Hexaphenylphenylene dendronised pyrenylamines for efficient organic light-emitting diodes

Article abstract of DOI:10.1039/b509325b

A series of blue-emitting triarylamines containing pyrene and hexaphenylbenzene dendrons were successfully synthesized by employing the Diels-Alder and palladium catalyzed C-N coupling reactions. They display high glass transition temperatures (>140°C) in

Full text of DOI:10.1039/b509325b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Hexaphenylphenylene dendronised pyrenylamines for efficient organic
light-emitting diodes
K. R. Justin Thomas,a M. Velusamy,a Jiann T. Lin,*ab C. H. Chuena and Yu-Tai Tao*a
Received 1st July 2005, Accepted 15th August 2005
First published as an Advance Article on the web 7th September 2005
DOI: 10.1039/b509325b
A series of blue-emitting triarylamines containing pyrene and hexaphenylbenzene dendrons were
successfully synthesized by employing the Diels–Alder and palladium catalyzed C–N coupling
reactions. They display high glass transition temperatures (.140 uC) in differential scanning
calorimetry and facile reversible oxidation couple in cyclic voltammetry. They are also thermally
stable exhibiting decomposition temperature above 385 uC. Efficient blue-emitting
electroluminescent devices were fabricated using these novel amines as the hole transporting layer
and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the electron transporting layer.
Colour mixing or green emission was observed when tris(8-hydroxyquinoline)aluminium (Alq3)
was used as the electron transporting layer. However insertion of a thin layer of TPBI or
1,3,5-tris(4-tert-butylphenyl-1,3,4-oxadiazolyl)benzene (TPOB) in between the HTL and Alq3
layers led to pure blue emission owing to the confinement of recombination inside the HTL layer
containing the compounds.
are readily available from the Diels–Alder reaction of alkynes
and cyclopentadienone.12 These polyphenyl dendrimers
Introduction
It is necessary to develop organic or organometallic com-
possessed unique photophysical properties and thermal
pounds with appropriate colour (blue, green and red)
stability. Small molecules based on polyphenylbenzene motifs
chromaticity for efficient translation of organic light-emitting
have been utilized to arrive at electroluminescent materials
emitting blue13 and red14 colours. In this article we broaden
diode (OLED) technology into practical flat-panel display
applications.1,2 Though organic small molecules and polymers
the application of the hexaphenylbenzene core in the con-
have been intensively sought for applications in OLED for a
struction of organic materials suitable for OLED fabrication
decade now, recent phosphorescent devices fabricated using
by tethering it with pyrenylamine which is a strong blue
triplet emitters such as cyclometallated iridium(III) complexes
emitter. We observe a narrow blue emission for these
decreased the dominance of organic compounds in the OLED
conjugates and they additionally display marked thermal
field. Though the organometallic triplet emitters are perform-
robustness. These two properties make them suitable for
ing efficiently their cost is high and only green and red emitters
electroluminescent applications.
have been realized so far with exceptional performance.3 Blue-
emitting organometallic complexes are scarce and their device
Experimental
fabrication is also limited by the non-availability of suitable
triplet hosts.4 Hence blue-emitting organic compounds that
General information
display colour purity and good performance statistics in
Unless otherwise specified, all reactions and manipulations
electroluminescent devices (EL) are attractive.5 Organic
were performed under nitrogen atmosphere using standard
materials reported to emit blue colour in EL are mostly
Schlenk techniques. All chromatographic separations were
polymeric in nature.6 Organic small molecules derived from
carried out on silica gel (60 M, 230–400 mesh). Dichloro-
fluorene7 or bifluorene8 motifs have been found to perform
methane and toluene were distilled from calcium hydride and
efficiently in OLED with bright blue emission. Triarylamines
Na/benzophenone respectively under nitrogen atmosphere.
containing polyaromatics such as pyrene,9 carbazole10 and
anthracene11 have been demonstrated as efficient emitting hole
Instrumentation
transporters. Only anthracene based amines exhibited promis-
ing blue EL performance while others displayed red-shifted
emission with green or yellow colours. Recently Mu¨llen et al.
demonstrated that large molecular architectures can be
assembled utilizing hexaphenylbenzene building blocks that
All the reactions were performed under inert atmosphere
unless otherwise specified. 1H spectra were recorded on a
Bruker AC300 spectrometer operating at 300.135 MHz. Mass
spectra (FAB) were recorded on a Hewlett-Packard GC/MS
5995 spectrometer. Elemental analyzes were performed on a
Perkin-Elmer 2400 CHN analyzer. Electronic absorption
spectra were measured on a Cary 50 Probe UV-visible
spectrometer. Emission spectra were recorded by a Hitachi
fluorescence spectrometer (F4500). Emission quantum yields
aInstitute of Chemistry, Academia Sinica, 115 Taipei, Taiwan
E-mail: jtlin@chem.sinica.edu.tw; Fax: +886-2-27831357;
Tel: +886-2-27898522
bDepartment of Chemistry, National Central University, 320 Chungli,
Taiwan
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were obtained by the method of Demas and Crosby15 using
Coumarin-I (WF 5 0.99 in ethyl acetate) as reference. Cyclic
voltammetry experiments were performed with a BAS-100
electrochemical analyzer. All measurements were carried out
at room temperature with a conventional three-electrode
configuration consisting of a glassy carbon working electrode,
a platinum auxiliary electrode and a non-aqueous Ag/AgNO3
reference electrode. The E1/2 values were determined as
1/2(Ep + Ep ), where Epaand Ep are the anodic and cathodic
peak potentials, respectively. All potentials reported are
referenced to ferrocene which was used as internal standard
toward the end of each experiment. The solvent in all
experiments was CH2Cl2 and the supporting electrolyte was
0.1 M21 tetrabutylammonium hexafluorophosphate. DSC
measurements were carried out on a Perkin-Elmer differential
scanning calorimeter at a heating rate of 10 uC min21 and a
cooling rate of 30 uC min21 under nitrogen atmosphere. TGA
measurements were performed on a TA-7 series thermogravi-
metric analyzer at a heating rate of 10 uC min21 under a flow
of nitrogen. The OLED fabrication procedure is elaborted in
detail in an earlier report.10a
6.37–6.41 (m, 6 H), 6.63–6.68 (m, 8 H), 6.74–6.78 (m, 4 H),
6.82–6.87 (m, 6 H), 6.95–6.98 (m, 2 H).
6. Obtained in 77% from 3,4-bis(4-tert-butylphenyl)-2,5-
diphenylcyclopenta-2,4-dienone and 1-(2-(4-tert-butylphenyl)-
ethynyl)-4-bromobenzene as described above. Colourless
solid. MS (FAB): m/z: 782.3 (M+). 1H NMR (CDCl3, d):
1.07 (s, 18 H), 1.11 (s, 9 H), 6.60–6.66 (m, 8 H), 6.77–6.85 (m,
16 H), 6.91–6.95 (m, 2 H).
a
c
c
7. Synthesized in 88% yield from 3,4-bis(4-methoxyphe-
nyl)-2,5-diphenylcyclopenta-2,4-dienone and 1-(2-(4-tert-butyl-
phenyl)ethynyl)-4-bromobenzene as described above for 5.
Colourless solid. MS (FAB): m/z: 756.3 (M+). 1H NMR
(CDCl3, d): 1.11 (s, 9 H), 3.60 (s, 6 H), 6.37–6.40 (m, 4 H),
6.62–6.68 (m, 8 H), 6.76–6.87 (m, 12 H), 6.91–6.94 (m, 2 H).
2,5-Bis(4-bromophenyl)-3,4-bis(4-methoxyphenyl)cyclopenta-
2,4-dienone (8). Yield: 72%. Purple solid. MS (FAB): m/z:
602.0 (M+). 1H NMR (CDCl3, d): 3.78 (s, 6 H), 6.69–6.73
(m, 4 H), 6.79–6.82 (m, 4 H), 7.06–7.10 (m, 4 H), 7.33–7.37 (m,
4 H).
Synthesis
2,5-Bis(4-bromophenyl)-3,4-bis(4-tert-butylphenyl) cyclo-
penta-2,4-dienone (9). Yield: 84%. Purple solid. MS (FAB):
m/z: 654.1 (M+). 1H NMR (CDCl3, d): 1.25 (s, 18 H), 6.74–6.78
(m, 4 H), 7.07–7.11 (m, 4 H), 7.12–7.16 (m, 4 H), 7.32–7.36 (m,
4 H).
The precursors 1,3-bis(4-bromophenyl)propan-2-one,16 1,2-bis-
(4-tert-butylphenyl)ethyne,17 1-bromo-4-((4-methoxyphenyl)-
ethynyl)benzene18 and 1,2-bis(4-methoxyphenyl)ethyne19 were
prepared by following the literature procedures. The syn-
thesis of B7 was described earlier.14
10. Yield: 89%. Colourless solid. MS (FAB): m/z: 812.6
(M+). Anal. Calcd. for C46H36Br2O4: C, 67.99; H, 4.47. Found:
C, 67.78; H, 4.36%. 1H NMR (CDCl3, d): 3.62 (s, 12 H), 6.38–
6.43 (m, 8 H), 6.60–6.64 (m, 12 H), 6.96–6.99 (m, 4 H).
3,4-Bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone
(3). A mixture of 1,2-bis(4-methoxyphenyl)ethane-1,2-dione
(2.70 g, 10 mmol), 1,3-diphenylpropan-2-one (2.10 g, 10 mmol)
and ethanol (50 mL) was heated to 80 uC and KOH (0.56 g,
10 mmol) added at once. An exothermic reaction was followed
by a dark purple solid precipitation. The solids were collected
by filtration, washed thoroughly with ethanol (3 6 30 mL)
and dried. Yield: 3.84 g (86%). Dark purple solid. MS (FAB):
m/z: 444.2 (M+). 1H NMR (CDCl3, d): 3.77 (s, 6 H), 6.68–6.71
(m, 4 H), 6.82–6.85 (m, 4 H), 7.21–7.23 (m, 10 H).
11. Yield: 80%. Colourless solid. MS (FAB): m/z: 916.9.
Anal. Calcd. for C58H60Br2: C, 75.98; H, 6.60. Found: C,
1
76.01; H, 6.54%. H NMR (CDCl3, d): 1.10 (s, 36 H), 6.59–
6.62 (m, 8 H), 6.65–6.69 (m, 4 H), 6.79–6.83 (m, 8 H), 6.92–
6.96 (m, 4 H).
12. Yield: 76%. Colourless solid. MS (FAB): m/z: 864.7
(M+). Anal. Calcd.for C52H48Br2O2: C, 72.22; H. 5.59. Found:
C, 72.05; H, 5.37%. 1H NMR (CDCl3, d): 1.10 (m, 18 H), 3.63
(s, 6 H), 6.39–6.44 (m, 4 H), 6.57–6.60 (m, 4 H), 6.63–6.67 (m,
8 H), 6.79–6.82 (m, 4 H), 6.94–6.97 (m, 4 H).
3,4-Bis(4-tert-butylphenyl)-2,5-diphenylcyclopenta-2,4-die-
none (4). Prepared from 1,2-bis(4-tert-butylphenyl)ethane-1,2-
dione as described above in 89% yield. Dark purple solid.
MS (FAB): m/z: 496.3 (M+). 1H NMR (CDCl3, d): 1.22 (s,
18 H), 6.79–6.82 (m, 4 H), 7.11–7.15 (m, 4 H), 7.18–7.23 (m,
10 H).
B1. A mixture of 5 (1.40 g, 2 mmol), N-phenylpyren-1-amine
(0.65 g, 2.2 mmol), potassium tert-butoxide (0.288 g, 3 mmol),
Pd(dba)2 (11.4 mg, 0.02 mmol), tri-tert-butylphosphine (6 mg,
0.03 mmol) and toluene (20 mL) was heated at 80 uC for 8 h.
After it cooled, water and diethyl ether were added and the
organic layer was separated. It was dried over anhydrous
MgSO4 and evaporated to dryness to leave the crude product.
It was purified further by column chromatography on silica gel
using a hexane–dichloromethane mixture (2 : 3). Yield: 1.62 g
(88%). Yellow solid. MS (FAB): m/z: 995.5 (M + H). Anal.
Calcd. for C76H67N: C, 91.80; H, 6.79; N, 1.41. Found: C,
5. A mixture of 3,4-bis(4-methoxyphenyl)-2,5-diphenyl-
cyclopenta-2,4-dienone (4.45 g, 10 mmol), 1-bromo-4-(2-(4-
methoxyphenyl)ethynyl)benzene (3.16 g, 11 mmol) and
diphenyl ether (10 mL) were heated to 200 uC for 3 h during
which the dark purple colour vanished. It was cooled and
poured into cold methanol. The colourless precipitate formed
was filtered, washed with methanol and dried. Further
purification was performed by column chromatography on
silica gel and hexane–dichloromethane mixture (1 : 1) as
eluent. Yield: 6.54 g (93%). Colourless solid. MS (FAB): m/z:
1
91.72; H, 6.81; N, 1.34%. H NMR (CDCl3, d):1.09 (s, 18 H),
1
704.2 (M+). H NMR (CDCl3, d): 3.59 (s, 6 H), 3.61 (s, 3 H),
1.24 (s, 9 H), 6.57–6.68 (m, 8 H), 6.73–6.93 (m, 20 H), 7.09
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(t, J 5 7.8 Hz, 2 H), 7.58 (d, J 5 8.2 Hz, 1 H), 7.84 (d,
J 5 8.2 Hz, 1 H), 7.92–81.5 (m, 8 H).
B4. Obtained in 89% yield from 10 and N-phenylpyren-1-
amine (2.2 equiv.) as described above for B1. Yellow solid. MS
(FAB): m/z: 1342.8 (M + H). Anal. Calcd. for C102H88N2: C,
91.30; H, 6.61; N, 2.09. Found: C, 91.16; H, 6.52; N, 1.94%. 1H
NMR (CDCl3, d): 1.10 (s, 36 H), 6.57–6.72 (m, 20 H), 6.77–
6.86 (m, 10 H), 7.03 (t, J 5 7.8 Hz, 4 H), 7.56 (d, J 5 8.2 Hz,
2 H), 7.80–7.83 (m, 2 H), 7.90–8.06 (m, 12 H), 8.10 (d, J 5
7.3 Hz, 2 H).
B2. Prepared from N-phenylpyren-1-amine and
6 as
described above. Yield: 75%. Yellow solid. MS (FAB): m/z:
917.1 (M + H). Anal. Calcd. for C67H49NO3: C, 87.84; H, 5.39;
N, 1.53. Found: 87.55; H, 5.26; N, 1.48%. Found: H NMR
1
(CDCl3, d): 3.59 (s, 3 H), 3.61 (s, 3 H), 3.63 (s, 3 H), 6.35–6.47
(m, 7 H), 6.52–6.55 (m, 3 H), 6.62–6.72 (m, 7 H), 6.79–6.89 (m,
9 H), 6.91–6.95 (m, 4 H), 7.13 (t, J 5 7.7 Hz, 2 H), 7.58 (dd,
J 5 8.2 Hz, 1 H), 7.89–8.08 (m, 7 H).
B5. Synthesized from 11 as described above. Yield: 83%.
Yellow solid. MS (FAB): m/z: 1238.5 (M + H). Anal. Calcd.
for C90H64N2O4: C, 87.35; H, 5.21; N, 2.26. Found: C, 87.09;
H, 5.18; N, 2.11%. 1H NMR (CDCl3, d): 3.62 (s, 12 H), 6.45 (d,
J 5 8.5 Hz, 8 H), 6.58 (s, 8 H), 6.67–6.70 (m, 8 H), 6.79–6.88
(m, 5 H), 7.07–7.10 (t, J 5 7.7 Hz, 4 H), 7.59 (d, J 5 8.2 Hz,
2 H), 7.83–7.87 (m, 5 H), 7.96 (s, 2 H), 7.96–7.98 (m, 4 H),
8.03–8.12 (m, 6 H).
B3. Prepared from N-phenylpyren-1-amine and
7 as
described above for B1. Yield: 71%. Yellow solid. MS
(FAB): m/z: 943.2 (M + H). Anal. Calcd. for C70H55NO2: C,
89.23; H, 5.88; N, 1.49. Found: C, 89.01; H, 5.74; N, 1.38%. 1H
NMR (CDCl3, d): 1.10 (s, 9 H), 3.59 (s, 3 H), 3.60 (s, 3 H),
6.37–6.41 (m, 5 H), 6.58–6.62 (m, 3 H), 6.65–6.71 (m, 8 H),
6.76–6.86 (m, 8 H), 6.87–6.90 (m, 6 H), 7.07 (t, J 5 7.7 Hz,
2 H), 7.52 (s, 1 H), 7.88–8.04 (m, 7 H).
B6. Prepared from 12 as described above for B4. Yield: 71%.
Yellow solid. MS (FAB): m/z: 1290.7 (M + H). Anal. Calcd.
for C96H76N2O2: C, 89.41; H, 5.94; N, 2.17. Found: C, 89.26;
H, 5.83; N, 1.99%. 1H NMR (CDCl3, d): 1.14 (s, 18 H), 3.56 (s,
6 H), 6.43 (d, J 5 8.6 Hz, 4 H), 6.59–6.62 (m, 8 H), 6.67–6.71
(m, 8 H), 6.75–6.80 (m, 4 H), 6.87 (d, J 5 8.4 Hz, 6 H), 7.07 (t,
J 5 7.7 Hz, 4 H), 7.58 (d, J 5 8.2 Hz, 2 H), 7.82–7.97 (m,
10 H), 8.01 (s, 2 H), 8.06–8.12 (m, 4 H).
Results and discussion
The synthetic pathways used for the construction of the blue-
emitting chromophores are presented in Schemes 1 and 2.
These involve three reactions, namely: (i) cycloaddition of a
Chart 1 Structure of the amines.
Scheme 1 Synthesis of the monoamines, B1–B3.
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Fig. 1 Absorption and emission spectra of selected compounds (B3
and B6) recorded in dichloromethane solution.
shoulders. The most bathochromically shifted transition is
ascribed to that originating from the amine to pyrene charge
transfer state. On increasing the number of pyrenylamines
present in the molecule the optical density of all the transitions
increases significantly. This clearly indicates that most of
the transitions are associated with the pyrenylamine unit. In
comparison with the UV profile of similar compounds we
assign the transitions at ca. 325 nm to that arising from the
pyrene based p–p* transitions.21 The intense bands appearing
at ca. 250–280 nm probably stem from terphenyl chromo-
phoric axes present in the molecule.22 In fact there are three
different terphenyl axes locatable in a single molecule which
accounts for the multiple shoulders protruding in the higher
energy region.
Scheme 2 Synthesis of the diamines, B4–B6.
diarylated acetone with the corresponding a,b-diketone; (ii)
Diels–Alder reaction between the cyclopentadienone formed in
the previous reaction and an acetylene derivative; (iii) finally
the C–N coupling reaction20 of the bromo derivative of the
hexaphenylbenzene and N-phenylpyren-1-ylamine. As most of
the above mentioned reactions proceed with large conversion
rates, the desired final products (B1–B6) were obtained in
excellent yields. The compounds are yellow in colour and
soluble in most common organic solvents such as dichloro-
methane, toluene, tetrahydrofuran, etc. and insoluble in
alcohols. The NMR spectral data recorded in CDCl3, mass
spectral data and elemental analyses results are in accordance
with the proposed structures.
All the compounds exhibit identical emission profiles in
dichloromethane solutions. This clearly indicates that the
emission originates from a similar excited state in this class of
compounds. Molecular modelling calculations (SPARTAN,
PM3) reveal that in this series of compounds the HOMO
is located in the aminopyrene segment while the LUMO is
contributed by the pyrene unit. Evidently the amine to pyrene
charge transfer state is the excited state in this series of
compounds which will be somewhat polar in nature. Due to
this a strong medium effect is observed in the fluorescence
spectra of the compounds (B1–B6). For instance B2 displays a
blue-shifted emission profile in solid film (466 nm) when
compared to that observed for dichloromethane solution
(494 nm). This latter observation suggests that the polarity
Fig. 1 shows representative examples of the absorption and
emission profiles of the compounds recorded in dichloro-
methane. All the relevant parameters are collected in Table 1.
In the absorption spectra all the compounds display three
prominent bands with unsymmetrical shape and adjoining
Table 1 Physical parameters of the compoundsa
Compd
lmax/nm
lem/nm [WF (%)]
lem(film)/nm
Tg, Tm, Td/ uC
Eox, DEp/mV
HOMO/eV
LUMO/eV
B1
B2
B3
B4
B5
B6
a
405, 388, 317, 274
406, 388, 317, 274
406, 387, 317, 274
406, 389, 326, 318, 274
406, 387, 328, 318, 274
406, 387, 327, 319, 274
494 [27]
494 [51]
494 [49]
492 [43]
492 [58]
492 [48]
474
466
470
488
476
490
141, 272, 449
140, 268, 386
140, 240, 456
157, 313, 535
155, 308, 512
151, 333, 540
386, 70
389, 73
391, 69
381, 94
405, 97
389, 97
5.186
5.189
5.191
5.181
5.205
5.189
2.429
2.369
2.403
2.393
2.417
2.369
Absorption and emission spectra of the compounds were recorded in dichloromethane solutions. Potentials reported are referenced related to
the ferrocene internal standard. HOMO values were deduced from the relation: HOMO 5 4.8 + Eox. LUMO values were back calculated from
the relation band gap 5 HOMO 2 LUMO where the band gap values were input from those derived from the optical edge.
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of the solid film is less than that in the dichloromethane
solution. This may be due to the reduced coplanarity of the
molecules in the solid state due to the more restricted bond
rotation.
Thermal properties of the compounds were investigated
by differential scanning calorimetry and thermogravimetry
under a flow of N2. The glass transition temperatures and the
thermal decomposition onset temperatures are listed in Table 1.
These new giant molecules display impressive thermal stability
and amorphous propensity. The glass transition temperatures
(Tg) of the present compounds are significantly higher than
those reported for commonly employed hole-transporting
materials such as TPD (Tg
5 60 uC) and a-NPD
(Tg 5 96 uC).23 Also, the Tg of the present compounds are
higher than those of hexathienyllbenzene derivatives contain-
ing six peripheral amines24 and diamines based on biphenyl
bridges.23 The high Tg recorded for these compounds is
attributed to the presence of the rigid pyrene segment and
hexaphenylbenzene core in the molecular architecture.
Fig. 2 Energy level alignment for the Alq3 (a) and TPBI (b) based
devices.
The redox propensity of the materials is a very crucial factor
that affects the performance of the OLEDs fabricated using
them, as it will exert a direct impact on the charge injection
and mobility in the molecular layer.25 We have examined
the redox behaviour of the present compounds by the cyclic
voltammetric method. All the molecules underwent a quasi-
reversible one-electron oxidation originating from the pyrenyl-
amine segment. No reduction waves were noticed within the
observable potential window.
emission while the Alq3 devices produced intense green
emission originating from Alq3. The EL spectra of the TPBI
based devices are narrow with small full-width at half-maxima
values and were superimposable onto the PL spectra of the
vapour deposited thin (approx. 80 nm) film. A representative
EL and PL comparison is provided in Fig. 3 for the compound
B1. Though the CIE 1931 x, y coordinates realized for these
devices slightly deviate from the NTSC standard blue colour
(CIE, x, y: 0.14, 0.08), they still lie in the pure blue region.
We believe the comparatively larger LUMO barrier (0.9 eV)
at the interface of Alq3/HTL, when compared to the barrier at
the TPBI/HTL interface (y0.3 eV), plays an important role by
slowing down the electron injection into the HTL layer. As this
happens the holes that are more mobile can easily surpass the
HTL/Alq3 HOMO barrier (y0.8 eV) and leak into the Alq3
layer. To restrict the recombination zone within the HTL in
Alq3 based devices we have introduced a thin (10 nm) hole
blocking layer (HBL). Since the TPBI based devices yielded
blue emission originating from the HTL, it can be safely
assumed that the TPBI layer effectively blocks the holes in
HOMO values calculated from the oxidation potentials by
comparing with the ionization potential of ferrocene
(HOMO 5 4.8 2 Eox; where Eox is the oxidation potential
of the material with referene to ferrocene) and the LUMO
values deduced from the band gap calculated from the optical
edge are listed in Table 1. The orbital energy values are
favorable for charge injection and transport in a multi-layered
OLED configuration. The closeness of the HOMO of these
materials to the work function of ITO (Ef 5 4.7 eV) indicates
that hole injection into the molecular layer of these compounds
will be effective. Use of an appropriate electron transporting
layer (ETL) is essential to restrict recombination inside the
hole-transporting layer (HTL). This will ensure blue emission
of the present compounds. Inspection of the energy level
alignment for the molecules and electron transporting
materials such as Alq3 and TPBI depicted in Fig. 2 will help
to decide on this issue. It is evident that the passage of holes
from the compounds to TPBI will be retarded due to the large
HOMO energy barrier (y1.0 eV). However electrons can
easily cross over to the HTL from TPBI as the LUMO barrier
is small (y0.3 eV) when compared to the HOMO barrier.
However, use of Alq3 as electron-transporting material affects
both the HOMO and LUMO barriers at the HTL/ETL
junction. It is also inferred that a y0.6 eV increase in the
LUMO barrier and a y0.2 eV decrease in the HOMO barrier
when compared to that of the HTL/TPBI interface will
dramatically affect the positioning of the recombination zone
in the Alq3 based devices.
We have tested the both electron transporting materials
TPBI and Alq3 with the device structure ITO/B1–B3 or
B7/ETL/Mg:Ag. All the TPBI based devices led to bright blue
Fig. 3 Comparison of EL spectra observed for the B1 based devices
and PL spectra of the vapour deposited B1 film.
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Table 2 EL parameters for the devices fabricated using B1–B3 and B7
gex,max
(%)
lEL
(fwhm)/nm
L100
/
gex,100
(%)
gcu,100/
Device
VON/V
Lmax/cd m22
CIE 1931 x, y
cd m22
cd A21
ITO/B1/TPBI/Mg : Aga
ITO/B2/TPBIMg : Aga
ITO/B2/TPBI/Alq3/Mg : Agb
ITO/B2/TPOB/Alq3/Mg : Agb
ITO/B3/TPBI/Mg : Aga
ITO/B3/TPBI/Alq3/Mg : Agb
ITO/B3/TPOB/Alq3/Mg : Agb
ITO/B7/TPBI/Mg : Aga
a
3.0
3.5
3.0
3.5
3.0
3.5
4.0
3.5
21150 at 14 V
10800 at 14 V
10170 at 15 V
11070 at 14.5 V
20590 at 14 V
16210 at 15 V
7820 at 15 V
11970 at 15 V
2.7
2.0
1.7
1.7
2.6
2.2
1.4
2.5
b
474 (56)
464 (54)
468 (52)
466 (52)
472 (56)
474 (58)
476 (58)
472 (54)
0.13, 0.21
0.13, 0.15
0.13, 0.17
0.14, 0.16
0.13, 0.20
0.13, 0.22
0.13, 0.23
0.13, 0.20
3750c
2370d
1720e
2020f
3520g
3400h
1940i
2610j
2.5c
2.0d
1.3e
1.7f
2.4g
2.2h
1.2i
1.8j
3.8c
2.4d
1.7e
2.0f
3.5g
3.4h
1.9i
2.6j
d
c
Device structure: ITO/compd (40 nm)/TPBI (40 nm)/Mg : Ag. 40 nm HTL, 10 nm HBL and 30 nm ETL were used. At 6.8 V. At 7.7 V.
e
f
g
h
i
j
At 7.7 V. At 8.2 V. At 7.0 V. At 8.7 V. At 10.0 V. At 6.5 V.
such architectures while permitting electron injection into
HTL. So why not use TPBI as a HBL in Alq3 based devices?
Device engineering strategies using TPBI as a hole blocking
host have been successfully demonstrated earlier to result
performance enhancement and colour purity in blue emitting
doped devices.26 The compounds B2 and B3 were used to
optimize the Alq3 based devices further. Indeed the use of a
thin layer of TPBI (10 nm) effectively blocked the holes and led
to blue emission. Additionally we found that the use of a
oxadiazole based star-burst electron-transporting material,
TPOB,27 also produced the same result for which the LUMO
is located at 2.9 eV. Full details are collected in Table 2.
The I–V–L characteristics of the devices are displayed in
Fig. 4 and 5 and the essential performance parameters are
collected in Table 2. Generally, for the blue-emitting devices
the use of a hole-blocker often led to an improvement in
efficiency. In our cases we found that devices where TPBI was
used as the electron-transporting layer excel in performance
when compared to Alq3 based devices containing an additional
hole blocking layer. This clearly suggests that the electron
injection into the emissive HTL is the limiting factor. The
performance of the blue-emitting devices fabricated using B1
or B3 and TPBI as electron-transporting layer rivals certain
anthracene11,28 based blue-emitting materials and amine-
oxadiazole29 conjugates reported earlier. It is to be noted here
Fig. 5 I–V–L characteristics of Alq3 based devices of B2 and B3 with
an additional hole-blocking layer.
that fluorene based blue-emitting materials were found to facili-
tate electron injection from the metal cathode thus leading
to superior performance in organic light-emitting diodes.30
In conclusion, we have synthesized a new series of pyrene-
amines that contain hexaphenylbenzene chromophores. The
design leads to high Tg blue-emitting materials possessing good
hole transporting capability. Bright blue emitting devices
were realized for both TPBI and Alq3 based devices, while
the latter OLEDs required a thin hole-blocking layer to
achieve recombination inside the hole transporting layer. The
new materials described here represent a novel entry to the
blue-emitting OLEDs that may find potential application in
the fabrication of white OLEDs or as hosts in electrophos-
phorescent devices.
Acknowledgements
Financial support from Academia Sinica is gratefully
acknowledged.
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