Indolo[3,2-b]carbazole-based functional derivatives as materials for light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2009.10.022

Derivatives of indolo[3,2-b]carbazole containing reactive oxetanyl groups were characterized using ¹H NMR, IR spectroscopy and mass spectrometry. The synthesized materials were examined using various techniques including differential scanning calorimetry, thermogravimetry, electron photoemission spectrometry and xerographic time of flight technique. The electron photoemission spectra of layers of the derivatives showed ionisation potentials of 5.2-5.45 eV; hole drift mobilities in layers of the derivatives dispersed in the polymeric host Bisphenol Z polycarbonate (50%) ranged from 6.2 × 10^-6 to 6.9 × 10^-4 cm² V^-1 s^-1 at high electric fields at room temperature. The compounds were tested as hole transporting materials in bilayer OLED's using Alq₃ as emitter; devices comprising phenyl substituted indolo[3,2-b]carbazole exhibited best overall performance with a turn-on voltage of ～5 V, maximum luminance efficiency of 3.64 cd A^-1 and maximum brightness of ～5700 cd m^-2. The compounds were also used for the preparation of cross-linked hole transporting layers by photoinduced polymerization, the ensuing layers being employed in light emitting diodes.

Full text of DOI:10.1016/j.dyepig.2009.10.022

Dyes and Pigments 85 (2010) 183e188
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Indolo[3,2-b]carbazole-based functional derivatives as materials
for light emitting diodes
,
b
b
S. Lengvinaite a, J.V. Grazulevicius a, S. Grigalevicius a , R. Gu , W. Dehaen ,
*
V. Jankauskas c, B. Zhang d, Z. Xie d
a Department of Organic Technology, Kaunas University of Technology, Radvilenu plentas 19, LT50254 Kaunas, Lithuania
b Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
c Department of Solid State Electronics, Vilnius University, Sauletekio aleja 9, LT10222 Vilnius, Lithuania
d State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Derivatives of indolo[3,2-b]carbazole containing reactive oxetanyl groups were characterized using 1H
NMR, IR spectroscopy and mass spectrometry. The synthesized materials were examined using various
techniques including differential scanning calorimetry, thermogravimetry, electron photoemission
spectrometry and xerographic time of flight technique. The electron photoemission spectra of layers of
the derivatives showed ionisation potentials of 5.2e5.45 eV; hole drift mobilities in layers of the
derivatives dispersed in the polymeric host Bisphenol Z polycarbonate (50%) ranged from 6.2 ꢀ 10ꢁ6 to
6.9 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 at high electric fields at room temperature. The compounds were tested as hole
transporting materials in bilayer OLED's using Alq3 as emitter; devices comprising phenyl substituted
indolo[3,2-b]carbazole exhibited best overall performance with a turn-on voltage of w5 V, maximum
luminance efficiency of 3.64 cd Aꢁ1 and maximum brightness of w5700 cd mꢁ2. The compounds were
also used for the preparation of cross-linked hole transporting layers by photoinduced polymerization,
the ensuing layers being employed in light emitting diodes.
Article history:
Received 3 July 2009
Received in revised form
22 October 2009
Accepted 23 October 2009
Available online 11 November 2009
Keywords:
Indolo[3,2-b]carbazole
Cross-linked layer
Ionisation potential
Charge transporting properties
Light emitting diodes
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
a triphenyldiamine (TPD) core prevail [4,6]. With the purpose of
developing organic hole transporting layers, carbazole- and triar-
Organic light emitting diodes (OLED's) based on organic small
molecules and polymers have attracted much attention owing to
their potential use in flat panel displays and lighting [1]. Efficient
organic LEDs can be obtained only by building multilayer struc-
tures [2]. The main difficulty in the preparation of such devices by
spin-coating is the solubility of the material which forms the
bottom layer onto which the top layer has to be cast, because most
organic semiconductors are soluble in the same solvents. One
approach that has been employed to circumvent this problem is
the appliance of bi-functional electroactive derivatives, which
could be converted into insoluble networks by cross-linking
reactions. Several series of photo-cross-linkable polymers and
monomers for the fabrication of OLEDs have been reported [3e7].
Cross-linked, solvent resistant charge-transporting layers are also
useful in electrophotographic photoreceptors [8]. Among the
cross-linkable monomers reported earlier, derivatives having
ylamine-based derivatives were also studied [9e12]. Here we
describe syntheses and properties of substituted derivatives of
indolo[3,2-b]carbazole having reactive oxetanyl groups. It was
reviewed recently that indolo[3,2-b]carbazole-based compounds
are very promising electroactive materials [13].
2. Experimental
2.1. Instrumentation
1H NMR spectra were recorded using a Varian Unity Inova
(300 MHz) apparatus. Mass spectra were obtained on a Waters ZQ
2000 spectrometer. FTIR spectra were recorded using a Perkin
Elmer FT-IR System. UV spectra were measured with a Spectronic
GenesysTM 8 spectrometer. Fluorescence (FL) spectra were recorded
with a MPF-4 spectrometer. Differential scanning calorimetry (DSC)
measurements were carried out using a Bruker Reflex II thermo-
system. Thermogravimetric analysis (TGA) was performed on
a Netzsch STA 409. The TGA and DSC curves were recorded in
a nitrogen atmosphere at a heating rate of 10ꢂ C/min.
* Corresponding author.
E-mail address: saulius.grigalevicius@ktu.lt (S. Grigalevicius).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.10.022

184
S. Lengvinaite et al. / Dyes and Pigments 85 (2010) 183e188
The ionisation potentials of the layers of the compounds
1.67e1.19 (m, 14H, 2 ꢀ CH3 of oxetane and 4 ꢀ CH2 of alkyl chain),
synthesized were measured by the electron photoemission method
in air, which was described earlier [14]. The measurement method
was, in principle, similar to that described by Miyamoto et al. [15].
Hole drift mobility was measured by the xerographic time of
flight technique [16,17]. The samples for the charge carrier mobility
measurements were prepared by the procedure we have described
earlier [18].
Organic light emitting diodes were fabricated on glass
substrates and had the typical structure with the organic layers
sandwiched between a bottom ITO anode and a top metal cathode.
Before use in device fabrication, the ITO-coated glass substrates
were carefully cleaned before deposition of the organic layers. The
hole transporting layers were made by spin-coating a 60 nm layer
of the corresponding material (3e5) onto the substrates from
chloroform solution. Evaporation of the electroluminescent tris
(quinolin-8-olato)aluminium (Alq3) layer (60 nm) and a LiF/Al
electrode (1/150 nm) was done at a pressure of 3 ꢀ10ꢁ6 mbar in
vacuum evaporation equipment.
The cross-linked hole transporting layers were made by spin-
coating a 25e30 nm layer of the derivatives 3e5 containing initiator
onto the substrates and by following cross-linking as we described
earlier [10,19]. 90e100 nm layer of green light emitting polymer
(GEP)- poly(9,9-dioctylfluorene-2,7-diyl-co-2,5-di(phenyl-4-yl)-
2,1,3-benzothiadiazole) [10] was spin-coated from toluene solution
onto the cross-linked network. The current-voltage and luminance-
voltage characteristics of the devices were recorded as we described
earlier [20].
1.05 (t, 3H, CH3, J ¼ 3 Hz).
MS(APCIþ, 20 V), m/z (%): 653 ([M þ H]þ, 90), 582 ([M-C5H11]þ,
100).
IR (KBr,
n
, cmꢁ1): 3070(CeH, in Ar), 2959, 2854(CeH), 1460
(CeN, in Ar), 978(CeOeC), 731 (Ar).
2,8-Di(10-phenothiazinyl)-5,11-di(3-methyl-3-oxetanylmethyl)-
6-pentylindolo[3,2-b]carbazole (3). Pd2(dba)3 (5.5mg, 0.006mmol),
t-Bu3P (2.4 mg, 0.012 mmol) and toluene (8 ml) of were charged
undernitrogen into a two neck flask. The mixture was stirred at room
temperature for 10 min. Then 2,8-dibromo-5,11-di(3-methyl-3-
oxetanylmethyl)-6-pentylindolo[3,2-b]carbazole (0.4 g, 0.6 mmol),
10H-phenothiazine (0.36 g, 1.8 mmol) and t-BuONa (0.34 g, 3.6 mmol)
were added, and the mixture was stirred at 80 ꢂC for 8 h. The reac-
tion mixture was cooled down and quenched by the addition of ice
water. The product was extracted into ethyl acetate. The extract was
dried over anhydrous MgSO4, and the solvent was removed by
evaporation. The product was purified by silica gel column chro-
matography using ethyl acetate/hexane (vol. ratio 1:2) as an eluent.
Yield: 0.33g (61%) of yellow crystals. M.p.: 191 ꢂC (DSC).
1H NMR spectrum (300 MHz, CDCl3,
d, ppm): 8.27e7.93 (m, 3H,
Ar), 7.69e6.8 (m, 20H, Ar), 4.90e4.79 (m, 4H, 2 ꢀ CH2 of oxetane
ring), 4.51(s, 4H, 2 ꢀ CH2N), 4.42 (d, 2H, CH2 of oxetane ring,
J ¼ 6 Hz), 4.28 (d, 2H, CH2 of oxetane ring, J ¼ 6 Hz), 1.67- 1.19 (m,
14H, 2 ꢀ CH3 of oxetane and 4 ꢀ CH2 of alkyl chain), 0.76 (t, 3H, CH3
of alkyl chain, J ¼ 7 Hz).
MS(APCIþ, 20 V), m/z (%):890.4 ([M þ H]þ, 73), 889.4 (Mþ, 100).
IR (KBr,
n
, cmꢁ1): 3056(CeH, in Ar), 2955, 2870(CeH), 1492
(CeN, in Ar), 973(CeOeC), 747(Ar).
2,8-Di(diphenylamino)-5,11-di(3-methyl-3-oxetanylmethyl)-6-
pentylindolo[3,2-b]carbazole (4). Pd2(dba)3 (6.9 mg, 0.0076 mmol),
t-Bu3P (3 mg, 0.015 mmol) and toluene (10 ml) were charged under
nitrogen into a two neck flask. The resulting mixture was stirred at
room temperature for 10 min and 2,8-dibromo-5,11-di(3-methyl-3-
oxetanylmethyl)-6-pentylindolo[3,2-b]carbazole (0.5 g, 0.76 mmol),
diphenylamine (0.64 g, 3.8 mmol) and t-BuONa (0.44 g, 4.56 mmol)
were added, and the mixture was stirred at 80 ꢂC for 12 h. Then the
reaction mixture was cooled down and quenched by the addition of
ice water. The product was extracted into ethyl acetate. The extract
was dried over anhydrous MgSO4, and the solvent was removed by
evaporation. The product was purified by silica gel column chro-
matography using ethyl acetate/hexane (vol. ratio 1:2) as an eluent.
Yield: 0.25g (40%) of yellow crystals. M.p.: 163 ꢂC (DSC).
2.2. Materials
10H-Phenothiazine, diphenylamine, phenylboronic acid 1,3-pro-
panediol ester, tris(dibenzylideneacetone)dipalladium(0)[Pd2(dba)3]
(caution: air sensitive; moisture sensitive; incompatible withmoist
air or water, strong oxidizing agents), tri(tert-butyl)phosphine
(t-Bu3P) (caution: powder or liquid is pyrophoric; avoid ignition
sources, dust generation, exposure to air, strong oxidants), bis(tri-
phenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2) (caution:
avoid dust generation, moisture, excess heat; incompatible with
strong oxidizing agents), sodium tert-butoxide (t-BuONa), potassium
carbonate, tetrabutylammonium hydrogen sulphate (TBAHS), Alq3
and potassium hydroxide were purchased from Aldrich and used as
received. 3-Bromomethyl-3-methyloxetane was received from
Chemada Fine Chemicals and used without further purification.
2,8-Dibromo-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole (1)
was prepared by the earlier reported procedure [21].
1H NMR spectrum (300 MHz, CDCl3,
d, ppm): 8.08e7.76 (m, 4H,
Ar), 7.39e6.97 (m, 23H, Ar), 4.88e4.79 (m, 4H, 2 ꢀ CH2 of oxetane
ring), 4.5 (s, 4H, 2 ꢀ CH2N), 4.38 (d, 2H, CH2 of oxetane ring,
J ¼ 6 Hz), 4.25 (d, 2H, CH2 of oxetane ring, J ¼ 6 Hz), 1.68e1.13(m,
14H, 2 ꢀ CH3 of oxetane and 4 ꢀ CH2 of alkyl chain), 0.85 (t, 3H, CH3
of alkyl chain, J ¼ 3 Hz).
2,8-Dibromo-5,11-di(3-methyl-3-oxetanylmethyl)-6-pentylin-
dolo[3,2-b]carbazole (2) was prepared by the reaction of 2,8-
dibromo-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole (1) with an
excess of 3-bromomethyl-3-methyl oxetane under basic conditions
in the presence of a phase transfer catalyst-TBAHS. Compound 1
(1.5g, 3 mmol), 3-(bromomethyl)-3-methyl oxetane (1.79 g,
10.5 mmol), powdered potassium carbonate (2.48g, 18 mmol),
potassium hydroxide (1g, 18 mmol) and TBAHS (0.2 g, 0.6 mmol)
were stirred at 50 ꢂC for 18 hours. After TLC control, the mixture
was quenched with water. The product was extracted by ethyl
acetate and purified by silica gel column chromatography using
ethyl acetate/hexane (vol. ratio 1:3) as an eluent. Yield: 1.6g (79%)
of yellow crystals. M.p.: 220 ꢂC (DSC).
MS(APCIþ, 20 V), m/z (%):830.5 ([M þ H]þ,65), 829.5 (Mþ, 100).
IR (KBr,
n
, cmꢁ1): 3058(CeH, in Ar), 2954, 2869(CeH), 1459
(CeN, in Ar), 979(CeOeC), 745(Ar).
2,8-Diphenyl-5,11-di(3-methyl-3-oxetanylmethyl)-6-pentylin-
dolo[3,2-b]carbazole (5). 2,8-Dibromo-5,11-di(3-methyl-3-oxeta-
nylmethyl)-6-pentylindolo[3,2-b]carbazole
(0.4 g,
0.6 mmol),
phenylboronic acid 1,3-propanediol ester (0.2 ml, 1.8 mmol),
potassium hydroxide (0.23 g, 6 mmol) and Pd(PPh3)2Cl2 (0.03 g,
0.04 mmol) were charged into two neck flask. The reaction vessel
was evacuated and filed with argon. Degassed THF (6 ml) and
degassed distilled water (0.2 ml) were added, and the reaction
mixture was stirred at 80 ꢂC for 20 h. After TLC control, the reaction
mixture was cooled down and quenched by the addition of ice
water. The crude product was extracted into ethyl acetate. The
organic solution was dried over anhydrous MgSO4, and the solvent
was removed by evaporation. The product was purified by silica gel
1H NMR spectrum (300 MHz, CDCl3,
d, ppm): 8.33e8.31 (m, 2H,
Ar), 7.87 (s, 1H, Ar), 7.62e7.57 (m, 2H, Ar), 7.34e7.17 (m, 2H, Ar),
4.84 (d, 2H, CH2 of oxetane ring, J ¼ 6 Hz), 4.74 (d, 2H, CH2 of
oxetane ring, J ¼ 6 Hz), 4.55(s, 4H, 2 ꢀ CH2N), 4.43 (d, 2H, CH2 of
oxetane ring, J ¼ 6 Hz), 4.26 (d, 2H, CH2 of oxetane ring, J ¼ 6 Hz),

S. Lengvinaite et al. / Dyes and Pigments 85 (2010) 183e188
185
column chromatography using ethyl acetate/hexane (vol. ratio 1:2)
as an eluent. Yield: 0.22g (56%) of yellow crystals. M.p.: 275 ꢂC
(DSC).
excess of 3-bromomethyl-3-methyloxetane under basic conditions
in the presence of a phase transfer catalyst TBAHS to obtain 2,8-
dibromo-5,11-di(3-methyl-3-oxetanylmethyl)-6-pentylindolo[3,2-b]
carbazole (2). Compound 2 was treated by Buchwald-Hartwig
[22,23] procedure with 10H-phenothiazine or diphenylamine
to yield phenothiazinyl- or diphenylamino-substituted derivatives
1H NMR spectrum (300 MHz, CDCl3,
d, ppm): 8.52e8.26 (m, 2H,
Ar), 8.06e7.32 (m, 15H, Ar), 4.96e4.83 (m, 4H, 2 ꢀ CH2 of oxetane
ring), 4.78e4.26 (m, 6H, 2 ꢀ CH2N and CH2 of oxetane ring), 4.31 (d,
2H, CH2 of oxetane ring, J ¼ 6 Hz), 1.72 (s, 3H, CH3 of oxetane), 1.67
(s, 3H, CH3 of oxetane), 1.66e1.27(m, 8H, 4 ꢀ CH2 of alkyl chain),
1.03 (t, 3H, CH3 of alkyl chain, J ¼ 7 Hz).
3
and 4 respectively. 2,8-Diphenyl-5,11-di(3-methyl-3-oxeta-
nylmethyl)-6-pentylindolo[3,2-b]carbazole (5) was obtained by
Suzuki- Miyaura [24,25] coupling of compound 2 with an excess of
phenylboronic acid 1,3-propanediol ester.
MSðAPCID; 20 VÞ; m=zð%Þ : 647:4ð½M D HꢃD; 50Þ:
IR (KBr, n
, cmꢁ1): 3058(CeH, in Ar), 2954, 2866(CeH),1470,1455
(CeN, in Ar), 979(CeOeC), 753, 762(Ar).
All the newly synthesized compounds were identified by mass
spectrometry, IR-, and 1H NMR spectroscopy. The data were found
to be in good agreement with the proposed structures. The mate-
rials (3e5) were soluble in common organic solvents, such as
chloroform and THF at room temperature. Transparent thin films of
these materials could be prepared by spin-coating from solutions.
The behaviour on heating of compounds 3e5 was studied by
DSC and TGA under a nitrogen atmosphere. The values of glass
transition temperatures (Tg), melting points (Tm) and the temper-
atures at which 5% loss of mass was observed (TID) are summarized
in Table 1. Compounds 3e5 demonstrate high thermal stability. The
mass loss occurs at a temperature ranging from 382 ꢂC to 400 ꢂC as
confirmed by TGA with a heating rate of 10o C/min. It is evident that
the phenothiazinyl-substituted derivative 3 exhibits slightly lower
3. Results and discussion
The synthesis of indolo[3,2-b]carbazole-based derivatives with
reactive oxetanyl groups (3e5) was carried out by a multi-step
synthetic route as shown in Scheme 1. 2,8-Dibromo-6-pentyl-5,11-
dihydroindolo[3,2-b]carbazole (1) as a key compound was obtained
by one-pot synthesis starting from 5-bromoindole with n-hexanal
catalysed by hydroiodic acid as described earlier [17]. Compound 1
containing two secondary amine moieties was then treated with an
O
S
H
N
N
Br
N
N
N
N
H
3
Br
S
O
1
H
N
b
O
a
Br
S
O
H
N
Br
O
N
N
N
N
Br
b
2
N
N
O
4
O
c
O
B
O
O
N
N
5
O
Scheme 1. a) KOH, K2CO3, b) Pd2(dba)3, t-Bu3P, t-BuONa, c) Pd(PPh3)2Cl2, KOH.

186
S. Lengvinaite et al. / Dyes and Pigments 85 (2010) 183e188
Table 1
corresponding spectra of 5,11-di(2,3-epoxypropyl)-6-pentyl[3,2-b]
Thermal characteristic of compound 3e5.
carbazole (DInCz) [26] are given in the Figure. All the compounds
(3e5) exhibit a broad absorption with lmax in the range of
265e360 nm and demonstrate a red shift of the UV absorption edge
with respect of that of DInCz. The strongest red shift of the
absorption band is observed for compound 4. These red shifts show
that the molecules synthesized have an interaction between the
Compound
Tg (ꢂC)
Tm (ꢂC)
TID (ꢂC)
3
4
5
155
123
172
191
163
275
382
398
400
chromophores and that
molecules.
p electrons are de-localized over these
thermal stability than diphenylamino- or phenyl-substituted
compounds 4 and 5.
The FL emission spectra of the synthesized derivatives also show
a red shift with respect of the DInCz spectrum (Fig. 2b). The fluo-
rescence spectra of 3 and 5 have similar shape compared to that of
DInCz, however they are red shifted by ca 10 nm. The spectrum of
diphenylamino-substituted derivative 4 shows a considerable
bathochromic shift with the emission maximum at 462 nm. The
highest Stokes shift of 4 can apparently be explained by the highest
relaxation energy in the excited state, which is determined by the
possibility of an independent rotation of the two phenyl groups at
the nitrogen atom.
Compounds 3e5 all were obtained as crystalline materials.
When the crystalline samples were heated, endothermic peaks due
to melting were observed in the region from 163 ꢂC to 275 ꢂC by
DSC (Table 1). However, the melt samples readily formed glasses
when they were cooled on standing in air or with liquid nitrogen.
When the amorphous samples were heated again, glass-transitions
were observed at 155ꢂC, 123ꢂC and 172ꢂC, respectively, for
compounds 3e5, and on further heating no peaks due to crystal-
lization and melting appeared.
UV absorption and FL spectra of dilute solutions of the
compounds synthesized are shown in Fig. 1. For comparison, the
Electron photoemission spectra of the amorphous layers of
compounds 3e5 as well as the values of their Ip are presented in
Fig. 2. The Ip values of amorphous layers of the materials synthe-
sized range from 5.2 to 5.45 eV. The layers of 4 show the lowest Ip
probably due to the most extended conjugation of
p electrons,
a
which was confirmed by comparison of UV absorption and FL
spectra of the materials.
4
The Ip values of these materials demonstrate that their layers or
cross-linked systems would be suitable for application in opto-
electronic devices. Positive charges could be easily injected into the
layers of 3e5 from compounds widely used in electrophotograpy as
pigments, such as titanyl phthalocyanines, perylene derivatives,
and bisazo pigments, which have Ip in the range of 5.1e5.6 eV [27].
The Ip values of compounds 3e5 are also close to that of indium-tin
oxide which is widely used as an anode in electroluminescent
devices [28]. Compounds 3e5 or their cross-linked networks can be
suitable as hole transporting layers in multilayer devices.
3
5
DInCz
Xerographic time of flight measurements were used to charac-
terize the magnitudes of hole drift mobility (mh) for compounds
3e5 molecularly dispersed in polymer host bisphenol Z poly-
carbonate (PC-Z). For the molecular dispersions of compounds 3e5
in PC-Z (50%) the room temperature mh shows the linear depen-
dencies on the square root of the electric field (Fig. 3). This char-
acteristic dependence is observed for the majority of amorphous
270
300
330
360
390
420
450
Wavelength (nm)
b
4
3
5
4 Ip= 5.2 eV
3 Ip= 5.45 eV
5 Ip= 5.33 eV
DInCz
390
420
450
480
510
540
5,0
5,2
5,4
5,6
hv (eV)
5,8
6,0
6,2
6,4
Wavelength (nm)
Fig. 1. UV absorption (a) and FL emission (b) spectra of dilute THF solutions of 3e5 and
DInCz.
Fig. 2. Electron photoemission spectra of the layers of 3e5.

S. Lengvinaite et al. / Dyes and Pigments 85 (2010) 183e188
187
10-3
1x10-4
1x10-5
10-6
a10000
1000
100
=0.0065 (cm/V)1/2
=0.0064 (cm/V)1/2
10-7
4
3
5
10
5 + PC-Z
4 + PC-Z
3 + PC-Z
1
10-8
=0.0052 (cm/V)1/2
10-9
0
200
400
600
800
1000
1200
1400
1/2 (V/cm)1/2
4
6
8
10
12
14
16
18
20
E
Voltage(V)
Fig. 3. Electric field dependencies of hole drift mobility in charge transport layers of
compounds 3e5 molecularly doped in PC-Z.
5
4
3
2
1
b
4
3
5
organic systems and can be attributed to the effects of disorder on
charge transport [29].
A
mh value of 6.2 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1 was observed for the
amorphous layer of compound 3 dispersed in PC-Z at an electric
field of 1.93 ꢀ 106 Vcmꢁ1 at 25 ꢂC. A layer containing a molecular
dispersion of compound
4 showed a mh well exceeding
10ꢁ5 cm2 Vꢁ1 sꢁ1 at high electric fields at 25 ꢂC. The highest mh was
observed in the layers of phenyl-disubstituted derivative 5. The
charge mobility reached almost 10ꢁ3 cm2 Vꢁ1 sꢁ1 at high electric
field. It should be mentioned that these charge mobilities were
estimated in layers having only 50 wt.% of the electroactive mate-
rials 3e5. Charge mobilities higher by at least one order of
magnitude can be predicted for amorphous films of the pure
compounds. The different charge transporting properties of the
synthesized compounds 3e5 are predetermined probably by
different chemical structures of the materials as well as by different
positional and energetic disorder in their layers.
0,1
1
10
100
1000
Current density (mA/cm2)
Fig. 4. OLED characteristics of the devices with the configuration: ITO/3, 4 or 5/Alq3/
LiF/Al.
The compounds 3e5 were also tested in OLEDs as hole trans-
porting (HT) materials. The two layer OLED devices were prepared
using these compounds for the HT layers and Alq3 for the electro-
luminescent/electron transporting layers. The cathode used was
aluminium with a thin LiF electron injection layer. When a positive
voltage was applied the bright green electroluminescence of Alq3
was observed with an emission maximum at around 520 nm
[30,31]. This implies that the hole mobility in the HT layers of 3e5 is
fully sufficient for an effective charge carrier recombination
occurring within the Alq3 layer. No exciplex formation at the
interface between HT and Alq3 layer was observed.
the derivatives 3e5 containing initiator onto PEDOT and by
following cross-linking as we described earlier [10,19].
When a positive voltage was applied to the devices bright green
electroluminescence of GEP emitter was observed with an emission
maximum at around 528 nm. This implies that the hole mobility in
the hole transporting networks is sufficient for an effective charge
carrier recombination occurring within the GEP layer. No exciplex
formation at the interface between hole transporting and emitting
layer was observed.
Fig. 4 shows luminance e voltage (a) and efficiency e current
density (b) characteristics for the OLEDs containing the HT layers of
3e5. These devices in general exhibit turn-on voltages of 5e8 V
(defined as the voltage where electroluminescence becomes
detectable) and a maximum brightness of 1090e5670 cd/m2 (at
13e17 V). Current efficiency of the OLEDs range from 2.21 to
3.64 cd/A. Among these devices, the OLED using derivative 5 as the
HT material exhibits the best overall performance, i.e. a turn-on
voltage of w5 V and maximal photometric efficiency of 3.64 cd/A.
The efficiency shows an only a moderate drop in the observed
current density window up to 120 mA/cm2, for the technically
important brightness of 100 cd/m2, an efficiency above 3.4 cd/A is
detected. These findings are rather promising also in comparison to
similar Alq3-based bilayer devices [1,32].
Turneon voltages of the devices were 2.5e3 V and maximum
brightness was 3500e7790 cd/m2. The best performance was
achieved from the device having cross-linked HT layer of derivative
5 (Fig. 5), which demonstrated the highest charge drift mobility. EL
efficiency of the device reached 2.8 cd/A and the maximum
brightness exceeded 7700 cd/m2. It should be pointed out that
these characteristics were obtained in a non-optimized test device
under ordinary laboratory conditions. The device performance may
be further improved by an optimization of the layer thicknesses and
processing conditions [33].
In conclusion, cross-linkable materials with 2,8-disubstituted
indolo[3,2-b]carbazole core as hole transport agent, and oxetanyl
groups as reactive moieties have been synthesized and character-
ized. The materials show high thermal stability and form glasses
with glass transition temperatures in the range of 123e155 ꢂC. The
ionization potential values (5.2e5.45 eV) of amorphous layers of the
derivatives and the hole drift mobility studies show that these
compounds are potential materials for the preparation of hole
Cross-linked layers of the material 3e5 were also tested as hole
transporting networks in two-layer OLEDs of the configuration ITO/
PEDOT/cross-linked material/GEP emitter/(Ca/Al). The hole trans-
porting layers were made by spin-coating from solution a layer of

188
S. Lengvinaite et al. / Dyes and Pigments 85 (2010) 183e188
References
3
2
1
0
[1] Mullen K, Scherf U, editors. Organic light emitting devices
properties and applications. Weinheim: Wiley-VCH; 2005.
[2] Mitschke U, Bauerle P. J Mater Chem 2000;10:913.
[3] Muller CD, Falcou A, Reckefuss N, Rojahn M, Wiederhirn V, Rudati P, et al.
Nature 2003;421:829.
[4] Nuyken O, Jungermann S, Wiederhirn V, Bacher E, Meerholz K. Monatsh Chem
2006;137:811.
[5] Huang F, Cheng Y-J, Zhang Y, Liu MS, Jen AK-Y. J Mater Chem 2008;18:4495.
[6] Wen G-A, Zhu X-R, Wang L-H, Feng J-C, Zhu R, Wei W, et al. J Polym Sci Part A:
Polym Chem 2007;45:388.
[7] Meerholz K, Muller CD, Nuyken O. In: Mullen K, Scherf U, editors. Organic light
emitting devices e synthesis, properties and applications. Weinheim: Wiley-
VCH; 2005.
[8] Daskeviciene M, Getautis V, Grazulevicius JV, Stanisauskaite A, Antulis J,
Gaidelis V, et al. J Imag Sci Technol 2002;46:467.
[9] Lengvinaite S, Grazulevicius JV, Grigalevicius S. Synth Met 2009;159:91.
[10] Lengvinaite S, Grazulevicius JV, Grigalevicius S, Zhang B, Yang J, Xie Z, et al.
Synth Met 2008;158:213.
e synthesis,
with cross-linked 4
with cross-linked 3
with cross-linked 5
[11] Du P, Zhu W-H, Xie Y-Q, Zhao F, Ku C-F, Cao Y, et al. Macromolecules 2004;37:
4387.
[12] Ning Z, Tian H. Chem Commun 2009:5483.
[13] Wakim S, Aich B-R, Tao Y, Leclerc M. Polym Rev 2008;48:432.
[14] Kirkus M, Lygaitis R, Tsai M-H, Grazulevicius JV, Wu C-C. Synth Met
2008;158:226.
0
50
100
150
200
250
300
350
Current Density (mA/cm2)
[15] Miyamoto E, Yamaguchi Y, Yokoyama M. Electrophotography 1989;28:364.
[16] Montrimas E, Gaidelis V, Pazera A. Lith J Phys 1966;6:569.
[17] Vaezi-Nejad SM. Int J Electron 1987;62:361.
Fig. 5. Current efficiency of the devices ITO/cross-linked HT layer/GEP/(Ca/Al).
[18] (a) Balionyte A, Grigalevicius S, Grazulevicius JV, Garsva G, Jankauskas V.
J Photochem Photobiol A: Chem 2004;162:249;
(b) Grigalevicius S, Lideikis E, Grazulevicius JV, Gaidelis V, Antulis J,
Jankauskas V, et al. Polymer 2002;43:5693.
[19] Lengvinaite S, Grazulevicius JV, Grigalevicius S, Zhang B, Xie Z. Mol Cryst Liq
Cryst 2008;497:496.
transporting layers for multilayer optoelectronic devices. These
derivatives have been studied as hole transporting materials in
bilayer OLEDs with Alq3 as an emitter. The devices with phenyl
substituted indolo[3,2-b]carbazole exhibited the best overall
performance (turn-on voltage: w5 V; maximum luminance effi-
ciency: 3.64 cd/A; maximum brightness: approx. 5700 cd/m2).
Cross-linked networks of these derivatives have also been tested as
hole transporting structures in multilayer OLEDs with green light
emitting polymer. The device containing cross-linked layer of
phenyl substituted indolo[3,2-b]carbazole demonstrated maximum
photometric efficiency of 2.8 cd/A and maximum brightness of
7790 cd/m2.
[20] Grigalevicius S, Ma L, Qian G, Xie Z, Forster M, Scherf U. Macromol Chem Phys
2007;208:349.
[21] Gu R, Hameurlaine A, Dehaen W. J Org Chem 2007;72:7207.
[22] Louie J, Hartwig JF. Tetrahedron Lett 1995;36:3609.
[23] Wolfe JP, Buchwald SL. Org Synth 2004;10:423.
[24] Miyaura N, Suzuki A. Chem Rev 1995;95:2457.
[25] Suzuki AJ. Organomet Chem 1999;576:147.
[26] Kirkus M, Grazulevicius JV, Grigalevicius S, Gu R, Dehaen W, Jankauskas V. Eur
Polym J 2009;45:410.
[27] Blazys G, Grigalevicius S, Grazulevicius JV, Gaidelis V, Jankauskas V,
Kampars V. J Photochem Photobiol A: Chem 2005;174:1.
[28] Tao XT, Zhang YD, Wada T, Sasabe H, Suzuki H, Watanabe T, et al. Adv Mater
1998;10:226.
[29] Borsenberger M, Weiss DS. Photoreceptors for imaging systems. New York:
Marcel Dekker; 1993.
Acknowledgements
[30] Hosokawa C, Higashi H, Kusumoto T. Appl Phys Lett 1992;60:1220.
[31] Kepler RG, Beeson PM, Jacobs SJ, Anderson RA, Sinclair MB, Valencia VS, et al.
Appl Phys Lett 1995;66:36118.
[32] Li W, Jones RA, Allen SC, Heikenfeld JC, Steckl AJ. J Disp Technol 2006;2:143.
[33] Jou JH, Hsu MF, Wang WB, Liu CP, Wong ZC, Shyue JJ, et al. Org Electron
2008;9:291.
Financial support of this research by the Lithuanian Science and
Studies Foundation and by the Alexander von Humboldt Founda-
tion (Germany) is gratefully acknowledged. Habil. dr. V. Gaidelis is
thanked for the help in ionisation potential measurements.