Blue and white light electroluminescence in a multilayer OLED using a new aluminium complex

Article abstract of DOI:10.1007/s12039-010-0073-0

Synthesis, structure, optical absorption, emission and electroluminescence properties of a new blue emitting Al complex, namely, bis-(2-amino-8- hydroxyquinolinato), acetylacetonato Al(III) are reported. Multilayer OLED using the Al complex showed blue emission at 465 nm, maximum brightness of ～ 425 cd/m² and maximum current efficiency of 0.16 cd/A. Another multilayer OLED using the Al complex doped with phosphorescent Ir complex showed 'white' light emission, CIE coordinate (0.41, 0.35), maximum brightness of ～ 970 cd/m² and maximum current efficiency of 0.53 cd/A. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-010-0073-0

J. Chem. Sci., Vol. 122, No. 6, November 2010, pp. 847–855. © Indian Academy of Sciences.
Blue and white light electroluminescence in a multilayer OLED using a
new aluminium complex
1
PABITRA K NAYAK , NEERAJ AGARWAL , FARMAN ALI , MEGHAN P PATANKAR ,
1,3
1
2
2, 1,
K L NARASIMHAN * and N PERIASAMY *
1
2
Department of Chemical Sciences, Department of Condensed Matter Physics and Materials Science,
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005
3
Centre for Excellence in Basic Sciences, University of Mumbai Kalina Campus, Santa Cruz (E),
Mumbai 400 098
e-mail: peri@tifr.res.in; kln@tifr.res.in
MS received 21 May 2010; revised 22 July 2010; accepted 19 August 2010
Abstract. Synthesis, structure, optical absorption, emission and electroluminescence properties
of a new blue emitting Al complex, namely, bis-(2-amino-8-hydroxyquinolinato), acetylacetonato
Al(III) are reported. Multilayer OLED using the Al complex showed blue emission at 465 nm,
2
maximum brightness of ~ 425 cd/m and maximum current efficiency of 0⋅16 cd/A. Another multilayer
OLED using the Al complex doped with phosphorescent Ir complex showed ‘white’ light emission, CIE
2
coordinate (0⋅41, 0⋅35), maximum brightness of ~ 970 cd/m and maximum current efficiency of
0⋅53 cd/A.
Keywords. Alq3; photoluminescence; electroluminescence; blue OLED; WOLED; DFT.
1. Introduction
over the ring containing oxygen and the LUMO is
9
on the part of the ring that contains nitrogen (also
Development of new electroluminescent organic
1–4
materials is an active area of research and a large
see supporting information). There are two possible
ways to increase the HOMO–LUMO gap of the
molecule – by attaching an electron withdrawing
group in the O-ring at 5 and 7 positions or an elec-
tron donating group in the N-ring at 2 or 4 position.
There have been numerous attempts to design the
structure of an aluminium complex of 8-hydroxy-
effort is under way to make organic light emitting
device (OLED)-based technology to develop blue
electroluminescence for full colour displays and
5–7
white light illuminant applications. Performance
of blue emitting OLEDs is not as efficient as that of
green or red and the main problems are their colour
purity, stability and lifetime and hence the search for
new molecules for blue electroluminescence (EL).
Among the best performing light emitting materials,
tris(8-hydroxyquinolinato) Al(III), (Alq3) has been
used widely because of its strong green emission at
~ 515 nm, ability to act as electron transport material
as well as host material for several phosphorescent
quinoline derivative as ligand with blue photolumi-
9–13
nescence (PL) emission.
Tuning the energy levels
by attaching various substituents in the 5 position
was carried out and the emission spectrum shifted
significantly, but the shortest wavelength attained
9,13
thereby was 479 nm.
Theoretical calculations indicated that a larger
blue shift in PL is possible for an Al complex of
8-hydroxyquinoline with an electron donating group
(NH2) at 2 position. In this paper, we report that blue
shift of PL and EL maximum to 465 nm is achieved
for a mixed ligand, Al complex of 2-amino-
8
dopants. It is known that substitution of electron
withdrawing/donating functional groups in specific
position in the ligand, 8-hydroxyquinoline, would
change the emission property of the Al complex. In
Alq3, the HOMO of 8-hyrdoxyquinoline is mainly
8-hydroxyquinoline.
Synthesis,
photophysical,
electrochemical, PL and EL properties, includ-
ing blue and white light EL, of bis-(2-amino-8-
hydroxyquinolinato), acetylacetonato Al(III) are
described.
*For correspondence
847

848
Pabitra K Nayak et al
(2-amino-8-hydroxuquinolinaato)
2. Experimental
acetylacetonato
aluminum(III) (1) are shown in scheme 1. 8-
Hydroxyquinoline was reacted with hydrogen perox-
ide and acetic acid mixture for three days. Removal
of acetic acid followed by extraction and purifica-
tion by column chromatography produced 8-hydroxy-
quinoline-N-oxide (2) as yellow solid in 38% yield.
2 was reacted with dimethyl sulphate and liquid
ammonia to afford 3 in 52% yield. Solutions of 3
(2⋅0 mmol) and Al(acac)3 (1⋅0 mmol) in ethanol
were mixed and stirred at 60°C. White precipitate of
Al complex (1) was formed, which was filtered and
washed several times with ethanol to remove any
soluble impurity, and dried in vacuum to get 1 as
white solid in 95% yield. 1 is stable in air and
showed high thermal stability. The melting point of
1 was determined by differential thermal analysis to
be 321°C. Single crystal of 1 was grown from
CH2Cl2 and methanol mixture. Figure 1 shows the
crystal structure of 1 (see supporting information for
details). It can be seen from the structure that two
Oxygen atoms of the 2-amino-8-hydroxyquinoline
ligand are at the axial position in the structure. It
may be noted that 1 crystallizes as one type of iso-
mer though other geometrical isomers are possible
in the synthesis. We mention in passing that tris(2-
amino-8-hydroxyquinolinato) Al(III) could not be
synthesized primarily due to steric constraints. Thus,
one ligand was replaced by an ancillary non-
luminescent ligand, acetyl acetone and 1 was syn-
thesized.
2.1 Materials and methods
2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyano-p-quinodi-
methane (F4TCNQ) was purchased from Lumetech
(Taiwan). 8-hydroxyquinoline, N-N′-diphenyl-N,N′-
(bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine
(TPD) and 4,7-diphenyl-1,10-phenanthroline (BCP)
were purchased from Sigma-Aldrich (USA) and
were used as received. All the solvents were ob-
1
tained from SD Fine Chemicals (India). H and
13
C
NMR spectra were recorded using Bruker NMR
spectrometer (AVANCE WB 500), with working
1
frequency 500 MHz for H NMR and 125 MHz for
13
C NMR. XRD and crystal structure data were ob-
tained using Nonius MACH3 (IIT Mumbai). Cyclic
voltammetry experiments were done with CH In-
strument 600 (USA). Absorption spectra of mole-
cules in solution and as thin film were recorded on
UV/Vis spectrophotometer (Lambda 25, Perkin
Elmer). Steady state fluorescence spectra were re-
corded using a spectrofluorimeter (SPEX model
1681T). Time resolved fluorescence decays were
obtained by the time correlated single photon count-
14,15
ing method.
OLED devices were fabricated on
pre-patterned, pre-cleaned ITO-coated glass sub-
strates. The substrates were patterned using standard
lithography and then cleaned by mechanical scrub-
bing with detergent, followed by ultrasonic cleaning
in hot water and finally blow drying using dry N2
gas. The patterned ITO substrate is UV exposed for
10 min. Organic and metal layers were then deposited
-6
by thermal evaporation in a high-vacuum (~ 10 torr) 3. Results and discussions
chamber. A thick layer of LiF (100 nm) was depos-
ited on top of Al cathode to act like a capping layer.
The thickness of various layers was monitored using
a quartz crystal thickness monitor.
3.1 Theoretical calculations
Figure 2 shows the distribution of HOMO and
The current, voltage and EL intensity charac- LUMO over the 2-amino-8-hydroxyquinoline ligand
teristics of the devices were measured in vacuum in the Al complex 1. When compared with the dis-
using a Keithley 617 programmable electrometer. tribution over 8-hydroxyquinoline in Alq3 (see Sup-
EL measurement was done with a calibrated silicon porting information) it is seen that the amino group
photodiode. EL spectra were recorded using a mono- influences the electron density distribution of both
chromator and photomultiplier Tube (S-20 response) HOMO and LUMO of the molecule. The influence
operated at 720 V. The EL intensity was converted on the LUMO is larger than that on the HOMO. This
2
to cd/m and cd/A using calibrated silicon detector is evident from comparison of quantitative values of
and device and detector geometry. Density Functional ionization potential and electron affinity of 1 and
Theory (DFT) was used in all the calculations based Alq3. Ionization potential and electron affinity were
on the hybrid B3LYP functional using Gaus- calculated using DFT at 6-311++G(d, p) level. In the
16,17
sian03.
gas phase, the ionization potential of 1 and Alq3 are
Synthesis of 2-amino-8-hydroxyquinoline (3) by a nearly equal, 6⋅55 eV and 6⋅63 eV, respectively,
18
slightly modified method and the Al complex bis- suggesting little perturbation of the HOMO level by

Blue and white light electroluminescence in a multilayer OLED using a new aluminium complex
849
Scheme 1. Synthetic scheme for Al complex 1.
ence between ionization potential and electron affin-
ity, 4⋅75 eV is the transport gap. TD-DFT
calculation of 1 at 6-311++G(d, p) level gave the op-
tical band gap of 1 in solid state as 3⋅23 eV. The
corresponding values for Alq3 in solid state are as
follows: ionization potential, electron affinity, trans-
port gap and optical gap are 5⋅82, 1⋅66, 4⋅16 and
2⋅83 eV, respectively. Thus, the increase in optical
gap by 0⋅4 eV and transport gap by 0⋅59 eV in 1 is
essentially due to decrease of electron affinity due to
the 2-amino group. Experimental results are in
agreement with the theoretical calculations.
3.2 Optical absorption and photoluminescence
properties
The absorption spectra of 1 and Alq3 were recorded
in chloroform and are shown in Figure 3. The
absorption peak corresponding to π–π* transition in
1 (~ 358 nm, molar extinction coefficient is 4076
Figure 1. Crystal structure of 1.
–1
–1
M cm ) is significantly blue shifted compared to
that of Alq3 (~ 386 nm). The dilute solution of 1 in
chloroform showed emission maximum at 460 nm
when excited at the absorption peak, whereas the
emission peak of Alq3 is 516 nm. A smooth thin film
of 1 and Alq3 on quartz were obtained by vacuum
deposition and used to record the emission proper-
ties in solid state. The emission of 1 in thin film
showed peak at 465 nm, a substantial blue shift
(~ 53 nm) of emission maximum compared to Alq3
(figure 4). The optical gap of 1 in solid state was de-
termined to be 3⋅08 eV from the intersection of
normalized excitation and emission spectra of 1 in
thin film. The optical gap for Alq3 is 2⋅70 eV.
the 2-amino group in 1. On the other hand, electron
affinity of 1 is less at 0⋅28 eV while that of Alq3 is
0⋅91 eV.
Energy levels of molecules in solid state are
important in organic electronics. The ionization po-
tential, electron affinity and optical transition ener-
gies of 1 and Alq3 were calculated in solid state
using DFT, TD-DFT at 6-311++G(d, p) level and
polarizable continuum model (PCM) by a method
19,20
reported recently.
and electron affinity of 1 in the solid state was found
to be 5⋅79 eV and 1⋅04 eV, respectively. The differ-
The vertical ionization potential

850
Pabitra K Nayak et al
Figure 2. Geometry optimized structure of 1 showing the contribution of
(a) HOMO and (b) LUMO.
Irrespective of the photophysical origin of biexpo-
nential decay, the PL quantum yield is proportional
to the area under the PL decay and the average life-
time is a measure of the area under PL decay. The
PL quantum yield of 1 in solid state is expected to
be same as in solution because of the absence of self
quenching indicated by similar average fluorescence
lifetime in solution and solid state, which is ~ 1⋅5 ns.
It is well established that HOMO (ionization
potential) and LUMO (electron affinity) energy
levels of organic molecules in solid state determined
by electron spectroscopy correlate linearly with the
cyclic voltammetric oxidation and reduction peak
22,23
potentials in dichloromethane.
The shift of redox
potential of 1 is thus a measure of shift of HOMO/
LUMO level with respect to Alq3. Cyclic voltam-
metry of 1 showed irreversible oxidation peak in
dichloromethane ([1] ~ 1 mM, tetrabutyl ammonium
hexafluorophosphate, 0⋅1 M as supporting electro-
lyte). The peak potential was observed at 0⋅58 V
with respect to ferrocene. The peak potential for
Alq3 was observed at 0⋅7 V. The difference in the
oxidation potential is 0⋅12 V whereas the difference
is 0⋅03 eV in ionization potential calculated theo-
retically. This difference is within the accuracy limit
( 0⋅15 eV) of theoretical value. 1 was not reducible
in the electrochemical range accessible in dichloro-
methane and therefore it was not possible to com-
pare the corresponding difference in the electron
affinity of 1 and Alq3.
Figure 3. Absorption spectra of Alq3 and 1 in chloro-
form.
The PL quantum yield of 1 in chloroform was
measured to be 0⋅08, which is comparable to that of
21
Alq3 in solution. Absence of self-quenching in
Alq3 in solid film was proven based on PL decay
data, and the PL quantum yield is 0⋅36 in crystalline
21
film. The PL decay of 1 in solution or solid state
was biexponential (see supporting information for
PL decays and results of analysis), a property that is
21
common with that of Alq3. Since the material is
pure, the origin of biexponential PL decay is either
due to intramolecular relaxation and/or intermolecu-
lar relaxation (solvent–solute interaction) or hetero-
geneity (for example, defect sites in solid state).
The absolute value of EHOMO of an organic mole-
cule in solid state is important for organic electron-

Blue and white light electroluminescence in a multilayer OLED using a new aluminium complex
851
ics. The usefulness of the electrochemical redox in dichloromethane, and therefore +0⋅72 V vs
25
potentials determined in cyclic voltammetry is es- NHE. The IUPAC recommended value of absolute
22,23
26
However, the value derived thus is potential for NHE is 4⋅44 0⋅02 eV. Using these
tablished.
only indicative because the polarization energy of data, the EHOMO of ferrocene and 1 are estimated to
the oxidized cation species is expected to be differ- be –5⋅16 eV and –5⋅74 eV, respectively. The value
ent in solid state and the common polar solvent used for 1 obtained thus is closer to the theoretically
in electrochemistry. Nevertheless, a linear relation calculated value of 5⋅79 eV.
between EHOMO and Vox, determined using ferrocene
22
The value ELUMO for 1 could not be obtained from
electrochemical data. The lower limit for this level
(Fc) as internal reference molecule, is established:
+
EHOMO = –(1⋅4 0⋅1) qVox (vs Fc/Fc ) – 4⋅6 ( 0⋅08) is obtained as –2⋅66 eV by addition of optical band
eV, where q is the electronic charge and Vox is the gap energy (3⋅08 eV) to the EHOMO
oxidation peak potential. The cyclic voltammogram Thus, the experimental values of oxidation poten-
of 1 is shown in supporting information. Using the tial and optical band gap are consistent with the
above relation, we obtain EHOMO of 1 as –5⋅41 theoretical values calculated using DFT, TD–DFT
.
15 eV. In an alternate method, EHOMO may be placed and PCM. Table 1 gives a comparison of photo-
at 0⋅58 eV less than that of EHOMO of ferrocene. The physical properties and energy levels of 1 and Alq3.
latter value may be calculated from the reported
oxidation potential of +0⋅52 V vs Ag/AgCl (ref. 24)
3.3 Electroluminescence properties
The electroluminescence properties (EL) of 1 were
studied in the following multilayer OLED structure:
ITO/F4TCNQ(20 Å)/TPD(600 Å)/1(600 Å)/BCP(60
Å)/LiF(15 Å)/Al. F4TCNQ, TPD, 1, BCP, LiF and
Al were successively deposited under high vacuum
2
onto the patterned ITO. The device area was 2 mm .
The energy levels of all the materials used in the
device are shown in figure 5 in the same order as
arranged in the device. The HOMO and LUMO
27
levels for F4TCNQ, TPD (ref. 28) and BCP (ref.
29) are taken from literature. The function of the
molecules in each layer as determined by the rela-
tive energy levels is as follows. The LUMO level of
F4TCNQ is approximately aligned with that of ITO
anode (–4⋅8 to –5⋅2 eV) and HOMO level of TPD.
Thus, the 20 Å thick F4TCNQ helps in efficient hole
Figure 4. Peak normalized emission spectra of 1 and
injection from ITO to TPD. BCP serves as an elec-
Alq3 in solid thin films on quartz.
tron transport layer from LiF/Al cathode and also as
a hole blocking layer due to the deep HOMO level
at –6⋅50 eV. That is, holes are confined to the doped
and undoped layers of 1 as they are blocked by BCP
layer to reach cathode. Thus, the device architecture
facilitates electron-hole recombination in the doped
and undoped layers of 1.
The device characteristics, namely the plots of
current (I) vs voltage (V), EL intensity vs V, EL vs
I, and EL efficiency vs I are shown in figures 6A
and B. The current and EL turn-on voltages were
observed at about 6 V. The sharp rise in the current
after turn-on indicates efficient hole and electron in-
jection in the device. Holes are transported by TPD
Figure 5. Energy level diagram of molecules.
and electron by BCP in the device and electron-hole

852
Pabitra K Nayak et al
Table 1. Peak wavelengths of absorption and emission, fluorescence lifetime and energy levels of 1 and Alq3.
Absorbance
peak (nm)
PL
PL
Oxidation
a
b
a
b
d
Compounds
peak (nm)
peak (nm)
τ (ns)
τ (ns)
potential (V vs Fc)
Eopt (eV)
d
e
Alq3
386
516
518
16⋅67
11⋅43
0⋅70
0⋅58
2⋅70
2⋅83
c
e
1
358
460
465
1⋅46
1⋅47
3⋅08
3⋅23
c
a
b
c
d
In CHCl3, In thin film, Calculated, (ref. 21), Experimental
e
2
was calculated as the ratio of EL intensity (cd/m )
2
and current density (A/m ). The current efficiency
vs current and EL intensity vs current plots are
shown in Figure 6B. It is seen that the EL intensity
is not saturated at the maximum current or current
density and further improvement in performance is
possible. The maximum current efficiency is
2
0⋅16 cd/A at 80 cd/m which declines to 0⋅08 cd/A at
2
425 cd/m . The CIE coordinates for the blue EL
were calculated to be 0⋅19, 0⋅23. To our knowledge,
the blue OLED with EL peak at 465 nm reported
here is the shortest wavelength, blue emitting OLED
using an Alq3-type molecule reported so far.
3.4 Near white light device using 1 and an iridium
complex
The blue OLED described above using 1 has the
potential to generate other colours of lower photon
energy or white light by addition of dopant mole-
cules in or adjacent to the emitting layer. The appli-
cations of a variety of heteroleptic and homoleptic
Iridum complexes as dopant molecules in OLED,
their synthesis and purification have been re-
30,31
viewed.
Here we describe a near-white light
emitting device (WOLED) using 1 and an orange-
red phosphorescent iridium complex, namely, (bis
[1-(4,6-difluorophenyl)isoquinolinato], acetylaceto-
nato Ir (III)). The structure of the Ir complex is
shown in figure 9 (inset). The emissive layer of the
white light device consists of two layers, (i) a layer
of 1 of thickness 300 Å and (ii) a layer of 1 doped
Figure 6. (A) Current and EL intensity vs applied volt-
2
age for the device (2 mm area) ITO/F4TCNQ/TPD/
1/BCP/LiF/Al. (B) EL intensity cs current and EL effi-
ciency vs current for the device.
recombination occurs in 1. The EL spectrum of the
blue OLED and PL spectrum of 1 are shown in fig-
ure 7. The PL and EL spectra match well and hence with the Ir complex. These layers were grown under
the electroluminescence is from 1. Figure 6A shows high vacuum sequentially without breaking vacuum.
that the forward and reverse current-voltage and EL The device structure is ITO/F4TCNQ (20 Å)/TPD
intensity–voltage plots overlapped well upon forward
and reverse voltage scans and the hysteresis is neg-
ligible. This indicates that trapping of carriers (hole
(400 Å)/1 (300 Å)/[1 + Ir complex] (300 Å)/BCP
(60 Å)/LiF (15 Å)/Al. The [1 + Ir complex] layer
was prepared by co-evaporation of a mixture of 1
or electron) in the organic layers is negligible in this and Ir complex in the ratio of 1 : 0⋅003 mole/mole
device. The current efficiency (cd/A) of the OLED from the same boat.

Blue and white light electroluminescence in a multilayer OLED using a new aluminium complex
853
The current vs voltage and EL intensity vs voltage improvement in performance is possible. The maxi-
plots of the white light OLED are shown in figure mum current efficiency (cd/A) is 0⋅53 at ~ 130 cd/
2
8A. The current efficiency vs current and EL inten- m , which declines moderately to 0⋅39 at ~ 970 cd/
2
sity vs current plots are shown in figure 8B. It is m . The EL spectrum of the device is shown in fig-
seen that the EL intensity is not saturated and further ure 9. The EL spectrum matches well with those of
the PL spectra of 1 and Ir complex. It is pertinent to
note that the spectrum spans the blue, green and red
region of the visible spectrum. The relative EL
emission intensity contribution of 1 and Ir complex
to the EL spectrum can be adjusted by varying the
dopant concentration. For a true white light device
the CIE coordinates are 0⋅33, 0⋅33. The CIE coordi-
nates for the EL spectrum as shown in figure 9 were
calculated to be 0⋅41, 0⋅35. The CIE coordinates
shift to 0⋅34, 0⋅33, very close to true white light,
when the intensity contribution of Ir complex is de-
creased by 50%.
The origin of the EL emission spectra matching
with those of 1 and Ir complex may be explained as
follows. The energy level diagram for the device is
identical to that shown in figure 5 since the mixed
layer of 1 is only lightly doped with Ir complex.
Figure 7. EL spectrum device ITO/F4TCNQ/TPD/1/
From the energy levels of the molecules it is
BCP/LiF/Al and PL of 1.
expected that the electron-hole recombination zone
is near the TPD: 1 interface as well as the layer of 1,
which results in the generation of singlet and triplet
excited state of 1 in the ratio of 1 : 3. While the
singlet state of 1 emits in blue, the triplet excitons
diffuse through the bulk of the 300 Å thick layer of
pure 1 to reach the doped layer, where the triplet
Figure 8. (A) Current vs voltage and EL vs voltage
2
Figure 9. EL spectrum of the device ITO/F4TCNQ/
TPD/1/[1 + Ir complex]/BCP/LiF/Al. Inset shows the
structure of Ir complex.
diagram for device (2 mm area), ITO/F4TCNQ/TPD/1/
[1 + Ir complex]/BCP/LiF /Al. (B). EL intensity vs cur-
rent and EL efficiency vs current for the same device.

854
Pabitra K Nayak et al
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4. Conclusions
A new blue emitting Al complex, bis-(2-amino-8-
quinolinolato), acetylacetonato Al(III) was synthe-
sized and its photophysical, elctrochemical and
energy level properties were studied and compared
with tris(8-hydroxyquinolato) Al(III). We have
demonstrated blue EL using this molecule. We have
also demonstrated a white light OLED using the Al
complex by incorporation of an additional layer
doped with an orange-red emitting Iridium complex.
The maximum brightness and maximum current
efficiency of the blue and white OLEDs were
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