Novel star-shaped host materials for highly efficient solution-processed phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c0jm00692k

Two novel star-shaped host materials for solution processed blue phosphorescent organic light-emitting devices, 9-(5′,5?-diphenyl[1, 1′:3′,1″:3″,1?:3?,1-quinquephenyl] -5″-diyl)-9H-carbazole (DQC) and 9,9′-(5′-phenyl[1,1′: 3′,1″-terphenyl]-3,5-diyl)bis-9H-carbazole (PTC), were synthesized by the Suzuki coupling reaction. These compounds both exhibited high glass-transition temperatures (T_g ≥ 128 °C) and excellent film-forming ability. The nonplanar star-shaped configuration of DQC and PTC limited the effective extension of their π-conjugation, leading to the same triplet energy of 2.81 eV. The solution processed single layer devices using DQC and PTC as the host for the phosphorescence emitter iridium(iii) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) showed the maximum luminance efficiencies of 9.2 and 12.8 cd A^-1, respectively. By introducing a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) electron-transporting and exciton-confining layer, the maximum efficiencies of the solution processed devices based on DQC and PTC were further improved to 21.7 and 25.7 cd A^-1 with the maximum external quantum efficiencies up to 9.2% and 11.9%, respectively. Furthermore, the DQC- and PTC-based devices showed significantly high performance compared with the corresponding devices based on 1,3-bis(9-carbazolyl)benzene (mCP).

Full text of DOI:10.1039/c0jm00692k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel star-shaped host materials for highly efficient solution-processed
phosphorescent organic light-emitting diodes
*
Wei Jiang, Lian Duan, Juan Qiao, Deqiang Zhang, Guifang Dong, Liduo Wang and Yong Qiu
Received 14th March 2010, Accepted 29th April 2010
DOI: 10.1039/c0jm00692k
Two novel star-shaped host materials for solution processed blue phosphorescent organic light-
emitting devices, 9-(50,5%-diphenyl[1,10:30,100:300,1%:3%,1&-quinquephenyl]-500-diyl)-9H-carbazole
(DQC) and 9,90-(50-phenyl[1,10:30,100-terphenyl]-3,5-diyl)bis-9H-carbazole (PTC), were synthesized by
the Suzuki coupling reaction. These compounds both exhibited high glass-transition temperatures (Tg
ꢀ
$ 128 C) and excellent film-forming ability. The nonplanar star-shaped configuration of DQC and
PTC limited the effective extension of their p–conjugation, leading to the same triplet energy of 2.81 eV.
The solution processed single layer devices using DQC and PTC as the host for the phosphorescence
emitter iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) showed the maximum
luminance efficiencies of 9.2 and 12.8 cd Aꢁ1, respectively. By introducing a thin 1,3,5-tris(1-phenyl-1H-
benzo[d]imidazol-2-yl)benzene (TPBI) electron-transporting and exciton-confining layer, the
maximum efficiencies of the solution processed devices based on DQC and PTC were further improved
to 21.7 and 25.7 cd Aꢁ1 with the maximum external quantum efficiencies up to 9.2% and 11.9%,
respectively. Furthermore, the DQC- and PTC-based devices showed significantly high performance
compared with the corresponding devices based on 1,3-bis(9-carbazolyl)benzene (mCP).
efforts have been devoted to develop OLEDs with the emitting
layer prepared by solution-processing techniques.12,13 Recently,
solution processed phosphorescent OLEDs using small organic
molecules and conjugated polymers as host materials have been
reported.14–20 It is known that the chemical purity and structural
uniformity are very important factors that have a dramatic
influence on the performance of phosphorescent OLEDs, the
impurities, especially the metal catalyst used in polymer
synthesis, could result in exciton quenching and device failure. In
contrast to polymer counterparts, small-molecule host materials
are advantageous, as they can easily synthesized and purified.21
However, the extent of conjugation and the molecular size of the
small molecule host materials must be constrained to achieve
high triplet energy. This requirement makes it difficult for the
molecules to acquire the morphological stability that is necessary
for forming stable and uniform amorphous thin films.22–25
Compared with linear organic conjugated oligomers and poly-
mers, star-shaped conjugated molecules have a number of
advantages, including better solubility, easy formation of quality
film, structural uniformity and high degree of purity. These
unique features render this kind of material rather promising for
use as host materials for blue phosphorescent OLEDs.
Introduction
Organic lighting-emitting diodes (OLEDs) have attracted a great
deal of attention due to their potential applications in full-color
flat-panel displays and lighting sources.1–5 Recently, the effi-
ciencies of OLEDs have been dramatically improved because of
the development of efficient phosphorescent molecules contain-
ing transition metals that can harvest both singlet and triplet
excitons for emission.6,7 For highly efficient blue phosphorescent
OLEDs, suitable host materials that should possess a high triplet
energy gap are necessary and remain a challenge. Several
carbazole-based host materials have been frequently used in
phosphorescence devices. For example, 4,40-bis(9-carbazolyl)-
2,20-biphenyl (CBP) is commonly used as a host material, but not
for blue triplet emitters because of its lower triplet energy gap
(ET) of 2.56 eV relative to those of blue triplet emitters, such as
iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic)
(ET ¼ 2.62 eV) and iridium(III) bis(40,60-difluorophenylpyr-
idinato)tetrakis(1-pyrazolyl)borate (FIr6) (ET ¼ 2.70 eV).8,9 One
of the earliest hosts used for blue electrophosphorescence devices
is 1,3-bis(9-carbazolyl)benzene (mCP) (ET ¼ 2.90 eV).10 As an
effective host of the blue triplet emitter, the triplet energy is
required to be higher than 2.75 eV in order to prevent the reverse
energy transfer from the triplet emitter to the host.11
It is conceivable that the overall cost will be reduced by a more
simplified fabrication such as fully solution processing and
a simple structure with only one or two active layers. In our
previous paper, we have demonstrated that a single layer solution
processed blue phosphorescence OLED, based on 9,9-bis[4-(3,6-
di-tert-butylcarbazolel-9-yl)phenyl]fluorine (TBCPF) as the host,
exhibited a high efficiency.26 In this paper, we used benzene
as the core structure to connect carbazole and m-terphenyl
units, and synthesized 9-(50,5%-diphenyl[1,10:30,100:300,1%:3%,1&-
quinquephenyl]-500-diyl)-9H-carbazole (DQC) and 9,90-(50-phe-
nyl[1,10:30,100-terphenyl]-3,5-diyl)bis-9H-carbazole (PTC), which
Concerning the fabrication of devices and displays, vapor
deposition requires vacuum processing, which is a relatively
costly process. In order to simplify the fabrication process of
OLEDs and prepare cheaper larger-area displays, many research
Key Lab of Organic Optoelectronics and Molecular Engineering of
Ministry of Education, Department of Chemistry, Tsinghua University,
Beijing, 100084, China. E-mail: qiuy@mail.tsinghua.edu.cn; Fax: +86 10
62795137; Tel: +86 10 62782197
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were employed as the host materials for single layer solution
processed blue phosphorescence devices. In addition, the unique
star-shaped structure of the molecules greatly enhances the
thermal stability of these materials and the nonplanar structure
helps keep the triplet energy gap at a very high level. The use of
these materials as hosts for blue triplet emitters showed the
maximum luminance efficiencies of 12.8 cd Aꢁ1 for single layer
devices and 25.7 cd Aꢁ1 for double layer devices. The excellent
performances of the solution processed devices are outstanding
with respect to other work for fully solution processed OLEDs
with a similar structure.
characteristics of the devices were characterized with Keithley
4200 semiconductor characterization system. The electrolumi-
nescent spectra were collected with a Photo Research PR705
Spectrophotometer. All measurements of the devices were
carried out in ambient atmosphere without further encapsula-
tions.
Materials
Synthesis of 4-bromo-50-pheyl-1,10:30,100-terphenyl (M1). 1,3,5-
tribromobenzene (10 g, 31 mmol), phenylboronic acid (8.25 g, 68
mmol), and Pd(PPh3)4 (1.80 g, 1.7 mmol) were added to 200 mL
of 1,4-dioxane solution, then 2 M K2CO3 solution (30 mL),
which was dissolved in H2O, was added to the reaction mixture.
The mixture was refluxed under nitrogen for 24 h. After the
reaction finished, the reaction mixture was extracted with
dichloromethane and water. The organic layer was dried over
anhydrous MgSO4 and filtered. The product was isolated by
silica gel column chromatography using petroleum ether (60–
Experimental
General information
All reactants and solvents, unless otherwise stated, were
1
purchased from commercial sources and used as received. H
NMR and 13C NMR spectra were measured on a Bruker
ARX600 NMR spectrometer with tetramethylsilane as the
internal standard. Elemental analysis was performed on an Ele-
mentar Vario EL CHN elemental analyzer. Mass spectrometry
was performed with a Thermo Electron Corporation Finnigan
LTQ mass spectrometer. Absorption spectra were recorded with
a UV–vis spectrophotometer (Agilent 8453) and PL spectra were
recorded with a fluorospectrophotometer (Jobin Yvon, Fluo-
roMax-3). TGA was recorded with a Netzsch simultaneous
thermal analyzer (STA) system (STA 409PC) under a dry
ꢀ
90 C)/ethyl acetate (20 : 1) eluent to afford a white solid (M1)
1
(4.84 g, 50.4%). H-NMR (600 MHz, CDCl3, d): 7.70 (s, 3 H),
7.61 (d, 4 H), 7.47 (t, 4 H), 7.40 (t, 2 H).
Synthesis of 4,4,5,5-tetramethyl-2-[1,10:30,100-terphenyl]-50-yl-
1.3.2-dioxaborolane (M2). A mixture of M1 (4.84 g, 15.6 mmol),
bis(pinacolato) diborate (4.7 g, 18.5 mmol), [1,10-bis(diphenyl-
phosphino)ferrocene]dichloropalladium(II) (0.57 mg, 0.78
mmol), and potassium acetate (4.08 g, 40 mmol) were dissolved
in anhydrous 1,4-dioxane (150 mL) and refluxed under nitrogen
for 24 h. Upon cooling, the mixture was poured into water
(20 mL) and extracted with dichloromethane. The combined
organic layers were dried with anhydrous MgSO4 and concen-
trated under reduced pressure. The crude product was purified by
column chromatography with petroleum ether/ethyl acetate
nitrogen gas flow at a heating rate of 10 C minꢁ1. Glass-tran-
ꢀ
sition temperatures were recorded by DSC at a heating rate of
10 ꢀC minꢁ1 with a thermal analysis instrument (DSC 2910
modulated calorimeter). Cyclic voltammetry was performed on
a Princeton Applied Research potentiostat/galvanostat model
283 voltammetric analyzer in CH2Cl2 solutions (10ꢁ3 M) at
a scan rate of 100 mV sꢁ1 with a platinum plate as the working
electrode, a silver wire as the pseudo-reference electrode, and
a platinum wire as the counter electrode. The supporting elec-
trolyte was tetrabutylammonium hexafluorophosphate (0.1 M)
and ferrocene was selected as the internal standard. The solutions
were bubbled with argon for 10 min before measurements. The
film surface morphology was measured with AFM (Seiko
Instruments, SPA-400).
1
(10 : 1) eluent to afford a white solid (M2) (4.5 g, 81.0%). H-
NMR (600 MHz, CDCl3, d): 8.03 (s, 2 H), 7.90 (s, 1 H), 7.69 (d, 4
H), 7.46 (t, 4 H), 7.36 (t, 2 H), 1.40 (s, 12 H).
Synthesis of 9-(3,5-Dibromophenyl)-9H-carbazole (M3). The
mixture of 1,3,5-tribromobenzene (10 g, 31 mmol), carbazole
(5.6 g, 35 mmol), copper(I) iodide (0.06 g, 3 mmol), 1,10-phe-
nanthroline (0.06 g, 3 mmol), K2CO3 (4.9 g, 35 mmol), and
dried DMF 100 mL was refluxed under an argon atmosphere
for 12 h. After cooling to room temperature, the solvent was
removed under vacuum and the residue was extracted with
dichloromethane. The product was then obtained by column
chromatography on silica gel with petroleum ether/ethyl acetate
(10 : 1) as the eluent, to yield a white solid (M3) (7.78 g, 62.6%).
1H-NMR (600 MHz, CDCl3, d): 8.13 (d, J ¼ 8.2 Hz, 2 H),
7.77 (s, 1 H), 7.70 (s, 2 H), 7.44–7.42 (m, 4 H), 7.32 (t, J ¼ 8.2
Hz, 2 H).
Device fabrication and performance measurements
In a general procedure, indium-tin oxide (ITO)-coated glass
substrates were carefully pre-cleaned and treated by UV ozone
for 4 min. A 40 nm poly(3,4-ethylenedioxythiophene) doped with
poly(styrene-4-sulfonate)(PEDOT:PSS) aqueous solution was
spin coated onto the ITO substrate and baked at 200 ꢀC for
10 min. The substrates were then taken into a nitrogen glove box,
where FIrpic-doped host: 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxi-
diazolyl]phenylene (OXD-7) layer was spin coated onto the
PEDOT:PSS layer from the 1,2-dichloroethane solution and
annealed at 80 ꢀC for 30 min. The substrate was then transferred
into an evaporation chamber, where the TPBI was evaporated at
Synthesis of 9,90-(5-bromo-1,3-phenylene)bis-9H-carbazole
(M4). The mixture of 1,3,5-tribromobenzene (10 g, 31 mmol),
carbazole (11.7 g, 70 mmol), copper(I) iodide (0.13 g, 7 mmol),
1,10-phenanthroline (0.13 g, 7 mmol), K2CO3 (9.7 g, 70 mmol),
and dried DMF 100 mL was refluxed under an argon atmosphere
for 12 h. After cooling to room temperature, the solvent was
removed under vacuum and the residue was extracted with
an evaporation rate of 1–2 A sꢁ1 under a pressure of 4 ꢂ 10ꢁ4 Pa
ꢀ
and the Cs2CO3/Al bilayer cathode was evaporated at evapora-
ꢁ1
ꢀ
tion rates of 0.2 and 10 A s for Cs2CO3 and Al, respectively,
under a pressure of 1 ꢂ 10ꢁ3 Pa. The current–voltage–brightness
6132 | J. Mater. Chem., 2010, 20, 6131–6137
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Scheme 1 Synthetic routes toward DQC and PTC.
dichloromethane. The product was then obtained by column
chromatography on silica gel with petroleum ether/ethyl acetate
(10 : 1) as the eluent, to yield a white solid (M4) (6.43 g, 42.6%).
1H-NMR (600 MHz, CDCl3, d): 8.13 (d, J ¼ 8.2 Hz, 4 H), 7.84 (s,
2 H), 7.77 (s, 1 H), 7.53 (d, J ¼ 8.2 Hz, 4 H), 7.45 (t, J ¼ 7.2 Hz, 4
H), 7.32 (t, J ¼ 7.2 Hz, 4 H).
137.5, 129.0, 127.5, 126.4, 123.1, 122.9, 122.7, 120.6, 120.5, 109.8.
MS (EI) m/z 699.2 [M+]. Anal. calcd for C54H33N (%): C, 92.67;
H, 5.33; N, 2.00. Found: C, 92.60; H, 5.40; N, 1.95.
Synthesis of 9,90-(50-phenyl[1,10:30,100-terphenyl]-3,5-diyl)bis-
9H-carbazole (PTC). A mixture of M4 (2 g, 4.1 mmol), M2
(1.61 g, 4.5 mmol), and Pd(PPh3)4 (0.21 g, 0.2 mmol) were added
to 50 mL of 1,4-dioxane solution, then 1 M K2CO3 solution
(10 mL), which was dissolved in H2O, was added to the reaction
mixture. The mixture was refluxed under nitrogen for 24 h. After
the reaction finished, the reaction mixture was extracted with
diethyl ether and water. The organic layer was dried by anhy-
drous MgSO4 and filtered. The product was isolated by silica gel
column chromatography using petroleum ether/ethyl acetate
(20 : 1) eluent to afford a white solid (PTC) (1.48 g, 56.6%). 1H-
NMR (600 MHz, CDCl3, d): 8.18 (d, J ¼ 7.6 Hz, 4 H), 8.05 (s,
2 H), 7.88 (s, 2 H), 7.86 (d, J ¼ 7.6 Hz, 2 H), 7.69 (d, J ¼ 7.6 Hz, 4
H), 7.63 (d, J ¼ 8.2 Hz, 4 H), 7.47–7.45 (m, 8 H), 7.40 (t, J ¼ 7.6
Hz, 2 H), 7.34 (t, J ¼ 7.6 Hz, 4 H). 13C-NMR (600 MHz, CDCl3,
d): 143.9, 142.8, 141.4, 140.9, 139.0, 129.0, 127.8, 127.4, 126.2,
126.0, 125.6, 125.3, 125.2, 123.6, 120.5, 120.2, 110.0. MS (EI) m/z
636.0 [M+]. Anal. calcd for C48H32N2 (%): C, 90.54; H, 5.07; N,
4.40. Found: C, 90.30; H, 5.10; N, 4.45.
Synthesis of 9-(50,5%-diphenyl[1,10:30,100:300,1%:3%,1&-quin-
quephenyl]-500-diyl)-9H-carbazole (DQC). A mixture of M3 (2 g,
5 mmol), M2 (1.96 g, 5.5 mmol), and Pd(PPh3)4 (0.26 g,
0.25 mmol) was added to 50 mL of 1,4-dioxane solution, then
1 M K2CO3 solution (5 mL), which was dissolved in H2O, was
added to the reaction mixture. The mixture was refluxed under
nitrogen for 24 h. After the reaction had finished, the reaction
mixture was extracted with dichloromethane and water. The
organic layer was dried by anhydrous MgSO4 and filtered. The
product was isolated by silica gel column chromatography using
petroleum ether/ethyl acetate (20 : 1) eluent to afford a white
1
solid (DQC) (1.48 g, 42.5%). H-NMR (600 MHz, CDCl3, d):
8.19 (d, J ¼ 7.6 Hz, 2 H), 8.10 (s, 1 H), 7.94 (s, 2 H), 7.90 (s, 4 H),
7.85 (s, 2 H), 7.72 (d, J ¼ 7.6 Hz, 8 H), 7.57 (d, J ¼ 8.2 Hz, 2 H),
7.49–7.43 (m, 10 H), 7.40 (t, J ¼ 6.9 Hz, 4 H), 7.33 (t, J ¼ 7.6 Hz,
2 H). 13C-NMR (600 MHz, CDCl3, d): 143.1, 141.8, 139.2, 138.6,
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Fig. 1 TGA traces of DQC and PTC recorded at a heating rate of 10 ꢀC
minꢁ1. Inset: DSC measurement recorded at a heating rate of 10 ꢀC minꢁ1
Fig. 3 Cyclic voltammograms of DQC and PTC with a concentration of
10ꢁ3M in CH2Cl2 solutions.
.
Thermal analysis
The thermal properties of DQC and PTC were investigated by
thermal gravimetric analyses and differential scanning calorim-
etry. The star-shaped molecular structures of the compounds
impacted by the carbazole and m-terphenyl units are strongly
beneficial to the thermal stability, as indicated by the high
decomposition temperature (Td, corresponding to 5% weight
loss) between 402 ꢀC and 407 ꢀC (Fig.1). Notably, the DQC and
PTC show high Tg values above 128 ꢀC, which is twice as high as
that of mCP (60 ꢀC). The results demonstrate that they could
form morphologically stable and uniform amorphous films upon
solution-process, which is very important in improving the effi-
ciency and lifetime of OLEDs. The morphologies of DQC and
PTC were characterized by atomic force microscopy (AFM)
using standard tapping mode. The films were prepared through
spin-coating from
a
1,2-dichloroethane solution onto
PEDOT:PSS layer and then heated at 80 ꢀC for 30 min to remove
the solvent. As we can see from Fig. 2, the surface of neat DQC
and PTC films are pinhole-free and quite smooth, and the root-
mean-square (RMS) values are 0.342 and 0.385 nm, respectively.
For spin-coated, blue-emitting DQC and PTC films doubly
doped with 10 wt% FIrpic and 30 wt% OXD-7, we calculated the
RMS surface roughness to be 0.486 and 0.495 nm, respectively.
There was no phase separation or any aggregated domains.
These results demonstrate that DQC and PTC are capable of
forming amorphous films through solution processing, with
organometallic triplet emitters dispersed homogeneously.
Fig. 2 AFM topographic images of (a) neat DQC, (b) neat PTC, (c)
DQC doped with 10 wt% FIrpic and 30 wt% OXD-7 and (d) PTC doped
with 10 wt% FIrpic and 30 wt% OXD-7. The films were prepared through
spin-coating from 1,2-dichloroethane solutions onto ITO/PEDOT:PSS.
Results and discussion
Synthesis and characterization
Scheme 1 illustrates the synthetic procedures for DQC and PTC.
First, 4-bromo-50-pheyl-1,10:30,100-terphenyl (M1) was prepared
from 1,3,5-tribromobenzene and phenylboronic acid catalyzed
by Pd(PPh3)4 in 1,4-dioxane, which then converted to the
aryboronic ester of 4,4,5,5-tetramethyl-2-[1,10:30,100-terphenyl]-
50-yl-1.3.2-dioxaborolane (M2) catalyzed by PdCl2(dppf) in the
presence of KOAc in 1,4-dioxane. The 9-(3,5-dibromophenyl)-
9H-carbazole (M3) and 9,90-(5-bromo-1,3-phenylene)bis-
9H-carbazole (M4) were prepared from carbazole and
1,3,5-tribromobenzene by a classic Ullmann reaction. The
Suzuki cross-coupling reactions of M3 and M4 with the ary-
boronic ester M2 led to DQC and PTC, respectively. All of the
compounds were purified by silica chromatography and recrys-
Electrochemical analysis
The electrochemical properties of DQC and PTC were studied in
solution through cyclic voltammetry (CV) using tetrabuty-
lammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte and ferrocene as the internal standard. As shown in
Fig. 3, during the anodic scan in dichloromethane, an oxidation
peak is observed at 1.55 V for DQC and two oxidation peaks are
observed at 1.47 and 1.75 V for PTC. The oxidation peaks at
relatively low potentials of 1.55 and 1.47 V are ascribed to the
oxidation of the carbazole units of DQC and PTC, respectively.
No reduction waves were detected. On the basis of the onset
potentials for oxidation, we estimated the HOMO energy of
DQC and PTC to be ꢁ5.67 and ꢁ5.59 eV, respectively, with
1
tallization, producing very pure powders. H-NMR, 13C-NMR,
mass spectrometry, and elemental analysis were employed to
confirm the chemical structures of the above-mentioned
compounds, as described in the experimental section.
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Table 1 The thermal, electrochemical, and photophysical data of DQC and PTC
Tg/Td/ꢀC
lmax,abs/nmab
lem/nma
lem/nmb
lem (77 K)/nmc
ET/eV
HOMO/LUMO/eVd
DQC
PTC
128/402
131/407
340a/343b
339a/341b
388
389
371
373/401/423
442/471/500
442/471/500
2.81
2.81
5.67/2.18
5.59/2.12
a
In CH2Cl2. b In solid film. c In 2-MeTHF glass. d The HOMO and LUMO energies were determined from cyclic voltammetry and absorption data.
Fig. 4 Optimized geometries and calculated HOMO and LUMO density
maps for DQC and PTC according to DFT calculations at the B3LYP/6-
31* level.
regard to ferrocene (ꢁ4.8 eV below vacuum). The highest occu-
pied molecular orbital (HOMO) energy levels are clearly raised
and approach the work function of poly(3,4-ethyl-
enedioxythiophene) (PEDOT, ꢁ5.2 eV). In contrast, the HOMO
energy level of mCP is at ꢁ6.15 eV.27 These results reveal that the
introduction of m-terphenyl unit can lead to the reduction of the
barrier height for hole injection, thereby facilitating the transfer
of holes. These, together with absorption spectra, are then used
to obtain the lowest unoccupied molecular orbital (LUMO)
energy levels (Table 1).
Fig. 5 (a) Normalized absorption, emission and phosphorescence (77 K)
spectra of DQC, PTC and FIrpic in CH2Cl2 and (b) normalized
absorption and emission spectra of solid films of DQC and PTC.
Theoretical calculations
Density functional theory (DFT) calculations have also been
performed to characterize the three-dimensional geometries, the
frontier molecular orbital energy levels and the ET energies of
DQC and PTC by using the Gaussian 03 program.28 The calcu-
lated geometries of DQC and PTC in Fig. 4 show that the
carbazole and m-terphenyl units are significantly twisted against
the phenyl core, resulting in a non-planar structure in each
molecule. These geometrical characteristics can effectively
prevent intermolecular interactions between p–systems and,
thus, suppress molecular recrystallization and limit the extent of
conjugation between the central core and branches, which
improves the morphological stability of the thin film and keeps
the triplet energy gap at a very high level in these molecules. The
calculation clearly indicated that the expanded p–conjugation
with the incorporation of a m-terphenyl unit into the skeletal
structure of mCP leads to decreases in the ET energy, from
3.32 eV (mCP) to 3.02 eV (DQC) and 3.05 eV (PTC). However,
DQC and PTC are expected to have higher ET than that of the
common host material, CBP (ET ¼ 2.66 eV), from the calcula-
tion. Calculated HOMO and LUMO density maps of DQC and
PTC are also included in Fig. 4. The HOMOs and LUMOs of
DQC are localized predominantly on the carbazole and m-ter-
phenyl fragments, respectively, and similar results are obtained
for PTC. Although the HOMOs and LUMOs of DQC and PTC
are similarly localized in the same units, there was a difference in
the HOMO and LUMO levels between the two materials. The
calculated HOMO and LUMO energies of the DQC are lower
than those of the PTC, which is in agreement with the results
measured by electrochemical CV.
Photophysical properties
Fig. 5 depicts the UV-vis absorption and photoluminescent (PL)
spectra of DQC and PTC in CH2Cl2 and as thin films formed on
quartz substrates by spin-coating. Table 1 contains a more
detailed listing of the photophysical properties of the
compounds. Both the absorption spectra showed vibronic
structures typical of a carbazole moiety at 275–350 nm and the
PL spectra have lost the vibronic structure and red-shifted to
388–389 nm with respect to the carbazole monomer emission. As
shown in Fig. 5(b), the absorption spectra of neat DQC and PTC
films show similar photophysical behaviors to those observed in
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Table 2 The device performance of the electrophosphorescent OLEDs
Von
V
/
Bmax
/
hc,max
/
hext,max
CIE
(x, y)
a
b
Device
A
Host
cd mꢁ2
cd Aꢁ1
(%)c
DQC
PTC
mCP
DQC
PTC
mCP
5.0
4.8
7.0
5.2
4.9
6.9
11 200
14 000
5600
19 100
25 500
8900
9.2
12.8
1.8
21.7
25.7
6.2
4.6
6.5
1.0
9.2
11.9
3.1
(0.16, 0.32)
(0.16, 0.32)
(0.16, 0.31)
(0.17, 0.42)
(0.16, 0.38)
(0.16, 0.33)
Bd
a
b
Maximum luminance. Maximum current efficiency. Maximum
c
d
external quantum efficiency. A 30 nm thick TPBI layer was inserted
between the light-emitting layer and the Cs2CO3 layer.
a value higher than reported for the common triplet blue-emitter
FIrpic (2.62 eV). Using host materials that possess high triplet
energies is a provision for effective confinement of the triplet
excitons on the guest and, consequently, for prevention of back
energy transfer between the host and dopant molecules. In this
case, the triplet energy of DQC and PTC are sufficiency high to
serve as appropriate hosts for FIrpic.
Electroluminescent properties
To assess the utility of those solution-processable compounds as
host materials in OLEDs, we fabricated single-layer blue elec-
trophosphorescent devices with DQC or PTC as the host and
10 wt% FIrpic as the dopant. The electron-transporting material,
OXD-7 was mixed into host materials to facilitate the electron
transport in the light-emitting layer.29–31 The single-layer devices
with
the
configuration
ITO/PEDOT:PSS/Host:OXD-
7(30 wt%):FIrpic(10 wt%)/Cs2CO3/Al have been fabricated by
spin-coating. The properties of these devices are compared to the
electroluminescence (EL) properties of mCP, which have been
usually chosen as host material for blue electrophosphorescence
devices. As shown in Fig. 6(a), the EL spectra of DQC- and PTC-
based devices are almost identical, with the emission of FIrpic,
corresponding to the CIE coordinates of (0.16, 0.32). When we
increased the voltage to 12 V, the CIE coordinates of the emis-
sion color remained almost constant and the EL spectrum barely
charged. Furthermore, no additional emission coming from
DQC or PTC is observed, indicative of efficient energy transfer
from the host to FIrpic. Fig. 6(b) presents the current density–
voltage–luminance (I–V–L) curves of DQC- and PTC-based
devices, which was turned on at 5.0 and 4.8 V (corresponding to
1 cd mꢁ2) and had a maximum brightness of 11200 and 14000 cd
mꢁ2. As revealed in Fig. 6(c), the maximum luminance efficiencies
of these single-layer blue-emitting devices were 9.2 and 12.8 cd
Aꢁ1, and the corresponding external quantum efficiencies (max,
hext) were 4.6 and 6.5%. In contrast, the single-layer device based
on mCP shows a maximum luminance of only 5600 cd mꢁ2 and
Fig. 6 (a) EL spectra of the DQC- and PTC-based devices (b) I–V–L
characteristics and (c) luminance efficiency versus current density plots of
the DQC-, PTC- and mCP-based devices.
CH2Cl2. In contrast, the PTC film displays a vibronic feature
with particular emission peaks at 373, 401 and 423 nm, which are
not observed in dilute solution, likely because of various degrees
of intermolecular aggregation and the excimer formation in the
film. However, the DQC film exhibits only one peak at 371 nm,
indicating that the emissions from aggregates and excimer are
effectively suppressed. The PL emission peak of the host mate-
rials was well overlapped with the metal-to-ligand charge-
transfer absorption peaks of the FIrpic around 380 and 420 nm,
a maximum luminance efficiency of 1.8 cd Aꢁ1
.
Generally, for single-layer devices, severe exciton quenching
exists at the cathode interface, resulting in degraded device
performance. To improve the performance of the device based on
DQC and PTC further, a thin TPBI electron-transporting and
exciton-confining layer was inserted between the light-emitting
layer and the cathode.32,33 Devices with configurations of ITO/
€
indicating that the Forster energy transfer from the host mate-
rials to the FIrpic would be efficient. Fig. 5(a) also depicts the
phosphorescence spectra of DQC and PTC measured from
a frozen 2-methyltetrahydrofuran matrix at 77 K. The highest
energy 0–0 phosphorescent emission located at 2.81 eV was used
to calculate the triplet energy gap of DQC and PTC, giving
PEDOT:PSS/Host:OXD-7(30
wt%):FIrpic(10
wt%)/
TPBI(30 nm)/Cs2CO3/Al have been fabricated. As shown in
6136 | J. Mater. Chem., 2010, 20, 6131–6137
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Fig. 6(b), the brightness of the devices with a TPBI layer was
significantly increased compared with those of single-layer devices.
After the insertion of the TPBI layer, the maximum efficiency of
the DQC- and PTC-based device was largely improved to 21.7 and
25.7 cd Aꢁ1, and the corresponding maximum external quantum
efficiencies were 9.2 and 11.9%. The performance of DQC- and
PTC-based devices is far superior to those of the corresponding
mCP-based devices (the EL data are summarized in Table 2). The
star-shaped structure of molecules led to forming morphologically
stable and uniform amorphous films upon solution-process,
resulting in a dramatic improvement of the EL performance of
DQC and PTC as compared to that of mCP.
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Conclusions
In summary, we have designed and synthesized two novel star-
shaped host materials, DQC and PTC, for solution processed
blue phosphorescent organic light-emitting devices that contain
carbazole and m-terphenyl units. The calculated geometries of
DQC and PTC show that the carbazole and m-terphenyl units
are significantly twisted against the phenyl core, effectively sup-
pressing molecular recrystallization and limit the extent of
conjugation, which improves the morphological stability of the
thin film and keeps the triplet energy gap at a very high level.
Utilizing those new compounds as hosts, single layer solution-
processed blue phosphorescence OLEDs with FIrpic as a dopant
show a maximum current efficiency of 12.8 cd Aꢁ1, and
a maximum external quantum efficiency of 6.5%. After a thin
TPBI layer was inserted between the light-emitting layer and the
cathode, the maximum efficiency of the devices was further
improved to 25.7 cd Aꢁ1, and a maximum external quantum
efficiency of 11.9%. The performance of DQC- and PTC-based
devices is far superior to those of the corresponding mCP-based
devices, which is outstanding for a solution-processed blue
phosphorescent OLED. Our work suggests that highly efficient
solution processed blue phosphorescence OLEDs can be ach-
ieved in the design of such host materials with high triplet energy,
excellent film-forming ability and morphological stability.
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We thank the National Nature Science Foundation of China
(Grant no. 50990060) and the National Key Basic Research and
Development Program of China (Grant no. 2006CB806203,
2009CB623604) for support.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 6131–6137 | 6137