Effect of intermolecular interaction on the characteristics of organic light emitting diodes with TPBB derivatives

Article abstract of DOI:10.1080/15421401003608337

We synthesized four new emitting materials of triphenylamine derivatives, 1,3,5-tris(4′-(1″-phenyl-benzimidazol-2′-yl)phenyl)amine (TPBB), MeO-TPBB, Br-TPBB, and Bu-TPBB, and investigated the effects of substituent on their intermolecular interaction and

Full text of DOI:10.1080/15421401003608337

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Effect of Intermolecular Interaction
on the Characteristics of Organic Light
Emitting Diodes with TPBB Derivatives
Man Su Kim a , Sun-Hee Lee a , Hanil Lee b , Kwang-Hyun Ahn b
Young Chul Kim a
&
a Department of Chemical Engineering and RIC-CAMID , Kyung Hee
University , Yongin-si, Gyeonggi-do, Korea
b Department of Applied Chemistry , Kyung Hee University , Yongin-
si, Gyeonggi-do, Korea
Published online: 19 Apr 2010.
To cite this article: Man Su Kim , Sun-Hee Lee , Hanil Lee , Kwang-Hyun Ahn & Young Chul Kim
(2010) Effect of Intermolecular Interaction on the Characteristics of Organic Light Emitting Diodes
with TPBB Derivatives, Molecular Crystals and Liquid Crystals, 520:1, 28/[304]-35/[311], DOI:
10.1080/15421401003608337
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Mol. Cryst. Liq. Cryst., Vol. 520: pp. 28=[304]–35=[311], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003608337
Effect of Intermolecular Interaction on the
Characteristics of Organic Light Emitting
Diodes with TPBB Derivatives
MAN SU KIM,1 SUN-HEE LEE,1 HANIL LEE,2
KWANG-HYUN AHN,2 AND YOUNG CHUL KIM1
1Department of Chemical Engineering and RIC-CAMID,
Kyung Hee University, Yongin-si, Gyeonggi-do, Korea
2Department of Applied Chemistry, Kyung Hee University,
Yongin-si, Gyeonggi-do, Korea
We synthesized four new emitting materials of triphenylamine derivatives, 1,3,5-tris
(40-(100-phenyl-benzimidazol-200-yl)phenyl)amine (TPBB), MeO-TPBB, Br-TPBB,
and Bu-TPBB, and investigated the effects of substituent on their intermolecular
interaction and luminescence characteristics. UV-vis absorption and photolumines-
cence (PL) spectra of the TPPB derivatives measured in dilute solution were almost
the same but, in solid state, the PL spectra exhibited large differences depending
on the substituent. Especially, the PL spectra of Br-TPPB redshifted remarkably
in solid state. We also prepared electroluminescent devices with the multilayer
structure of [ITO=NPB (50 nm)=EML (30 nm)=TPBI (20 nm)=Alq3 (10 nm)=
LiF (1 nm)=Al] using one of the TPBB derivatives as an emitting layer. The TPBB
derivatives showed much different electroluminescence (EL) characteristics
depending on the nature of the substituent. This suggests that each substituent
present different degree of intermolecular electronic interaction in the TPBB
derivatives.
Keywords 1,3,5-tris(40-(100-phenyl-benzimidazol-200-yl)phenyl)amine; heavy atom
effect; intermolecular interaction; organic light emitting diodes
Introduction
Organic light emitting diode (OLED) is now considered as a prime display device of
the next generation because of its potential for thin, light-weight, low-power, and
also flexible displays [1,2]. Although much progress has been made over the past
two decades, researches for new materials with high luminescent efficiency and stab-
ility are still going on [3,4] with the efforts to develop ingenious device fabrication
techniques [5,6].
In search of a new emitting material, many derivatives of luminescent molecules
were synthesized with various functional substituents [7] because the photo-
electronic and physical properties can be modified depending on the substituent
Address correspondence to Prof. Young Chul Kim, Department of Chemical Engineering,
Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Korea. E-mail: kimyc@khu.ac.kr
28=[304]

Effect of Intermolecular Interaction on the Electroluminescence 29=[305]
groups. It has been reported that the optical and electrical properties of conjugated
materials are dependent on the nature of the substituents (electron-donating or
electron-withdrawing) of conjugated systems as well as on the p-conjugation length
and interchain distance of the conjugated segments [8,9]. In addition to the effects of
electron-donating or -withdrawing substituents on the electronic structure of the
molecule, the effects of substituents on the intermolecular interaction has also been
studied [10]. The emitting materials which have different intermolecular interaction
tend to show different photo- and electro-luminescence characteristics [11]. Ku et al.
has confirmed that intermolecular dipole interactions are detrimental to the photo-
luminescence efficiency in solid state, retarding their uses as solid state emitters
in OLED devices [12]. They also reported that, in solid state, a slight red-shift of
the absorption and emission maxima occurred, compared with those obtained in
dilute solution.
In this work, we synthesized four new emitting materials of triphenylamine
derivatives, 1,3,5-tris(40-(100-phenyl-benzimidazol-200-yl)phenyl)amine (TPBB), MeO-
TPBB, Br-TPBB, and Bu-TPBB, and investigated the effects of substituents on their
intermolecular interaction and luminescence characteristics.
Experimental
Synthesis of the TPBB Derivatives and Fabrication of OLEDs
As presented in Scheme 1, the TPBB derivatives were synthesized from the reaction
between 4,40,400-nitrotribenzaldehyde and N-phenyl-2,3-phenylenediamine in the pres-
ence of Na2S2O5 with an yield of 95%. Methoxy, bromine, and n-butyl groups were
substituted at the para-positions of the phenyl rings to prepare the compounds b–d.
The OLED devices were fabricated on the pre-patterned indium-tin-oxide
(ITO)-coated glass substrates with a sheet resistance of 10 X=&. The size of pre-
patterned ITO coated glass substrates was 3 ꢀ 3 cm2 and each substrate has four
pixels with the active area of 3 ꢀ 3 mm2. The substrate was sequentially cleaned with
trichloroethylene, acetone, deionized water, and isopropyl alcohol in an ultrasonic
bath each for about 10 min followed by rinsing in deionized water. The cleaned
substrate was treated with oxygen plasma for 2 min at a power of 600 W.
Scheme 1. Synthetic route to the TPBB derivatives: (a) BuLi=DMF, THF, ꢁ78ꢂC;
(b) N-phenyl-2,3-phenylenediamine, Na2S2O5, DMF, 150ꢂC.

30=[306]
M. S. Kim et al.
Figure 1. Energy band diagram of the materials used in the OLED devices.
Four OLED devices with the multilayer structure of [ITO=NPB (50 nm)=EML
(30 nm)=TPBI (20 nm)=Alq3 (10 nm)=LiF (1 nm)=Al] were prepared by using one
of the TPBB derivatives as an emitting layer (EML). In the devices, N,N0-bis(1-
naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (NPB) was used as a hole trans-
port layer (HTL), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as a hole
blocking layer (HBL), tris-(8-hydroxyquinoline) aluminum (Alq3) as an electron
transport layer (ETL), and the bilayer of lithium fluoride (LiF) and aluminum
(Al) as a cathode. All organic and metal layers were deposited by thermal evapor-
ation under ꢃ10ꢁ6 torr onto the ITO substrate and the deposition rates were 1.0,
˚
0.1, 3 ꢃ 4 A=s for the organic, LiF, and Al layers, respectively. The intersection area
of cathode and anode defines the active area of the device. The energy band diagram
of the devices is shown in Figure 1.
Measurements
UV-vis absorption and photoluminescence (PL) spectra were measured using an
Agilent 8453 UV-visible spectrophotometer and a PTI QuantaMasterTM spectro-
fluorometer, respectively. Cyclic voltammograms were recorded on a Bioanalytical
System Bas 100B in 0.1 M Bu4NPF6=CH2Cl2 solution with a platinum working elec-
trode, a platinum auxiliary electrode, and a saturated Ag=AgCl reference electrode.
Differential scanning calorimetric analysis (DSC) were performed on a Perkin Elmer
N536-0003 at a scan rate of 10ꢂC=min under nitrogen atmosphere. Electrolumines-
cence characteristics of the devices were studied by using a JBS I-V-L 300 EL
characterization system equipped with a Keithley 2400 sourcemeter.
Results and Discussion
We measured the highest occupied molecular orbital (HOMO) and the lowest unoc-
cupied molecular orbital (LUMO) energy levels of the TPBB derivatives in order to
investigate the effect of substituent on the electronic states of the materials. HOMO
level was calculated by using the oxidation potential onset measured from cyclic vol-
tammograms and LUMO level by adding the optical bandgap to the HOMO level.
However, all of the TPBB derivatives showed almost the same HOMO and LUMO
levels of 5.34 and 2.30 eV, respectively, without reference to the substituent. It
implies that the Br, n-Bu, and MeO substituents could hardly alter the energy levels
of TPBB although they have different electron donating ability.

Effect of Intermolecular Interaction on the Electroluminescence 31=[307]
Figure 2. Normalized UV-vis absorption- and PL-spectra of the TPBB derivatives measured
(a) in the solution and (b) in the solid states.
Normalized UV-vis absorption and PL spectra of the four new luminescent
materials measured in dilute toluene solution and thin solid film states are shown
in Figure 2. The absorption peak wavelengths of the materials measured in solution
state are located at practically the same wavelength of 368 nm and also the PL peaks
fell within 425–430 nm. On the other hand, the materials in solid state exhibited sig-
nificantly different PL spectra while the absorption spectra were much similar. The
PL peak wavelengths of Br-, MeO-, and Bu-TPBB in solid state shifted remarkably
to the longer wavelengths than in solution state. The PL peak wavelengths of TPBB,
Br-TPBB, Bu-TPBB, and MeO-TPBB in solid state were observed at 432, 489, 478,
and 484 nm, respectively. The degree of redshift was dependant on the substituent
although the substituents could not change the bandgap of TPBB. Therefore these
spectral shifts in the solid state spectra are attributed to the different degree of
intermolecular interactions provided by the different substituents.
DSC thermograms of the materials could give some information on their
molecular mobilities in melt state and the intermolecular interactions in solid state
as shown in Figure 3. The materials were cooled from 350ꢂC at a rate of 10ꢂC=min
after the initial heating in order to erase the previous thermal history. During the

32=[308]
M. S. Kim et al.
Figure 3. (a) DSC thermograms of the TPBB derivatives and (b) summary of thermal
characteristics.
second heating scan, TPBB showed only glass transition but the Br-, Bu-, and
MeO-TPBB showed the crystallization and melting behavior as well as glass tran-
sition. This suggests that the substitution increased the mobility of TPBB and the
derivatives with substituent could have intermolecular interactions strong enough
to form crystals. As indicated by the heat of fusion (DHm), MeO-TPBB has the high-
est crystallizability, followed by Br-TPBB and Bu-TPBB.
We fabricated the OLEDs with the multilayer structure of [ITO=NPB (50 nm)=
EML (30 nm)=TPBI (20 nm)=Alq3 (10 nm)=LiF (1 nm)=Al] using one of the TPBB
derivatives as an emitting layer in order to confirm the effect of intermolecular
interaction on the electroluminescence performances. Previously, we fabricated the
OLED devices with various structures using TPBB as an emitting layer and obtained
the best EL properties with abovementioned structure, which will be reported else-
where. Figure 4 shows the current density-voltage-luminance (J-V-L) characteristics
and luminous efficiencies of the devices with the TPBB derivatives. The device with
unsubstituted TPBB showed the best performance with the maximum luminance of
10,000 cd=m2 and a luminous efficiency of 2.9 cd=A at a current density of 100 mA=cm2
but the devices with Br-, Bu-, and MeO-TPBB gave lower luminance and luminous
efficiency.
As shown in Figure 5, the emission of Br-, Bu-, and MeO-TPBB redshifed not
only in the solid state photoluminescence but also in the electroluminescence spectra.
The effect of substituent is considered to make the TPBB molecules more mobile so
that the aggregates are formed from several neighboring molecules to have stronger
intermolecular interaction. DSC study of the materials supports this tendency. It is
generally understood that excimer (MꢄM) or exciplex (MꢄN) can be formed by strong
interactions between the molecular segments in excited state (Mꢄ) and the same
molecules (M) or a different molecules (N) in the ground state, respectively, when
aggregated. And the emission spectrum from the excimer or exciplex has a broad
peak or shoulder in the longer wavelength region [13]. In this work, neither a broad
peak due to an excimer emission was observed, nor the redshift of the TPBB deriva-
tives reached that of the excimer or exciplex but, the redshift is considered to
originate from the increased intermolecular interaction in the densely packed
molecules, which caused by the molecular mobility provided by the substituent.

Effect of Intermolecular Interaction on the Electroluminescence 33=[309]
Figure 4. (a) The current density-voltage-luminance (J-V-L) characteristics and (b) luminous
&
efficiency of the devices fabricated with the TPBB derivatives ( : TPBB, 4: Br-TPBB,
:
ꢂ
Bu-TPBB, ꢅ: MeO-TPBB).
In the case of the Br-TPBB based device, however, it is not enough to explain the
remarkable redshift and significant decrease of luminance and efficiency as just
the result of the increased intermolecular interaction. The low fluorescence quantum
yield of the Br-substituted compound can be ascribed to the high non-radiative
Figure 5. EL-spectra of the devices fabricated with the TPBB derivatives.

34=[310]
M. S. Kim et al.
Table 1. Summary of EL characteristics of the OLEDs fabricated with TPBB
derivatives
Turn on Luminous
EL
CIE
Emitting
layer
Max luminance
[cd=m2]
voltage efficiency E.Q.E kmax Coordinates
[V]a
[cd=A]b
[%]b [nm]b
[X, Y]b
TPBB
Br-TPBB
10000 at 440 mA=cm2
230 at 200 mA=cm2
8
16
2.93
0.17
1.7 444 0.15, 0.15
0.05 511 0.33, 0.53
520
MeO-TPBB 4500 at 280 mA=cm2
Bu-TPBB
3700 at 280 mA=cm2
16
12
2.17
2.05
0.9
1.1
486 0.16, 0.27
481 0.16, 0.21
aDefined as the voltage required to reach a luminance of 100 cd=m2.
bMeasured at 100 mA=cm2.
deactivation that occurs during intersystem crossing from S1 to T1, which is
often referred to as the ‘‘heavy atom effect’’ observed in the presence of
bromine atom [14,15]. The EL characteristics of all the devices are summarized in
Table 1.
The reason for the great performance of the device with TPBB is that the mol-
ecular mobility of TPBB is not enough to form aggregates. On the other hand, the
TPBB derivatives with Bu- and MeO-substituents can gain enough molecular
mobility for a dense packing to have a stronger intermolecular interaction, leading
to the decreased EL performances and the redshifted spectra. Although Br-TPBB
has low molecular mobility comparable to TPBB, it has strong intermolecular attrac-
tion force enough to crystallize upon heating. The poor EL characteristics of
Br-TPBB are considered to result from both the strong intermolecular attraction
force and the heavy atom effect of bromine.
Conclusion
We studied the effects of intermolecular interaction on the electroluminescence
properties of the OLED devices fabricated with the TPBB derivatives. The TPBB
derivatives substituted with Br, n-butyl, and methoxy groups showed enhanced mol-
ecular mobility than unsubstituted TPBB resulting in more dense molecular packing.
Therefore the PL spectra of solidified Br-, Bu-, and MeO-TPBB redshifted compared
with those obtained in solution state. Unsubstituted TPBB showed a better EL per-
formance than the substituted TPBBs and the substituted TPBBs exhibited poor EL
properties due to the increased intermolecular interaction. The heavy atom effect of
bromine also deteriorated the EL performance of the device fabricated with
Br-TPBB.
Acknowledgments
This research was supported by Basic Science Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science
and Technology (Grant No. 2009-0090177), and the ‘‘GRRC’’ Project of Gyeonggi
Provincial Government, ROK.

Effect of Intermolecular Interaction on the Electroluminescence 35=[311]
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