Efficient non-doped blue light emitting diodes based on novel carbazole-substituted anthracene derivatives

Article abstract of DOI:10.1016/j.orgel.2011.10.004

In this study, we synthesized three anthracene derivatives featuring carbazole moieties as side groups - 2-tert-butyl-9,10-bis[4-(9-carbazolyl) phenyl]anthracene (Cz⁹PhAnt), 2-tert-butyl-9,10-bis{4-[3,6-di-tert- butyl-(9-carbazolyl)]phenyl}anthracene (tCz⁹PhAnt), and 2-tert-butyl-9,10-bis{4′-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl} anthracene (tCz⁹Ph₂Ant) - for use in blue organic light emitting devices (OLEDs). The anthracene derivatives presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited strong blue emissions in solution, with high quantum efficiencies (91%). We studied the electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL provided superior EL performance than did those lacking the HTL. The highest brightness (6821 cd/m²) was that for the tCz⁹PhAnt-based device; the greatest current efficiency (2.1 cd/A) was that for the tCz⁹Ph₂Ant-based device. The devices based on these carbazole-substituted anthracene derivatives also exhibited high color purity.

Full text of DOI:10.1016/j.orgel.2011.10.004

Organic Electronics 13 (2012) 43–52
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient non-doped blue light emitting diodes based on novel
carbazole-substituted anthracene derivatives
Ying-Hsiao Chen a, Shin-Lei Lin b, Yu-Chen Chang a, Yung-Chung Chen c, Jiann-Tsuen Lin c,
d
a,e,
Rong-Ho Lee a, , Wen-Jang Kuo , Ru-Jong Jeng
⇑
⇑
a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
b Material and Chemical Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan
c Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
d Department of Applied Chemistry, National University of Kaohsiung, Kaohsiung 811, Taiwan
e Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 June 2011
Received in revised form 25 August 2011
Accepted 3 October 2011
Available online 20 October 2011
In this study, we synthesized three anthracene derivatives featuring carbazole moieties as
side groups – 2-tert-butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz9PhAnt), 2-tert-
butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbazolyl)]phenyl}anthracene (tCz9PhAnt), and 2-
tert-butyl-9,10-bis{40-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}anthracene (tCz9Ph2
Ant) – for use in blue organic light emitting devices (OLEDs). The anthracene derivatives
presenting rigid and bulky tert-butyl-substituted carbazole units possessed high glass-
transition temperatures (220 °C). Moreover, the three anthracene derivatives exhibited
strong blue emissions in solution, with high quantum efficiencies (91%). We studied the
electroluminescence (EL) properties of non-doped OLEDs incorporating these anthracene
derivatives, with and without a hole-transporting layer (HTL). OLEDs incorporating an HTL
provided superior EL performance than did those lacking the HTL. The highest brightness
(6821 cd/m2) was that for the tCz9PhAnt-based device; the greatest current efficiency
(2.1 cd/A) was that for the tCz9Ph2Ant-based device. The devices based on these carbazole-
substituted anthracene derivatives also exhibited high color purity.
Keywords:
Organic light-emitting diodes
Carbazole group
Anthracene derivative
Electroluminescence properties
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
being readily injected into the light-emitting layer of the de-
vice because of the large band gap energy of such materials.
Since Tang et al. first applied a light-emitting layer doped
with a guest fluorescent dye to organic light-emitting
devices (OLEDs), OLEDs have attracted growing interest for
use in full-color flat-panel display applications and solid
state lighting [1–6]. Although the photo-electronic proper-
ties of OLEDs have been investigated thoroughly and great
advances have been made in developing novel organic
light-emitting materials, high brightness and high current
efficiency remain difficult to achieve for OLEDs based on
blue emissive materials – due primarily to the carriers not
In addition to high brightness and high current efficiency,
high color purity is also required for the full-color display
applications [7–9]. Amorphous materials possessing high
thermal stability are also needed to obtain OLEDs exhibiting
excellent operational stability [10–12].
Several small-molecule emitters, including terfluorene
[13], anthracene [14], distyrylarylene [15,16], carbazole,
[17] and pyrene [18] derivatives, have been reported for
applications in blue OLEDs. For example, a device based
on 4,40-bis(2,2-diphenylvinyl)-1,10-biphenyl (DPVBi) has
exhibited a high luminous efficiency and a blue-greenish
emission [15]. Unfortunately, the low glass transition tem-
perature (Tg) of DPVBi led to the formation of crystals in the
light-emitting layer, thereby resulting in poor operational
⇑
Corresponding authors.
E-mail addresses: rhl@nchu.edu.tw (R.-H. Lee), rujong@ntu.edu.tw
(R.-J. Jeng).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.10.004

44
Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
stability of the device [16]. A device based on another
well-known blue-emitting material, 2-methyl-9,10-di
(2-naphthyl)anthracene (MADN) [14], has exhibited supe-
rior thermal stability and higher color purity than those
of the devices based on DPVBi, but its low luminance
efficiency inhibits its potential application in full-color
displays. Therefore, the development of novel organic
light-emitting materials with high brightness, high current
efficiency, high color purity, and good operational stability
remains an important challenge for realizing full-color
OLED applications. Among the several types of blue
emitters that have been reported, anthracene derivatives
have emerged as one of most relevant electroluminescent
materials because of their high photoluminescence (PL)
and electroluminescence (EL) quantum yields and good
thermal stabilities [19–37]. Several anthracene derivatives
presenting electron-donating or -withdrawing moieties at
their 9 and 10 positions display hole-transporting/light-
emitting [20,28,31,33,35–37] or electron-transporting/
light-emitting [24,27] properties. A direct substitution of
either strong electron-withdrawing or electron-donating
groups at 9 and 10 positions changes the electron density
on the anthracene ring and consequently enhances the con-
jugation length of the anthracene derivative [27]. Apart
from that, the attachment of bulky side-groups to the
anthracene (tCz9Ph2Ant) – for use as blue emitters in
OLEDs. For tCz9PhAnt and tCz9Ph2Ant, we expected attach-
ment of the non-coplanar 3,6-di-tert-butyl-substituted car-
bazole moieties onto the 9 and 10 positions of the central
anthracene core to inhibit molecular close packing in the
solid state and thereby enhance the EL performance. We
positioned the 3,6-di-tert-butyl-(9-carbazolyl)phenyl and
3,6-di-tert-butyl-(9-carbazolyl)biphenyl units on the 2-
tert-butyl-substituted anthracene core to study the effect
of the conjugation length of the side groups on the EL per-
formance. Moreover, we also synthesized the 4-(9-carbaz-
olyl)phenyl carbazole-substituted anthracene derivative
Cz9PhAnt for the sake of comparison. We expected to ob-
serve excellent EL performance from these carbazole-
substituted anthracene derivatives. In addition, we ana-
lyzed the influence of the attached carbazole moieties on
the thermal stability, electro-chemical properties, photo-
physical behavior, and EL performances of the anthracene
derivative-based devices.
2. Experimental section
2.1. Materials
All reactions and manipulations were performed under a
N2 atmosphere using standard Schlenk techniques. All
chromatographic separations were performed using SiO2.
9H-Carbazole, copper powder, potassium carbonate,
[18]crown-6 (18C6), 1,4-dibromobenzene, 4,40-dibromobi-
phenyl, 2-chloro-2-methylpropane, nitromethane, zinc
chloride (ZnCl2), and n-butyl lithium (n-BuLi) were obtained
from Aldrich and used as received. Potassium iodide and
sodium hypophosphite monohydrate were purchased from
Acros. Acetic acid and o-dichlorobenzene (DCB) were
obtained from TEDIA. Tetrahydrofuran (THF) was purified
through distillation from Na in the presence of benzophe-
none. The syntheses of compounds 1–4 and carbazole-
substituted anthracene derivatives (Cz9PhAnt, tCz9PhAnt,
tCz9Ph2Ant) are presented in Scheme 1.
anthracene core can prevent p–
p⁄ stacking interactions of
the molecules in the solid state and reduce self-quenching
effects [35,36].
Various electron-donating and -accepting groups have
been attached to tert-butyl substituted anthracene deriva-
tives for use as blue emitters in OLEDs [19–21,34–37]. High
morphological stability of thin films of anthracene deriva-
tives has been observed after introduction of a tert-butyl
unit at the C2 position of the anthracene moiety [36]. More-
over, attachment of electron-donating/hole-transporting
groups on the electron-acceptor/electron-transporter
anthracene core can enhance the PL performance of anthra-
cene derivatives. Excellent EL performances have been
observed for such OLEDs based on bipolar anthracene
derivatives [35,36]. Nevertheless, the attachment of 4-(9-
carbazolyl)phenyl carbazoles and 3,6-di-tert-butyl-substi-
tuted carbazole moieties on 2-tert-butyl-substituted
anthracene cores have not been synthesized previously.
We have reported carbazole-based fluorophores that exhi-
bit both blue emissions and hole-transporting properties
[38]. Incorporating a carbazole moiety into a molecular
scaffold can significantly improve the glassy-state durabil-
ity and thermal stability of an organic compound. More-
over, the 3, 6, and 9 positions of the carbazole moiety are
readily functionalized, allowing fine-tuning of the electro-
optical properties of the molecule [39–43]. N-Arylated
carbazoles, in which a phenyl or naphthyl group is attached
at the 9 position of the carbazole unit, have exhibited excel-
lent thermal stability and EL properties in small-molecule
OLEDs [44–46]. In this study, we synthesized three
2.2. Synthesis of monomers
2.2.1. 3,6-Di-tert-butyl-9H-carbazole (1)
2-Chloro-2-methylpropane (9.8 mL, 90 mmol) was
added dropwise to a suspension of 9H-carbazole (4.9 g,
30 mmol) and ZnCl2 (12.2 g, 90 mmol) in nitromethane
(100 mL) and then the mixture was stirred for a further
6 h at room temperature, followed by partition between
ethyl acetate and water. The organic layer was dried
(MgSO4) and the volatiles evaporated under reduced pres-
sure. The residue was poured into MeOH and filtered off, to
give a white solid (6.6 g, 81%). 1H NMR (600 MHz, CDCl3): d
(ppm) 8.20 (d, J = 2.1 Hz, 2H), 7.58 (br s, 1H), 7.56 (dd,
J = 8.55, 1.80 Hz, 2H), 7.25 (d, J = 8.4 Hz, 2H), 1.57 (s,
carbazole-substituted anthracene derivatives
–
2-tert-
18H). 13C NMR (125 MHz, CDCl3):
d (ppm) 142.14,
butyl-9,10-bis[4-(9-carbazolyl)phenyl]anthracene (Cz9Ph
Ant), 2-tert-butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-carbaz-
olyl)] phenyl} anthracene (tCz9PhAnt), and 2-tert-butyl-
9,10-bis{40-[3,6-di-tert-butyl-(9-carbazolyl)]biphenyl-4-yl}
137.93, 123.46, 123.14, 116.07, 110.07, 34.66, 32.09. HRMS
(m/z): calcd for C20H25N 279.1987; found 279.1980. Anal.
Calcd for C20H25N: C, 85.97; H, 9.02; N, 5.01. Found: C,
85.85; H, 9.01; N, 5.46.

Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
45
N
N
H
N
(b)
Br
N
73%
Cz9PhAnt (61%)
O
2
(a)
81%
O
N
N
Br
N
(b)
(d)
H
78%
N
3
t
Cz9PhAnt (67%)
1
(c)
Br
N
75%
N
N
4
t
Cz9Ph2Ant (62%)
-DCB, reflux, 16 h.
n
-BuLi,THF, -78 oC, over night (ii) KI, NaH2PO2, AcOH, reflux, 5
(a) 2-Chloro-2-methylpropane, ZnCl2, CH3NO2, RT, 6 h.
(c) 4,4'-dibromobiphenyl, K2CO3, Cu, 18C6, -DCB, reflux, 16 h.
(b) 1,4-dibromobenzene, K2CO3, Cu, 18C6,
o
(d) (i)
h.
o
Scheme 1. Synthetic routes toward Cz9PhAnt, tCz9PhAnt, and tCz9Ph2Ant.
2.2.2. 9-(4-Bromo-phenyl)-9H-carbazole (2)
CDCl3): d (ppm) 143.13, 138.90, 137.24, 132.93, 128.24,
123.71, 123.46, 120.22, 116.30, 108.96, 34.72, 31.98. HRMS
(m/z): calcd for C26H28BrN 433.1405; found 433.1409. Anal.
Calcd for C26H28BrN: C, 71.89; H, 6.50; N, 3.22. Found: C,
71.94; H, 6.23; N, 3.55.
A mixture of 9H-carbazole (5.02 g, 30 mmol), 1,4-dibro-
mobenzene (14.13 g, 60 mmol), K2CO3 (16.56 g, 120 mmol),
Cu powder (1.94 g, 30 mmol), and 18C6 (3.71 g, 15 mmol)
in o-DCB (120 mL) was stirred and deoxygenated under
N2 for 1 h and then the mixture was heated under reflux
for a further 16 h. After cooling to room temperature, the
crude mixture was filtered, and washed with CH2Cl2. The
combined filtrates were evaporated to dryness and then
the residues were subjected to column chromatography
(SiO2; hexane) to give a white solid (7.0 g, 73%). 1H NMR
(600 MHz, CDCl3): d (ppm) 8.13 (d, J = 7.8 Hz, 2H), 7.72 (d,
J = 8.7 Hz, 2H), 7.40–7.45 (m, 3H), 7.35–7.38 (m, 3H),
7.26–7.31 (m, 2H). 13C NMR (125 MHz, CDCl3): d (ppm)
140.60, 136.79, 133.10, 128.71, 126.07, 123.47, 120.87,
120.37, 120.20, 109.53. HRMS (m/z): calcd for C18H12BrN:
321.0153. Found: 321.0162. Anal. Calcd for C18H12BrN: C,
67.10; H, 3.75; N, 4.35. Found: C, 67.45; H, 3.73; N, 4.21.
2.2.4. 3,6-Di-tert-butyl-9-(40-bromobiphenyl-4-yl)-9H-
carbazole (4)
A mixture of 1 (5.6 g, 20 mmol), 4,40-dibromobiphenyl
(12.5 g, 40 mmol), K2CO3 (11.1 g, 80 mmol), Cu powder
(0.85 g, 13 mmol), and 18C6 (1.7 g, 6 mmol) in o-DCB
(100 mL) was stirred and degassed under N2 for 1 h and
then heated under reflux with stirring for 12 h. After cool-
ing to room temperature, the crude mixture was filtered
and the collected solids washed with CH2Cl2. The com-
bined filtrates were evaporated to dryness and the residue
subjected to column chromatography (SiO2; hexane/
CH2Cl2, 3:1) to give a white solid (6.6 g, 64%). 1H NMR
(600 MHz, CDCl3): d (ppm) 8.18 (d, J = 1.2 Hz, 2H), 7.74
(d, J = 8.4 Hz, 2H), 7.63 (dd, J = 6.00, 3.60 Hz, 4H), 7.54 (d,
J = 8.40 Hz, 2H), 7.50 (dd, J = 8.5, 2.10 Hz, 2H), 7.42 (d,
J = 8.4 Hz, 2H), 1.49 (s, 18H). 13C NMR (125 MHz, CDCl3):
d (ppm) 142.98, 139.20, 139.07, 138.38, 137.72, 132.00,
128.62, 128.16, 126.95, 123.63, 123.45, 121.80, 116.27,
109.19, 34.73, 32.00. HRMS (m/z): calcd for C32H32BrN
509.1718; found 509.1707. Anal. Calcd for C32H32BrN: C,
75.29; H, 6.32; N, 2.74. Found: C, 75.46; H, 6.28; N, 2.66.
2.2.3. 3,6-Di-tert-butyl-9-(4-bromo-phenyl)-9H-carbazole (3)
A mixture of 1 (5.6 g, 20 mmol), 1,4-dibromobenzene
(15.7 g, 67 mmol), K2CO3 (11.1 g, 80 mmol), Cu powder
(0.85 g, 13 mmol), and 18C6 (1.7 g, 6 mmol) in o-DCB
(100 mL) was stirred and degassed under N2 for 1 h and
then heated under reflux for 12 h. After cooling to room
temperature, the crude mixture was filtered, and washed
with CH2Cl2. The combined filtrates were evaporated to
dryness and then the residue was subjected to column
chromatography (SiO2; hexane/CH2Cl2, 7:3) to give a white
solid (6.8 g, 78%). 1H NMR (600 MHz, CDCl3): d (ppm) 8.18
(d, J = 1.50 Hz, 2H), 7.70 (dd, J = 8.70, 2.70 Hz, 2H), 7.49 (dd,
J = 8.70, 1.80 Hz, 2H), 7.45 (dd, J = 8.70, 2.10 Hz, 2H), 7.33
(d, J = 8.4 Hz, 2H), 1.49 (s, 18H). 13C NMR (125 MHz,
2.2.5. 2-tert-Butyl-9,10-bis[4-(9-
carbazolyl)phenyl]anthracene (Cz9PhAnt)
A solution of 2 (3.22 g, 10 mmol) in dry THF (100 mL)
was stirred at ꢀ78 °C and then n-BuLi (2.5 M in hexane,
4.8 mL) was added slowly over 1 h. A solution of 2-tert-

46
Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
butylanthraquinone (1.32 g, 5.0 mmol) in THF (20 mL) was
then added dropwise to the mixture at ꢀ78 °C. The result-
ing mixture was warmed to room temperature and stirred
for overnight. After the reaction had finished, cold water
was added to the mixture, which was then partitioned be-
tween EtOAc and water. The combined organic fractions
were dried (MgSO4) and the volatiles evaporated under re-
duced pressure. The residue was added to a mixture of KI
(3.32 g, 20 mmol), sodium hypophosphite monohydrate
(4.25 g, 40 mmol), and AcOH (50 mL) and then the mixture
was heated under reflux for 5 h. After the reaction had fin-
ished, the mixture was poured into water and the precipi-
tate filtered off and washed with plenty of water. The
yellowish crude product was purified through column
chromatography (SiO2;hexane/CH2Cl2, 7:3) to give a white
solid (2.2 g, 61%). 1H NMR (600 MHz, CDCl3): d (ppm) 8.25
(d, J = 7.80 Hz, 4H), 7.91–7.99 (m, 3H), 7.84–7.88 (m, 4H),
7.78–7.81 (m, 4H), 7.63–7.76 (m, 6H), 7.49–7.58 (m, 6H),
7.39 (t, J = 7.8 Hz, 4H), 1.41 (s, 9H). 13C NMR (125 MHz,
CDCl3): d (ppm) 147.74, 140.97, 140.88, 138.47, 138.21,
137.07, 136.98, 136.16, 135.98, 132.84, 132.80, 130.09,
130.06, 129.69, 128.64, 127.04, 126.90, 126.84, 126.58,
126.06, 126.03, 125.31, 125.12, 123.55, 120.95, 120.43,
120.10, 120.08, 109.92, 109.80, 35.06, 30.72. HRMS (m/z):
calcd for C54H40N2 716.3191; found 717.3287 [M+1]+. Anal.
Calcd for C54H40N2: C, 90.47; H, 5.62; N, 3.91. Found: C,
90.60; H, 5.44; N, 3.22.
[M+1]+. Anal. Calcd for C82H80N2: C, 90.06; H, 7.37; N,
2.56. Found: C, 90.17; H, 6.98; N, 2.55.
2.3. Measurements
1H and 13C NMR spectra were recorded using a Varian
Gemini NMR spectrometer operated at 600 and 125 MHz,
respectively. Elemental analysis was performed using an
elemental analyzer (Elementar Vario EL III). High-resolu-
tion mass spectra were recorded using a Finnigan/Thermo
Quest MAT mass spectrometer. Glass transition tempera-
tures were measured under N2 using a differential scanning
calorimeter (TechMax Instruments DSC 6220) operated at a
heating rate of 10 °C minꢀ1. Thermogravimetric analysis
(TGA) was performed under a N2 atmosphere using a ther-
mogravimetric analyzer (VersaTherm thermogravimetric
analyzer) operated at a heating rate of 10 °C minꢀ1. UV–
vis absorption and PL spectra were recorded using a Shima-
dzu UV-1240 spectrophotometer and an Acton Research
Spectra Pro-150 spectrometer, respectively. Cyclic voltam-
metry (CV) was conducted using a CHI model611D appara-
tus and a three-electrode cell, in which an indium tin oxide
(ITO) sheet, a Pt wire, and silver/silver nitrate (Ag/Ag+) were
used as the working electrode, counter electrode, and refer-
ence electrode, respectively. All electrochemical experi-
ments were performed using deoxygenated MeCN
solutions containing 0.1 M tetrabutylammonium perchlo-
rate (TBAP) as the electrolyte.
2.2.6. 2-tert-Butyl-9,10-bis{4-[3,6-di-tert-butyl-(9-
2.4. EL device fabrication and electro-optical characterization
carbazolyl)]phenyl}anthracene (tCz9PhAnt)
tCz9PhAnt was obtained as a white solid (3.1 g, 67%)
after using a procedure similar to that described above
for Cz9PhAnt. 1H NMR (600 MHz, CDCl3): d (ppm) 8.27 (s,
4H), 7.85–7.98 (m, 7H), 7.74–7.80 (m, 5H), 7.59–7.69 (m,
9H), 7.48–7.53 (m, 2H), 1.50 (s, 36H), 1.35 (s, 9H). 13C
NMR (125 MHz, CDCl3): d (ppm) 147.97, 143.33, 143.30,
139.64, 139.56, 138.24, 137.98, 137.89, 137.76, 136.55,
136.37, 133.03, 132.98, 130.41, 130.03, 128.97, 127.16,
126.93, 126.76, 125.55, 125.35, 124.03, 124.01, 123.87,
123.84, 121.33, 116.65, 109.71, 109.59, 35.37, 35.08,
32.37, 31.06. HRMS (m/z): calcd for C70H72N2 940.5696;
found 941.5776 [M+1]+. Anal. Calcd for C70H72N2: C,
89.31; H, 7.71; N, 2.98. Found: C, 89.67; H, 7.53; N, 3.05.
Fabrication of OLEDs was conducted through high-vac-
uum thermal evaporation of the organic materials onto
ITO-coated glass (sheet resistance: 15
X/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 depos-
ited onto the organic material layer, followed by Al metal
deposition as the top layer in a high-vacuum chamber.
The OLED configurations in this study were (type-A) ITO/
anthracene derivative (70 nm)/1,3,5-tris(N-phenyl benzi-
midizol-2-yl)benzene (TPBI, 30 nm)/LiF (0.5 nm)/Al
(150 nm) and (type-B) ITO/4,40-bis[N-(1-naphthyl)-N-phe-
nylamino]biphenyl (NPB, 40 nm)/anthracene derivative
(30 nm)/TPBI (30 nm)/LiF (0.5 nm)/Al (150 nm), where
NPB, the anthracene derivative, and TPBI were the hole-
transporting layer (HTL), blue emitting layer, and elec-
tron-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). UV–vis spectra of the organic thin films
were measured using a Hewlett–Packard 8453 spectrome-
ter equipped with a photodiode array detector. PL and EL
spectra were recorded using a Hitachi F-4500 fluorescence
spectrophotometer. Current–voltage characteristics were
measured using a programmable electrometer equipped
with current and voltage sources (Keithley 2400).
Luminance was measured using a Newport 1835C optical
2.2.7. 2-tert-Butyl-9,10-bis{40-[3,6-di-tert-butyl-(9-
carbazolyl)]biphenyl-4-yl}anthracene (tCz9Ph2Ant)
tCz9Ph2Ant was obtained as a white solid (3.4 g, 62%)
after using a procedure similar to that described above
for Cz9PhAnt. 1H NMR (600 MHz, CDCl3): d (ppm) 8.18 (s,
4H), 7.91–8.01 (m, 8H), 7.78–7.84 (m, 3H), 7.61–7.75 (m,
9H), 7.45–7.53 (m, 9H), 7.36–7.40 (m, 2H), 1.49 (s, 36H),
1.31 (s, 9H). 13C NMR (125 MHz, CDCl3): d (ppm) 147.50,
142.95, 139.35, 139.31, 139.26, 139.11, 138.57, 138.48,
137.53, 136.52, 136.33, 132.01, 131.95, 130.16, 129.94,
129.69, 128.60, 128.36, 128.33, 127.06, 127.03, 127.00,
126.90, 126.74, 125.02, 124.83, 124.76, 123.67, 123.49,
121.19, 116.28, 109.32, 35.01, 34.76, 32.05, 30.83. HRMS
(m/z): calcd for C82H80N2 1092.6322; found 1093.9395

Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
47
meter equipped with
photodiode.
a
Newport 818-ST silicon
temperatures are desirable for a light-emitting material
to be applied in OLEDs. Fig. 1 presents DSC and TGA ther-
mograms of the carbazole-substituted anthracene deriva-
tives; Table
1
summarizes their values of Tg,
3. Results and discussion
crystallization temperature (Tc), melting temperature
(Tm), and Td The glass transition, and re-crystallization
and melting peaks were clearly observed in the heating-
scan, while only glass transition was observed during the
cooling-scan for three anthracene derivatives. tCz9Ph2Ant
exhibited values of Td and Tg of 505 and 220 °C, respec-
tively; among our three materials, these values were the
highest, presumably because of the higher molecular
weight of tCz9Ph2Ant [20]. The values of Tg, Tc, Tm, and Td
of Cz9PhAnt were 168, 244, 355, and 490 °C, respectively.
In contrast, tCz9PhAnt exhibited only values of Tg and Td
(153 and 353 °C, respectively), but no signs of crystalliza-
tion or melting. Presumably, the tight molecular stacking
of Cz9PhAnt in the absence of the tert-butyl groups on
the 3 and 6 positions of carbazole unit meant that it exhib-
ited thermal properties superior to those of tCz9PhAnt. Fur-
thermore, the presence of the sterically bulky carbazole
side groups meant that these carbazole-substituted
anthracene derivatives exhibited higher values of Tg than
those of the well-known blue-emitting materials DPVBi
(64 °C) [12] and MADN (120 °C) [14]. Therefore, we ex-
pected these carbazole-substituted anthracene derivatives
to form the morphologically stable and uniform amor-
phous films required to improve the efficiencies and life-
times of OLEDs.
3.1. Synthesis and characterization
Scheme 1 illustrates the routes that we followed for the
syntheses of the anthracene derivatives containing carba-
zole moieties. We obtained 3,6-di(tert-butyl)-9H-carbazole
(1) through Friedel–Crafts alkylation of 9H-carbazole [47]
and compounds 2–4 through Ullman coupling reactions
[39]. The carbazole-containing anthracene derivatives
Cz9PhAnt, tCz9PhAnt, and tCz9Ph2Ant were synthesized
through reactions of 2-tert-butylanthraquinone with vari-
ous carbazole derivatives [20]. 1H and 13C NMR spectros-
copy, elemental analysis, and high-resolution mass
spectrometry confirmed the chemical structures of these
compounds. The relative intensities of the signals in the
various spectra were in agreement with the proposed
structures of these anthracene derivatives.
The operational stability of an OLED is directly related
to the thermal stability of its light-emitting layer. Thus,
high glass transition (Tg) and thermal degradation (Td)
(a)
2nd heating
2nd cooling
Cz9PhAnt
3.2. Photophysical properties
Cz9PhAnt
Cz9Ph2Ant
Fig. 2 presents UV–vis absorption and PL spectra of the
carbazole-substituted anthracene derivatives in dilute tol-
uene solutions and as solid films. The UV–vis absorption
spectra of the emitters in solution were measured at a con-
centration of 1 mg mLꢀ1; the thickness of the emitter layer
for the UV–vis absorption measurements was approxi-
mately 100 nm. Table 1 summarizes the maximal absorp-
tion and PL emission wavelengths of the 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]), with
moderately weak absorptions in the ranged from 350 to
425 nm (characteristic vibronic patterns attributed to the
t
t
100
200
300
°
400
500
Temperature ( C)
(b)
Cz9PhAnt
Cz9PhAnt
Cz9Ph2Ant
100
t
t
p–
p⁄ transition of anthracene [26]). Relative to the absorp-
tion behavior of the anthracene derivatives in solution, the
maximal absorption wavelengths corresponding to the
carbazole-centered transitions were slightly red-shifted
for the three anthracene derivatives in thin film state, pre-
sumably because of the interactions and aggregation of the
molecules in thin film state.
80
60
40
20
0
Upon excitation at 350 nm, these anthracene derivatives
exhibited deep-blue emissions with PL maxima at ca. 433–
436 nm in dilute toluene solution. The maximum emission
PL
max
wavelength of the PL (k ) and the full width at half-
maximum (FWHM) of the PL were dependent on the chem-
ical structure of the carbazole-substituted anthracene
derivative. Relative to the PL spectra of the dilute solutions,
however, the emission spectra of these anthracene deriva-
tives exhibited red-shifts of 10–12 nm in the thin film state.
100
200
300
400
500
600
700
800
°
Temperature ( C)
Fig. 1. DSC and TGA thermograms of Cz9PhAnt, tCz9PhAnt, and
tCz9Ph2Ant.

48
Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
Table 1
Thermal and optical properties of the anthracene derivatives.
Tg/Tc/Tm/Td (°C)a
Ufl (%)e
k
(nm)c
FWHM of k
(nm)c
k
(nm)d
FWHM of k
(nm)d
PL
max
PL
max
PL
max
PL
max
k
(nm)c
k
(nm)d
abs
max
abs
max
Compound
Cz9PhAnt
tCz9PhAnt
tCz9Ph2Ant
168/244/355/490
153/n.a.b/n.a./353
220/322/468/506
293
298
295
433
433
436
53
55
54
299
299
300
443
444
448
62
55
63
78
70
91
a
b
c
All of the transition temperatures were determined form the second heating scan.
n.a.: Not available.
Measured in dilute toluene solution.
d
e
Measured in solid film.
PL quantum efficiency (Ufl) of the compounds, measured in toluene solution using 9,10-diphenylanthracene as a standard.
PL
max
the value of k
of tCz9Ph2Ant was slightly red-shifted (by
Cz9PhAnt
Cz9PhAnt
Cz9Ph2Ant
(a)
4 nm) relative to that of tCz9PhAnt. The FWHM of the PL
emission band was larger for tCz9Ph2Ant than it was for
tCz9PhAnt, presumably because tCz9Ph2Ant has one more
benzene ring in its carbazole-substituted group than does
tCz9PhAnt; that is, the longer conjugation length led to the
red-shift of the PL emission band of tCz9Ph2Ant. Moreover,
the presence of the biphenyl groups made tCz9Ph2Ant more
bulky, which would reduce conformation uniformity. As a
result, the FWHM of tCz9Ph2Ant was larger than that of
tCz9PhAnt.
PL
t
t
UV-vis
We measured the fluorescence quantum yields (Ufl) of
the three anthracene derivatives in dilute toluene solution
by comparing their emissions with that of a standard solu-
tion of 9,10-diphenylanthracene in cyclohexane (Ufl = 0.90)
at room temperature [37]. Our blue emitters Cz9PhAnt,
tCz9PhAnt, and tCz9Ph2Ant exhibited high quantum yields
of 0.78, 0.70, and 0.91, respectively. We suspect that the va-
lue of Ufl of tCz9PhAnt was smaller than that of Cz9PhAnt
because the vibration of the tert-butyl groups on the
peripheral carbazole moiety resulted in the PL emission
quenching of carbazole-substituted anthracene derivative.
Nevertheless, the effect of the tert-butyl group was
minimized after incorporation of the biphenyl group in
the carbazole moiety. The greater conjugation length of
the biphenyl-containing carbazole moiety led to the higher
value of Ufl for tCz9PhAnt relative to those of Cz9PhAnt and
tCz9PhAnt. In addition, the presence of bipolar structure in
these carbazole-attached 2-tert-butyl-substituted anthra-
cenes would lead to larger Ufl values when compared with
2-tert-butyl-substituted anthracene derivatives [20,21,36].
300
350
400
450
500
550
600
650
Wavelength (nm)
(b)
Cz9PhAnt
PL
UV-vis
t
t
Cz9PhAnt
Cz9Ph2Ant
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 2. UV–vis absorption and PL spectra of Cz9PhAnt, tCz9PhAnt, and
tCz9Ph2Ant (a) in toluene solutions and (b) as thin films.
3.3. Electrochemical properties of carbazole-substituted
anthracene derivatives
The differences in the PL spectra of the compounds in solu-
tion and in the form of thin films suggested the presence of
intermolecular interactions or aggregation in thin film
states. In the thin film, with the incorporation the tert-butyl
We employed CV to investigate the electrochemical
behavior of our compounds and to estimate the energy lev-
els of the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of the
anthracene derivatives. The HOMO energy levels of the
substituents at the 3 and 6 positions of the carbazole unit,
PL
the value of k
of tCz9PhAnt was similar to that of
anthracene derivatives were calculated from the onset po-
max
ox
onset
Cz9PhAnt, but the FWHM of tCz9PhAnt was smaller than
that of Cz9PhAnt. The PL emission intensity of tCz9PhAnt in
the long wavelength range (450–550 nm) was weaker than
that of Cz9PhAnt. The steric hindrance of the tert-butyl
groups on the peripheral carbazole group reduced the de-
tential of oxidation (E
) by assuming the absolute en-
ergy level of ferrocene was ꢀ4.8 eV below the vacuum
level. Moreover, we calculated the LUMO energy levels
from the HOMO energy levels and the absorption edges
[48–50]. Fig. 3 presents the CV traces of 9,10-diphenylan-
thracene (DPA), Cz9PhAnt, tCz9PhAnt, and tCz9Ph2Ant,
whereas Table 2 lists the band gap energies and LUMO
gree of intermolecular
to the lower FWHM of the PL emission band. In addition,
p–p
stacking of tCz9PhAnt, leading

Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
49
tCz9PhAnt, and tCz9Ph2Ant were ꢀ5.47, ꢀ5.48, and
ꢀ5.48 eV, respectively. The HOMO energy levels of carba-
zole-substituted anthracene derivatives are slightly higher
than those of 9,10-di(2-naphthyl)anthracene (ADN,
ꢀ5.8 eV) and 2-methyl-9,10-di(20-naphthyl)anthracene
(MADN, ꢀ5.5 eV) [24,33]. The incorporation of electron-
rich carbazole moieties onto the 9 and 10 positions of
anthracene enhances the HOMO energy levels of anthra-
cene derivatives [38]. The high HOMO energy levels of
carbazole-substituted anthracene derivatives facilitate
hole-injection from an ITO transparent anode to the
light-emitting layer [53]. As a result, we expected good
EL performances from OLEDs based on our carbazole-
substituted anthracene derivatives. The LUMO energy
levels of Cz9PhAnt, tCz9PhAnt, and tCz9Ph2Ant were
ꢀ2.52, ꢀ2.53, and ꢀ2.57 eV, respectively. The difference
between the LUMO energy levels of the anthracene deriv-
atives and TPBI (2.70 eV) was smaller than that between
the anthracene derivatives and Alq3 (3.30 eV) [20]. TPBI
exhibits superior electron-injection characteristics than
those of Alq3. Moreover, the difference between the HOMO
energy levels of the anthracene derivatives (5.48 eV) and
TPBI (6.20 eV) is larger than that between the anthracene
derivatives and Alq3 (5.70 eV). TPBI shows a better hole-
blocking characteristic than Alq3 [54]. Consequently,
superior EL performance could be realized if TPBI were to
be used as the ETL in OLEDs based on these anthracene
derivatives. The band gap energies of Cz9PhAnt, tCz9PhAnt,
and tCz9Ph2Ant were 2.95, 2.95, and 2.91 eV, respectively.
We suspect that the anthracene derivative tCz9Ph2Ant
had a lower band gap energy than Cz9PhAnt or tCz9PhAnt
because its carbazole moiety-based side groups featured
a greater conjugation length [35]. Moreover, we expected
good color purities for the blue emissions from OLEDs
fabricated from anthracene derivatives with such large
band gap energies.
DPA
Cz9PhAnt
t
Cz9PhAnt
t
Cz9Ph2Ant
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V vs. Ag/AgNO3)
Fig. 3. CV spectra of DPA, Cz9PhAnt, tCz9PhAnt, and tCz9Ph2Ant.
Table 2
Electrochemical properties of the anthracene derivatives.
Eox (V)a
HOMO (eV)
LUMO (eV)
Eogpt (eV)b
Compound
Cz9PhAnt
tCz9PhAnt
tCz9Ph2Ant
0.80
0.85
0.86
ꢀ5.47
ꢀ5.48
ꢀ5.48
ꢀ2.52
ꢀ2.53
ꢀ2.57
2.95
2.95
2.91
a
Onset oxidation potential versus Ag/AgNO3.
Calculated from the UV–vis absorption edge in dilute toluene
b
solution.
and HOMO energy levels. For DPA, we found an oxidation
peak and a reduction peak at about 1.25 and ꢀ1.25 V,
respectively. The redox peaks were attributed to the oxida-
tion/reduction of phenyl-substituted anthracene for DPA.
Workentin et al. reported that a radical cation formed dur-
ing the oxidation of DPA [51]. In addition, the carbazole-
substituted anthracene derivatives exhibited two quasi-
reversible oxidation peaks in the positive potential region.
We attribute the oxidation peak at a lower potential to the
oxidation of anthracene-linked carbazole unit. This is be-
cause the carbazole moiety is an electron-rich group [52].
Moreover, the oxidation peak of DPA showed up at a higher
potential than the first oxidation peak of carbazole-substi-
tuted anthracene derivatives. Yet, the oxidation potential
of the second oxidation peak was slightly higher than that
of the oxidation peak of DPA. Therefore, we assigned the
second oxidation peak, occurring at a more positive poten-
tial, to oxidation of the other carbazole unit in the anthra-
cene derivative. This is because the second oxidation
potential of carbazole-substituted anthracene would be
enhanced as the first oxidation of carbazole moiety had
been occurred. Two oxidation peaks were not assigned to
the oxidation of two carbazole units and anthracene,
respectively. This is because the current density ratio of
two oxidation peaks was not 2:1. On the other hand, only
one reduction peak was observed at about ꢀ1.25 V for
the carbazole-substituted anthracene derivatives. The
presence of the reduction peak in CV indicates that such
anthracene derivatives exhibited the electron-accepting
capacity, and could transport electrons when used as the
emitter layer. The HOMO energy levels of Cz9PhAnt,
3.4. EL properties of devices based on carbazole-substituted
anthracene derivatives
According to their photophysical and electrochemical
properties, the fluorophores Cz9PhAnt, tCz9PhAnt, and
tCz9Ph2Ant appeared to be promising blue-emitting mate-
rials for OLED applications. To determine the EL properties
of these fluorophores, we fabricated non-doped devices
possessing double- and triple-layer configurations. In the
double-layer devices (type-A devices; devices A-I, A-II,
and A-III), the anthracene derivatives functioned as both
the HTL and the blue emitting layer and TPBI functioned
as both the hole-blocking material and the ETL. In the tri-
ple-layer devices (type-B devices; devices B-I, B-II, and B-
III), NPB and the anthracene derivatives functioned as the
HTL and blue emitting layer, respectively, while TPBI func-
tioned as both the hole-blocking material and the ETL.
Fig. 4 presents the configurations and energy levels of the
double- and triple-layer devices, revealing that the OLEDs
based on the anthracene derivatives possessed good
matching of the energy levels at the interfaces between
the HTL and the blue emitting layer and between the blue
emitting layer and the TPBI layer.

50
Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
(a)
10000
1000
100
10
800
Cz9PhAnt
tCz9PhAnt
tCz9Ph2Ant
600
400
200
Fig. 4. Energy levels of the multilayer devices incorporating the fluoro-
phores Cz9PhAnt, tCz9PhAnt, and tCz9Ph2Ant as emitting layers, NPB as
the hole transporting layer, TPBI as the electron-transporting layer, and
LiF as the buffer layer.
0
1
0
2
4
6
8
10
12
14
16
Voltage (V)
2.5
2.0
1.5
1.0
0.5
0.0
(b)
1400
(a)
Cz9PhAnt
tCz9PhAnt
tCz9Ph2Ant
Cz9PhAnt
1000
1200
tCz9PhAnt
tCz9Ph2Ant
1000
100
10
1
800
600
400
200
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
10
12
14
16
Voltage (V)
Voltage (V)
0.5
0.4
0.3
0.2
0.1
0.0
Fig. 6. EL performances of Cz9PhAnt-, tCz9PhAnt-, and tCz9Ph2Ant-based
type-B devices.
(b)
Cz9PhAnt
tCz9PhAnt
tCz9Ph2Ant
voltages of the Cz9PhAnt-, tCz9PhAnt-, and tCz9Ph2Ant-
based devices were 2.0, 5.0, and 5.5 eV, respectively. The
tCz9Ph2Ant-based device (Device B-III) exhibited a maxi-
mum current efficiency of 2.10 cd/A and a maximum
brightness of 1576 cd/m2; for the tCz9PhAnt-based device
(Device B-II), these values were 1.98 cd/A and 6821 cd/
m2, respectively; for the Cz9PhAnt-based device (Device
B-I), they were 0.42 cd/A and 3042 cd/m2, respectively.
The Cz9PhAnt-based device exhibited the lowest turn-on
voltage among the three type-B devices. The presence of
the tert-butyl groups reduced the charge transporting
capacities of tCz9PhAnt and tCz9Ph2Ant and resulted in
higher turn-on voltages for their devices [23]. Moreover,
the high value of Ufl and high coplanarity of Cz9PhAnt
led to the high current density and high brightness of the
Cz9PhAnt-based device. Nevertheless, the high coplanarity
of Cz9PhAnt in thin film state resulted in EL quenching in
the device. As a result, the lowest current efficiency was
that of the Cz9PhAnt-based OLED. In addition, the
tCz9PhAnt-based devices exhibited higher brightnesses
and larger current efficiencies relative to those of the
Cz9PhAnt-based device. The presence of tert-butyl groups
on the carbazole moiety inhibited the aggregation and
2
4
6
8
10
12
14
16
18
20
Voltage (V)
Fig. 5. EL performances of Cz9PhAnt-, tCz9PhAnt-, and tCz9Ph2Ant-based
type-A devices.
Figs. 5 and 6 present the EL properties of the OLEDs
based on the carbazole-substituted anthracene deriva-
tives; Table 3 summarizes the key EL performance data
of the non-doped devices. The EL performance of the tri-
ple-layer devices (type-B devices) was superior to that of
the double-layer devices (type-A devices). The holes could
be injected into the blue emitter layer more efficiently
through the HTL (NPB layer) for the type-B devices. Never-
theless, the effect of the carbazole moieties on the EL per-
formance of the type-A devices was the same as that for
the type-B devices. For the type-B devices, the turn-on
p–
p⁄ stacking of the anthracene derivatives, thereby
improving the EL performance of the OLEDs. As a result,

Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
51
Table 3
EL properties of OLEDs incorporating the anthracene derivatives.
Von (V)a
Max. luminance (cd/m2)
Max. efficiency (cd/A)
CIE (x, y)b
EL
max
Compound
Device type
FWHM of EL at 10 V (nm)
k
(nm)
Cz9PhAnt
Cz9PhAnt
tCz9PhAnt
tCz9PhAnt
tCz9Ph2Ant
tCz9Ph2Ant
Device A-Ic
Device B-Id
Device A-II
Device B-II
Device A-III
Device B-III
440
446
440
446
446
452
61
58
56
58
52
62
6.0
2.5
6.5
5.0
9.0
5.5
1030
3042
1120
6821
714
0.08
0.42
0.27
1.98
0.45
2.10
(0.18, 0.15)
(0.16, 0.11)
(0.19, 0.14)
(0.16, 0.12)
(0.20, 0.17)
(0.17, 0.16)
1576
a
b
c
Turn-on voltage at 1 cd/m2.
Commission Internationale de L’Eclairage coordinates; under an applied voltage of 10 V.
Device A: ITO/EML (70 nm)/TPBI (30 nm)/LiF/Al.
d
Device B: ITO/NPB (40 nm)/EML (30 nm)/TPBI (30 nm)/LiF/Al.
the current efficiency of the tCz9PhAnt-based device was
much higher than that of the Cz9PhAnt-based device. On
the other hand, the maximum current efficiency of the
tCz9Ph2Ant-based device was greater than that of the
tCz9PhAnt device, while the current density and brightness
followed the opposite trend. We attribute the higher cur-
rent efficiency of the tCz9Ph2Ant-based device to the great-
er conjugation of the biphenyl group linking the carbazole
moiety to the anthracene derivative. The EL performances
were, however, not only affected by the degree of conjuga-
tion of the molecule but also by the conformation and
packing density of the emitter molecules in thin film state
[23,25]. The lower current density and lower brightness of
the tCz9Ph2Ant-based device might have been due to
poorer packing of these molecules. In addition, the pres-
ence of bipolar structure would bring about better charge
balance of electron and hole, and further resulted in the
higher EL performances for these carbazole-attached
anthracene derivatives based devices [55,56].
(a)
Cz9PhAnt
Cz9PhAnt
Cz9Ph2Ant
1.0
0.8
0.6
0.4
0.2
0.0
t
t
380 400 420 440 460 480 500 520 540 560 580 600
Wavelength (nm)
Cz9PhAnt
(b) 1.0
t
t
Cz9PhAnt
Fig. 7 presents the EL spectra of the type-A (Devices A-I,
A-II, and A-III) and type-B (Devices B-I, B-II, and B-III) de-
Cz9Ph2Ant
0.8
0.6
0.4
0.2
0.0
vices. Table 3 summarizes the maximum emission wave-
EL
max
length of the EL (k ), the FWHM of the EL signal, and
the CIE coordinates of the devices based on these carba-
zole-substituted anthracene derivatives. We observed blue
EL
max
light with a value of k
near 440–446 nm for the type-A
devices. The values of FWHM for our three type-A devices
ranged from 52 to 61 nm. These EL spectra are quite simi-
lar to the PL spectra of the carbazole-substituted anthra-
cene derivatives in thin film state, apart from the FWHMs
of device A-III. For compound tCz9Ph2Ant, the FWHM
(63 nm) of PL emission band was larger than that
(52 nm) of EL emission band. The deviation of EL from its
PL might be attributed to the electric-field or micro-cavity
effects on the EL emission of tCz9Ph2Ant-based device
(Device A-III) [12,57]. Thus, the EL emissions arose pre-
380 400 420 440 460 480 500 520 540 560 580 600
Wavelength (nm)
Fig. 7. EL spectra of Cz9PhAnt-, tCz9PhAnt-, and tCz9Ph2Ant-based devices
under an applied voltage of 10 V (a) type-A devices; (b) type-B devices.
dominantly from the carbazole-substituted anthracene
EL
of the PL peaks of the anthracene derivatives, suggesting
that the EL spectra of the OLEDs were affected by not only
the chemical structure of blue emitter but also the thick-
ness of the light emitting layer and the nature of the HTL
and ETL materials [12]. For the type-A devices, the CIE
color coordinates of Cz9PhAnt-, tCz9PhAnt-, and
tCz9Ph2Ant-based devices were (0.18, 0.15), (0.19, 0.14),
and (0.20, 0.17), respectively. The CIE color coordinates of
the Cz9PhAnt-, tCz9PhAnt-, and tCz9Ph2Ant-based type-B
derivatives. Notably, the values of k
of the type-B
max
devices were blue-shifted by 6 nm relative to those of the
type-A devices. The influence of the chemical structure
EL
on the values of k
of the type-B devices was consistent
max
PL
with the variations in the values of k
of the anthracene
max
derivatives. The FWHMs of the type-B devices differed
slightly from those of the type-A devices. In this case, the
effect of the chemical structure on the FWHM of the EL
peaks was not consistent with the variation of the FWHMs

52
Y.-H. Chen et al. / Organic Electronics 13 (2012) 43–52
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devices were (0.16, 0.11), (0.16, 0.12), and (0.17, 0.16),
respectively – quite close to those of the standard blue
emission (0.15, 0.15). Thus, the color purities of the type-
B devices were higher than those of the type-A devices.
[20] K. Danel, T.H. Huang, J.T. Lin, Y.T. Tao, C.H. Chuen, Chem. Mater. 14
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4. Conclusion
We have synthesized and characterized three blue-
emitting anthracene derivatives – Cz9PhAnt, tCz9PhAnt,
and tCz9Ph2Ant – featuring carbazole-substituted side-
groups. The presence of 3,6-di-tert-butyl-9H-carbazole
moieties prevented molecular aggregation of the anthra-
cene derivatives, resulting in amorphous solid state struc-
tures. These anthracene derivatives exhibit high Tgs and,
therefore, good thermal stability. Moreover, Cz9PhAnt,
tCz9PhAnt, and tCz9Ph2Ant exhibit strong blue emissions
in the form of thin films. From OLEDs fabricated using the
carbazole-substituted anthracene derivatives as blue emit-
ting layers, the brightnesses and current efficiencies for the
tCz9PhAnt- and tCz9Ph2Ant-based devices were greater
than those of the Cz9PhAnt-based device. The presence of
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p⁄ stacking of the anthracene deriv-
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tCz9Ph2Ant-based OLEDs displayed high color purity. The
EL performances of the devices based on these anthracene
derivatives were strongly dependent on the chemical struc-
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anthracene derivatives bearing carbazole moieties are
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