Effect of methyl substituents on the N-diaryl rings of anthracene-9,10- diamine derivatives for OLEDs applications

Article abstract of DOI:10.1016/j.orgel.2011.01.020

A series of N-diaryl-anthracene-9,10-diamine derivatives with methyl substituents at meta or para position of N-diaryl rings were synthesized and used as dopants for Organic Light-Emitting Devices (OLED). The effects of substituted methyl substituents hav

Full text of DOI:10.1016/j.orgel.2011.01.020

Organic Electronics 12 (2011) 694–702
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Effect of methyl substituents on the N-diaryl rings of
anthracene-9,10-diamine derivatives for OLEDs applications
Yuan-Hsiang Yu a, , Chien-Hsun Huang b,d, Jui-Ming Yeh b, Ping-Tsung Huang c
⇑
a Department of Innovations in Digital Living, Lan Yang Institute of Technology, Yilan County 261, Taiwan
b Department of Chemistry, Center for Nanotechnology at CYCU and R & D Center for Membrane Technology, Chung-Yuan Christian University,
Chung Li 32023, Taiwan
c Department of Chemistry, Fu Jen Catholic University, Hsinchuang, Taipei County 24205, Taiwan
d Department of Intelligent Materials Technology, Plastics Industry Development Center, Taichung, 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 N-diaryl-anthracene-9,10-diamine derivatives with methyl substituents at meta
or para position of N-diaryl rings were synthesized and used as dopants for Organic Light-
Emitting Devices (OLED). The effects of substituted methyl substituents have been com-
pared based on both materials and solid device evaluation. The performance of solid state
devices in the configuration of ITO/N,N0-bis(naphthalene-1-yl)-N,N0-bis(phenyl)benzidine
(NPB) (40 nm)/3% dopant: 9,10-di(2-naphthyl)anthracene (AND) (40 nm)/tris(8-hydroxy-
quinoline)aluminum (Alq3) (20 nm)/LiF (1.5 nm)/Al (200 nm) have been studied. As com-
pared to the non-substituted dopant N,N,N0,N0-tetraphenyl-anthracene-9,10-diamine,
current efficiency was promoted to approximately 2.9, 6.33, and 7.94 cd/A, and power effi-
ciency was improved by 0.74, 2.26 and 3.02 lm/W, for dopants with the substituted methyl
groups at meta, para and both meta/para positions, respectively. The device, prepared by
using the dimethyl-substituted dopant at meta/para position, showed the best performance
when the luminance of the device reached 24,991 cd/m2 with CIE (x, y) = (0.38, 0.59), and
the efficiency of the device reached 24.99 cd/A and 7.48 lm/W at a driving current of
100 mA/cm2. As compared with a device made of a coumarin (C545T)/Alq3-based emitting
layer, the dimethyl-substituted dopant/ADN-based device developed in this study, exhib-
ited great improvement in device performance.
Received 19 November 2010
Received in revised form 10 January 2011
Accepted 25 January 2011
Available online 23 February 2011
Keywords:
Organic electronics
OLED
Anthracene derivatives
Dopant
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
potential for fluorescent materials [6–8]. A variety of bulky
functional groups have been designed to prevent the inter-
Since the first discovery of organic light-emitting de-
vices (OLEDs) by Tang and co-workers [1], materials and
devices of OLEDs have drawn much attention due to their
potential application in flat panel display [2,3]. Recently,
organic light-emitting diodes based on guest/host emitters
were found to give extremely excellent high luminance as
well as current and power efficiency [4,5]. OLEDs using
anthracene derivatives as guest/host emitters have been
reported to exhibit very excellent performance and have
molecular stacking of anthrancene to increase fluorescent
quantum yield, device lifetime and color purity [9,10]. In
this study, we fabricate the OLEDs based on a guest/host
system utilizing different anthracene-9,10-diamine deriva-
tives as dopants, which were incorporated with strategi-
cally placed ‘‘methyl’’ steric groups in the structure.
The methyl groups were introduced to the aryl ring of
N-aryl-substituted anthracene-9,10-diamine derivatives
to prevent molecular aggregation of the dopant emitters
and reduce self-quenching through steric hindrance of
the molecular structure. In addition, the constructed
OLEDs were based on a wide band gap host material
⇑
Corresponding author. Tel.: +886 3 9771997x231.
E-mail address: yuyh@mail.fit.edu.tw (Y.-H. Yu).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.01.020

Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
695
9,10-di(2-naphthyl)anthracene (AND) [11], which is de-
glass subtrates with surface resistance 15
bought from Ritek Co., Ltd.
X
/h were
signed to be used as a host material because it also has
the anthracene structure identical to the dopants. Here,
both the host and guest contain the anthracene moieties
which are supposed to increase the compatibility between
guest and host as to have well-dispersed dopants in the
host matrix, and lead to an increase in the emission
efficiency.
The synthesized compounds were characterized by Nu-
clear Magnetic Resonance (NMR) and Mass spectrometer.
NMR measurements were made using Bruker AMX 400
and 500 NMR spectrometer. Mass spectra were obtained
with a MAT-95XL HRMS mass spectrometer. The UV–visi-
ble spectra were recorded using a Perkin Elmer Lambra
35 UV–visible spectrophotometer. Fluorescence spectra
were recorded using a Hitachi F-4500 fluorescence spec-
trophotometer. Photo-electron spectra were obtained by
a Riken Keiki Photo-electron Spectrometer model AC-2.
Quantum yields were recorded using an integrating sphere
The dopants (with sample code) for OLEDs fabrication
were N,N,N0,N0-tetraphenyl-anthracene-9,10-diamine (TAD),
N,N,N0,N0-tetra-m-tolyl- anthracene-9,10-diamine (TmAD),
N,N,N0,N0-tetra-p-tolyl-anthracene-9,10-diamine
(TpAD)
and N,N,N0,N0-tetrakis-(3,4-dimethyl-phenyl)-anthracene-
9,10-diamine (TmpAD). Among these compounds, the
physical properties and device performances of TAD and
TpAD were already reported as the host emitters [12].
Here, we present the first reported case in which the N-dia-
ryl amino anthracenes were used as green dopant in ADN
host system for OLED devices. We are reporting on the syn-
thesis, characterization, fabrication and performance of
OLED devices that incorporate the four dopant molecules
described above in the emitting layer. The effects of substi-
tuted methyl group at different positions of aromatic N-
aryl-substituted anthracene-9,10-diamine have been com-
pared based on both materials and solid device evaluation.
Moreover, the device performance of the dimethyl-substi-
tuted dopants/AND-based devices was also compared to a
general guest/host system based on coumarin (C545T) as
dopant and Alq3 as host [13]. The improvement effect of
guest/host both based on anthracence derivatives could
be observed in this study.
and
a HAMAMATSU photonic multi-channel analyzer
C10027. All data were averaged from at least five raw data
to ensure statistical significance. Device properties were
monitored using a Keithley 2400 power supply with a
400 nm cut-off filter. An optical meter of model PR705
was used in this study. All of the products were purified
by sublimation method with a temperature programmed
condition using an upper opened sublimation system
made by ULVAC. The high vacuum evaporation system
was made by Branchy Technology Co., Ltd. for organic thin
film deposition. Thin films were prepared by vacuum evap-
oration at a base pressure of 2 ꢀ 10ꢁ5 Torr or lower. The
film thickness was measured with a calibrated quartz-
crystal microbalance during deposition.
2.2. General procedure for the synthesis of diaryl-anthracene-
9,10-diamine derivatives
The structure of as-synthesized diaryl-anthracene-9,10-
diamine derivatives are illustrated in Fig. 1 with compound
code TAD, TmAD, TpAD and TmpAD, respectively. The dia-
ryl-anthracene-9,10-diamine derivatives were prepared
in high yields, by palladium-catalyzed C–N cross-coupling
[12,14–16] of the 9,10-Dibromoanthracence with the
amine (Diphenylamine, Di-m-tolyl-amine, Di-p-tolyl-
amine or Bis-(3,4-dimethyl-phenyl)-amine). 9,10-Dib-
romoanthracence (1 equiv.), Pd(OAc)2 or Pd2(dba)3
(1 mol%), t-BuONa (2.5 equiv.), the amine (Diphenylamine,
Di-m-tolyl-amine, Di-p-tolyl-amine or Bis-(3,4-dimethyl-
phenyl)-amine) (2.4 equiv.) were weighted into a 500 ml
double-neck round-bottom flask connected with a con-
denser under nitrogen. Dry toluene (200 mL) was then
added under nitrogen with stirring to give a homogeneous
mixture. A solution containing P(t-Bu)3 (5 mol%) and dry
toluene (3.4 mol%) was then added to the mixture and re-
acted under nitrogen for some hours at 130 °C. The end-
point of reaction was monitored by thin-layer
chromatography. After complete consumption of starting
materials, the resulting solution was cooled to room tem-
perature; and n-hexane was added to precipitate the prod-
ucts. The products were collected by filtering and then
dried under vacuum for overnight at 100 °C. The crude
products were purified by sublimation method under the
programmated condition (e.g. 280 °C for 90 min, 300 °C
for 90 min, 320 °C for 90 min and then 100 °C). The de-
tailed synthetic procedures of compounds TAD, TmAD,
TpAD and TmpAD were described as follows:
2. Experimental
2.1. Materials and Instrumentations
Sodium tert-butoxide (t-BuONa, Merck), Tri-tert-butyl-
posphine (P(t-Bu)3, Strem), Palladium(II) acetate
(Pd(OAc)2, Strem), Tris(dibenzylideneacetine)dipalladium
(Pd2(dba)3, Strem), 9,10-Dibromoanthracene (98%, Alfa),
Diphenylamine (98%, Alfa), Di-m-tolyl-amine (98%, Alfa),
Di-p-tolyl-amine (97%, Adrich), 4-Bromo-o-xylene (98%,
Alfa), Benzylamine (99%, Acros), Palladium (10% on carbon,
Alfa), Anthracene (98%, Acros), Ethanol (Merck), n-Hexane
(Merck), Methanol (Merck) were used as received without
further purification. Tetrahydrofuran (THF, Merck), Tolu-
ene (SHOWA), Dichloromethane (CH2Cl2, Merck) were
dried and distilled under standardard procedure (THF and
Toluene dried using Na/Ph2CO; CH2Cl2 dried using CaH2)
and stored under nitrogen. For device fabrication, 9,10-
di(2-naphthyl)anthracene (ADN), copper phthalocyanine
(CuPc), N,N0-bis(naphthalene-1-yl)-N,N0-bis(phenyl)benzi-
dine
(NPB),
2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-
[9,9a,1gh]
1H,5H,11H-10(2-benzothiazolyl)quinolizine-
coumarin (C545T), tris(8-hydroxyquinoline)aluminum
(Alq3) were received from a commercial source (Summit-
Tech Co., Ltd.) and purified by a temperature programmat-
ed sublimation system. Tungsten boat (Summit-Tech Co.,
Ltd.), Alumina (Alfa) and LiF (Alfa) were used as received
from commercial sources. Indium tin oxide (ITO)-coated

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Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
(a)
(b)
Fig. 1. (a) Schematic OLED device setup. (b) Molecular structure of as-synthesized dopants and relative energy diagram of materials used in all the devices.
2.2.1. N,N,N0,N0-tetraphenyl-anthracene-9,10-diamine (TAD)
From 9,10-Dibromoanthracence (10 g, 29.8 mmol) and
Diphenylamine (12.09 g, 71.4 mmol), compound TAD was
obtained as a yellow solid. The crude product was purified
twice by sublimation to afford compound TAD in 78% yield.
1H NMR (400 MHz, CDCl3): d = 6.89–6.92 (t, J = 3.2 Hz,
J = 7.2 Hz, 4 H), 7.10–7.12 (d, J = 4.4 Hz, 8 H), 7.18–7.22 (t,
J = 7.2 Hz, J = 8.4 Hz, 8 H), 7.33–7.36 (q, J = 3.2 Hz, 4 H),
8.17–8.20 (q, J = 3.2 Hz, 4 H); 13C NMR (100 MHz, CDCl3):
d = 120.3, 121.4, 125.1, 126.8, 129.3, 131.8, 137.4, 147.7;
EI MS (m/e): 512 (M+).
was obtained as a yellow solid. The crude product was
purified twice by sublimation to afford compound TmAD
in 79% yield. 1H NMR (400 MHz, CDCl3): d = 2.19 (s, 12
H), 6.70–6.72 (d, J = 7.2 Hz, 4 H), 6.86–6.88 (d, J = 9.84 Hz,
4 H), 6.94 (s, 4 H), 7.04–7.08 (t, J = 8.0 Hz, J = 7.6 Hz, 4 H),
7.33–7.34 (q, J = 3.2 Hz, 4 H), 8.17–8.17 (q, J = 3.2 Hz, 4
H); 13C NMR (100 MHz, CDCl3): d = 21.6, 117.7, 120.9,
122.1, 125.1, 126.6, 129.0, 131.9, 137.5, 138.9, 147.9; EI
MS (m/e): 568 (M+).
2.2.3. N,N,N0,N0-tetra-p-tolyl-anthracene-9,10-diamine (Tp
AD)
From 9,10-Dibromoanthracence (5 g, 14.9 mmol) and
Di-p-tolyl-amine (7.05 g, 35.7 mmol), compound TpAD
was obtained as a yellow solid. The crude product was
purified twice by sublimation to afford compound TpAD
2.2.2. N,N,N0,N0-tetra-m-tolyl-anthracene-9,10-diamine (Tm
AD)
From 9,10-Dibromoanthracence (5 g, 14.9 mmol) and
Di-m-tolyl-amine (7.05 g, 35.7 mmol), compound TmAD

Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
697
in 82% yield. 1H NMR (500 MHz, CDCl3): d = 2.24 (s, 12 H),
6.98–6.99 (d, J = 1.2 Hz, 16 H), 7.30–7.34 (m, 4H), 8.16–
8.19 (m, 4H); 13C NMR (125 MHz, CDCl3): d = 20.6, 120.1,
121.1, 126.6, 129.8, 130.3, 131.9, 137.5, 145.6; EI MS (m/
e): 568 (M+).
lower. The deposition rate was 1 Å/s for all organic films.
The LiF film, with thickness of 15 Å, was evaporated at a
rate of 0.1 Å/s. The Al cathode, with thickness of 200 nm,
was deposited at a rate of 5 Å/s.
2.2.4. N,N,N0,N0-tetrakis-(3,4-dimethyl-phenyl)-anthracene-
9,10-diamine (TmpAD)
3. Results and discussion
For the synthesis of TmpAD, the Bis-(3,4-dimethyl-phe-
nyl)-amine was first synthesized as the following proce-
dure: 4-Bromo-o-xylene (2 g, 10.8 mmol), Benzylamine
(2.54 g, 23.7 mmol), Pd2(dba)3 (0.1 g, 1 mol%), t-BuONa
(2.4 g, 2.3 equiv.) were weighted into a 250 ml double-
neck round-bottom flask connected with a condenser un-
der nitrogen. About 50 mL of toluene was added and then
stirred to give a homogeneous mixture. A solution of
5 mol% P(t-Bu)3 in toluene (2.6 mL) was then added to
the mixture and reacted at 130 °C. After overnight, the
resulting solution was cooled to room temperature, and
the solution was evaporated to dryness to afford a powder,
which was adsorbed onto 4 g of silica gel and chromato-
graphed with hexane to give 2 g (yield: 80%) of Benzyl-
bis-(3,4-dimethyl-phenyl)-amine as a red-brown liquid.
The as-prepared Benzyl-bis-(3,4-dimethyl-phenyl)-amine
was further reduced to form Bis-(3,4-dimethyl-phenyl)-
amine. A mixed solvent of ethanol and THF was added to
a 100 mL double-neck round-bottom flask which con-
3.1. Photo-physics studies
The photo-physics properties of as-synthesized methyl
groups substituted N-aryl anthracene-9,10-diamine deriv-
atives TAD, TmAD, TpAD and TmpAD were studied by UV–
visible and fluorescence spectrophotometer. The absorp-
tion and emission spectra as illustrated in Fig. 2a and Table
1 show that these compounds in form of solid film have
major absorption bands with wavelength maxima (kmax
)
within 258–268, 296–297, 360–380 and 470–490 nm.
The potoluminence (PL) emission band has kmax located
around 519–532 nm. As a result, compound TAD and
TmAD have similar emission spectra with kmax ꢂ 520 nm
in solid state, whereas there are obvious bathochromic
shift observed at kmax ꢂ 530 nm for compound TpAD and
TmpAD. The compound TmpAD, with methyl groups
substituted in both para/meta position, also has similar
emission spectra to that of TpAD. The result indicates the
methyl group substituted at the para position of aromatic
phenyl diamine has a greater electronic effect than meta
position on the anthracene moiety. There is a strong sol-
vent effect for these anthracene materials as illustrated
in Fig. 2b and Table 1. Basically the emission color of these
compounds exhibit a green color in non-polar solvents, i.e.,
toluene, and red shift to yellow-green color in polar sol-
vents, i.e., CH2Cl2, for compound with methyl substuents.
The solvent effect indicates there is an observable dipole
moment between phenyl diamine and anthracene moie-
ties. The dipole moment is strongly dependent on the posi-
tion of methyl groups at the phenyl diamine, especially for
the compound TpAD and TmpAD. The effects of methyl
substituents for the comparison of kem in solvent phase
showed the following sequence of TmpAD, TpAD and then
TmAD.
tained
Benzyl-bis-(3,4-dimethyl-phenyl)-amine
(2 g,
6.34 mmol) and Pd/C (0.025 g, 2 mol%) and stirred under
nitrogen. After the starting materials had dissolved, hydro-
gen gas was then bubbled into the solution and further re-
acted overnight. The reaction was monitored by thin-layer
chromatography. After the reaction had completed, the
resulting brown solution was evaporated to dryness to af-
ford 0.83 g (yield: 58%) of Bis-(3,4-dimethyl-phenyl)-
amine as a brown solid which was used as the starting
material for the synthesis of compound TmpAD. Compound
Benzyl-bis-(3,4-dimethyl-phenyl)-amine was identified by
1H NMR (400 MHz, CDCl3): d = 2.08–2.19 (m, 12 H), 4.30 (s,
2 H), 6.40–6.43 (q, J = 2.4 Hz, 2H), 6.49–6.50 (d, J = 2.0 Hz,
2H), 6.93–6.95 (d, J = 80 Hz, 2H), 7.25–7.38 (m, 5H). Com-
pound bis-(3,4-dimethyl-phenyl)-amine was identified by
1H NMR (400 MHz, CDCl3): d = 2.19–2.20 (d, 12 H), 5.43
(s, 1 H), 6.78–6.82 (m, 4 H), 6.99–7.01 (d, 2H).
From 9,10-Dibromoanthracence (5 g, 14.9 mmol) and
Bis-(3,4-dimethyl-phenyl)-amine (8.05 g, 35.7 mmol),
compound TmpAD was obtained as an orange solid. The
crude product was purified twice by sublimation to afford
compound TmpAD in 81% yield. 1H NMR (400 MHz, CDCl3):
d = 2.11 (s, 12 H), 2.15 (s, 12H), 6.77–6.80 (q, J = 2.4 Hz, 4
H), 6.91–6.93 (m, 8 H), 7.31–7.33 (q, J = 3.2 Hz, 4 H),
8.17–8.19 (q, J = 3.2 Hz, 4 H); 13C NMR (100 MHz, CDCl3):
d = 18.9, 20.1, 117.8, 121.4, 125.2, 126.4, 129.0, 130.2,
132.0, 137.2, 137.5, 145.96; EI MS (m/e): 624 (M+).
A photoelectron spectrometer was used to evaluate
the energy of the highest occupied molecular orbital
(HOMO). The energy gap (Eg) was calculated by the for-
mula: Eg (eV) = hc/konset = ꢂ1241/konset
,
where
h
is
Planck’s constant and c is speed of light. The wavelength
onset, konset, was obtained by absorption spectra as illus-
trated in Table 1. The lowest unoccupied molecular orbi-
tal (LUMO) was derivated from the values of HOMO and
Eg, and the data are listed in Table 1. The relative energy
diagram of materials used for devices fabrication is com-
pared in Fig. 1 The compound TmpAD has relatively the
smallest value of HOMO, i.e. 5.37 eV, due to its higher
donor ability than others. The delocalization degree of
anthracene relatively decreased along with the donor
ability of phenyl diamine moiety, and the energy band
gaps become smaller. The energy band gaps follow the
2.3. OLED fabrication
Indium tin oxide (ITO)-coated glass substrates were
thoroughly cleaned and treated with oxygen plasma.
OLEDs were fabricated by using a multiple source thermal
evaporation system at a base pressure of 2 ꢀ 10ꢁ5 Torr or
sequence:
compound
TAD > TmAD > TpAD > TmpAD,
where the Eg values are 2.46, 2.44, 2.39 and 2.36 eV,
respectively.

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Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
Fig. 2. (a) The absorption and PL spectra of TAD, TmAD, TpAD, and TmpAD in solid film. (b) Solvent effect studies of PL spectra for compound TAD, TmAD,
TpAD and TmpAD in toluene and CH2Cl2.
Table 1
Relationship of the as-prepared anthracene-9,10-diamine derivatives TAD, TmAD, TpAD, and TmpAD with the kabs, konset, kem, Q.Y., HOMO, LUMO and Band Gap
measured by UV–vis spectrophotometer, fluorescence spectrophotometer, photoelectron spectrometer.
Sample code
kabs (nm) Film
konset (nm) Film
kem (nm)
Q.Y.a
HOMO (eV)
LUMO (eV)
Band Gap (eV)
Film
Toluene
CH2Cl2
TAD
259, 297, 360, 379, 480
258, 296, 360, 379, 469
258, 296, 360, 379, 486
268, 296, 360, 379, 486
504
509
519
526
520
519
531
532
510
521
530
535
524
530
546
553
0.95
0.92
0.94
0.82
5.57
5.49
5.45
5.37
3.11
3.05
3.06
3.01
2.46
2.44
2.39
2.36
TmAD
TpAD
TmpAD
a
Measured in a CH2Cl2 solution by using a HAMAMATSU Photonic multi-channel analyzer C10027.
3.2. Devices fabrication
with devices fabricated as follows: ITO/CuPc (15 nm)/NPB
(40 nm)/TpAD:AND (40 nm)/Alq3 (20 nm)/LiF (1.5 nm)/Al
(200 nm), where the doping concentration of TpAD was
changed between 1 and 4 mol%. It can be seen that the
luminance efficiency and brightness of devices were rela-
tively increased by changing the concentration from 1 to
4 mol% as the curves show in Fig. 3. The results show that
The as-prepared anthrancene-9,10-diamine materials
TAD, TmAD, TpAD, and TmpAD were used as dopants of
the emitting layer for the application of organic electrolu-
minescent devices. Before device fabrication, compound
TpAD was used as a dopant for doping concentration tests

Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
699
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
(a)
24
22
20
18
16
14
12
10
1%
2%
3%
4%
0
50
100
150
200
250
300
350
400
450
500
Current Density (mA/cm2)
(b)
Fig. 3. The doping concentration test with devices fabricated as: ITO/CuPc (15 nm)/NPB (40 nm)/TpAD:AND (40 nm)/Alq3 (20 nm)/LiF (1.5 nm)/Al (200 nm),
where the concentration of TpAD is changed between 1 and 4 mol%.
the devices have good luminance efficiency below a cur-
rent density of 100 mA/cm2. As the current density in-
creased, the optimized concentration of TpAD doping in
AND could be compared and selected, where the doping
concentration of 3 mol% was the optimum value for con-
sidering the driving voltage, luminance efficiency and
brightness over a wide range of current density. The result
could be observed from the J–V curves for the doping con-
centration test as shown in Fig. 3b, in which 3 mol% of dop-
ant shows relatively lower driving voltage, when compared
at the same current density. Therefore, in this study, a ser-
ies of solid state devices were fabricated by using 3 mol%
dopant for the comparison in the effects of methyl substit-
uents on the dopants.
The structure of devices using the as-prepared anthra-
cene-9,10-diamine derivatives as dopants was designed
and fabricated as ITO/NPB (40 nm)/3% dopant:AND
(40 nm)/Alq3 (20 nm)/LiF (1.5 nm)/Al (200 nm) and sche-
matic shown as Fig. 1 Herein, NPB was used as the hole
transport layer (HTL). The wide band gap material of
AND, which has a high energy band gap of 3.02 eV, was
used as the host material because it might restrict the elec-
tron and hole in the emitting layer and was expected to
improve the efficiency of emission during recombination.
Moreover, the same structure of anthracene moieties for
both dopant and the host AND was expected to have more
compatible properties and be suitable as a good guest/host
combination for the emitting layer. The electron transport
layer (ETL) was tris(8-hydroxyquinoline)aluminum (Alq3).
3.3. Devices properties studies
A comparative device based on a general guest/host sys-
tem with coumarin (C545T)/Alq3-based emitting layer was
fabricated and named as device 1, which has the structure
of ITO/NPB (40 nm)/2% C545T:Alq3 (40 nm)/Alq3 (20 nm)/
LiF (1.5 nm)/Al (200 nm). The devices based on the as-pre-
pared compound TAD, TmpAD, TmAD, and TpAD were fab-
ricated and named as device 2, device 3, device 4 and
device 5, respectively. The device properties are summa-
rized in Table 2. As the results show, device 3 exhibits
the best performance with maximal external quantum

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Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
efficiency (E.Q.E.) reached to 8.2%. The E.Q.E. values of de-
vices 1–5, which evaluated at 100 mA/cm2, follow the se-
quence: device 3 > device 5 > device 4 > device 2 > device
1, where the E.Q.E. values are 6.91%, 6.37%, 5.40%, 5.33%
and 3.52%, respectively. The effects of substituted methyl
group at both para/meta positions of aromatic N-aryl-
methyl-substituted 9,10-diaminoanthracene by fabricating
the device using the TmpAD/AND based guest/host system
have been observed with the power efficiency of 9.24 lm/
W and current efficiency of 28.17 cd/A, evaluated at lumi-
nance of 10,000 cd/m2. All of the devices have color coordi-
nates located at the green region in the Commission
internationale de l’éclairage (CIE) chromaticity diagram.
The electroluminescent spectra of devices 1–5 are shown
in Fig. 4. Evidently, the trends of emission for these devices
are similar to that of solid films as shown in Fig. 2a. For the
devices 1–5, the relationships of luminance (B) versus cur-
rent density (J) are shown in Fig. 5. As shown in B–J curves,
the brightness of devices using the dopants with methyl
groups at both para/meta or para positions of N-aryl-
methyl-substituted 9,10-diaminoanthracene, i.e., the de-
vices 3 and 5, were better than that at meta position or
non-substituted, i.e., devices 4 and 2. However, all of these
developed devices based on the anthracene-9,10-diamine
derivatives/AND based guest/host system were greatly en-
hanced in brightness as compared at the same current den-
sity to the system based on coumarin (C545T)/Alq3 of
device 1. Furthermore, as we study the current efficiency
(cd/A) or power efficiency (lm/W) related to luminance
(cd/m2), we can find that the curve trends show the device
efficiency as follows: device 3 is better than device 5, de-
vice 5 is better than devices 4 and 2; device 1 is the worst,
as shown in Fig. 6. The improvement in optoelectronic
properties of the guest/host system based on anthracene-
9,10-diamine derivatives/9,10-di(2-naphthyl)anthracene
(AND) as compared to the coumarin (C545T)/Alq3 is obvi-
ous. In addition, the dopant TmpAD illustrated the effect
of methyl groups substituted at both the para/meta posi-
DEVICE1
DEVICE2
DEVICE3
DEVICE4
DEVICE5
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 4. The electroluminescent spectra of devices fabricated in this study,
where device 2–5 with structure of ITO/NPB (40 nm)/dopants: AND
(40 nm)/Alq3 (20 nm)/LiF (1.5 nm)/Al (200 nm), and device
1 (as a
compared device) with structure of ITO/NPB (40 nm)/C545T:
Alq3(40 nm)/Alq3 (20 nm)/LiF (1.5 nm)/Al (200 nm). The dopant of device
2, TAD; device 3, TmpAD; device 4, TmAD; Device 5, TpAD.
tions could greatly improve in device performance. This
may be attributed to the relative amorphous property of
the emission film and the reduction of the self-quenching
phenomena. The steric effect of the two methyl groups
substituted at the meta and para position of phenyl dia-
mine may make the molecules relatively difficult to aggre-
gate with each other. Therefore the emission efficiency can
be improved by developing these serials of 9,10-diamino-
anthracene derivatives as the dopant TmpAD. However,
the methyl substituents may decrease the carrier mobility
and lead to a higher driving voltage as compared with the
current density (mA/cm2) versus driving voltage (V) curves
of the fabricated devices as shown as Fig. 7. The driving
voltage of device 1 is lower than device 2–5 because of
the relatively lower energy barrier between the host
Table 2
The device properties of the as-synthesized anthracene-9,10-diamine derivatives acted as the dopants in the device structure of device 2–5: ITO/NPB (40 nm)/
3% dopant:ADN (40 nm)/Alq3 (20 nm)/LiF (1.5 nm)/Al (200 nm); and compared with the device 1: ITO/NPB (40 nm)/2% C545T:Alq3 (40 nm)/Alq3 (20 nm)/LiF
(1.5 nm)/Al (200 nm).
Device 1
Device 2
Device 3
Device 4
Device 5
Dopant
EL, nm
C545T
526
TAD
514
TmpAD
542
TmAD
518
TpAD
534
CIE (x, y)
fwhm, nm
Va, volt
(0.34, 0.62)
66
9.90
13,190
4.19
13.19
3.52; 9.90
4.12; 8.0
74.41
4.45
(0.26, 0.61)
69
10.27
17,048
5.21
17.05
5.33; 10.27
6.94; 9.0
51.81
6.22
19.30
(0.38, 0.59)
71
10.49
24,991
7.48
24.99
6.91; 10.49
8.20; 9.0
35.5
9.24
28.17
(0.29, 0.62)
68
9.93
19,955
6.31
19.95
5.4; 9.93
5.82; 9.0
49.01
6.96
(0.35, 0.61)
68
10.23
23,378
7.18
23.38
6.37; 10.23
6.93; 9.0
39.63
8.48
25.23
Ba, cd/m2
Epa, lm/W
Eca, cd/A
E.Q.E.a (%; V)
E.Q.E.b (%; V)
Ic,mA/cm2
Epc, lm/W
Ecc, cd/A
13.44
20.41
B, luminance; I, current density; V, driving voltage; Ep, power efficiency; Ec, current efficiency; fwhm, full width at half maximum; E.Q.E., external quantum
efficiency.
a
evaluated at a current density of 100 mA/cm2.
b
the maximum value of E.Q.E.
c
evaluated at luminance of 10,000 cd/m2.

Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
701
300000
250000
200000
150000
100000
50000
0
DEVICE1
DEVICE2
DEVICE3
DEVICE4
DEVICE5
0
1000
2000
J (mA/cm2)
3000
4000
Fig. 7. The current density (mA/cm2) versus driving voltage (V) measured
from the fabricated devices.
Fig. 5. The relationships of luminance (B) versus current density (J) for
the fabricated devices.
higher driving voltage is acceptable as compared to the
voltages illustrated in Table 2, e.g., 10.23 volt of device 5
as compared to 9.9 V of device 1.
(a) 32
DEVICE1
DEVICE2
DEVICE3
DEVICE4
DEVICE5
30
28
26
24
From the discussion above, we demonstrate the as-pre-
pared compound TAD, TmAD, TpAD, and TmpAD, especially
TmpAD, could be used as good dopants in the AND based
host material for OLEDs fabrication, with excellent electro-
luminescent properties. In general, their performances are
better than coumarins/Alq3 based devices, although the
driving voltage is a little worse. The methyl substituents
on the aromatic ring of phenyl diamine also induce the
emission red-shift to closer to the range of eye sensitive
wavelength near 550 nm, e.g., 542 nm for TmpAD. There-
fore, these methyl-substituted dopants are suitable when
used as highly efficient green dopant materials for OLEDs.
As compared with device 2 for the effects of methyl
substituents in devices efficiency, current efficiency was
promoted about 2.9, 6.33, and 7.94 cd/A for dopants with
the substituted methyl groups at meta, para and both
meta/para position, for device 4, 5 and 3, respectively. In
addition, the power efficiency was also significantly in-
creased with 0.74, 2.26 and 3.02 lm/W for device 4, 5,
and 3, respectively. When comparing the results, device
3, fabricated by TmpAD/AND-based emitter system, exhib-
its the best E.Q.E., luminance efficiency, power efficiency,
and the most saturated green with CIE x = 0.038; y = 0.59
for OLED applications. We demonstrate that the as-pre-
pared compound TAD, TmAD, TpAD and TmpAD, especially
TmpAD, could be used as good dopants in the AND based
host material for OLEDs fabrication, with excellent electro-
luminescent properties. In general, their performances are
better than coumarins/Alq3 based devices.
22
20
18
16
14
12
10
8
6
4
2
0
0
50000
100000 150000 200000 250000 300000
B Luminance (cd/m2)
12
10
8
(b)
DEVICE1
DEVICE2
DEVICE3
DEVICE4
DEVICE5
6
4
2
0
0
50000
100000 150000 200000 250000 300000
B Luminance (cd/m2)
Fig. 6. The relationships of (a) current efficiency (cd/A) and (b) power
efficiency (lm/W) relate to luminance (cd/m2) for the fabricated devices.
4. Conclusions
The methyl substuents on the N-diarryl rings of anthra-
cene-9,10-diamine showed obvious effects in emission
spectra as demonstrated in photo-studies, which observed
a significant bathochromic shift followed by the sequence
of compound TmpAD, TpAD, and TmAD as compared to
TAD. The energy band gaps are relatively decreased as
emitter layer and the electron transport layer (ETL) as illus-
trated in Fig. 1. Device 1 has both a host emitter layer and
an ETL using material of Alq3, while devices 2–5 had been
fabricating the host emitter by using AND as to improve
the properties of the organic devices. However, a little

702
Y.-H. Yu et al. / Organic Electronics 12 (2011) 694–702
the methyl groups substituted at the N-diarryl rings, espe-
cially significant for both on para/meta positions of com-
pound TmpAD. Here we demonstrate the as-prepared
compound TAD, TmAD, TpAD, and TmpAD, especially
TmpAD, could be used as good dopants in the wide band
gap AND based emitting layers for OLEDs farication, which
have excellent performances better than coumarins/Alq3
based devices. The effect of methyl groups substituted at
both the para/meta positions of the N-diarryl rings of
anthracene-9,10-diamine could show great improvement
in device performance as compared to the non-substituent
compound TAD.
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Acknowledgements
The financial support of this research by the NSC-96-
2622-E-267-001-CC3 is gratefully acknowledged. We
thank Professor Ken-Tsung Wong and Miss Shu-Hua Chou
at Department of Chemistry of National Taiwan University
for the measurement of quantum yields.
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