Green-light–emitting electroluminescent device
based on a new cadmium complex
Rahul Kumar, Ritu Srivastava, Akshay Kumar,
M. N. Kamalasanan and K. Singh
EPL, 90 (2010) 57004
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EPL, 90 (2010) 57004
Green-light–emitting electroluminescent device based on a new
Rahul Kumar1, Ritu Srivastava2, Akshay Kumar1, M. N. Kamalasanan2and K. Singh1(a)
1School of Physics and Material Science, Thapar University - Patiala-147004, India
2Centre for Organic Electronics, National Physical Laboratory - New Delhi-110012, India
received 13 January 2010; accepted in final form 30 May 2010
published online 1 July 2010
PACS 78.55.-m – Photoluminescence, properties and materials
PACS 78.60.Fi – Electroluminescence
PACS 74.25.Gz – Optical properties
Abstract – A new cadmium complex is synthesized to investigate its stability and applicability for
a luminescent device. The as-prepared Cd(Bpy)q sample is characterized by Fourier-transformed
infra-red spectroscopy (FTIR), thermal gravimetric analyzer (TGA) and photoluminescence (PL).
The prepared sample shows excellent thermal stability up to 380◦C. A maximum is observed at
240nm in absorption spectra which is attributed to the π-π∗transition. An organic-light–emitting
diode (OLED) has been fabricated using this material. The fundamental structures of the device
exhibit ITO/α-NPD/Cd(Bpy)q/BCP/Alq3/LiF/Al. The electroluminescence (EL) device emits
bright green light with maximum luminescence 1683cd/m2at 20V.
Copyright c ? EPLA, 2010
Introduction. – Luminescent organometallic com-
pounds have been attracting much attention in recent
years because of their potential application as organic-
exhibit higher sensitivity and efficiency than inorganic-
light–emitting diodes due to the requirement of low drive
voltage with variable color emission. Therefore, a lot of
efforts has been focused to synthesize new coordination
compounds to improve the efficiency and thermal stability
of OLEDs. The properties can further be enhanced by
development of better material or more efficient device
structures. Ma et al.  have reported a highly efficient
device based on osmium complexes. However, the perfor-
mance of the device is less than 2cd/A. On the other
hand, Li et al.  have reported a highly efficient device
based on rhenium complexes. They have reported that
the maximum brightness and efficiency are 7.15cd/A
and 3686cd/m2, respectively. Among all these materials,
cadmium complexes could play an important role to fabri-
cate variable OLED due to their wide spectral response.
So, extensive research work is going on to synthesize new
cadmium complexes containing new ligands to produce a
number of novel luminescent cadmium complexes [5–7].
These new complexes can be used as emitters and electron
transporters in OLEDs [8–13]. Basically, the coordination
numbers of cadmium complexes are variable, which
can be exploited to synthesize new emitter materials
with varying optoelectronic properties. Additionally, Cd
complexes exhibit good thermal, chemical and photo-
chemical stability. In the present study, a new cadmium
complex (2,2′bipyridine) 8-hydroxyquinoline (Cd(Bpy)q)
is synthesized using a chemical processing route. As-
prepared materials are characterized for their structural,
optical and photoluminescence (PL) properties. The new
Cd complexes are used as an emitter layer to fabricate a
device which emits green light with good efficiency and
higher brightness than osmium-containing device.
Experimental details. – The synthesis process of the
Cd(Bpy)q complex is shown in fig. 1(a). Bipyridine (Bpy),
8-hydroxyquinoline (q) and cadmium acetate (metal
ligand) was taken in 1:1:1 molar ratio in ethyl alcohol
to synthesize the Cd(Bpy)q complex. The solution of
bipyridine and 8-hydroxy quinoline were mixed and kept
at 90◦C for 2h. After that, mixture was allowed to cool
up to 70◦C. At this stage, a solution of cadmium acetate
(0.267g in 3ml of deionized water), was also added
dropwise in the reaction mixture. After 2h of stirring a
yellowish precipitate was observed. The yellow precipitate
was separated, filtered and dried at 90◦C in a vacuum
Rahul Kumar et al.
Fig. 1: (Colour on-line) (a) Synthesis process of the Cd complex. (b) Configuration of the EL device.
The schematic diagram of the device is shown in
fig. 1(b) indium-tin oxide (ITO) coated glass was used
as a substrate. Resistance of substrate of deposition was
20Ω/m. Before film deposition, the ITO substrate was
ultrasonically cleaned and treated with oxygen plasma
for 5min. On the substrate, the hole transport and
emitting layers were deposited sequentially under a high
vacuum (1×10−5torr). During emitter layer deposition
the rate was 0.2–0.5˚A/s. However, in case of the LiF
layer, the deposition rate was 0.1–0.2˚A/s. The thick-
nesses of the deposited layers were controlled by a moni-
tor of quartz crystal. The cathode was deposited on
the top of the structure through a shadow mask. A
40nm N,N diphenyl-N′N′-bis(1-naphthyl)-1,1′-biphenyl-
4,4′-diamine(α-NPD) (Sigma Aldrich) was used as a hole
transport layer. While Cd(Bpy)q and aluminum tris-8-
hydroxyquinoline (Alq3) (Sigma Aldrich) was used as an
emitter layer and electron transport layer, respectively.
The electron injection was facilitated using a 1nm thin
LiF (Merck, Germany) layer followed by a thick layer of
aluminum. The size of each pixel was 5×3mm2.
As-prepared samples of Cd(Bpy)q were characterized
using FTIR of the Nicolet 5700 spectrometer. The spectra
were recorder in the region of 4500–400cm−1. Thermal
stability of Cd(Bpy)q was checked by thermo-gravimetric
analysis (TGA) using SDTA
system. During the experiment, the heating rate was
10◦C/min. UV-visible (UV-Vis) absorption spectra were
recorded by a Shimadzu UV-2401 spectrophotometer.
The excitation and emission spectra of the material were
measured by a Fluorolog Spectrofluorometer (Horiba
Jobin YVON Fluolog Model FL 3-11) at room tempera-
ture. Both the spectra were taken in a solution of toluene.
The electroluminescent (EL) spectrum was recorded on a
high-resolution spectrometer (Ocean Optics, HR-2000CG
UV-NIR). The current-voltage-luminance (I-V -L) charac-
teristics were measured using a luminance meter (LMT
1009) and a Keithley 2400 programmable voltage-current
digital source meter. Time-resolved PL decay spectra
were recorded by a time-correlated single-photon counting
(TCSPC) system from IBH (UK). During the experiment,
the excitation wavelength and repetition rate were 341nm
and 1µHz, respectively. The instrument response time
Result and discussion. – FTIR spectra of Cd(Bpy)q
are shown in fig. 2 (a). Bands at 1580cm−1and 2925cm−1
can be assigned to C=C and C–H stretching, respec-
tively. Apart from these bands, some other bands are
also observed, i.e. 1280 and 1385cm−1. These bands are
attributed due to the C–N–C bond stretching. Addi-
tionally, the IR absorption band at 1570–1620 might be
attributed due to C=N stretching . The FTIR spec-
trum also exhibits the characteristics peaks of a quinolinic
aromatic ring at 738, 743, 793, and 829cm−1. Cadmium
ions form a stable complex with quinoline by a metal-
nitrogen bond which is observed at 490cm−1. Addition-
ally, the band at 505cm−1arose due to the stretching of
the Cd–O bond. FTIR spectra indicated that cadmium
metal is well connected with the polymeric host.
Green-light–emitting electroluminescent device based on a new cadmium complex
Fig. 2: (Colour on-line) Cd complex: (a) FTIR spectra,
(b) TGA curve.
The TGA curve of Cd(Bpy)q is shown in fig. 2(b). The
TGA curve does not show any weight loss up to 380oC
which is a clear manifestation of the excellent thermal
stability of the new Cd complex. Above 380oC, the sample
exhibits a drastic weight loss. This might be ascribed to
the decomposition of the polymeric host.
The UV-Vis absorption and photoluminescence spec-
trum of Cd(Bpy)q are shown in fig. 3. The absorption
spectrum shows a maximum at ∼240nm, which corre-
sponded to a π-π∗transition of the ligands. In addition to
this, two moderate absorption peaks are also observed at
280 and 400nm. These bands might also arise due to the
π-π∗transition of the aromatic ring. Similar absorption
spectra were observed for Zn(Bpy)q complexes . The
PL spectra of Cd(Bpy)q emit green light with a maximum
peak at 500nm, which is consistent with the absorption
spectra. The small red shift of EL vs. PL spectra has been
observed in the system. It is probably due to a combi-
nation of inhomogeneous band broadening and the fact
that charges can explore a much larger number of lumine-
sent sites than excitons . Additionally, a weak shoulder
at 540nm is also observed. In comparison to PL and EL
spectra, there is a little difference in peak maximum. This
phenomenon may be associated to the complex interaction
between (Bpy)q and cadmium under the influence of an
electric field. This interaction causes the3π-π∗transition
increase in the EL excitation process to the PL excitation
process . Interestingly, there are no characteristic
peaks of complex, which indicate the effective energy
transfer from the host excitation to the dopant.
The time-resolved PL spectrum is given as inset in fig. 3.
There are two components in the decay curve. The curve
is fitted by I =a1e−t/τ1+a2e−t/τ2, where τ1 and τ2 are
the lifetime of the components with amplitude a1 and
a2 . The lifetimes of the first and second component
Fig. 3: (Colour on-line) UV-Vis and photoluminescence spectra
Fig. 4: (Colour on-line) EL spectra of an OLED based on the
are 4.14ns and 12.7ns, respectively. The sample shows a
faster decay than AlQ3which is well fitted with single
exponential functions . The bi-exponential behaviour
of the decay curve can be explained by a) a fast process
which might be attributed due to relaxation of the carriers
into the unoccupied ground state, b) the slow process
can be correlated with radiative recombination from the
excited state. However, both the processes contribute to
the energy transfer rate which depends on the acceptor
concentration . The decay curve also indicates that
the triplet energy transfer is completely suppressed in the
present sample. Electroluminescence spectra with different
injection voltages are given in fig. 4. As the ejection voltage
increases the intensity of the spectra also increases. The
device emitted green light with an emission maximum at
510nm. Moreover, the present sample could not show any
shift as reported in AlGaInP systems .
The current-density–voltage (J-V ) characteristic of the
fabricated device is recorded by applying the voltage
Rahul Kumar et al. Download full-text
Fig. 5: (Colour on-line) Current-luminescence vs. voltage curve
of an OLED.
across the device using ITO as anode and aluminum as
cathode (forward bias). The onset of light emission starts
at about 11.5V (threshold voltage) as shown in fig. 5.
Above this voltage, the current rises non-linearly due to
the space charge effects. Above the threshold voltage, the
device emits a greenish light. The maximum brightness
and current density are observed to be 1681cd/m2and
1790A/m2at 20V, respectively. Below this voltage, the
J-V characteristics showed Ohmic current indicating the
presence of a thermally generated carrier. The device
shows a maximum current efficiency of 4.1cd/A and a
maximum power efficiency of 1.0lm/W at 14V, which is
higher than commonly used (PVK:PBD) materials .
Conclusion. – In summary, a new Cd complex is
synthesized by the reaction of 8-hydroxyquinoline and
2,2′bipyridine with cadmium acetate. Cd complexes
exhibit good thermal stability up to 380◦C. The as-
prepared materials emit bright green light in EL and PL
spectra. The fabricated Cd-complex–based device shows
a maximum brightness of 1683cd/m2at 20volt and a
current efficiency of 4.1cd/A at 14volt. The thermal
stability and electroluminescence properties of Cd(Bpy)q
suggest a promising green-light–emitting material for
organic-light–emitting device application.
The authors are very grateful for Dr A. K. Gupta,
Dr S. S. Bawa, Dr S. K. Dhawan for their suggestions
and discussion. The authors also gratefully recognize the
financial support from the Department of Information
Technology (DIT) India.
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