1.54 μm electroluminescence from erbium (III) tris(8-hydroxyquinoline) (ErQ)-based organic light-emitting diodes

Article abstract of DOI:10.1063/1.124700

A 1.54 μm room temperature electroluminescence from organic light emitting diodes (OLED) using ErQ as the emitting layer is demonstrated. The possibility of a route producing a silicon based organic light emitting diode operating at this wavelength is highlighted. Some preliminary measurements of electroluminescence intensity against drive current were also performed.

Full text of DOI:10.1063/1.124700

1.54 μm electroluminescence from erbium (III) tris(8-hydroxyquinoline) (ErQ)-based
organic light-emitting diodes
R. J. Curry and W. P. Gillin
Citation: Applied Physics Letters 75, 1380 (1999); doi: 10.1063/1.124700
View online: http://dx.doi.org/10.1063/1.124700
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APPLIED PHYSICS LETTERS
VOLUME 75, NUMBER 10
6 SEPTEMBER 1999
1.54 ␮m electroluminescence from erbium „III… tris„8-hydroxyquinoline…
„ErQ…-based organic light-emitting diodes
R. J. Curry and W. P. Gillina)
Department of Physics, Queen Mary and Westfield College, University of London, E1 4NS, United Kingdom
͑Received 9 February 1999; accepted for publication 12 July 1999͒
Organic light-emitting diodes have been fabricated using erbium tris͑8-hydroxyquinoline͒ as the
emitting layer and N, N -diphenyl-N,N -bis͑3-methylphenyl͒-1,1 -biphenyl-4,4 -diamine as the
Ј
Ј
Ј
Ј
hole-transporting layer. Room-temperature electroluminescence was observed at 1.54 ␮m due to
4
4
intra-atomic transitions between the I13/2 and I15/2 levels in the Er3ϩ ion. These results suggest a
possible route to producing a silicon-compatible 1.54 ␮m source technology. © 1999 American
Institute of Physics. ͓S0003-6951͑99͒02336-0͔
Erbium-doped materials have for many years been the
subject of much interest due to their application in optical
fiber communications. The Er3ϩ ion has a sharp lumines-
cence centered at 1.54 ␮m due to an intra-4f shell transition
between the first excited state (4I13/2) and the ground state
(4I15/2). Given that silicon is transparent at 1.54 ␮m, it has
been doped with erbium, often with other dopants such as
oxygen and fluorine, with the hope of producing a silicon-
based 1.54 ␮m emitter technology.1 There are, however,
problems with this approach as the erbium-related lumines-
cence tends to quench at room temperature.
Organic light-emitting diodes ͑OLEDs͒ have been the
subject of much research since Tang and VanSlyke demon-
strated electroluminescence from aluminum tris͑8-
hydroxyquinoline͒ ͑AlQ͒.2 Since then, considerable work has
been done on improving OLED characteristics such as life-
time, brightness, and efficiency. In this letter, we report the
fabrication of an OLED emitting electroluminescence at 1.54
␮m using erbium tris͑8-hydroxyquinoline͒ ͑ErQ͒ as the emit-
emitted under a driving voltage of 25 V along with the pho-
toluminescence previously reported. While there is little
change in the main peak positions of the two spectra, it can
be seen that the electroluminescence does not have the
higher-energy peak which appears in the photoluminescence
spectra. The electroluminescence spectra peaks at ϳ600 nm
and has a full width at half maximum ͑FWHM͒ of 127 nm. It
was also considerably weaker in intensity than the electrolu-
minescence, observed under the same operating conditions,
from OLEDs with other group III chelates ͑such as AlQ͒
used as the emitting layer. Figure 2 shows the erbium-related
electroluminescence, obtained under the same conditions as
the ‘‘band-edge’’ electroluminescence, along with the photo-
luminescence for the same region. The two spectra are mark-
edly different with the electroluminescence having a FWHM
of ϳ33 nm compared to ϳ76 nm for the photoluminescence.
ting layer and N,N -diphenyl-N,N -bis͑3-methylphenyl͒-
Ј
Ј
1,1 -biphenyl-4,4 -diamine ͑TPD͒ as the hole-transporting
Ј
Ј
layer.
The ErQ was produced by mixing erbium ͑III͒ chloride
in aqueous solution with 8-hydroxyquinoline in methanol.
Following purification, it was placed into a boron nitride
crucible in an UHV chamber for sublimation. First, 500 Å of
TPD was evaporated onto cleaned indium–tin–oxide-coated
glass ͑ITO͒, with a sheet resistance of 20 ⍀/ᮀ, followed by
600 Å of ErQ. 1100 Å of aluminum was then evaporated to
form the top electrode. The I–V characteristics were ob-
tained using a Keithley 236 current–voltage source. The
electroluminescence was dispersed in a 1 m spectrometer and
recorded using a 0.5 ␮m Blaze grating with an S-20 photo-
multiplier for the ‘‘band-edge’’ luminescence and a 1 ␮m
Blaze grating with a liquid-nitrogen-cooled Ge detector for
the erbium-related luminescence. The photoluminescence
obtained has been described in a previous paper.3 All spectra
were obtained at room temperature.
FIG. 1. The 300 K ‘‘band-edge’’ photoluminescence of ErQ excited using
the 351 nm line from an argon-ion laser and the ‘‘band-edge’’ electrolumi-
nescence obtained from an ITO/TPD͑500 Å͒/ErQ͑600 Å͒/Al OLED. Note
that the solid lines are a guide to the eye.
Figure 1 shows the ‘‘band-edge’’ electroluminescence
a͒
Electronic mail: w.gillin@qmw.ac.uk
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0003-6951/99/75(10)/1380/3/$15.00 1380 © 1999 American Institute of Physics
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Appl. Phys. Lett., Vol. 75, No. 10, 6 September 1999
R. J. Curry and W. P. Gillin
1381
FIG. 2. The 300 K erbium-related photoluminescence excited using the 457
nm line from an argon-ion laser and the erbium-related electroluminescence
obtained from an ITO/TPD͑500 Å͒/ErQ͑600 Å͒/Al OLED. Note that the
solid lines are a guide to the eye.
FIG. 3. Current–voltage characteristics for an ITO/TPD͑500 Å͒/ErQ͑600
Å͒/Al OLED. The inset shows the same data plotted on a linear scale with
the vertical axis in mA.
Furthermore, the main luminescence peak of the electrolumi-
nescence is at 1533 nm, which is close to the emission seen
from erbium in other materials,1 this is significantly shifted
from the peak of the photoluminescence of ErQ which is at
1525 nm. Close investigation of Fig. 2 does show the pres-
ence of a shoulder at ϳ1533 nm in the erbium-related pho-
toluminescence and the peak at ϳ1558 nm is present in both
spectra. The most obvious difference between these two
spectra is that the photoluminescence has appreciable emis-
sion between 1450 and 1520 nm, which is very much re-
duced in the electroluminescence.
These differences between the photoluminescence and
the electroluminescence may be indicative of different forms
of ErQ. It is known that the Stark splitting of the energy
levels in Er3ϩ depends on the local electric field seen by the
atom. Furthermore, it is known that for AlQ there are two
isomers of the molecule, the fac (C3v symmetry͒ and mer
(C2v symmetry͒.4 It is possible that the material used for the
photoluminescence, which has not been sublimed, could be a
mixture of these two isomers resulting in emission from er-
bium atoms in different local environments. However, the
sublimation required to form the OLED may have resulted in
this material having a higher concentration of one of the
isomers.
as the contact electrode. There is no reason to suppose that
this turn-on voltage cannot be reduced as has been achieved
with AlQ-based diodes. The device shows excellent diode
characteristics, however, and does not break down under re-
verse biases as high as Ϫ30 V ͑Fig. 3͒. We have performed
some preliminary measurements of electroluminescence in-
tensity against drive current and these indicate that for drive
voltages up to 30 V we have not saturated the erbium-related
emission, and so, it should be possible to obtain brighter
luminescence. It should be noted, however, that our drive
current at 30 V is an order of magnitude greater than that
observed in TPD/AlQ diodes of similar geometry. These
greater currents may be indicative that the ErQ is a better
hole transporter than AlQ, with the possibility that a number
of the injected carriers are traversing the diode without re-
combination and are thus not contributing to the electrolumi-
nescence. Thus, by increasing the thickness of the ErQ layer
or introducing hole-blocking layers, it may be possible to
increase the emission intensity and maybe to saturate the
erbium luminescence.
It has been demonstrated by Zhou et al.5 and Kim et al.6
that OLEDs can be fabricated using p-type doped silicon as
the anode with TPD and AlQ as the organic layers. Given the
results presented here, we suggest that this provides a pos-
sible route to producing a 1.54 ␮m silicon-integrated emitter
technology.
In conclusion, we have demonstrated 1.54 ␮m room-
temperature electroluminescence from OLEDs using ErQ as
the emitting layer and have highlighted the possibility of a
route to producing a silicon-based organic light-emitting di-
ode operating at this wavelength.
The differences in the high-energy side of the ‘‘band-
edge’’ luminescence between the photoluminescence and
electroluminescence may be due to the presence of these
structural differences or to other impurities, and further work
will be necessary to understand this.
Figure 3 shows the current–voltage characteristics for
the device and the erbium-related electroluminescence inten-
sity as a function of driving current. From the I–V charac-
teristics it can be seen that the device turn-on voltage is 12
V. This high turn-on voltage is indicative of poor injection
The authors would like to thank M. Somerton for his
help in producing and purifying the ErQ.
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efficiencies and is probably caused by the use of aluminum
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1382
Appl. Phys. Lett., Vol. 75, No. 10, 6 September 1999
R. J. Curry and W. P. Gillin
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