Efficient green phosphorescent tandem organic

Journal of Luminescence 154 (2014) 345–349
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Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin
Efﬁcient green phosphorescent tandem organic light emitting diodes
with solution processable mixed hosts charge generating layer
N.A. Talik a,b, K.H. Yeoh a,b, C.Y.B Ng a,b, B.K. Yap c, K.L. Woon a,n
a
Low Dimensional Research Center, Department of Physics, University Malaya, 50603 Kuala Lumpur, Malaysia
ItraMAS Corporation. Sdn. Bhd., 542A-B Mukim 1, Lorong Perusahaan Baru 2, Kawasan Perindustrian, Perai 13600, Penang, Malaysia
c
Center of Microelectronic and Nanotechnology Engineering (CeMNE), College of Engineering, Universiti Tenaga Nasional, Jln. Uniten-Ikram, 4300 Kajang,
Selangor, Malaysia
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 14 October 2013
Received in revised form
9 April 2014
Accepted 23 April 2014
Available online 14 May 2014
A novel solution processable charge generating layer (CGL) that consists of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN6)/Poly(N-vinylcarbazole) (PVK): 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC) for a tandem green phosphorescent organic light emitting diode (PHOLED) is
demonstrated. The use of orthogonal solvent to dissolve HATCN6 and PVK:TAPC is the key to overcome
the interface erosion problem for the solution processed CGL. The current efﬁciency of the 2 wt% TAPC
mixed with PVK is the highest at 24.2 cd/A, which is more than three-folds higher than that of the single
device at 1000 cd/m2.
& 2014 Elsevier B.V. All rights reserved.
Keywords:
Charge generation layer
Solution-processable
Tandem organic light emitting diodes
Current efﬁciency
1. Introduction
Organic light emitting diode (OLED) has attracted considerable
attention due to its applications in large panel display and lighting
applications [1–3]. However, achieving high brightness and long
operation lifetime simultaneously remains a challenge for OLEDs
development. In conventional OLED, the luminance increased
proportionally with current density. Devices that operate with
high current density are prone to operational lifetime issues due to
thermal degradation of the organic material [4,5]. A commonly
used method to overcome this problem is by using tandem OLED
which consists of vertically stacked two or multiple light-emitting
unit (LEU) connected by CGL. In contrast to the conventional OLED,
the presence of CGL allows emission of multiple photons from a
pair of injected electron and hole, thus achieving higher luminance
at lower current density [3]. Over the years, many research works
have been carried out in designing high performance tandem
OLED with efﬁcient CGL. One of the widely used approach is the
use of p–n junction CGL structure [6–12]. Recently, Liu et al.
demonstrated a stable CGL based on a p-doped monolayer using
2,3,5,6-tetraﬂuoro-7,7,8,8-tetracyanoquinodimethane
(F4-TCNQ)
doped N,N0 -Di-[(1-naphthyl)-N,N0 -diphenyl]-1,10 -biphenyl)-4,40 diamine (NPB) with enhanced lifetime [13]. Most of these CGLs
rely heavily on a vacuum deposition method. Forming a bi-layer
ﬁlm using a solution process method is not an easy task due to
intermixing problem [1,14,15]. In order to reduce the deviceprocessing cost and to simplify the fabrication process, it is highly
desirable to reduce the vacuum deposition process to the minimum. Most of the research done for the solution processed OLED
focus on the emissive layer [16–20], electron/hole transport layer
[21–24] but not much on the CGL. Only the latest research by
Chiba et al. [25] utilized a hybrid CGL consisting of solution
processed poly(4-butylphenyl)-diphenyl-amine (Poly-TPD) with
vacuum deposited molybdenum tri-oxide (MoO3).
In this paper, we demonstrate solution processable green LEU
tandem PHOLED connected using an n-type/p-type CGL. The
CGL consists of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile
(HATCN6) as the n-type layer and 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC) with mixture of poly(N-vinylcarbazole) (PVK) as the mixed host p-type layer. Vacuum deposition
method is only used to deposit LiF/Al, which act as the (electron
injection layer) EIL for the 1st emitting unit and also for the device
electrode. A two-folds efﬁciency, up to 24 cd/A (7.5 lm/W) at
1000 cd/m2 was achieved, compared to the single emitting unit.
2. Experiment
n
Corresponding author. Tel.: þ 60 3 79674281.
E-mail address: [email protected] (K.L. Woon).
http://dx.doi.org/10.1016/j.jlumin.2014.04.027
0022-2313/& 2014 Elsevier B.V. All rights reserved.
PEDOT:PSS (AI 4083) was purchased from H.C. Starck. TAPC,
HATCN and iridium fac-tris(2-phenylpyridine)iridium (Ir(ppy)3)
346
N.A. Talik et al. / Journal of Luminescence 154 (2014) 345–349
were purchased from Luminescence Technology (Taiwan). PVK
and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)
were obtained from Sigma-Aldrich. All materials were purchased
and used as received without further puriﬁcation. The device
structure consists of indium tin oxide (ITO)/PEDOT:PSS/ PVK:PBD
(70:30 w/w): Ir(ppy)3 (1 wt%)/LiF /Al/HATCN,/TAPC: PVK/PVK: PBD
(70:30 w/w): Ir(ppy)3 (1 wt%)/LiF (1 nm) /Al. Fig. 1 shows the
schematic diagram of device structure and the chemical structures
of the material used in this work. Mixed hosts of PVK:TAPC were
dissolved in dichloroethane with varying TAPC concentrations of
0 wt%, 1 wt% and 2 wt%. ITO coated glass substrates with sheet
resistance were patterned using a standard lithography method.
The substrates were ultrasonically cleaned using deionized (DI)
water, acetone, isopropyl alcohol sequentially and DI water again
for 20 min, followed by oxygen plasma treatment for 5 min. 40 nm
of PEDOT:PSS was spin coated on the substrates and immediately
baked in N2 environment for 10 min at 150 1C. The emissive
material dissolved in chlorobenzene was subsequently spin coated
onto the PEDOT:PSS to produce a 80 nm thick ﬁlm and baked at
80 1C for 30 min. 1 nm thick LiF and 10 nm thick Al layers were
sequentially vacuum deposited under 2.4 10 4 mbar. HATCN6,
dissolved in acetonitrile, was spin-coated on top of the vacuum
deposited aluminum to give a 10 nm thick layer. HATCN6 is
insoluble in dichloroethane. Therefore, PVK:TAPC could be spincoated on top of HATCN6. PVK:TAPC was spin-coated on top of
HATCN6 to form a 30 nm thick ﬁlm. The ﬁlm was later baked at
100 1C for 30 min. The second emissive material (PVK: PBD (70:30
w/w): Ir(ppy)3 (1 wt%)) was then spin-coated on top of PVK:TAPC
to produce a 80 nm thick emissive layer. Finally, LiF (1 nm)/Al
(200 nm) were sequentially vacuum deposited at a base pressure
of 2.4 10 4 mbar. The devices were encapsulated using UV
curable epoxy and glass lid. The current–brightness–voltage characteristics were measured via Konica Minolta CS-200. All solution
processed ﬁlm thicknesses were measured using a P-6 proﬁlermeter (KLA-Tencor). The surface morphology of the ﬁlms was
investigated via Atomic Force Microscopy (AFM, NT-MDT NTEGRAPrima). The EL spectra of the devices were observed using a Fiber
Optics Spectrometer using SM32Pro software.
3. Results and discussions
Fig. 2(a) shows the brightness–voltage characteristic of both
single unit and tandem devices. The turn-on voltage (at 1 cd/m2)
in the single device is 5.9 V. The turn-on voltages for the tandem
OLED with 0 wt%, 1 wt%, and 2 wt% TAPC are 5.2 V, 5.8 V and 5.4 V,
respectively. Typically, tandem OLED shows higher turn-on voltage
compared to the single device [25]. In our case, the turn-on
voltages of the tandem OLED are lower than a single unit.
Although the efﬁcient charge extraction and charge transfer at
the CGL organic–organic interface might result in insigniﬁcant
voltage drop across CGL, it is still insufﬁcient to explain the lower
turn on voltage observed in our devices. This phenomenon is also
observed by Chen et al. using LiF/ZnPc:C60/MoO3 as a CGL [27]
where the turn on voltages for the tandem OLED are actually lower
than single unit. Investigation of the underlying mechanism of
such phenomenon is being carried out and results will be
discussed elsewhere.
The brightness–voltage and current density–voltage characteristics
of the fabricated devices are illustrated in Fig. 2(a) and (b) respectively.
As observed in Fig. 2(a), the brightness of all devices is higher than
1000 cd/m2 at 11 V. Tandem OLED with 1 wt% TAPC shows the highest
brightness compared to other devices. In Fig. 2(b), the tandem OLED
with 0% TAPC shows the highest current density compared to the rest
of devices at 11 V. The current density of the tandem OLED with 2 wt%
TAPC is the lowest at 5.02 mA/cm2, compared to 10.8 mA/cm2,
Fig. 1. Device structures and the chemical structures of the material used in
this work.
13.7 mA/cm2 and 12.3 mA/cm2 for single OLED, 0 wt% TAPC and
1 wt% TAPC respectively.
Fig. 3(a) illustrates the current efﬁciency of the tandem OLEDs
compared to single OLED. When the ratio of TAPC in PVK increased
from 1 wt% to 2 wt%, the current efﬁciency at 1000 cd/m2
increased from 19.3 cd/A to 24.2 cd/A. The current density of these
tandem devices is more than double compared to the single device
that exhibits only 10.7 cd/A. Fig. 3(b) shows that the power
efﬁciencies of the tandem OLED with 2 wt% TAPC are the highest
at 7.3 lm/W which is 2.9 folds of that of single OLED at 2.5 lm/W.
Such magnitude of improvement is signiﬁcant compared to that
reported for vacuum deposited CGL devices [22]. These results
show that the tandem OLED structure possesses excellent charge
generation, transport as well as extraction and injection
N.A. Talik et al. / Journal of Luminescence 154 (2014) 345–349
347
Fig. 2. (a) Brightness vs. voltage and (b) current density vs. voltage of single unit, tandem with 0% TAPC, 1% and 2% TAPC.
Fig. 4. Energy level of the tandem OLED.
Fig. 3. (a) Current efﬁciency vs. brightness and (b) power efﬁciency vs. brightness
of single and tandem OLEDs (1 wt% and 2 wt% of TAPC concentration in PVK).
capabilities that would result in negligible voltage drop across the
CGL [3].
Fig. 4 shows the close match between the HOMO level of PVK at
5.8 eV and the LUMO level of HATCN6 at 5.7 eV that could facilitate the
electron transfer. The generated electrons and holes could be effectively extracted out from CGL and injected into an adjacent emissive
layer. Under the inﬂuence of an external electric ﬁeld, electrons tunnel
from the HOMO level of PVK to the LUMO level HATCN6 via a narrow
depletion region at the organic–organic interface, and are injected
into the 1st emissive layer assisted by the LiF/Al as EIL [26].
Generated holes in PVK:TAPC are injected into the 2nd emission
layer. The current efﬁciency of the tandem OLED improved by almost
3 folds, compared to the vacuum deposited CGL reported [26]. It is
most likely that the signiﬁcant performance improvement in the
tandem OLED is due to efﬁcient charge generation and extraction
that occurred.
For tandem OLED with 0% TAPC, an accumulation of holes is
expected to occur at the HATCN6 and PVK interfaces. Holes can be
easily injected into a PVK transporting layer. However, the HOMO
level of TAPC is at 5.5 eV [28] while the PVK HOMO level is at
5.8 eV [29]; doping of 1% TAPC in PVK is expected to generate traps
for hole transport. Hence, in Fig. 2(b), the current density of
tandem OLED doped with TAPC is lower than device without
doping. The trapped and de-trapping holes could occur at higher
applied voltages. Hence at high voltages, for example at 11 V, the
current density is the highest for 0%, followed by 1% and then 2%
TAPC for tandem devices. However, for 2% TAPC, it shows an
unusual shape for a diode at a lower voltage. The HOMO level of
TAPC is 0.2 eV higher than the LUMO level of HATCN6 while the
HOMO level of PVK is 0.1 eV lower than the LUMO level of
HATCN6. Since TAPC has a better hole donating property than
the PVK, doping of TAPC can increase the hole generation at the
HATCN6/PVK interface. Hence, our hypothesis is that there are
two mechanisms taking place when sufﬁcient amount of TAPC is
doped. The ﬁrst is the increase of hole generation and the second
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N.A. Talik et al. / Journal of Luminescence 154 (2014) 345–349
Fig. 5. AFM images of (a) Al/HATCN6, Ra=7.138 nm and (b) Al/HATCN6/TAPC:PVK (2%), Ra=0.289 nm.
4. Conclusions
In summary, we have successfully fabricated solution processable tandem OLEDs. HATCN6 as the n-type CGL layer has a good
solubility in acetonitrile and insoluble in all other types of organic
solvent. Hence, another layer could be spin coated on top of
HATCN6 layer without intermixing problem. The tandem OLED
device with two hosts, PVK:TAPC as the p-type CGL layer, exhibits
high efﬁciency with 24.2 cd/A, which is more than double compared
to the single OLED that exhibits only 10.7 cd/A at 1000 cd/m2. The
combination of HATCN6 and PVK:TAPC as the CGL could be a very
promising step towards fully solution processable tandem OLED for
the applications of display and lightings.
Acknowledgments
Fig. 6. show the normalized electrolumonescent (EL) spectra of the single unit and
the tandem OLED with 2% TAPC viewed in the normal direction at a brightness at
12 V.
This research was partially supported by IPPP (PG071-2013A),
HIR Chancellery (UM.C/625/1/HIR/195) and ItraMas Technology
Sdn. Bhd (PV002-2013).
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