Colour tuning of blue electroluminescence using bipolar carbazole-oxadiazole molecules in single-active-layer organic light emitting devices (OLEDs)

Article abstract of DOI:10.1039/c2jm31825c

A synthetically versatile strategy has been employed for luminescence colour tuning in a new series of bipolar carbazole-2,5-diaryl-1,3,4-oxadiazole hybrid molecules 1-7. Their syntheses, solution absorption and emission properties and cyclic voltammetric data are reported. Calculations using DFT (density functional theory) establish that they possess molecular orbitals which favour bipolar charge-transport. Single-active-layer organic light emitting devices (OLEDs) have been fabricated by thermal evaporation using the bipolar compounds as the emitters in the architecture ITO:PEDOT-PSS:X:Ca/Al (X = 1-7). The structure-property relationships within the series of compounds are assessed with emphasis on the OLED performance and emission colour. The HOMO-LUMO gap has been varied by systematic modifications of the molecular subunits of 1-7, allowing the colour of the electroluminescence to be tuned from deep blue (CIE x,y 0.157, 0.079) through to green (CIE x,y 0.151, 0.096). These materials are very attractive for further development due to the combination of good processability of the molecules, their bipolar structure, colour tunability and efficient performance of OLEDs using a simple device architecture.

Full text of DOI:10.1039/c2jm31825c

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PAPER
Colour tuning of blue electroluminescence using bipolar carbazole–oxadiazole
molecules in single-active-layer organic light emitting devices (OLEDs)†
a
Katharine E. Linton, Alison L. Fisher, Christopher Pearson, Mark A. Fox, Lars-Olof Palsson,
b
b
a
a
ꢀ
a
Martin R. Bryce and Michael C. Petty
b
*
*
Received 23rd March 2012, Accepted 1st May 2012
DOI: 10.1039/c2jm31825c
A synthetically versatile strategy has been employed for luminescence colour tuning in a new series of
bipolar carbazole–2,5-diaryl-1,3,4-oxadiazole hybrid molecules 1–7. Their syntheses, solution
absorption and emission properties and cyclic voltammetric data are reported. Calculations using DFT
(density functional theory) establish that they possess molecular orbitals which favour bipolar charge-
transport. Single-active-layer organic light emitting devices (OLEDs) have been fabricated by thermal
evaporation using the bipolar compounds as the emitters in the architecture ITO:PEDOT-PSS:X:Ca/Al
(X ¼ 1–7). The structure–property relationships within the series of compounds are assessed with
emphasis on the OLED performance and emission colour. The HOMO–LUMO gap has been varied by
systematic modifications of the molecular subunits of 1–7, allowing the colour of the
electroluminescence to be tuned from deep blue (CIE x,y 0.157, 0.079) through to green (CIE x,y 0.151,
0.096). These materials are very attractive for further development due to the combination of good
processability of the molecules, their bipolar structure, colour tunability and efficient performance of
OLEDs using a simple device architecture.
OLEDs that do not need additional charge injection layers.
Moreover, they do not require doping into host molecules –
a strategy which presents problems of dopant–host phase sepa-
ration. The devices with bipolar fluorophores are, therefore,
easier to fabricate and manufacturing costs are reduced.7–15
Molecules with comparable hole- and electron-transporting
abilities are also termed ambipolar.9,15
Introduction
Organic light emitting devices (OLEDs) are attracting great
attention within academia and industry due to their emerging
applications in full-colour flat panel displays1–3 and solid-state
lighting technologies.4,5 Much work with phosphorescent and
fluorescent OLEDs has concerned multilayer structures which
can afford very bright and efficient devices by using an emitter
layer in combination with additional hole-transport and elec-
tron-transport layers. However, there are limitations in the
practical applicability of multilayer devices as they require
tedious sequential layer-by-layer fabrication procedures. Exci-
plex formation at the interfaces of the stacked organic layers can
reduce operating lifetimes and result in broad red-shifted emis-
sion.6a,b This has led to the search for small molecules in which
the HOMO and LUMO levels can be adjusted for efficient
injection and transport of both holes and electrons (i.e. bipolar
molecules).6c,d These can serve as emitters in single-active-layer
Carbazole (Cz) is a popular electron donor component for
optoelectronic materials: it is easy to functionalise, thermally
very stable and has good hole-transporting ability.16,17 On the
other hand, oxadiazole (OXD) derivatives – more specifically
2,5-diaryl-1,3,4-oxadiazoles – are widely used as versatile elec-
tron-transporting materials.18–20
The aim of the present work is to study the series of fluorescent
Cz–OXD donor–acceptor dyad molecules 1–7 in which the
bipolar characteristics and HOMO–LUMO levels are systemat-
ically varied by chemical modification and to exploit them as
emitters in OLEDs. We use a simple device architecture
comprising a single-active-layer, without additional layers for
charge injection or transport. Compound 1 has been reported in
a recent communication.21 The Cz–OXD dyads 1–7 are struc-
turally different from those reported by Thomas et al.22 and
Guan et al.,23 and the Cz–OXD–Cz triad systems studied by Ma
et al.24,25 Furthermore, these authors22–25 did not report system-
atic variations in order to probe structure–property
relationships.
aDepartment of Chemistry, and Centre for Molecular and Nanoscale
Electronics, Durham University, South Road, Durham, DH1 3LE, UK.
E-mail: m.r.bryce@durham.ac.uk
bSchool of Engineering and Computing Sciences, and Centre for Molecular
and Nanoscale Electronics, Durham University, South Road, Durham,
DH1 3LE, UK. E-mail: m.c.petty@durham.ac.uk
† Electronic supplementary information (ESI) available: Experimental
procedures, copies of NMR spectra, solution absorption and
photoluminescence spectra, AFM images, cyclic voltammograms; DFT
calculations. See DOI: 10.1039/c2jm31825c
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Results and discussion
Molecular design, synthesis and characterisation
Molecules 1–7 (Chart 1) each possess a donor carbazole and an
acceptor diaryloxadiazole unit. 9,9-Dihexylfluorene was chosen
as the spacer component in 1–6 as fluorene derivatives are
highly fluorescent26 and serve as efficient p-conjugating units.27
The hexyl chains at the C(9) position of fluorene improve the
solubility of the molecules in organic solvents. The molecular
structures in the series 1–7 were systematically varied in the
following ways to probe structure–property relationships. (i)
The acceptor diarylOXD was changed, with phenylene (1),
pyridyl (2) and thienyl (3) units in the backbone to modify the
HOMO and LUMO levels of this portion of the molecules. (ii)
The electron donor ability and electrochemical stability of the
carbazole unit was enhanced with the attachment of dimethoxy
substituents (4) and tert-butyl substituents (5). (iii) The number
of fluorene units was changed from one (1–5) to two (6) to
none (7) to probe the effects of the conjugation length of the
spacer.
Scheme 1 Reagents and conditions: (i) Pd(PPh3)2Cl2, NaOH (aq), THF,
reflux, 15 h (1, 78%; 2, 72%; 3, 84%).
Cyclic voltammetry studies
Electrochemical measurements for 1–7 are presented in Table 1
and cyclic voltammograms are shown in the ESI†. All
compounds showed oxidation waves due to the electron-
donating carbazolyl groups. Compounds 4 and 5 with substitu-
ents at the 3- and 6-positions of the carbazolyl groups revealed
reversible oxidation waves at 0.48 and 0.75 V, respectively,
whereas compounds 1–3, 6 and 7 showed irreversible oxidation
waves at high sample concentrations. The latter waves appear
more ‘reversible’ as the sample concentrations are lowered and at
faster scan rates. A carbazolyl group couples with a second
carbazolyl group on oxidation via the 3- (or 6-) positions and
thus a dimer is formed:30 this process cannot occur with 4 and 5.
This process is favoured on increasing the concentration of the
sample and the length of time the sample is in the oxidised state.
The dimer contains a redox centre with a lower oxidation
potential so a second cathodic wave is observed at ca. +0.55 V in
1–3, 6 and 7. A second irreversible oxidation wave in compound
4 is probably due to the oxidation of the electron-donating
methoxy group.
The synthetic route to 1–3 is shown in Scheme 1. Cross-
coupling of 821 with the halo-diarylOXD derivative 9,28 10 or 11
under palladium-catalysed Suzuki–Miyaura conditions gave 1–3,
respectively, in 72–84% yields.
The synthesis of 4 and 5 is shown in Scheme 2. Copper-cata-
lysed C–N cross-coupling of 1229 with the appropriate carbazole
derivative gave 13 and 14 in 93 and 78% yields, respectively.
Palladium-catalysed cross-coupling of 13 or 14 with 15 then gave
the final products 4 and 5.
The synthesis of bifluorene derivative 6 from 167 is shown in
Scheme 3.
Compound 7 was synthesised by a Buchwald–Hartwig C–N
coupling reaction (Scheme 4).
Compound 6, which has a bifluorenyl link, showed a second
oxidation wave that is only 0.23 V higher than the first oxidation
Chart 1 The structures of bipolar molecules 1–7 studied in this work.
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Scheme 2 Reagents and conditions: (i) 3,6-dimethoxycarbazole or 3,6-di-
tert-butylcarbazole, CuI, 1,10-phenanthroline, K2CO3, DMF, 120 ꢁC,
40 h, (13, 93%; 14, 73%); (ii) 15, Pd(PPh3)2Cl2, NaOH (aq), THF, reflux,
15 h (4, 73%; 5, 77%).
Scheme
4
Reagents and conditions: (i) carbazole, Pd(OAc)2,
PtBu3$HBF4, K2CO3, toluene, reflux, 24 h (54%).
Table 1 Electrochemical measurements for 1–7
E1/2(Ox1)a
E1/2(Ox2)a
E1/2(Red)c
ab
Ec
wave. Fig. 1 shows the CV traces for 6 at high and low sample
concentrations. The second oxidation may take place at one of
the fluorene units, presumably the unit furthest from the oxidised
carbazolyl group, and this fluorene redox process has a degree of
reversibility.
1
2
3
4
5
6
7
0.86d
0.89d
0.85d
0.48
0.54
0.57
0.56
ꢀ2.19
ꢀ2.28
1.19e
1.03d
0.75
Reversible reduction waves at ꢀ2.19 and ꢀ2.28 V were
observed for the pyridylene and thienylene compounds, 2 and 3,
respectively. No reduction waves were present within the THF
solvent window for 1 and 4–7. The reduction processes in 2 and 3
are likely to occur at the pyridylene and thienylene units,
respectively.
0.80d
1.00d
0.54
0.50f
a
Scan between 1.5 V and ꢀ0.5 V vs. ferrocenium/ferrocene couple (FcH+/
FcH ¼ 0.0 V) in DCM. b From cathodic wave of the ‘dimer’ formed after
c
oxidation. Scan between ꢀ0.5 V and ꢀ2.5 V vs. FcH+/FcH (0.0 V) in
d
THF. Estimated from ‘reversible wave’ at low sample concentration.
e
f
Irreversible wave, anodic wave value given. Third cathodic wave
observed at ꢀ0.40 V after oxidation.
Photophysical studies
Absorption data for 1–7 in cyclohexane and acetonitrile solu-
tions are summarised in Table 2. All compounds have strong
bands at 345–370 nm and additional bands are observed at
312 nm for 4, 321 nm for 5 and 285 nm for 7. The extinction
coefficients for these bands correlate with the number of fluorene
units present, 3 ¼ 47 000–63 000 for 1–5, 3 ¼ 100 000 for 6 and
3 ¼ 27 000 for 7. There are significant solvatochromic shifts on
going from cyclohexane to acetonitrile solutions for 2, 4, 5 and 7
and negative solvatochromism is observed for these compounds.
Such solvatochromism, typically found for zwitterions, indicates
that for these compounds their ground states are more polar than
their excited states.
effects observed in the absorption data. Fig. 2 shows the emission
bands for 4 and 6 in solvents of different polarities. Compounds
2, 4, 5 and 7 display the largest solvatochromic effects, whereas
the smallest solvatochromic shift is observed for compound 6
with two fluorene units. Compound 3, containing a thiophene
unit, has the second smallest solvatochromic shift. The charge
transfer character in the excited states of 2, 5, 7 and especially 4
must be relatively strong, whereas the CT character in 6 and 3 is
weaker. The Stokes shifts in cyclohexane solutions for
compounds 1–6 are consistent in the range of 2040–3010 cmꢀ1
,
whereas for 7 it is small at 720 cmꢀ1. The excited state geometry
in 7 must be quite similar to its ground state geometry and thus
there is a substantial blue shift in its emission compared to 1.
Small geometry rearrangements are likely in the excited states for
1–6 and presumably involve the fluorene units present in these
molecules.
Emission data for 1–7 in cyclohexane and acetonitrile solu-
tions are listed in Table 3. Blue emission bands with vibronic
structures with high fluorescence quantum yields (Ff) in the
range of 0.80 to 0.99 were observed in cyclohexane for all
compounds. While compounds 1–6 have blue emission maxima
between 383 nm (for 1) and 412 nm (for 3), the deep blue emis-
sion maximum for 7 is at 358 nm. The modifications on the
parent molecule 1 result in red shifts of the emissions for 2–6.
Compound 7 is blue shifted relative to 1; thus the absence of
a fluorene unit increases the energy gap between the excited and
ground state in 7 compared to 1.
Thin films of compounds 1–7 were obtained by thermal
evaporation and showed broad featureless emissions with the
maxima values listed in Table 3. The emission maxima are
consistently red-shifted by 2870–3550 cmꢀ1 (Table 4) compared
to emission maxima from cyclohexane solutions. The PLQY
values (Ff) are between 0.17 and 0.38 for thin films and low
compared to the PLQY values in cyclohexane solutions. They
indicate that devices, which are ‘solid state’ in nature, from 1–7
The trend in the positive solvatochromic shifts in the emissions
listed in Table 4 follow the trend in the negative solvatochromic
Scheme 3 Reagents and conditions: (i) ICl, DCM, RT, 1 h, 87%; (ii) 8, Pd(PPh3)2Cl2, NaOH (aq), THF, reflux, 15 h (91%).
11818 | J. Mater. Chem., 2012, 22, 11816–11825 This journal is ª The Royal Society of Chemistry 2012

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Fig. 1 Cyclic voltammograms showing two oxidation waves for 6 in low (left) and high sample concentrations.
Table 2 Absorption data for 1–7
Abs cyclohexane/
nm (cmꢀ1
Extinction coefficient
(3) cyclohexane (Mꢀ1 cmꢀ1
Abs acetonitrile/
nm (cmꢀ1
Solvatochromic
shift/cmꢀ1
)
)
)
1
2
3
4
345 (28 986)
363 (27 548)
370 (27 027)
363 (27 548)
312 (32 051)
353 (28 329)
321 (31 153)
357 (28 011)
349 (28 653)
339 (29 498)
285 (35 088)
284 (35 211)
61 510
63 180
56 710
46 660
60 450
57 770
43 360
99 540
26 520
30 770
34 560
51 560
344 (29 070)
353 (28 329)
370 (27 027)
351 (28 490)
311 (32 154)
350 (28 571)
321 (31 153)
358 (27 933)
339 (29 498)
331 (30 211)
291 (34 364)
284 (35 211)
ꢀ84
ꢀ745
0
ꢀ942
5
ꢀ242
6
7
78
ꢀ845
Table 3 Emission data for 1–7
lmax cyclohexanea/
lmax acetonitrileb/
nm (cmꢀ1
lmax thin filmc/
nm (cmꢀ1
lmax ELd/
nm (cmꢀ1
PLQY
cyclohexanee (Ff)
PLQY thin
filmf (Ff)
nm (cmꢀ1
)
)
)
)
1
2
3
4
5
6
7
383, 405 (26 110, 24 691)
392, 415 (25 510, 24 096)
412, 435 (24 272, 22 988)
402, 424 (24 876, 23 585)
395, 417 (25 316, 23 981)
400, 420 (25 000, 23 810)
358, 377 (27 933, 26 525)
464 (21 552)
505 (19 802)
472 (21 186)
583 (17 153)
498 (20 080)
439 (22 779)
439 (22 779)
430 (23 256)
450 (22 222)
467 (21 413)
465 (21 505)
450 (22 222)
450 (22 222)
410 (24 390)
435 (22 988)
453 (22 075)
484 (20 661)
470 (21 277)
453 (22 075)
447 (22 371)
415 (24 096)
0.88
0.99
0.86
0.80
0.96
0.99
0.95
0.38
0.36
0.17
0.17
0.20
0.21
0.29
a
Excitation at the lowest energy labs of the compound in cyclohexane. b Excitation at the lowest energy labs of the compound in acetonitrile. c Excitation
wavelengths (lex) for 1 at 350 nm, for 2, 3 and 4 at 370 nm and for 5, 6 and 7 at 340 nm. d Electroluminescence (EL) data from devices, see Table 7 for
details. Photoluminescence quantum yields (PLQY) measured in cyclohexane solution using an integrating sphere with excitation lex at the lowest
e
f
energy labs of the compound in cyclohexane. Estimated errors ꢂ5%. PLQY data obtained using an integrating sphere as described in the
literature;31 estimated errors ꢂ5%.
would have EQE values more closely related to PLQY values
from thin films than to the higher PLQY values from cyclo-
hexane solutions. Atomic force microscope (AFM) images of
films of 1–7 revealed a granular-type morphology with grain sizes
of 125–278 nm (ꢂ10%). Details are given in the ESI†.
are denoted by the suffix a, i.e. 1a–7a. Electronic structure
calculations on these geometries 1a–7a show that the HOMOs
are located, as expected, at the carbazolyl groups as shown in the
molecular orbitals for 1a and 6a (Fig. 3). Fig. 4 summarises the
energy levels of the HOMO and LUMO along with selected
orbitals that are close to the HOMO or LUMO in energies. There
is excellent agreement between the trend in HOMO energies for
1a–7a and the trend in the oxidation potentials for 1–7 (Table 5).
The second oxidation waves observed in the cyclovoltammo-
grams for 4 and 6 are also in accord with the relatively high
HOMO+1 energies in 4a and 6a. While the second oxidation
Molecular orbital computations
Ground state (S0) geometry optimisations were carried at the
B3LYP/6-31G* model chemistry without symmetry constraints.
Ethyl groups were used instead of hexyl groups in compounds
1–7 to reduce computational efforts and these model geometries
wave observed experimentally in
4 is assigned to the
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Table 4 Stokes, solvatochromic and phase shift data for 1–7
Stokes shift
cyclohexane (cmꢀ1
Stokes shift
acetonitrile (cmꢀ1
Solvatochromic
shift (cmꢀ1
Film-solution
shift (cmꢀ1
EL-solution
shift (cmꢀ1
Film-EL
shift (cmꢀ1
)
a
b
)
c
d
e
f
)
)
)
)
1
2
3
4
5
6
7
2880
2040
2760
2670
3010
3010
720
7520
8530
5840
11 340
8490
5150
4560
5710
3090
7720
5240
2220
5150
2850
3290
2860
3370
3100
2780
3550
3120
3440
3610
3600
3240
2630
3840
270
140
750
230
140
ꢀ150
6720
290
a
c
Difference between the lowest energy labs and the highest energy lem in cyclohexane. b Difference between the lowest energy labs and lem in acetonitrile.
Difference between the highest energy lem in cyclohexane and lem in acetonitrile. d Difference between the highest energy lem in cyclohexane and lem as
thin film. e Difference between the highest energy lem in cyclohexane and lem as device. f Difference between the highest energy lem as thin film and lem as
device.
Fig. 2 Emission spectra for 4 (left) and 6 (right) showing solvatochromism in different solvents.
1/2
) .
Fig. 3 Selected molecular orbitals for 1a (left) and 6a (right) contours drawn at ꢂ0.04 (e bohrꢀ3
dimethoxycarbazolyl group, the corresponding observed wave in
6 is due to oxidation at the fluorene ring next to the oxadiazole
ring where the HOMOꢀ1 for 6a is depicted in Fig. 3.
accepting) and 3 (increased conjugation) and the electron-
donating OMe and butyl groups at the carbazolyl groups in 4
and 5 lower the HOMO–LUMO gap compared to 1 according to
Fig. 4.
The two reduction waves observed only for 2 and 3 are in
accord with the lowest LUMO energies calculated for 2a and 3a
in the series 1a–7a. These LUMOs are located on the hetero-
aromatic rings. The heteroaromatic linkers in 2 (electron
The characters of LUMO and LUMO+1 are the phenylene
rings next to the oxadiazole ring for 1a–5a and 7a, rather than the
oxadiazole unit itself as viewed in Fig.
3 for 1a. The
11820 | J. Mater. Chem., 2012, 22, 11816–11825
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Fig. 4 Molecular orbital energy diagram for the important orbitals in 1a–7a.
diaryloxadiazole unit as a whole is a relatively strong p-electron
acceptor. The LUMO+1 in 6a is located on the fluorene unit near
the carbazolyl group rather than on the phenylene ring with the
butyl group. Thus 6a has a different orbital make-up where the
two fluorene units contribute to the LUMO+1 and HOMO-1 of
6a in Fig. 3.
TD-DFT computations were carried out on 1a–7a to predict
the absorption maxima and the nature of the transition bands.
TD-DFT data are summarised in Table 6 along with observed
absorption data. The predictions generally underestimate the
absorption maxima energies which can be attributed to a number
of factors, for example a mixture of many conformers exists in
the solution state whereas only the static ‘gas-phase’ (most
planar) conformer is computed. The trend of the absorption
maxima, nevertheless, is in broad agreement with the observed
maxima. The lowest energy bands observed are assigned as
charge-transfer transitions from the carbazolyl donor unit to the
acceptor ring unit directly linked to the oxadiazole ring. The
considerable orbital contribution of the oxadiazole character in
all the LUMOs suggests that the diaryloxadiazole unit is a more
appropriate description of the acceptor in these molecules. The
higher energy bands observed in 4, 5 and 7 are assigned to local
p–p* transitions.
The different solvatochromic effects observed for 1–7 can be
linked to the amount of carbazolyl character in the HOMO, as
evident from Table 5. The largest carbazolyl group contribution
to the HOMO at 86% is for 4a and the smallest at 61% is for 6a.
The carbazolyl group contributions to the LUMOs in 1a–7a
are negligible thus low energy charge transfer transitions are
expected in 1–7.
Table 5 Comparison of calculated HOMO energies for 1a–7a vs.
observed oxidation potentials, and carbazolyl character in HOMO vs.
observed solvatochromic shifts
Carbazolyl
contribution Emission
Electroluminescence properties of OLEDs
Absorption
HOMO
(calc., eV) (obs., V)a (calc., %)
HOMO in HOMO solvatochromic solvatochromic
b b
In a previous communication we reported that compound 1
could be successfully integrated into a simple and efficient, single-
active-layer device that emitted deep-blue electroluminescence.21
The Commission Internationale de l’Eclairage (CIE) chroma-
ticity diagram (x,y) co-ordinates were (0.16, 0.07), close to the
National Television System Committee (NTSC) standard blue
co-ordinates of (0.14, 0.08). The blue device possessed a high
external quantum efficiency for a single-active-later device (EQE
– over 4% in the case of OLEDs produced by thermal evapora-
tion). Here, it was anticipated that the systematic modifications
to the molecular structure of 1 would enable a degree of tuning
on the OLED output colour, turn-on voltage and efficiency. The
current versus voltage, I–V, and EL (measured by the photo-
current in the photodiode detector) versus voltage, L–V, char-
acteristics of OLEDs based on 1–7 are contrasted in Fig. 5. The
device architecture in all cases is: ITO/PEDOT:PSS/X/Ca/Al
(X ¼ 1–7). The corresponding EQEs and EL spectra are shown in
Fig. 6 and 7, respectively, while the CIE co-ordinates of the
various devices are depicted in Fig. 8. Important electro-optical
characteristics of the various OLEDs are collated in Table 7.
The electrical data in Fig. 5 reveal similar I–V characteristics
for 2 and 3 (i.e. within experimental error), but with currents
significantly higher than measured for the OLED based on 1. The
predicted HOMO levels for these three compounds are approx-
imately the same (Fig. 4 and Table 5) while the LUMO levels of 2
shift (cmꢀ1
)
shift (cmꢀ1
)
1a ꢀ5.28
2a ꢀ5.28
3a ꢀ5.26
4a ꢀ4.89
5a ꢀ5.14
6a ꢀ5.22
7a ꢀ5.45
ꢀ5.26
ꢀ5.29
ꢀ5.25
ꢀ4.88
ꢀ5.15
ꢀ5.20
ꢀ5.40
73
73
63
86
77
61
81
4560
5710
3090
7720
5240
2220
5150
ꢀ80
ꢀ780
0
ꢀ940
ꢀ240
80
ꢀ850
For 1–7, based on E(HOMO) ¼ ꢀ4.4 V ꢀ E1/2(Ox1). b For 1–7.
a
Table 6 TD-DFT data for 1a–7a and comparison with observed
absorption maxima
Calc. abs.
(nm)
Oscillation
strength (f)
Obs. abs
(nm)
Transitions
1a
2a
3a
4a
394
424
421
436
348
407
344
397
378
303
0.741
0.681
0.995
0.423
1.459
0.659
1.534
1.544
0.482
0.591
p(Cz) > p*(C6H4)
p(Cz) > p*(C5H3N)
p(Cz) > p*(C4H2S)
p(Cz) > p*(C6H4)
p(fluorene) > p*(C6H4)
p(Cz) > p*(C6H4)
p(fluorene) > p*(C6H4)
p(Cz) > p*(C6H4)
p(Cz) > p*(C6H4)
345
363
370
363
312
353
321
357
349
285
5a
6a
7a
t
p(C6H4 Bu) > p*(C6H4)
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Fig. 8 CIE diagram indicating the colour emission of each device
compared to standard white light.
enhanced EL output and a reduced turn-on voltage for EL
(Fig. 5), although the overall device efficiencies for OLEDs based
on 2 and 3 are both significantly lower than that measured for
OLEDs based on 1. Lowering the LUMO level of the active
molecule in our OLED architectures resulted in the peak EL
emission moving from the blue end of the spectrum (Fig. 7), so
the output colour of the OLED was no longer confined to the
deep-blue region of the CIE diagram (Fig. 8). This is consistent
with the reduced LUMO–HOMO separations of 2 and 3 (Fig. 4).
Compound 4 has increased donor strength relative to
compound 1 due to the methoxy groups to the 3,6-positions of
the carbazole rings. The predicted LUMO levels of 1 and 4 are
similar, but the HOMO level of 4 is significantly increased
(Fig. 4). The OLED based on compound 4 exhibited a green
rather than blue EL as expected from a reduced LUMO–HOMO
separation, with an EQE of 1.4%. This device possessed the
lowest turn-on voltage (2.8 V) of all the OLEDs studied.
Compound 5 has a higher HOMO–LUMO gap compared to 4,
and therefore 5 retains the blue emission. Moreover, the current
and EL output are both increased for the OLEDs manufactured
with 5 compared to 1 (Fig. 5), leading to an EQE of 1.9% – the
second highest of all the compounds studied in this work.
Compounds 6 and 7 provided the means to study the influence
of the fluorene group on the OLED behaviour. Fig. 7 shows that
the additional fluorene group in 6 has no impact on the device
colour as supported by DFT predictions. In contrast, 7, which
does not contain fluorene, provides the deepest blue OLEDs of
all the compounds synthesised for this study again supported by
HOMO–LUMO predictions here. However, the EQE of OLEDs
based on this compound is low (0.36%). The above results
suggest that the fluorene group is not itself responsible for the
deep blue emission in some of our compounds but it is necessary
for high efficiency. The deep blue colour must therefore arise
from the presence of the carbazole group. This is consistent with
results obtained in a previous study of two compounds,21 where
a diarylamine donor was replaced with carbazole and the colour
of emission moved from greenish-blue to deep-blue. Fig. 5 and 6
reveal that OLEDs based on 6 and 7 possess similar I–V and L–V
characteristics. The key difference is the much deeper blue EL
from devices based on 7. Although the predicted greater LUMO–
HOMO separation explains the deeper blue EL for 7, it cannot
account for their electrical behaviour. It is possible that the more
difficult charge carrier injection into 7 (raised LUMO and
Fig. 5 (a) Current–voltage and (b) photocurrent–voltage characteristics
of the devices incorporating the new materials. Device architecture:
ITO/PEDOT:PSS/X/Ca/Al (X ¼ 1–7).
Fig. 6 External Quantum Efficiency of devices incorporating materials
1–7.
and 3 are lowered with respect to that for 1. This suggests that the
increased currents measured for devices based on 2 and 3 are
electron currents. However, only the device based on 3 reveals an
Fig. 7 Electroluminescence spectra of each device.
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Table 7 Electroluminescence data for OLEDs based on compounds 1–7
Compound
1a
2
3
4
5
6
7
Turn on Vb (V)
4.18
4.71
431
81
3.87
0.20
453
66
3.04
0.38
484
87
2.80
1.48
455
63
3.03
1.89
453
73
3.22
0.54
452
81
3.68
0.36
428
70
0.161
0.049
6.312
0.124
0.075
EQEc (%)
Wavelength of maximum emissiond (nm)
FWHM (nm)
CIEe x at 1 mA
0.157
0.158
0.181
0.150
0.151
0.164
CIEe y at 1 mA
0.079
72.25
1.419
0.665
0.120
1.739
0.034
0.014
0.273
35.81
0.703
0.409
0.177
7.511
0.147
0.125
0.096
99.60
1.956
0.960
0.114
7.767
0.153
0.084
Brightnessf (cd mꢀ2
)
Current efficiencyf (cd Aꢀ1
)
Power efficiencyf (lm Wꢀ1
)
a
d
. Peak emission of 5 and 7
Data for 1 are taken from ref. 21. b Turn on voltage given at 1 nA of photocurrent. c EQE measured at 100 ꢂ 30 cd mꢀ2
measured under an applied current of 10 mA and 2 mA, respectively. All other OLEDs measured at 20 mA (J z 103 A mꢀ2). e CIE coordinates measured
at an applied current of 1 mA. Brightness, current efficiency and power efficiency measured under an applied current of 1 mA.
f
lowered HOMO in relation to 1) is compensated by much higher
carrier mobilities for thin films of 7.
originates from the carbazole group; (iii) the presence of a fluo-
rene group adjacent to the carbazole donor, and separated from
the oxadiazole moiety by a phenylene ring spacer, is required to
achieve a high device efficiency (i.e. compounds 1, 4 and 5). The
links between other electro-optical parameters measured for the
OLEDs (e.g. device currents and EL turn-on voltages) and their
predicted electronic band structures are less clear. This indicates
that different factors, for example, electron and hole carrier
mobilities are a major influence in controlling the electro-optical
parameters of these single-active-layer OLEDs.
The most significant correlation that has emerged from our
studies of OLEDs based on this series of bipolar carbazole–
oxadiazole molecules is that between the EL maxima and the
bandgap Eg (¼ ELUMO ꢀ EHOMO) predicted by molecular orbital
calculations. Fig. 9 shows a plot of Eg versus the reciprocal of the
CIE(y) co-ordinate. The OLEDs based on molecules with the
largest LUMO–HOMO separations generally show the largest
values of 1/CIE(y) for the deepest blue EL. Two exceptions are
compounds 3 and 6. A possible explanation is that these
compounds are the least bipolar based on solvatochromic and
computed data. Fig. 7 shows that OLEDs using compound 3
possess the most red-shifted EL spectra of all the molecules
studied. The peaks in the EL emissions from 3 and 4 are quite
similar; however, the broad shoulder on the low energy side of
the peak emission for 3 results in dissimilar CIE co-ordinates. A
comparison of the PL emission maxima of the thin films and EL
of devices in Tables 3 and 4 shows similar data in these molecules
except for 3. Compound 3 with the thiophene unit is unusual in
that its emission maxima vary considerably and do not follow
trends of other compounds in the series.
To place our work in a broader context it should be noted that
the OLEDs from compounds 1 and 5 are competitive with the
most efficient deep-blue OLEDs based on all-organic small-
molecule fluorophores. There are very few literature reports of
OLEDs of this type with emission close to the NTSC standard
blue co-ordinates of (0.14, 0.08) that achieve EQE values >1% or
current efficiencies >1 cd Aꢀ1 32
.
Conclusions
The syntheses, solution absorption and emission properties, and
cyclic voltammetric data of a new series of bipolar carbazole–2,5-
diaryl-1,3,4-oxadiazole hybrid molecules, 1–7, are reported.
Density functional theory calculations establish that these
molecules possess molecular orbitals which favour bipolar
charge-transport. Single-active-layer organic light emitting
devices have been fabricated by thermal evaporation using the
bipolar compounds as the emitters in the architecture ITO/
PEDOT:PSS/X/Ca/Al (X ¼ 1–7). The results reveal an excellent
Important design rules that have emerged from our study are:
(i) the most balanced carrier transport in the OLEDs (i.e. similar
electron and hole currents, providing a high device efficiency) is
achieved with a 2,5-diphenyl-1,3,4-oxadiazole acceptor and
a carbazole donor; (ii) the deep blue emission from the molecules
correlation between the emitted EL spectrum and the ELUMO
–
EHOMO separation predicted by the molecular orbital calcula-
tions. OLEDs based on 1, incorporating a 2,5-diphenyl-1,3,
4-oxadiazole acceptor group and a carbazole donor unit emitted
deep-blue electroluminescence, with CIE (x,y) co-ordinates of
(0.16, 0.08), close to the NTSC standard blue co-ordinates of
(0.14, 0.08). Important design rules that have emerged from
our study are that the deep blue emission from the molecules
originates from the carbazole group and that the presence of
a fluorene group adjacent to the carbazole donor, and separated
from the oxadiazole acceptor by
a phenylene ring (i.e.
compounds 1, 4 and 5) is required to achieve a high device
efficiency (EQE > 1%). These materials are very attractive
for further development due to the combination of good
Fig. 9 Dependence of band gap, Eg (from Fig. 4), on 1/CIE(y) for
OLEDs based on the different molecules.
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were measured using an Ocean Optics USB2000 Miniature Fibre
Optic Spectrometer and CIE coordinates measured using
a SpectraScan PR-655 Spectroradiometer.
Acknowledgements
Fig. 10 OLED architecture.
We thank EPSRC for funding.
processability of the molecules, their bipolar structure, colour
tuneability and efficient performance of OLEDs using a simple
non-doped device architecture.
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