Organic multilayer white light emitting diodes

Full text of DOI:10.1021/ja953302n

J. Am. Chem. Soc. 1996, 118, 1213-1214
Organic Multilayer White Light Emitting Diodes
1213
Marko Strukelj,*,1 Rebecca H. Jordan, and
Ananth Dodabalapur*
AT & T Bell Laboratories
Murray Hill, New Jersey 07974
ReceiVed September 28, 1995
Simple, low-power, electroluminescent (EL) devices based
on thin films of organic materials are promising candidates for
a host of applications, such as flat panel displays and backlights.
The need for white light emitting diodes (LEDs) and backlights
has spurred interest in the preparation of white light emitting
devices so that red, green, and blue would be conveniently
available from one device.2,3 The most stable LEDs use the
efficient green emitter, tris(8-hydroxyquinoline)aluminum (AlQ),
and a hole transporter such as N,N′-diphenyl-N,N′-bis(3-meth-
ylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD).4 The green/red
emission in such devices has been enhanced by incorporating
lower energy dopants.4 However, increased blue output cannot
readily be achieved in a similar manner. We report new bright
white and mixed color displays that are stable and exhibit fairly
good quantum efficiencies. This was achieved by modifying
the double-layer TPD/AlQ device structure to include a new
triaryl-substituted oxazole blue/green emitter layer and the red
fluorescent dye DCM1 as a dopant in a second AlQ layer. The
physics of our device operation is markedly different from that
recently reported by Kido and co-workers.5 The blue emission
in our devices comes from a triaryloxazole instead of from the
hole transporter, TPD. This difference not only provides another
degree of freedom in terms of device construction, but, more
importantly, it should also give an advantage in terms of device
stability, since exciton formation and subsequent emission from
TPD decreases device longevity.6
Figure 1.
The TPD/AlQ device structure consists of two layers of
organics sandwiched between electrodes (Figure 1a). It is
prepared by sequentially depositing onto an indium-tin oxide
anode (ITO) layer TPD (1, Figure 2), AlQ (2), and a low work
function metal cathode such as aluminum. Doping the AlQ with
small amounts of fluorescent dyes has been shown to lead to
changes in the EL spectrum and to higher external quantum
efficiency.4
Figure 2.
The emission spectrum of AlQ is weak in the blue and red
compared to green (λmax ) 540 nm) and is not completely
satisfactory for white LEDs. We have developed an alternate
approach to white light LEDs that enhances blue emission and
does not require a microcavity.3 It involves using a new blue/
green emitting compound that can be deposited between the
AlQ and the diamine TPD (Figure 1b). It was our intention to
broaden the emission profile of TPD/AlQ by first bolstering
the emission of blue light and subsequently enhancing emission
of the red component by adding a red emitting dye, if required.
Three new blue/green emitters, 3a-c (Figure 2), and com-
pound 3d, synthesized from readily available starting materials
using simple synthetic routes (one or two steps), were surveyed
for this application. Compound 3a7 was prepared in one step
by coupling 1,4-dibromo-2,5-dimethoxybenzene with pheny-
lacetylene in the presence of bis(triphenylphosphine)palladium-
(II) chloride and cupric acetate in THF (see supporting
information). The substituted oxazoles 3b8 and 3d9 were both
prepared in a two-step sequence by first esterifying 4,4′-
dimethoxybenzoin with the appropriate aryl acid chloride,
followed by ring closure in refluxing glacial acetic acid with
ammonium acetate (see supporting information). The substi-
tuted imidazole 3c10 was prepared similarly in a one-step
reaction from 4,4-dimethoxybenzil and 1-naphthaldehyde using
ammonium acetate (see supporting information). Preliminary
characterization of the photoluminescence (PL) profiles of 3a-c
showed bright blue emission in dilute solution, with maxima at
460 nm [420 nm (s)], 445 nm, and 430 nm, respectively. For
device applications, the compound must emit efficiently in the
solid state, and it must also form stable homogeneous adherent
films. For thin films (∼800 Å thick) prepared by evaporation,
(1) Current address: DuPont Central Research and Development,
Experimental Station, P.O. Box 80304, Wilmington, DE 19880-0304.
(2) Johnson, G. E.; McGrane, K. M. Proc. SPIE-Int. Soc. Opt. Eng.
1993, 1910, 6.
only 3b fulfilled both criteria (λmax(1) ) 440 nm and λmax(2)
)
500 nm). The imidazole 3c forms excellent thin films but
(3) Dodabalapur, A.; Rothberg, L. J.; Miller, T. M. Appl. Phys. Lett.
1994, 65, 2308.
(4) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. Chen J. Appl. Phys. 1989,
65, 3610.
(8) mp ) 137-139 °C. Anal. Calcd for C27H21NO3: C, 79.59; H, 5.19;
N, 3.44; O, 11.78. Found: C, 79.40; H, 5.15; N, 3.31; O, 11.40.
(9) mp ) 125-127 °C. Anal. Calcd for C23H15NO3: C, 77.29; H, 5.36;
N, 3.92; O, 13.43. Found: C, 77.33; H, 5.24; N, 3.85; O, 13.37.
(10) mp ) 259-261 °C. Anal. Calcd for C27H22N2O2: C, 79.78; H, 5.46;
N, 6.89; O, 7.87. Found: C, 79.72; H, 5.44; N, 6.77; O, 7.49.
(5) Kido, J.; Kimura, M.; Nagaim, K. Science 1995, 67, 1332.
(6) Tang, C. W. Private communication.
(7) mp ) 177-179 °C. Anal. Calcd for C24H18O2: C, 85.18; H, 5.36;
O, 9.46. Found: C, 84.97; H, 5.26; O, 9.24.
0002-7863/96/1518-1213$12.00/0 © 1996 American Chemical Society

1214 J. Am. Chem. Soc., Vol. 118, No. 5, 1996
Communications to the Editor
blue emission does not come from TPD, because substitution
of 3b by 3d, a species which is almost identical and does not
emit visible light, gives devices that lack the blue emission
component. As the thickness of the oxazole (3b) layer is
increased, the electroluminescence spectrum shifts toward blue.
Increasing the thickness of 3b beyond 150 Å causes no
appreciable change in the spectrum but reduces efficiency. In
addition to serving as one of the emitting layers, 3b also
transports one of the carriers preferentially.
To get as close as possible to white light emission, the device
architecture was modified further (Figure 1c) to increase the
contribution of the red component by incorporating small
amounts of the red emitting dye DCM1 into a second AlQ
layer.4 The amount of red light was modulated by incorporating
an undoped AlQ layer of thickness 100-300 Å between oxazole
3b and the doped AlQ (+DCM1) layer and by varying the
dopant concentration from 0.3 to 0.5%. The whitest EL
spectrum (Figure 3) was obtained for a 300 Å AlQ filler and a
200 Å doped AlQ (0.5% DCM1) layer capped by a 200 Å thick
AlQ undoped layer (Figure 1c).
Finally, in order to make a realistic and quantitative com-
parison between various white light emitting devices, it is
imperative to specify the color of the emitted light in photo-
metric terms, as standardized by the Commission Internationale
de l’Eclairage (CIE).14 When the radiometric EL spectra are
converted into chromaticity coordinates on a CIE diagram, the
TPD/AlQ device (CIE 0.39, 0.56) shows movement from green
in the basic system, to blue-green for TPD/oxazole 3b/AlQ
(0.28, 0.46), and then toward white for TPD/oxazole 3b/AlQ/
AlQ + DCM1/AlQ (0.32, 0.41). The CIE coordinates for
absolute white are (0.33, 0.33). The luminescence intensity and
external quantum efficiency of the devices are as high as 4000
cd/m2 and >0.5%, respectively, at an operating voltage of 15
V using a double-layer Li:Al cathode. Most importantly,
preliminary measurements indicate that the stability,15 operating
Voltage, and quantum efficiencies of these new devices are
comparable to those of the TPD/AlQ device. Furthermore, the
combination of EL materials reported here could also be
successfully employed in microcavity LEDs to selectively
enhance individual or multiple colors that lie within the emission
band. This would involve changing the thickness of a micro-
cavity containing a polyimide or a silicon nitride filler layer to
utilize Fabry-Perot cavity effects (for microcavity LEDs),
allowing red, green, and blue emission to be obtained.
Figure 3. Device electroluminescence spectra. (1) TPD/AlQ; (2) TPD/
oxazole 3b/AlQ; (3) TPD/oxazole 3b/AlQ/AlQ + DCM1/AlQ.
exhibits weak emission in the solid state (λmax ) 445 nm),
presumably due to self-quenching. While the linear species 3a
is chemically very stable in thin film form and emits strongly
(λmax ) 460 nm), it exhibits a propensity to crystallize after
evaporation to form large crystals (> 500 Å). It is thought that
organic solids that form large crystallites yield devices that
perform poorly, because grain boundaries function as quenching
sites. Oxazole 3d also forms excellent films; however, it was
specifically chosen so it would not emit in the blue (Vide infra).
Compounds 3b-d presumably form good films, because the
noncoplanar conformation of the pendant aryl rings hinders
crystallization. This reasoning is corroborated by CAChe
molecular modeling results, which predict that the pendant aryl
rings in 3b-d are, indeed, noncoplanar with the oxazole
heterocycle in the minimum energy confirmation.11 Such
deportment is also inherent in TPD, which forms amorphous
films,12 while AlQ has been reported to be microcrystalline
(<500 Å).4
The three-layer device structure with 3b as the blue/green
emitter is depicted in Figure 1b. This device exhibits a
significantly broader EL spectrum, spanning the range 400-
650 nm (Figure 3), than does TPD/AlQ, due to enhanced blue
emission.13 This device architecture allows exciton recombina-
tion to occur in more than one layer. In the TPD/AlQ device,
hole and electron recombination occurs in the AlQ layer,
presumably because of the energy levels and the transport
properties of AlQ and TPD. The output spectrum is fairly broad
and peaks in the green at 540 nm. In the new device, the green
emission comes from both AlQ and 3b, but nearly all of the
blue emission comes from the latter. We are confident that the
Acknowledgment. The work of M.S. was partially supported under
a Natural Sciences and Engineering Council of Canada postdoctoral
fellowship (1992-1994). We thank F. C. Schilling for the CAChe
molecular simulations and Dr. T. M. Miller for supplying the AlQ and
TPD. Useful comments on the manuscript by Drs. L. J. Rothberg, H.
E. Katz, E. A. Chandross, and R. E. Slusher are appreciated.
Supporting Information Available: Synthesis and characterization
of 3a-d (2 pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm version
of the journal, can be ordered from the ACS, and can be downloaded
from the Internet; see any current masthead page for ordering
information and Internet access instructions.
(11) Computer Aided Chemistry molecular modeling program (Cache,
Version 3.0; CAChe Scientific, 18700 NW Walker Rd., Bldg. 92-01,
Beaverton, OR 97006). Compound 3b: naphthalene group is twisted 20°
out of the plane of the oxazole ring, and the two pendant anisole moieties
are twisted 31° and 36.6°. For 3d, the anisole groups are similarly twisted,
and the phenyl group if coplanar (2°).
JA953302N
(12) Stolka, M.; Yanus, J. F.; Pai, D. M. J. Phys. Chem. 1984, 88, 4707.
(13) Typical thicknesses were TPD, 600 Å; oxazole 3b, 150 Å; AlQ,
600 Å; Al, 2000 Å.
(14) Colorimetry; Central Bureau of the CIE: Vienna, 1986.
(15) Devices have a shelf-life of at least 9 months.