
anthracene units. Therefore, the Ffl values of Cz3PhAn and
Cz3PhAnt were slightly larger than those of Cz3An and Cz3Ant,
respectively. Furthermore, the presence of bipolar structure in
these carbazole-attached 2-tert-butylesubstituted anthracenes
would lead to larger Ffl values when compared with other 2-tert-
butylesubstituted anthracene derivatives in literature [21,28,32].
3.3. Electrochemical properties of carbazole-substituted anthracene
derivatives
Fig. 4. Energy levels of the blue emitters (Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt),
hole-transporting layer (NPB), electron-transporting layer (Alq3 and TPBI), hole-
blocking layer BCP, and cathode (LiF/Al). (For interpretation of the references to co-
lor in this figure legend, the reader is referred to the web version of this article.)
We employed CV to investigate the electrochemical behavior
(Fig. 3) and to estimate the energy levels of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital

Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
583
Table 2
(ꢁ6.20 eV) was larger than that between the anthracene derivatives
and Alq3 (5.70 eV), implying that TPBI showed a better hole-
blocking characteristic than Alq3 [51]. Consequently, superior EL
performance could be realized if TPBI was to be used as ETL in
OLEDs based on these anthracene derivatives. The band gap en-
ergies of Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt were 2.80, 2.79,
2.79, and 2.78 eV, respectively. The band gap energies of these
anthracene derivatives were slightly decreased with the incorpo-
ration of a phenyl ring between the carbazole and anthracene units,
Electrochemical behaviors of the carbazole-substituted anthracence derivatives.
(eV)a
Eg (eV)b
HOMO (eV)c
LUMO (eV)d
ox
Compound
E
onset
Cz3An
Cz3Ant
Cz3PhAn
Cz3PhAnt
0.76
0.78
0.80
0.84
2.80
2.79
2.79
2.78
ꢁ5.48
ꢁ5.50
ꢁ5.52
ꢁ5.56
ꢁ2.68
ꢁ2.71
ꢁ2.73
ꢁ2.78
a
b
c
ox
E
1/2
¼ the onset potential of oxidation peak.
Determined from UVeVis absorption spectra.
Determined by cyclic voltammeter in 0.1 M TBAP/DCM.
LUMO ¼ HOMO ꢁ Eg.
d
carbazole moieties onto the C9 and C10 positions of anthracene
enhances the HOMO energy levels of anthracene derivatives [17].
The high HOMO energy levels of carbazole-substituted anthracene
derivatives would certainly facilitate hole-injection from an ITO
transparent anode to the light-emitting layer [50]. Because of this,
better EL performances from OLEDs based on the carbazole-
substituted anthracene derivatives in this study would be ach-
ieved. On the other hand, the LUMO energy levels of Cz3An, Cz3Ant,
Cz3PhAn, and Cz3PhAnt were ꢁ2.68, ꢁ2.71, ꢁ2.73, and ꢁ2.78 eV,
respectively. The difference of the LUMO energy levels between the
anthracene derivatives and TPBI (ꢁ2.70 eV) was smaller than that
between the anthracene derivatives and Alq3 (ꢁ3.30 eV) [21]. This
indicates that TPBI would exhibit superior electron-injection
characteristics to those of Alq3. Moreover, the difference of the
HOMO energy levels between the anthracene derivatives and TPBI
Fig. 5. Energy levels and configurations of the OLEDs (Device A: ITO/4,40-bis[N-(1-
naphthyl)-N-phenylamino]biphenyl (NPB, 30 nm)/anthracene derivative (30 nm)/tri-
(8-hydroxyquinoline)aluminum (Alq3, 30 nm)/LiF (0.5 nm)/Al (150 nm); Device B: ITO/
NPB (30 nm)/anthracene derivative (30 nm)/1,3,5-tris(N-phenyl benzimidizol-2-yl)
benzene (TPBI, 30 nm)/LiF (0.5 nm)/Al (150 nm); Device C: ITO/NPB (30 nm)/anthra-
cene derivative (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 10 nm)/
Alq3 (30 nm)/LiF (0.5 nm)/Al (150 nm), and Device D: ITO/NPB (40 nm)/anthracene
derivative (30 nm)/BCP (10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Al (150 nm)).
Fig. 6. Brightness, EL efficiency, and EL spectra of Cz3An, Cz3Ant, Cz3PhAn, and
Cz3PhAnt-based type-A devices (Devices A-I e A-IV).

584
Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
corresponding to the enhancement of the conjugation length of the
anthracene derivative. On the other hand, smaller band gap en-
ergies, lower LUMO levels, and higher HOMO levels were observed
for these 9-ethyl-9H-carbazol-3-yl or 4-(9-ethyl-9H-carbazol-3-yl)
phenyl groups substituted anthracene derivatives when compared
with the anthracene derivatives attached with the N-arylated
injection into ETL from cathode. As a result, better EL perfor-
mance was observed for the Device A-III in comparison with those
for Devices A-I and A-II. Furthermore, as compared to Devices A-I
and A-II, longer conjugation length of CzPhAn resulted in a larger
EL
max
value of the maximal EL wavelength ð
l
Þ of Device A-III
(Fig. 6(c)). The Cz3PhAn based device showed a larger FWHM value
of EL emission than did the Cz3An and Cz3Ant based devices, which
is attributed to the lower HOMO energy level of Cz3PhAn. Lower
HOMO energy level of Cz3PhAn favored the hole-transporting from
light-emitting layer to Alq3 ETL, leading to the EL emission partially
contributed from Alq3. Green emission from Alq3 at wavelength
ranged from 530 to 600 nm was observed not only for the Cz3PhAn
based devices, but for the Cz3An and Cz3Ant based devices as well.
Additionally, poor EL performance was obtained for the Cz3PhAnt
based Device A-IV as compared to the Cz3PhAn based Device A-III.
The incorporation of the tert-butyl group on the anthracene unit
disrupted the order packing of Cz3PhAnt, which was not favorable
for the charge transfer between the molecules [45,52]. Apart from
that, low HOMO energy level led to the hole-transporting from the
Cz3PhAnt layer into ETL. Consequently, lower brightness and
smaller EL efficiency were achieved for Device A-IV. In addition, a
lowest turn-on voltage was observed for the Device A-IV as
compared to those for the Devices A-I e A-III. This is due to the fact
that Cz3PhAnt exhibited the lowest HOMO energy level among
these carbazole-substituted anthracene derivatives.
on
carbazole moieties in literature [45]. The E values, band gap en-
ox
ergies, LUMO levels, and HOMO levels of the anthracene derivatives
attached with N-arylated carbazole moieties are summarized in
Table S1 (Supporting Information).
3.4. EL properties of devices based on carbazole-substituted
anthracene derivatives
The fluorophores Cz3An, Cz3Ant, Cz3PhAn, and Cz3PhAnt
appeared to be promising blue-emitting materials for OLED appli-
cations based on their photophysical and electrochemical proper-
ties. To determine the EL properties of these fluorophores, we
fabricated a series of non-doped devices using Alq3 (or TPBI) as ETL
and BCP as HBL. The energy levels and configurations of the Cz3An,
Cz3Ant, Cz3PhAn, and Cz3PhAnt based OLED devices are shown in
Fig. 5. Fig. 6 present the brightness, EL efficiency (current and
external quantum efficiency), and EL spectra of the OLEDs (Device
A-I e A-IV) based on the carbazole-substituted anthracene de-
rivatives; the key EL performance data of the non-doped devices
are summarized in Table 3. The Alq3 was used as ETL for the Devices
A-I e A-IV. For Devices A-I e A-IV, the Cz3Ant based Device A-II
exhibited higher brightness and larger EL efficiency (current and
external quantum efficiency) than the Cz3An based Device A-I
(Fig. 6(a,b)). The presence of tert-butyl groups on the anthracene
As TPBI was used as ETL, the EL properties were improved
significantly for the carbazole-substituted anthracene derivatives
based OLEDs (Fig. 7). The EL efficiency of the Devices B-I e B-IV
were much higher than those of the Devices A-I e A-IV. Moreover,
the FWHM values of the Devices B-I e B-IV were much smaller than
those of the Devices A-I e A-IV (Fig. 7(c)). The EL emissions of
Devices B-I e B-IV were blue-shifted as compared to those of the
Devices A-I e A-IV. Devices B-I e B-IV also exhibited better color
purities than those of the Devices A-I e A-IV. This is attributed to
the better blocking effect of TPBI for OLEDs. Low HOMO energy
level of TPBI would block the holes to transfer from blue emitter
layer to ETL. As a result, higher EL efficiency and better color purity
were obtained for Devices B-I e B-IV. Nevertheless, TPBI exhibits a
lower electron-transporting capacity than Alq3. Therefore, the
brightness values of Devices B-I e B-IV were slightly lower than
those of Devices A-I e A-IV. In addition, the brightness values of the
unit inhibited the aggregation and pep* stacking of the anthracene
derivatives, thereby improving the EL performance of the OLEDs. As
a result, the brightness and EL efficiency of the Cz3Ant based Device
A-II were much higher than those of the Cz3An based Device A-I. In
addition, higher brightness and larger efficiency were observed for
the Cz3PhAn based Device A-III as compared to the OLEDs based on
Cz3An and Cz3Ant (Devices A-I and A-II). CzPhAn with one more
phenyl ring between the carbazole and anthracene units exhibited
longer conjugation length than those of the Cz3An and Cz3Ant.
Moreover, CzPhAn exhibits a lower LUMO energy level than those
of the Cz3An and Cz3Ant, which is favorable for the electron-
Table 3
EL properties of the carbazole-substituted anthracence derivatives based OLEDs.
EL
max
Devicesa
EML
Von (V)
Max. luminance (cd/m2)
Max. efficiency (cd/A)
FWHMc (nm)
l
(nm)
CIE (x,y)d
b
Device A-I
Device A-II
Device A-III
Device A-IV
Device B-I
Device B-II
Device B-III
Device B-IV
Device C-I
Device C-II
Device C-III
Device C-IV
Device D-I
Device D-II
Device D-III
Device D-IV
Cz3An
Cz3Ant
Cz3PhAn
Cz3PhAnt
Cz3An
3.1
3.1
3.1
2.4
3.3
3.1
3.2
3.1
4.0
3.0
3.0
3.0
3.5
3.0
3.5
3.0
8937
12841
37423
25669
12270
5136
33118
19290
4847
1.88
3.35
4.02
2.94
5.09
5.37
5.84
4.35
3.38
3.92
4.52
4.65
4.80
5.74
5.37
5.89
110
110
120
114
68
68
72
70
68
72
82
72
80
74
80
74
489
494
504
500
468
468
470
470
470
474
472
470
474
474
472
474
(0.21,0.36)
(0.22,0.38)
(0.23,0.40)
(0.23,0.39)
(0.15,0.22)
(0.15,0.21)
(0.16,0.24)
(0.15,0.20)
(0.13,0.22)
(0.17,0.26)
(0.18,0.28)
(0.16,0.23)
(0.16,0.27)
(0.15,0.26)
(0.18,0.27)
(0.17,0.25)
Cz3Ant
Cz3PhAn
Cz3PhAnt
Cz3An
Cz3Ant
4324
Cz3PhAn
Cz3PhAnt
Cz3An
Cz3Ant
Cz3PhAn
Cz3PhAnt
25419
23049
9394
2987
30049
17977
a
Device A: ITO/NPB (30 nm)/anthracene derivative (30 nm)/Alq3 (30 nm)/LiF (0.5 nm)/Al (150 nm); Device B: ITO/NPB (30 nm)/anthracene derivative (30 nm)/TPBI (30 nm)/
LiF (0.5 nm)/Al (150 nm); Device C: ITO/NPB (30 nm)/anthracene derivative (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (0.5 nm)/Al (150 nm); Device D: ITO/NPB (30 nm)/
anthracene derivative (30 nm)/BCP (10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Al (150 nm).
b
Turn-on voltage at 1 cd/m2.
Under an applied voltage of 8 V.
Commission International de L’Eclairage coordinates; under an applied voltage of 8 V.
c
d

Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
585
Cz3Ant based Device B-II and Cz3PhAnt based Device B-IV were
lower than those of the Cz3An based Device B-I and Cz3PhAn based
Device B-III, correspondingly. This is because the incorporation of
the tert-butyl group on the anthracene unit reduced the charge
transfer capacity in light-emitting layer. Therefore, lower bright-
ness values were found for the Devices B-II and B-IV. On the other
hand, larger turn-on voltages were observed for the Devices B-I e
B-IV as compared to those for Devices A-I e A-IV. As shown in
Fig. 7(c), EL spectra were not changed significantly with increasing
applied voltage (8e12 V). This implies that these carbazole-
substituted anthracene derivatives based non-doped OLEDs
exhibited good blue color stability.
In order to improve the EL properties, BCP as HBL was inserted
between the blue emitter layer and ETL Alq3 (or TPBI) for the
carbazole-substituted anthracene derivatives based non-doped
OLEDs (Devices C-I e C-IV and Devices D-I e D-IV). Figs. 8 and 9
present EL properties of these OLEDs (Devices C-I e C-IV and De-
vices D-I e D-IV). EL efficiency and color purity were improved
significantly as the Alq3 based OLEDs (Devices C-I e C-IV) were
incorporated with HBL BCP (Fig. 8). In fact, the EL efficiency and
Fig. 7. Brightness, EL efficiency, and EL spectra of Cz3An, Cz3Ant, Cz3PhAn, and
Cz3PhAnt-based type-B devices (Devices B-I e B-IV).
Fig. 8. Brightness, EL efficiency, and EL spectra of Cz3An, Cz3Ant, Cz3PhAn, and
Cz3PhAnt-based type-C devices (Devices C-I e C-IV).

586
Y.-C. Chang et al. / Dyes and Pigments 99 (2013) 577e587
color purity of Devices C-I e C-IV were much better than those of
Devices A-I e A-IV. However, the brightness was slightly reduced
by the incorporation of BCP HBL between the blue emitter layer and
Alq3 for devices C-I e C-IV. The turn-on voltages of Devices C-I e C-
IV did not vary significantly as compared to those of Devices A-I e
A-IV. In addition, the improvement of EL properties was not sig-
nificant as BCP HBL was inserted between the blue emitter layer
and TPBI ETL for the carbazole-substituted anthracene derivatives
based OLEDs (Devices D-I e D-IV; Fig. 9). The insertion of BCP
reduced the electron-injection activities from TPBI to blue emitter
layer because of the presence of the energy barrier between the
light emitting layer and BCP. As shown in Fig. 9, the EL efficiency
was not further enhanced, while the brightness was reduced for the
Devices D-I e D-IV as compared to those for Devices B-I e B-IV. In
addition, the EL spectra of Devices C-I e C-IV and Devices D-I e D-IV
are shown in Figs 8(c) and 9(c). The green emission was slightly
enhanced with increasing applied voltage for Cz3An based Device
C-I and Cz3Ant based Device C-II. This implies that the charge
recombination was occurred at the Alq3 layer under higher applied
voltages (>10 V). The color purity of the Cz3PhAn based Device C-III
and Cz3PhAnt based Device C-IV was more stable than those of the
Cz3An based Device C-I and Cz3Ant based Device C-II. Furthermore,
the color stability was improved by inserting BCP HBL between the
blue emitter layer and TPBI ETL (Fig. 8(c)). As compared to the
Devices B-I e B-IV (Fig. 7(c)), the color purity was not improved by
inserting BCP HBL between the blue emitter layer and TPBI ETL for
Cz3An, Cz3Ant, and Cz3PhAn based Devices D-I e D-IV (Fig. 9(c)).
This is attributed to the presence of the microcavity effect in the
multilayer structure of the blue OLEDs [53]. The color purities of the
9-ethyl-9H-carbazol-3-yl or 4-(9-ethyl-9H-carbazol-3-yl)phenyl
groups containing anthracene derivatives based OLEDs were not
exactly matched the saturated deep-blue CIE chromaticity co-
ordinates. Nevertheless, the non-doped OLED based on the
anthracene derivatives attached with the carbazolyl groups at C3-
positions exhibited much better EL performance (brightness and
current efficiency) than the OLEDs based on the anthracene de-
rivatives attached with N-arylated carbazole moieties [45]. The EL
properties of the OLEDs fabricated from N-arylated carbazole
moieties attached anthracene derivatives are summarized in
Table S2 (Supporting Information).
4. Conclusion
Excellent EL performance was observed for the Cz3An, Cz3Ant,
Cz3PhAn, and Cz3PhAnt based nondoped sky-blue OLEDs, which is
attributed to the efficient conjugation of the anthracene unit
attached with the carbazolyl group at C3-position. Better conju-
gation results in the lower LUMO and higher HOMO energy levels of
the anthracene derivative, which is favorable for the electron- and
hole-injection from the electrodes to the light-emitting layer. As a
result, the non-doped OLED based on the anthracene derivatives
attached with the carbazolyl groups at C3-postions exhibited much
better EL performance than the OLEDs based on the anthracene
derivatives attached with N-arylated carbazole moieties. In addi-
tion to the efficient conjugation of the carbazole moieties
substituted anthracenes, the optimal design of the OLED architec-
ture is also a deciding factor for the EL performance.
Acknowledgment
We thank the National Science Council (NSC) and the Ministry of
Education, Taiwan, for financial support under the ATU plan.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.06.025.
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Fig. 9. Brightness, EL efficiency, and EL spectra of Cz3An, Cz3Ant, Cz3PhAn, and
Cz3PhAnt-based type-D devices (Devices D-I e D-IV).

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