Efficiency enhancement of solution processed single layer blue phosphorescent
organic light emitting diodes by using Polyethylene glycol mono [4-(1,1,3,3tetramethylbutyl)phenyl] ether (Triton X-100)
K.H. Yeoh1,a), K.L. Woon1,b)
1
Department of Physics, Faculty of Science, University Malaya, 50603 Kuala Lumpur, Malaysia
Abstract
We report on the efficiency enhancement of solution processed single layer blue phosphorescent organic light emitting
diodes (PhOLED) by blending Triton X-100 into the electroluminescne (EL) layer. With 3.2wt% Triton X-100 blending, the
power efficiency of the device can reach 8.5lm/W at brightness of 1000cd/m2, which corresponds to 2-fold increament
compared to the similar PhOLED without Triton X-100 doping. The efficiency enhancement is attributed to the
interfacial interaction between Al and the Triton X-100 which lowers the cathodic barrier injection of the PhOLED.
Keyword: Organic light emitting diodes, Triton X-100
(2 li
1. INTRODUCTION
Organic light emitting (OLED) diodes have attracted
much attention lately due to its potential applications for
large panel display and solid state lightings. Compared to
vaccum deposition method, solution processed method is
more promising because of its lower fabrication cost and
compatibility of roll-to-roll processing. However this
progress is heavily relied on the development of blue
OLED because typically blue OLED has lower lifetime and
efficiency due to the wide bandgap host that is required for
the commonly used phosphorescent blue dopant e.g.
Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium
(FIrpic).
Several groups have developed multi layer solution
processed blue OLED that yield efficiency of 9 to 10lm/W
at 1000cd/m2 [1,2]. In contrast to single layer OLED,
the fabricating solution processed multi layer OLED is very
challenging and high cost. The top layer need to form
orthogonal film with the underneath layer. To date the
development of multi layer OLED using wet process is
fairly slow mainly hampered by the intermixing of the
bottom layer with the top subsequent layer. For lighting
application, a simple and low cost solution processed single
layer OLED is highly desirable.
In this paper, we report that high efficiency solution
processed single layer PHOLED can be achieved by
blending Triton X-100 as shown in Figure 1 into the EL
layer. Our device shows 2 fold increament of luminous
efficacy compared to the similar device without Triton X100 doping.
i )
weight ratio (PVK:OXD-7:FIrpic:Triton X-100) where X
is varied from 0 to 10. The EL layer was spin coated on to
the PEDOT:PSS coated substrates to give a 70nm thick film
and then baked at 80°C for 30 minutes. Following that, 1nm
of Cs2CO3 and 100nm Al was vacuum deposited at base
pressure of 2X10-6 mbar without breaking the vacuum.
All the devices were encapsulated using UV curable epoxy
and glass lid purchased from Ossila. The devices current
density- voltage-luminance characteristics were measured
using Konica Minolta Cs-200 and Keithley 276 source
meter. Photocurrents of OLED were measured under
illumination of a simulated AM1.5 light source at
100mW/cm2.
Figure 1: Chemical structure of Triton X-100.
The current density and brightness -applied voltage curve is
shown in Figure 2. It is clearly shown that the current
injection increases with Triton X-100 doping concentration.
2. EXPERIMENTAL AND RESULTS
Indium tin oxide (ITO) coated glass substrates with sheet
resistance 25Ω/□ were patterned using standard lithography
method. The substrates were ultrasonically cleaned DI
water, acetone, IPA and DI water for 10mintues, followed
by oxygen plasma treatment for 5minutes. Next 50nm of
poly(ethylenedioxythiophene):polystyrenesulfonate
(PEDOT:PSS) (H. C. Starck, Clevios P VP Al 4083) was
spin coated on the substrates and immediately baked in N2
environment for 7 minutes at 150°C. The chlorobenzne
dissolved EL layer consists of poly(vinylcarbazole) ( PVK),
1,3-bis[(4-tertbutylphenyl) -1,3,4 oxidiazolyl] phenylene
(OXD-7), FIrpic and Triton X-100, blended in 100:40:10:X
a)
b)
Presenting author: [email protected]
Corresponding author: [email protected]
Figure 2: Current density, brightness – voltage (J-V) of
blue PhOLED with Triton X-100 concentrations varied
from 0wt% to 10wt%
With 3.2wt% Triton X-100 doping, the device turn on
voltage reduces to 3.5V at 1cd/m2 as shown in Figure 2.
Figure 3 shows the power efficiency of the blue PhOLED at
different brightness. The optimum Triton X-100 doping
was 3.2wt% which gave 8.5lm/W at 1000cd/m2. The
device efficiency varied between 7lm/W to 8.7lm/W from
100cd/m2 to 2500cd/m2. This variation is considerably
small compared to the best solution processed single layer
blue PhOLED device reported to date [3]. The efficiency
enhancement is due to the interaction of lone pair oxygen
atoms in Triton X-100 with Al which lead to the formation
of Al3+ and consequently excessive negative charge is
released at the cathode- emitting layer interface. These
results in the formation of dipole at the cathode interface
which effectively lower the cathode barrier for electron
injection [4].
Figure 3: Power efficiency-brightness of the PhOLED with
different concentrations of Triton X-100 doping.
Figure 4: Photovoltaic characteristic of the blue PhOLED
at 0wt% and 3.2wt% TritonX-100 doping.
We measured the OLED’s photovoltaic response [3] and
found that the open circuit voltage (VOC) for 3.5% TritonX doping is about 0.4V higher than the VOC of the control
device as shown in Figure 4. This shows that Triton X100 has lower the effective work function of the cathode,
which eases the electron injection.
3. SUMMARY
In summary, we have demonstrated that the efficiency of
a single layer solution processed blue PhOLED can be
significantly enhanced by using a simple and low cost
method by means of Triton X-100 doping. The efficiency
enhancement is attributed to the dipole formation at the
cathode interface due to the interaction of the lone pair
oxygen atoms in the Triton X-100 with Al cathode.
This
simple and cost effective method will pave way towards
large scale manufacturing of OLED for display and lighting
applications.
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