A field-dependent organic LED consisting of two new high Tg blue light emitting organic layers: A possibility of attainment of a white light source

Article abstract of DOI:10.1039/b209949g

Two new blue light emitting trimeric compounds of the Y-shape type having high glass transition temperatures were synthesized and EL behavior of LED devices consisting of bilayers of the two compounds was studied. One of the compounds is of hole-transporting type containing carbazole moieties, whereas the other is of electron-transporting type bearing phenyloxadiazole moieties. The bilayer LED devices exhibit a strong field-dependence and emit white light (simultaneous light-emittance in blue, green and red regions), at high applied electric fields. Increased interracial formation of exciplexes at stronger external fields appears to be responsible for this field-dependence.

Full text of DOI:10.1039/b209949g

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A field-dependent organic LED consisting of two new high Tg blue
light emitting organic layers: a possibility of attainment of a white
light source{
Soon Wook Cha and Jung-Il Jin*
Department of Chemistry and the Center for Electro- and Photo-Responsive Molecules, Korea
University, Seoul 136-701, Korea
Received 10th October 2002, Accepted 10th January 2003
First published as an Advance Article on the web 4th February 2003
Two new blue light emitting trimeric compounds of the Y-shape type having high glass transition temperatures
were synthesized and EL behavior of LED devices consisting of bilayers of the two compounds was studied.
One of the compounds is of hole-transporting type containing carbazole moieties, whereas the other is of
electron-transporting type bearing phenyloxadiazole moieties. The bilayer LED devices exhibit a strong field-
dependence and emit white light (simultaneous light-emittance in blue, green and red regions), at high applied
electric fields. Increased interfacial formation of exciplexes at stronger external fields appears to be responsible
for this field-dependence.
highly efficient phosphorescent organic light-emitting devices
are attracting a great deal of interest.28
Introduction
There have been extensive studies on organic light-emitting
diodes (LEDs)1–11 using low molecular mass organic materials,
dendrimers12,13 and polymers. The possibilities of making large
area LEDs and of tuning the colors, covering the whole range
of the visible spectrum, are especially attractive properties since
this would allow the construction of full color displays. In
addition, there is a strong demand for realization of efficient
white light sources that can be utilized for a variety of uses such
as in the application of large panel LEDs for back lighting in
liquid crystal displays. Several ways to produce white light
from polymeric and low molecular mass organic devices have
been presented.14–21 Polymers doped with an appropriate
combination of red, green and blue fluorescent dyes can
produce white light.14 A cell structure comprised of polymeric
layers each doped with blue, green and red fluorescent dyes can
produce white light emission.15 A luminescent conjugated
polymer overlaid with a luminescent low molar mass electron
transport material produced bluish-white light emission
through a combined electroluminescence from both emitting
layers as well as charge transfer complexes formed at the
polymer/low molar mass molecule interface.16
The thermal stability of organic materials and their glassy
state is an important factor governing device stability of
organic light emitting diodes (OLEDs). There are a number of
sources of thermal stress on OLEDs:22–24 Heat evolved from
deposition of the metal cathode during fabrication, Joule
heating generated during device operation, and sudden
exothermic crystallization of organic compounds employed
in the devices are representative examples. If an OLED is
heated above the glass transition temperature (Tg) of the
organic materials in the device, irreversible failure can occur. In
order to improve device performance, one often has to employ
electron transporting and/or hole transporting materials.
Poly(N-vinylcarbazole) (PVK)25 and carbazole derivatives26
are known to be excellent hole transporting materials, whereas
1,3,4-oxadizole containing compounds27 have been extensively
investigated as electron transporting materials. In addition,
In this paper, we describe characteristics of an LED device
consisting of two layers of new compounds, Cz3d and Oxa3d,
whose structures are shown in Schemes 1 and 2.
Results and discussion
Schemes 1 and 2 illustrate the synthetic procedures used for the
preparation of the two compounds. They could be readily
prepared via multistep synthetic routes utilizing known
reactions for each step. The structures of the intermediates
and the final compounds, Cz3d and Oxa3d, were confirmed by
1H NMR and IR spectroscopy and by elemental analysis as
described in the Experimental section and the Supporting
Information.{ Both compounds are of bent-shape type. Cz3d is
composed of three carbazole moieties whereas Oxa3d com-
prises three 1,3,4-oxadizole moieties. The three moieties in each
are connected through stilbene bridges in both compounds.
According to the thermal properties of these compounds
investigated by DSC analysis, the glass transition temperatures
(Tg) of Cz3d and Oxa3d are 250 and 156 uC, respectively
(Table 1). These values are very high when compared with
commonly used hole transporting materials such as 1,4-bis-
(1-naphthylphenylamino)biphenyl (a-NPD, Tg ~ 100 uC) and
1,4-bis(phenyl-m-tolyamino)biphenyl (TPD, Tg ~ 60 uC).
Okumoto and Shirota29 recently reported high Tg organic
compounds which were utilized as carrier transporting
materials in EL devices.
Fig. 1 compares the UV-Vis absorption and photolumines-
cence (PL) spectra of Cz3d and Oxa3d in dilute solutions and in
thin films. The solid thin films were prepared by vacuum
deposition. The PL spectra were taken at an excitation
wavelength of 300 nm. Fig. 1(a) shows that a dilute solution
of Cz3d (1025 mol L21) absorbs at 300 and 349 nm with the tail
extending to about 425 nm. The two absorptions are ascribed
to the carbazole and phenylcarbazole moieties, respectively.
The extended tail part originates from the p–p* transition of
the whole conjugated p-electron system. The two absorptions
show a slight red shift in the solid film and appear at 304 and
355 nm, respectively (Fig. 1(b)). In addition, the absorption
spectrum of the solid film exhibits a much longer tail extending
{Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/jm/b2/b209949g/
DOI: 10.1039/b209949g
J. Mater. Chem., 2003, 13, 479–484
This journal is # The Royal Society of Chemistry 2003
479

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Scheme 1 Reagents and conditions: (i) 4-bromotoluene, Cu, K2CO3, dibenzo-18-crown-6, o-dichlorobenzene, reflux; (ii) Br2, CCl4, room
temperature; (iii) N-bromosuccinimide (NBS), CCl4, reflux; (iv) triphenylphosphine, benzene, reflux; (v) 1 M NaOH; formaldehyde (37% water
solution), room temperature; (vi) (PPh3)4Pd (cat.), toluene/Na2CO3(2 M in H2O), 4-tert-butylphenylboronic acid; (vii) palladium(II) acetate, tri-o-
tolylphosphine, DMF, triethylamine, 90 uC.
beyond 550 nm with a shoulder clearly discernible at 395 nm.
The optical band gap energy estimated from the absorption
edge (420 nm) of the solid thin film is 2.96 eV. The two sharp
peaks at 414 and 434 nm (Fig 1(a)) in the PL spectrum of the
Cz3d solution also displayed a red-shift and moved to 426 and
444 nm with a longer tail (Fig. 1(b)) in the solid film.
Additionally, the emission intensity ratio between the two
peaks became reversed in the solid film, indicating the lower
energy deactivation or longer wavelength emission is more
preferred in the solid state.
We observe similar phenomena in UV-Vis absorption and
PL spectra of Oxa3d as shown in Fig. 1(c) and (d). Oxa3d as a
dilute solution (1025 mol L21) absorbs at 279, 334 and 337 nm
with a shoulder peak appearing at 368 nm (Fig. 1(c)). The first
three absorptions arise from the substituted oxadiazole
structures and the shoulder from the p–p* transition of the
extended p-electron system. The optical band gap energy for
Oxa3d is estimated to be 3.13 eV. The solution PL spectrum
shown in Fig. 1(c) exhibits an emission intensity maximum at
403 nm with a shoulder at 430 nm. The solid film reveals a
much less resolved absorption spectrum (Fig 1(b)), which
shows a strong absorption at 339 nm with a shoulder at
378 nm. The solid film shows a strong PL emission at 438 nm
with weak shoulders overlapped around at 495 and 518 nm.
Although it is not definitive, it appears that there is a shoulder
at about 420 nm. It is clear that both Cz3d and Oxa3d are blue-
light emitters. We observe the occurrence of a weak long tail in
the PL spectrum of Oxa3d, implying possible additional
emission from excimers and/or exciplexes. The PL quantum
yields in chloroform solutions were 0.28 for Cz3d and 0.93 for
Oxa3d, respectively (Table 1). The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) levels determined by cyclovoltametry and from
optical bandgaps are at 5.64 and 2.68 eV for Cz3d and 5.91
and 2.78 eV for Oxa3d, respectively. According to these values,
Oxa3d is expected to have a higher affinity for electrons and
also to act as a hole-blocker. For the sake of comparison, data
for the EL spectra of Cz3d and Oxa3d obtained from ITO/Cz3d
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Scheme 2 Reagents and conditions: (i) NH2NH2?H2O, ethanol, reflux; (ii) 4-tert-butylbenzoylchloride, THF, pyridine, 0 uC; (iii) POCl3, reflux; (iv)
N-bromosuccinimide (NBS), CCl4, reflux; (v) triphenylphosphine, benzene, reflux; (vi) 1 M NaOH; formaldehyde (37% water solution), room
temperature; (vii) 3,5-dibromobenzoylchloride, THF, pyridine, 0 uC; (viii) palladium(II) acetate, tri-o-tolylphosphine, DMF, triethylamine, 90 uC.
2.5 MV cm21, the emission intensity of the shoulder part
(424 nm) greatly increases in intensity and becomes the major
emission peak. This emission originates from the excitons
formed in the Cz3d and Oxa3d layers (see EL spectra shown in
Fig. 1(b) and (d)) and is thus an overlapped emission from the
two layers. When the external field is further increased,
Table 1 Physical data for Cz3d and Oxa3d
a
lmax /nm
a
lem /nm
Compound
Tg/uC
Tm/uC
Wfb
Cz3d
Oxa3d
250
156
NAc
218
334
339
444
438
0.28
0.93
aMeasured in film. bFluorescence quantum efficiency in chloroform.
cNA: Tm not detected.
or Oxa3d (50 nm)/Li : Al devices are included in Fig. 1.
Although there exist some differences in details the general
profile of their PL and EL spectra are very similar.
A double layer EL device was fabricated using Cz3d as a hole
transporting, emitting layer and Oxa3d as an electron
transporting, emitting layer, with the configuration ITO/Cz3d
(40 nm)/Oxa3d (20 nm)/Li–Al. The two organic layers were
consecutively vacuum deposited onto indium–tin oxide (ITO)
coated glass (anode) followed by vacuum deposition of Al–Li
alloy (cathode) containing 0.26% of Li.
Fig. 2(a) shows how the EL spectrum of the device depends
on the applied electric field and clearly demonstrates that the
EL spectrum of this device changes drastically when the applied
field was higher than 2.5 MV cm21. At electric fields of 1.5 and
1.83 MV cm21 the device emits at 400–550 nm with maximum
emission intensity being observed at 451 nm overlapped with a
shoulder at about 424 nm. As the applied field is increased to
Fig. 1 Absorption, PL and EL spectra of (a) Cz3d in solution (CHCl3),
(b) Cz3d film (50 nm), (c) Oxa3d in solution (CHCl3), and (d) Oxa3d
film (50 nm).
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Fig. 2 Dependence of EL spectrum on applied electrical field (device:
(a) ITO/Cz3d (40 nm)/Oxa3d (20 nm)/Li–Al and (b) ITO/Cz3d (20 nm)/
Oxa3d (40 nm)/Li–Al).
Fig. 4 Comparison of UV-Vis absorption ($, +, 6, 1) and PL
spectra (#, ') of the double layer and blend films.
as the applied field is increased to 2.5 MV cm21. However, it
then starts to decrease as the field is further increased. This
point occurs with the appearance of the red emission (see
Fig. 2). The external quantum efficiency was about 0.2% at an
applied field of 2.0 MV cm21 and it decreases steadily when the
surprisingly, two new peaks appear; one in the green light
region (526 nm) and the other in the red light region (592 nm)
region, in addition to the original blue light emission. Above an
applied electric field of 2.8 MV cm21 the emitted light appears
visually to be white. It is believed that the red light emission
…
field is higher than 2.8 MV cm21
.
originates from the exciplexes (Oxa3d* Cz3d) formed at the
interface between the excited species of Oxa3d and Cz3d in the
ground state. The peak in the green region appears to result
Recently, Jenekhe and coworkers30 reported photolumines-
cence properties of double layers of poly(p-phenylene benzo-
bisoxazole) (PBO) and tris(p-toyl)amine. They observed an
emission at 474 nm which was ascribed to the exciplexes formed
between the electron-donating amine compound and electron-
accepting PBO. Epstein and coworkers31 also reported PL and
EL emissions from exciplexes from blends of PVK and an
electron-accepting conjugated polymer. Separately, we con-
structed an LED device by codepositing a 60 nm thick film of
Cz3d and Oxa3d in 1 : 1 weight ratio onto an ITO glass and
tried to study the device characteristics. Unfortunately, the
device revealed very poor device performance due to very high
turn-on electric field and we failed in obtaining an acceptable
EL spectrum. Thus, we obtained only a PL spectrum (Fig. 4) of
the blend film and we compared it with the PL spectrum of the
double layered film. We note that the two PL spectra are very
different from each other. While the PL spectrum of the double
layered film is practically the same as its EL spectrum obtained
at low applied fields as shown in Fig. 1, the PL spectrum of the
blend exhibited a significant degree of red shifting and showed
an emission maximum at about 475 nm; the spectrum was also
much broader. A close examination of the PL spectrum of the
blend showed shoulders around 524 and 600 nm. The positions
of the shoulders coincide with the positions of the new EL
emission peaks for the double layered film at high applied
electric fields as shown in Fig. 2. This implies that the blend,
due to much greater contact area between Cz3d and Oxa3d
molecules, exhibit stronger emissions from exciplexes than the
bilayered film.
…
from the exciplexes (Cz3d* Oxa3d) formed between the
excited species of Oxa3d and Cz3d in the ground state. This
conclusion can be easily derived by comparing the EL spectra
(Fig. 2(a)) of this device with those of ITO/Cz3d (20 nm)/
Oxa3d (40 nm)/Li–Al presented in Fig. 2(b), considering the
relative thickness of the two emitting layers. The former device
favors the accumulation of more Oxa3d polarons on the
interface between the two layers, whereas in the latter
accumulation of more Cz3d polarons on the interface is
favored. Such a conjecture is based on the assumption that the
hole mobility in the Cz3d layer and the electron mobility in the
Oxa3d layer are comparable. Increasing the applied external
electrical field is expected to increase the number of carriers
injected, which, in turn, enhances the probability for the
formation of exciplexes at the interface. This is exactly what we
observe in the present devices.
Therefore, it is as expected to observe that the total intensity
of emitted light also depends on the applied electric field. Fig. 3
shows that the intensity of emitted light increases to 410 cd m22
Conclusion
We have synthesized thermally stable, high Tg, and bent-
shaped conjugated molecules, Cz3d and Oxa3d, both of which
are blue light emitting materials. A double layer EL device
consisting of Cz3d and Oxa3d layers exhibited a strong
dependence of its EL spectra on the applied electric field and
emitted white light at high field (w2.8 MV cm21). This is
ascribed to the increased formation of exciplexes at the
interface of the two layers. High applied electric fields are
expected to increase the number of the two opposite carriers
injected into the layers, which would confer a greater chance to
Fig. 3 Luminance versus applied electric field of the device ITO/Cz3d
(40 nm)/Oxa3d (20 nm)/Li–Al and EL spectra at two different applied
electric fields.
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form exciplexes at the interface between the two layers. Our
observation leads to a novel method to design a white light
source by employing two layer LEDs consisting of an electron-
donating layer and an electron-attracting layer both of which
are blue-emitting, but are able to form green and red-light
emitting exciplexes. This work also provides us with an
excellent example that reveals a color tuning by changing the
applied field. The light intensity and the device efficiency of the
present devices, however, require much further improvement to
be acceptable for practical application.
Acknowledgements
This work was supported by the Korea Science and Engineer-
ing Foundation through CMR of Korea University. S. W. Cha
is a recipient of the Brain Korea 21 scholarship from the
Ministry of Education and Human Resources, Korea.
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