Synthesis and characterization of a novel bipolar polymer for light-emitting diodes

Article abstract of DOI:10.1039/a807975g

A novel bipolar light-emitting polymer containing both efficient hole and electron injecting/transporting segments exhibiting high thermal stability (T(d) = 445 °C), good electrochemical reversibility, excellent thin film-forming and light-emitting proper

Full text of DOI:10.1039/a807975g

Synthesis and characterization of a novel bipolar polymer for light-emitting
diodes
Yunqi Liu, Hong Ma and Alex K-Y. Jen*
Department of Chemistry, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115, USA.
E-mail: ajen@lynx.neu.edu
Received (in Cambridge, UK) 14th October 1998, Accepted 10th October 1998
A novel bipolar light-emitting polymer containing both
efficient hole and electron injecting/transporting segments
exhibiting high thermal stability (Td = 445 °C), good
electrochemical reversibility, excellent thin film-forming
and light-emitting properties (bright yellow emission, a
rectification ratio greater than 105 and a low turn-on voltage
of 3.7 V) is reported.
no)ferrocene (dppf) and NaOBut, followed by the addition of
1-bromo-4-butylbenzene and additional NaOBut. Compound 3
was obtained (72% yield) from 2 by a demethylation reaction.12
Polymerization between 1 and 3 was carried out in the presence
of K2CO3 in a NMP–toluene solvent mixture.13 The resulting
viscous polymer solution was precipitated into MeOH, fol-
lowed by Soxhlet extraction with MeOH to afford polymer 4
(78% yield). The chemical structures of monomers and polymer
were confirmed by 1H NMR and elemental analyses.
For most of the conjugated polymers that are reported in the
literature, the hole injection/transport is more favorable than the
electron injection/transport due to the electron-richness of the
p-conjugated systems. To facilitate the electron injection,
metals with low work functions, such as calcium and magne-
sium, have been used as cathodes. However, the stability of the
resulting devices is severely compensated for by the sensitivity
of these metals to both oxygen and moisture. To circumvent
these problems, several approaches have been utilized, such as
synthesizing highly electron-affinitive polymers,1 inserting an
additional electron injecting/transporting layer between the
emitter and the cathode2 and blending small molecules into the
polymer matrix.3 Although some success has been achieved,
several deficiencies are associated with these methods. For
example, more tedious and complex multilayer structures were
needed to offset the poor hole injection of the highly electron-
affinitive polymers for achieving high device efficiency. In
addition, phase separation often occurs when the dopant
concentration is high. Recently, Bao et al.4 reported the results
of light-emitting polymers containing both electron and hole
transporting moieties that achieved efficient device perform-
ance. Thus, it is desirable to design and synthesize a bipolar
polymer that possesses both a hole injecting/transporting
segment and an electron-affinitive segment in order to enhance
the charge injecting/transporting ability. Polyquinolines (PQs)
are a class of heteroaromatic polymers that possess excellent
thermal stability, good thin film processibility, high electron
affinity and unique nonlinear optical properties.5,6 A blue
electroluminescent (EL) device using a fluorinated polyquino-
line (PQ-100) as the emitting layer has been reported that
possessed a very high turn-on voltage (50 V) due to difficulties
achieving efficient/balanced charge injection.7 On the other
hand, tetraphenyldiaminobiphenyl (TPD) has been widely used
as an excellent hole-transport material in fabricating small
molecule EL devices.8 In the hope of combining both the blue
light-emitting and good electron-affinitive properties of the bis-
quinoline and the efficient hole-injecting/transporting proper-
ties of the TPD into a single bipolar material to enhance the
overall performance of an EL device, we have copolymerized
these two functional moieties via a nucleophilic replacement
reaction/polycondensation. Here we report the excellent ther-
mal, electrochemical and EL results obtained from this
polymer.
TPD-PQE was a pale-grey fibrous solid. As a result of the
two n-butyl groups attached on the TPD moiety, the TPD-PQE
was readily soluble in common organic solvents, such as
CHCl3, THF and cyclopentanone. The weight average molec-
ular weight (Mw = 58400 with a polydispersity index of 5.76)
was determined by gel permeation chromatography using THF
as eluent and polystyrene as standard.
The thermal properties of TPD-PQE were determined by
thermal gravimetric analysis (TGA) and differential scanning
H2N
O
NH2
O
+
Ac
F
C
C
i
N
N
F
F
1
+
Br
Br
MeO
NH2
ii,iii
.
MeO
N
N
OMe
2
iv
HO
N
N
OH
3
+
1
3
v
N
N
O
N
N
O
n
The synthesis of the copolymer (TPD-PQE) is outlined in
Scheme 1. Compound 1 was synthesized (80% yield) from 4,4A-
diamino-3,3A-dibenzoyldiphenyl ether and 4-fluoroacetophe-
none.9 Compound 2 was synthesized (50% yield) from the
modified Buchwald coupling reaction10,11 between 4-anisidine
and 4,4A-dibromobiphenyl in the presence of tris(dibenzylide-
neacetone)dipalladium [Pd2(dba)3], 1,1A-bis(diphenylphosphi-
TPD-PQE
Scheme 1 Reagents and conditions: i, conc. H2SO4, AcOH; ii, Pd2(dba)3,
dppf, toluene, NaOBut, 90 °C; iii, 4-BrC6H4Bu, NaOBut; iv, BBr3·SMe2,
ClCH2CH2Cl; v, K2CO3, toluene, NMP.
Chem. Commun., 1998, 2747–2748
2747

Fig. 3 (2) Photoluminescence spectrum of TPD-PQE and electro-
luminescence spectra of (—) ITO/CuPc/TPD-PQE/Al A, (8) ITO/PVK/
TPD-PQE/Alq3/Al B and (Ω) ITO/TPD-PQE/Alq3/Al C.
Fig. 1 Cyclic voltammetry of TPD-PQE spun-coated on ITO glass in an
MeCN solution of TBAP (0.1 M
) at a scan rate of 40 mV s21: (a) reduction
and (b) oxidation.
(except for vacuum evaporation) and measurements were
performed in air and at room temperature.
calometry (DSC). The TPD-PQE possessed excellent thermal
stability with a glass transition temperature (Tg) of 196 °C, and
a Td (onset of the decomposition temperature) of 445 °C.
The energy levels of TPD-PQE were investigated by cyclic
voltammetry (CV). A thin film of the polymer was spin-coated
onto pre-cleaned indium-tin-oxide (ITO) glass as a working
electrode ( ~ 3 cm2) from a cyclopentanone solution (concentra-
tion ~ 3 mg ml21). The reference and counter electrodes were
Ag/Ag+ (non-aqueous) and Pt gauze, respectively. The experi-
ment was carried out under nitrogen with tetrabutylammonium
Fig. 2 shows the current–voltage characteristics of the LEDs.
Excellent diode behavior was found for these devices. The turn-
on voltage and rectification ratio were determined to be 3.7 V,
and 5.1 3 104 (at 4.5 V) for A, 21.0 V and 4.2 3 105 (at 30.0
V) for B, and 30.1 V and 6.0 3 104 (at 36.0 V) for C,
respectively. It is worth noting that, for these devices using PVK
or CuPc as hole injecting/transporting layers, the turn-on
voltages were decreased, while using Alq3 as an electron
injecting/transporting layer, the turn-on voltages were in-
creased. This indicated that the TPD-PQE polymer has a good
electron injecting/transporting ability. When the hole injecting/
transporting (PVK or CuPc) layers were introduced, the overall
charge injection/transport of the devices were balanced, which
led to the decrease of the turn-on voltages.
Fig. 3 shows the photoluminescence (PL) spectrum of a TPD-
PQE film and EL spectra of the LEDs. The PL spectrum of
TPD-PQE had an emission peak at 547 nm when it was excited
at 366 nm. The EL spectrum from device A is blue-shifted (lmax
= 530 nm) compared to the PL spectrum of the TPD-PQE. In
particular, its long wavelength side became much steeper. This
was due to partial absorption by CuPc in the 520–680 nm
region.15 The shape of the EL spectrum from device C is similar
to the PL spetrum of TPD-PQE, however, it is red-shifted to 568
nm. In the case of device B, the EL emission shows a peak at
570 nm with a shoulder at ~ 620 nm. The shoulder is probably
due to emission from aggregates of carbazole groups.19 Bright
light emission can clearly be seen under room light at forward
bias for all three types of LEDs.
perchlorate in anhydrous MeCN (TBAP, 0.1
M) as the
supporting electrolyte. A typical CV curve is shown in Fig. 1.
Two redox-active moieties were revealed for the copolymers. It
is worth pointing out that the oxidative (p-doping) process was
highly reversible and the reductive (n-doping) process was
quasi-reversible for TPD-PQE. As a matter of fact, such a
bipolar ability is beneficial to the performance of LEDs. The
energy values of the lowest unoccupied molecular orbital
(LUMO) and the highest occupied molecular orbital (HOMO)
for TPD-PQE were calculated using the ferrocence (FOC) value
of 24.8 eV below the vacuum level.14 The onset potentials of
oxidation and reduction of TPD-PQE (Fig. 1) were determined
as +0.39 and 22.07 V vs. Ag/Ag+, corresponding to +0.27 and
22.19 V vs. FOC (EFOC = 0.12 V vs. Ag/Ag+). Thus, the
HOMO and LUMO levels and the HOMO–LUMO gap should
be 25.07, 22.61 and 2.46 eV, respectively. The optical Eg
calculated from the onset (426 nm) of the UV–VIS absorption
spectra of the spin-coated films for TPD-PQE was 2.91 eV.
To fabricate LED devices, copper phthalocyanine (CuPc),
tris(8-hydroxyquinoline)aluminium (Alq3) and Al layers were
evaporated in a vacuum (1026 Torr). Thin films of poly(9-
vinylcarbazole) (PVK) and TPD-PQE were spin-coated from a
1,2-dichloroethane solution and from a CHCl3 solution, which
were filtered through a 0.2 m Teflon filter before spin-coating.
Three types of LEDs with the structure of ITO/CuPc/TPD-PQE/
Al (A), ITO/PVK/TPD-PQE/Alq3/Al (B) and ITO/TPD-PQE/
Alq3/Al (C) were employed in this study. The thickness of the
TPD-PQE was the same ( ~ 35 nm) for all three types of LEDs,
while the thickness of CuPc, PVK, Alq3 and Al were ~ 35,
~ 40, ~ 50 and ~ 100 nm, respectively. The active area of the
resulting devices was 7.07 mm2. All of the fabrication processes
In conclusion, a bipolar copolymer was synthesized and
characterized. This polymer showed high thermal stability,
good electrochemical reversibility and excellent thin film-
forming ability. In addition, very good LED performance was
obtained with a low turn-on voltage, high rectification ratio, and
bright light emission.
Notes and references
1
N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend and
A. B. Holmes, Nature, 1993, 365, 628.
2 R. Brown, D. D. C. Bradley, P. L. J. Burn, H. Burroughes, R. H. Friend,
N. Greenham and A. Kraft, Appl. Phys. Lett., 1992, 61, 2793.
3 Zhang, H. V. Seggern, B. Kraabel, H. W. Schmidt and A. J. Heeger,
Synth. Met., 1995, 72, 185.
4 H. Peng, Z. N. Bao and M. E. Galvin, Chem. Mater., 1998, 10, 2086.
5 Zhang, A. S. Shetty and S. A. Jenekhe, Acta Polym., 1998, 49, 52.
6 A. K-Y. Jen, X. M. Wu and H. Ma, Chem. Mater., 1998, 10, 471.
7 D. Parker, Q. Pei and M. Marrocco, Appl. Phys. Lett., 1994, 65, 1272.
8 A. VanSlyke and C. W. Tang, US Pat., 1991, 5,061,569.
9 A. Fehnel, J. Org. Chem., 1966, 31, 2899.
10 S. Thayumanavan, S. Barlow and S. R. Marder, Chem. Mater., 1997, 9,
3231.
11 J. P. Wolfe and S. L. Buchwald, J. Org. Chem., 1996, 61, 1113.
12 G. Williard and C. B. Fryhle, Tetrahedron Lett., 1980, 21, 3731.
13 L. Hedrick and J. W. Labadie, Macromolecules, 1990, 23, 1561.
14 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bassler, M.
Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
15 Q. Liu, M. S. Liu, X. C. Li and A. K-Y. Jen, Chem. Mater., in the
press.
Fig. 2 Current–voltage characteristic for the light emitting devices. (a) ITO/
CuPc/TPD-PQE/Al A, (b) ITO/PVK/TPD-PQE/Alq3/Al B and (c) ITO/
TPD-PQE/Alq3/Al C.
Communication 8/07975G
2748
Chem. Commun., 1998, 2747–2748