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Electrical and Optical Simulations of a Polymer-Based Phosphorescent
Organic Light-Emitting Diode with High Efficiency
Mahmoud Al-Sa’di,1 Frank Jaiser,1 Sergey Bagnich,1 Thomas Unger,1
James Blakesley,1 Andreas Wilke,2 Dieter Neher1
1
Institute of Physics and Astronomy, Soft Matter Physics, University of Potsdam, Karl-Liebknecht-Str. 24-25,
D-14476 Potsdam, Germany
2
€t zu Berlin, Brook-Taylor-Str. 6, D-12489 Berlin, Germany
Institute of Physics, Supramolecular Systems, Humboldt-Universita
Correspondence to: M. Al-Sa’di (E-mail: alsadi@uni-potsdam.de) or D. Neher (E-mail: neher@uni-potsdam.de)
Received 10 August 2012; accepted 13 August 2012; published online 11 September 2012
DOI: 10.1002/polb.23158
ABSTRACT: A comprehensive numerical device simulation of the
electrical and optical characteristics accompanied with experimental measurements of a new highly efficient system for polymer-based light-emitting diodes doped with phosphorescent
dyes is presented. The system under investigation comprises an
electron transporter attached to a polymer backbone blended
with an electronically inert small molecule and an iridium-based
green phosphorescent dye which serves as both emitter and
hole transporter. The device simulation combines an electrical
and an optical model. Based on the known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) levels of all components as well as the measured electrical and optical characteristics of the devices, we model the emissive layer as an effective medium using the dye’s HOMO as hole
transport level and the polymer LUMO as electron transport
level. By fine-tuning the injection barriers at the electron
and hole-injecting contact, respectively, in simulated devices,
unipolar device characteristics were fitted to the experimental
data. Simulations using the so-obtained set of parameters
yielded very good agreement to the measured current–voltage,
luminance–voltage characteristics, and the emission profile of
entire bipolar light-emitting diodes, without additional fitting parameters. The simulation was used to gain insight into the physical processes and the mechanisms governing the efficiency of
the organic light-emitting diode, including the position and
extent of the recombination zone, carrier concentration profiles,
and field distribution inside the device. The simulations show
that the device is severely limited by hole injection, and that a
reduction of the hole-injection barrier would improve the device
C 2012 Wiley Periodicals, Inc. J Polym
efficiency by almost 50%. V
Sci Part B: Polym Phys 50: 1567–1576, 2012
INTRODUCTION Electronic devices based on organic semiconductors, such as light-emitting diodes, field effect transistors, and solar cells have attracted much interest as possible
inexpensive and flexible alternatives to inorganic devices.
Since the seminal work of Tang and VanSlyke in the late
1980s where they reported the first green organic lightemitting diode (OLED) based on small organic molecules by
thermal vacuum deposition,1 OLEDs have been intensively
investigated as potentially promising candidates for the fabrication of thin and flexible displays and other novel optoelectronic applications. The prospects for wide angle viewing,
high efficiency, and the fast response compared to liquid
crystal displays have made OLED technology very attractive
for display applications. Also, the small active layer thickness
and the possibility to fabricate OLEDs on flexible substrates
enable lightning applications that are not yet realizable with
conventional light sources. Here polymer-based OLEDs are
particularly suited as they allow for low-cost deposition of
the active material on large areas. Besides optimization of
the device fabrication techniques, continuous improvement
of the OLED performance relies on the comprehensive
understanding of the electrical and optical properties of the
different components forming the OLED and the physical
processes underlying the device operation. Simulation and
modeling of OLEDs under various driving conditions can
provide the necessary information and can avoid the tedious
and costly process of device optimization by systematic
experimental approach.
KEYWORDS: conjugated polymers; high performance polymers;
organic electronics; organic light-emitting diode; simulations; TCAD
In an efficient OLED, the active layers need to combine different functions such as electron and hole transport and
radiative recombination. The simplest approach is to blend
materials with respective properties in one active layer.
Indeed, OLEDs where a single blend layer is sandwiched
between an anode and a cathode exhibited high efficiencies
when using phosphorescent emitters.2,7 However, such blend
layers might suffer from aggregation and/or phase separation. One simple route to avoid these problems is to
C 2012 Wiley Periodicals, Inc.
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FIGURE 1 Chemical structures of the materials used in the
EML, (a) electron transporting polymer, (b) conjugated small
molecule ‘‘filler,’’ and (c) green emitting Iridium-based phosphorescent dye.
covalently attach the active conjugated components to a nonconjugated backbone.8,10
It was also shown that the performance of polymer-based
OLEDs can be substantially improved when adding a thin
polymer interlayer between the anode and the active
layer.11,15 These interlayers are usually formed by depositing
a suitable high band-gap polymer onto the conducting polymeric anode, followed by annealing and washing. As a result,
a very thin (typically 2–10 nm thick) insoluble polymer
interlayer forms on top of the anode. It was proposed that
these interlayers improve device properties by either preventing excitons reaching the conducting polymer anode,11
by blocking electrons at the interface between the active film
and the interlayer13 or by improving hole injection.14
In this work, we investigate the electrical and optical properties of a new efficient system for OLEDs through comprehensive numerical device simulations using the Technology Computer Aided Design (TCAD) software from Silvaco.16 The
OLEDs under study consist of a hole-injecting/electron-blocking interlayer and an emissive layer (EML) sandwiched
between two metallic contacts. The EML under investigation
comprises an electron transporter attached to a polymer
backbone blended with an electronically inert small molecule
and an iridium-based green phosphorescent dye which
serves as both emitter and hole transporter. The chemical
structures of all three components are shown in Figure 1. A
detailed investigation of the electrical and optical properties
of devices made from these materials will be published elsewhere.17 This combination of polymer and small molecules
showed high efficiencies above 40 cd/A and 35 lm/W (see
Fig. 2). This compares quite well to other green phosphorescent devices based on soluble systems published
recently.8,18,20 While we observe an only weak roll-off of
luminance efficiency with increasing bias, the power efficiency decreases rapidly. This deterioration is mainly caused
by an only moderate increase in current with bias.
To gain a comprehensive understanding of the processes
determining the electrical and optoelectronic device characteristics, we have conducted an extensive simulation of the
characteristic properties of single and dual carrier devices.
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In addition to that, the simulation is used to extract physical
parameters such as the width and location of the recombination zone that are otherwise difficult to determine experimentally. The physical models used in our simulations comprise the key optical and electronic processes governing
OLED performance such as charge injection and transport,
exciton formation, diffusion and decay, and the outcoupling
of the generated photons to the outside. We find that the device performance is mainly limited by the barrier for hole
injection between the polymeric anode and the hole-injection
layer (HIL). Reducing the hole-injection barrier is predicted
to enhance the power efficiency of the device by up to 50%,
while lowering the barrier for electron injection will have an
only small effect on the device efficiency.
EXPERIMENTAL
The OLED devices presented here comprise the following
multilayer structure: Glass substrates (1 mm) are covered
with prepatterned indium-tin oxide (ITO,150 nm) and
PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) (Clevios P VP AI 4083, Heraeus Precious Metals,
60 nm) serving as transparent anode. The next layer is a 5
nm HIL formed via the ‘‘interlayer’’ method as described
FIGURE 2 (a) Current–voltage (closed symbols) and luminance–voltage characteristics (open symbols) of a device with
100 nm EML thickness. (b) Corresponding dependencies of the
luminous efficiency (gL, closed symbols) and of the luminous
power efficiency (gP, open symbols) on the current.
Straight lines indicate the operation conditions for a luminance
of 1000 cd/m2.
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FIGURE 3 Schematic band diagram of the investigated system.
(a) HOMO energies and work functions as measured by UPS
and LUMO energies as measured via cyclovoltammetry. (b)
Effective energy scheme used in simulation. In the EML,
dashed lines represent the green dye and full lines represent
the electron transporting polymer. The inert ‘‘filler’’ component
of the EML is neglected as described in the text.
above, using a conjugated polymer based on triarylamine
and fluorene-type units (polymer provided by Merck KGaA).
The EML comprises three components with different values
of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO). The first component is a polymer with a non-conjugated backbone and
conjugated side groups that act as electron transporter in
the system. The second component is a (conjugated) small
molecule and acts as ‘‘filler,’’ which due to its HOMO and
LUMO levels has no electronically active role in the device
operation. The third component is a green-emitting Iridiumbased phosphorescent dye. Polymer and ‘‘filler’’ are blended
in a 1:2 ratio by weight with the emitter added at 17 wt %.
The chemical structures of the organic materials used in the
EML are shown in Figure 1. The polymer was synthesized by
the group of Dr. Krueger at Fraunhofer Institute of Applied
Polymer Research, Potsdam-Golm. Details on the synthesis of
the electron transporting polymer will be published elsewhere.17 The small molecules were supplied by Merck KGaA.
The cathode (Ba covered by Al) was subsequently thermally
evaporated in high vacuum at pressures of 106 mbar. The
complete layer sequence of the finished device is ITO (150
nm)/PEDOT:PSS (60 nm)/HIL (5 nm)/EML (100 nm)/Ba
(5 nm)/Al with an active area of 16 mm2.
The luminance–voltage and current–voltage characteristics
as summarized in Figure 2 were measured in nitrogen
atmosphere using a Konica Minolta CS-100 ChromaMeter
and a Keithley 2400 SourceMeter. The OLED turns on slightly
above 2 V as indicated by the steep increase in both current
and luminance. A brightness of 1000 cd/m2 is reached at
about 7.3 V. The efficiency increases strongly after turn on,
reaching a maximum of 40 cd/A at a luminance of about
185 cd/m2. At 1000 cd/m2, there is only a small roll off of
luminance efficiency to 39 cd/A. However, the peak
power efficiency of 35 lm/W is reached at a luminance of
5.2 cd/m2 and then drops pronouncedly to only 17 lm/W at
1000 cd/m2.
Mobilities of electrons
measured by sensitized
A detailed description
published elsewhere.21
and holes in the active layer were
transient electroluminescence (TEL).
of this method and the setup is
To measure electron mobility, the
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OLED device structure was altered by replacing the HIL with
an insoluble sensing layer of poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-(1,4-phenylene-1,2-ethenylene)] (M3EH-PPV, H.H. H€
orhold, Jena) of 5
nm thickness formed by the ‘‘interlayer method.’’11,21 CsF
covered by Al was used as the cathode. For hole mobility
measurements, a red-emitting small molecule provided by
Merck KGaA was evaporated as a sensing layer (10 nm) on
top of the unaltered active layer stack and covered by Ba
and Al as cathode. All devices for TEL measurements had an
active area of 2 mm2.
For unipolar devices, the contacts were adapted to ensure
injection of only one type of charge carriers into the films.
Hole-only devices were fabricated by replacing the low work
function cathode (Ba) with high work function metal oxide
MoO3 (/MoO3 ¼ 6.6 eV) resulting in a layer stack
PEDOT:PSS/HIL/EML/MoO3. Similarly, electron-only devices
were fabricated by inserting a aluminum layer (/Al ¼ 4.2
eV) between PEDOT:PSS and EML so that the devices comprised PEDOT:PSS/Al/EML/Ba/Al. In both cases, the unaltered contact was used for carrier injection.
Steady-state spectra of the electroluminescence (EL) were
measured with an Ocean Optics HR2000 spectrometer. Absolute PL efficiencies were determined with a Hamamatsu
C9920 setup, including an integrating sphere combined with
a photonic multi-channel analyzer. Transient photoluminescence measurements were performed with excitation at 420
nm by an EKSPLA NT-242 Nd:YAG/optical parametric oscillator system with pulse widths below 6 ns. The phosphorescence was recorded by a monochromator and a Becker &
Hickl multiscaler PMS 400.
The HOMO energies of the electron transporting unit, the filler, and the dye as well as the PEDOT work function were
measured using ultraviolet photoemission spectroscopy
(UPS) in vacuum. Corresponding LUMO energies were
obtained from cyclovoltammetry measurements at Merck
KGaA. A schematic band diagram of the measured HOMO
and LUMO values is shown in Figure 3(a). The HOMO and
LUMO energy levels of the filler are 6.6 eV and 2.3 eV,
respectively, and have been neglected from the figure for
clarity reasons.
The real and imaginary parts of the refractive index of each
organic layer were derived from transmission and reflection
measurements of the respective materials on glass slides
with a Varian Cary 5000 spectrophotometer equipped with
an integrating sphere. The optical constants of the film have
been iteratively fitted point by point with the Newton–Raphson method until the measured and theoretical reflection
and transmission data converged.22 The measured refractive
index values of the EML and the HIL are shown in Figure 4.
The radiation pattern and spatial distribution of luminescent
power of the OLEDs were measured at Fraunhofer Institute
for Applied Optics and Precision Engineering IOF, Jena. The
polarized angular radiation patterns were recorded utilizing
a rotational stage, where the OLED is mounted, and a
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The physical equations are solved numerically to obtain solutions for the electron and hole current densities, carrier densities, electric field, electrostatic potential, and recombination
rate. The boundary conditions used in the calculations will
be discussed below together with a brief description of the
models used in the electrical and optical simulations.
FIGURE 4 Real and imaginary parts of the refractive index of
HIL (squares) and EML (circles) as determined from absorption
and reflection measurements of the organic layers.
detection system comprising a polarizer, a retarder, and a
fiber coupled spectrometer. During the measurements, the
number of charge carriers in the devices was controlled and
stabilized by driving the OLEDs at constant current (0.1 mA)
using a current source.23 This current corresponded to a
luminance of about 250 cd/m2.
DEVICE MODEL AND PARAMETERS
The Metal-Insulator-Metal model is used to describe the
investigated OLED devices. Accordingly, the device is considered to consist of semiconductor layer(s) sandwiched
between two metal contacts. From device measurements
obtained in our laboratory it is clear that the electron transport occurs via the electron transporting polymer, while the
hole transport is exclusively via the dye.17 This is supported
by the UPS measurements of the system which show that
the HOMO of both the electron transporter and the filler are
very deep and well below the dye’s HOMO. At the same
time, the polymer LUMO is the lowest energy state for excess
electrons. Hence we model the EML layer as an effective medium using the dye’s HOMO as hole transport level and the
polymer LUMO as electron transport level. UPS measurements showed a small vacuum level shift (D ¼ 0.1 eV) at
the EML/HIL interface due to interface dipoles. As the simulator cannot include such a shift, a flat vacuum level was
used. To restore the correct energy level alignment at the
HIL/EML interface, the EML HOMO and LUMO were shifted
by 0.1 eV accordingly. The effective work functions of the
contact materials (/PEDOT and /Ba) were optimized to give
optimum fits to the experimental current density–voltage
(J-V) characteristics of single-carrier devices. A schematic of
the band diagram used to simulate our system illustrating
the relevant LUMO and HOMO levels as well as the effective
work functions of the contact materials is shown in
Figure 3(b).
The OLED devices are simulated using a calibrated finite-element TCAD simulation software based on the drift-diffusion
transport model, coupled to Poisson’s equation and the current continuity equations for electrons and holes. In addition
to that, a Poole–Frenkel type field-dependent mobility and
Langevin bimolecular recombination model are employed.
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Electrical Model
Charge Transport Equations
Charge transport in organic devices is described by the general semiconductor drift-diffusion model, where the continuity equations for electrons and holes in the drift-diffusion
form for current density are coupled to Poisson’s equation.
Assuming a trap free system, the model consists of
rðerwÞ ¼ qðn pÞ
dn
¼ qðG RÞ
dt
*
dp
rJ p þ q ¼ qðG RÞ
dt
*
rJ n q
(1)
(2)
(3)
where w is the electrostatic potential, e is the product of the
vacuum permittivity e0 and the relative permittivity er of the
organic material, n(p) is the electron (hole) density, Jn (Jp) is
the electron (hole) current density, R (G) denotes the recombination (generation) rate, and q is the elementary charge.
The time derivatives in eqs 2 and 3 will be omitted due to
the steady-state nature of this work. In the drift-diffusion
model, the currents of electrons and holes are described as a
sum of two contributions; the drift component, proportional
to the electric field, and the diffusion component, proportional to the gradient of the charge density. The current
equations for electrons and holes are given by
*
*
J n ¼ qn ln E þ qDn rn
*
*
J p ¼ qp lp E qDp rp
(4)
(5)
with Dn (Dp) denoting the diffusion coefficient of electrons
(holes). Assuming that the Einstein relation holds for the
system, the diffusivities read
Dn ¼
kB T
l
q n
(6)
Dp ¼
kB T
l
q p
(7)
where kB is Boltzmann constant, T is the lattice temperature,
and ln (lp) is the electron (hole) mobility. The generation
rate is neglected since it is not relevant for materials with
an energy gap larger than 2 eV. The bimolecular recombination rate is described in the Langevin form, where recombination rate is given by
R ¼ cðnp n2i Þ
(8)
with the intrinsic carrier density ni and the reduced Langevin recombination rate
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c¼a
q ln ðEÞ þ lp ðEÞ
z
(9)
Assuming Maxwell–Boltzmann statistics, the electron and
hole concentrations are expressed as
EF; n ELUMO
n ¼ NLUMO exp
kB T
(10)
EHOMO EF;p
p ¼ NHOMO exp
kB T
(11)
and
where ELUMO and EHOMO are the energy levels of LUMO and
HOMO, EF,n (EF,p) is the quasi-Fermi level for electrons
(holes) and NLUMO (NHOMO) is the density of states in the
LUMO (HOMO).26 The densities of states are taken as 1.0 1020 cm3 for all organic layers in this work. The mobilities
are taken to be field dependent with the Poole–Frenkel form
(12)
(13)
where ln0 (lp0) is the zero field electron (hole) mobility, E0n
and E0p are constants related to the disorder in the material,
and E is the electric field.27,28
Boundary Conditions
The OLED structures are numerically simulated using Schottky
contact boundary conditions between the organic layer and
the anode or cathode metal, respectively. The barrier height
that governs carrier injection at the contacts is given by
/B ¼ /m ve
(14)
where /m is the contact work function, /B is the Schottky
barrier height over which the carriers have to be injected
and ve is the electron affinity of the organic material. The
built-in voltage is taken to be the difference between the
work functions of the two different contact materials used as
anode and cathode.
The injection is treated using the Scott–Malliaras modification of Schottky barrier boundary conditions.29 The net current density at the contact is given by
/B
exp f 1=2
J ¼ 4w2 N0 qlE exp
kB T
(15)
where N0 is the density of chargeable sites in the polymer, f
is the reduced electric field expressed by
f ¼
e3 E
4peðkB TÞ2
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and
wðf Þ ¼ f 1 þ f 1=2 f 1 ð1 þ 2f 1=2 Þ1=2
where a 1 is a model parameter.24,25
rffiffiffiffiffiffiffi
E
ln ðEÞ ¼ ln0 exp
En0
sffiffiffiffiffiffiffi!
E
lp ðEÞ ¼ lp0 exp
Ep0
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(16)
(17)
Finally, the interface between the organic materials (EML
and HIL) has been taken into account by employing thermionic and tunneling models at the organic materials interface
when solving the current density equations.16,27,30
Optical Model
Optical characteristics of the bipolar OLEDs were simulated
using the transfer-matrix approach for multilayer systems in
combination with the dipole emission source term method
and reverse ray-tracing technique. The dipole emission
model is based on the equivalence between photon emission
due to an electrical dipole transition and the radiation from
a classical electrical dipole antenna. The OLED is considered
as an emission source embedded in a microcavity. Because
the lateral dimensions are much larger than the thickness,
the device is assumed to be a one-dimensional structure,
and the excitons within the OLED are modeled assuming
randomly oriented point dipoles driven by the reflected electromagnetic waves inside a microcavity. The model includes
the emission of the dipole antenna into plane and evanescent
waves in the emitting layer with transverse electric (TE) and
transverse magnetic (TM) polarizations. The model accounts
for the wide-angle and multiple-beam interference caused by
partial reflection, total internal reflection, and absorption. A
detailed description of the dipole emission source term
method and the transfer-matrix approach has been published
elsewhere.31,35
Simulation Parameters
The main parameters required as input for the device simulation are the charge carrier mobilities and the energy levels
of each organic layer as well as the effective work functions
of the contacts. The effective work functions of the electrode
materials in contact with the respective organic material
were obtained by fitting experimental J(V) characteristics of
electron- and hole-only devices and found to be consistent
with literature values.36,38 For PEDOT:PSS, there is a small
difference of the work function which yielded the best simulation results (4.7 eV) and the value measured by UPS (4.8
eV). This deviation will be addressed in the discussion. The
mobility parameters were obtained by fitting the mobilities
measured by sensitized TEL (see below). A summary of the
parameters used in the electrical simulations are shown in
Tables 1 and 2.
The input parameters for the optical simulation comprised
the layers thickness, the real (n), and imaginary (k) parts of
the refractive index of each layer as a function of wavelength,
and the photoluminescence spectrum of the EML. The measured refractive index values of the EML and HIL shown in
Figure 4 are implemented in our simulator. The values of n
and k for ITO were taken to be 1.85 and 0.0065, respectively.39 For glass, n and k were set to 1.5 and 0, respectively.39 Table 3 summarizes the additional parameters
required for the optical simulations.40,41 The triplet radiative
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TABLE 1 Electrical Simulation Parameters
Parameter
er
EML
TABLE 3 Parameters Required for the Optical Simulation
HIL
3.0
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Units
3.0
–
Parameter
Triplet-polaron quenching constant
Value
Units
13
cm3 s1
12
cm3 s1
3 10
EHOMO
5.1
5.2
eV
Triplet-triplet annihilation constant
4 10
ELUMO
2.7
1.9
eV
Intersystem crossing constant
1 1013
s1
6
Eg
2.4
3.3
eV
Triplet radiative decay lifetime
1.25 10
s
NLUMO
1 1020
1 1020
cm3
80
%
NHOMO
1 1020
1 1020
cm3
Dye photoluminescence quantum
efficiency
ln0
9.534 108
7
cm2/Vs
0
4
lp0
1.12 10
2.5 10
cm2/Vs
En0
42,711.96
–
V/cm
Ep0
176,233.5
176,233.5
V/cm
The energy values have been determined by UPS and cyclovoltammetry. For the densities of states and the dielectric constant, typical values
for organic semiconductors have been taken. Mobility parameters were
determined by fitting measured mobilities to a Poole–Frenkel type
model (see text). The electron mobility in HIL is discussed in the main
text.
decay time was calculated from the measured triplet lifetime
of 1 106 s and the photoluminescence quantum efficiency of the dye in the EML (80%).
RESULTS AND DISCUSSION
Mobility parameters obtained experimentally by TEL on single-carrier devices are used as input to model the electrical
characteristics. The measured mobilities of electrons and
holes for the EML at room temperature are shown in Figure
5. The field dependence of ln and lp can be well explained
by Poole–Frenkel type field-assisted hopping transport.42
The fitted mobility parameters as summarized in Table 1
were used for the simulation of electron- and hole-only devices as well as bipolar OLED.
As in the experiments, the unipolar devices were obtained
by changing the appropriate contacts. Regrettably, the measured data for both types of unipolar devices suffers from a
relatively large uncertainty due to deviations between different pixels and devices. This limits the accuracy of the work
functions extracted from the simulation to 60.1 eV. For the
hole-only device with PEDOT:PSS anode, the best agreement
with the mean experimental data was obtained for /PEDOT:PSS
¼ 4.7 eV. For the system under investigation, UPS measurements have shown a PEDOT:PSS work function of 4.8 eV and
a barrier of 0.4 eV for hole injection into HIL [Figure 3(a)].
Considering the problems mentioned above, the PEDOT:PSS
work function used here to simulate the measured characteristics agrees well with these measurements. Also, it is known
that the work function of PEDOT:PSS decreases when
exposed to water vapor.43 Hence, the different environments—vacuum for UPS measurements and device preparation in nitrogen atmosphere—are an additional explanation
for this slight deviation. For the electron-only device with Ba
cathode, the best fit to the experimental data was obtained
with /Ba ¼ 2.8 eV. This value is slightly higher than the
standard Ba work function of 2.7 eV, which we attribute to
partial oxidation of the metal during device fabrication and
the above mentioned uncertainty of the measured data.
The effective work function of the contact materials and the
mobility parameters obtained from unipolar devices are
used without further adjustment as input to model the electrical and optical characteristics of the bipolar OLEDs. Figure
6 shows simulated and experimental J(V) curves at room
temperature for hole-only, electron-only, and bipolar devices.
The figure shows very good agreement between simulation
and experimental results, especially for the unipolar devices.
In addition to that, the simulation model correctly predicts
that the bipolar OLED current density is almost three orders
of magnitude larger than either of the unipolar current densities in the same applied voltage range. The deviation at
low currents (currents smaller than 103 mA/cm2) is
explained by the experimental leakage current which dominates at low bias.
In addition to the J(V) characteristics, the simulation yields
the spatial distribution of carrier density, electric field, and
recombination rate inside the devices. For the bipolar device
TABLE 2 Effective Work Function of the Contact Materials as
Obtained by Fitting Experimental J(V) Characteristics of
Electron- and Hole-Only Devices
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Parameter
Ba
PEDOT:PSS
Units
Effective work function
2.8
4.7
eV
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FIGURE 5 The measured (symbols) field-dependent mobilities
of electrons and holes for the EML and the corresponding fits
to a Poole–Frenkel type field dependence (lines). Experimental
mobilities were obtained by sensitized TEL (see text).
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FIGURE 6 J(V) characteristics for a hole-only device (circles),
electron-only device (squares), and bipolar device (triangles).
EML thicknesses are 200 nm for the unipolar devices and 100
nm for the bipolar device. The symbols denote experimental
data and the solid lines represent simulation results.
under forward bias, the density of electrons injected into the
EML is much higher than the density of the injected holes
due to the larger barrier offset at the anode side (0.5 eV)
compared to the cathode side (0.2 eV). There is a strong
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FIGURE 8 Comparison of simulated (lines) and measured
(symbols) normalized radiation pattern for the wavelength
range (450–850) nm for an OLED with EML thickness of 100
nm. TE and TM denote transverse electric and transverse magnetic modes, respectively.
electron accumulation at the EML/HIL interface, as the electrons cannot penetrate the HIL due to the large LUMO offset.
Simulations with an arbitrary, non-zero electron mobility in
the HIL did not change the overall behavior. Due to the
lower mobility of holes in the EML, the hole density in the
EML is much higher than in HIL. In the bulk of the EML,
both carrier densities are relatively low and almost constant,
with a factor of 10 lower hole density. The electric field is
found to be high across the HIL and at the EML/HIL interface, and uniformly distributed across the EML as shown in
Figure 7(a). In agreement with the charge distribution, we
found that the recombination zone is located close to the
EML/HIL interface and extends only slightly into the EML as
shown in Figure 7(b). The width of the recombination zone
obtained from the electrical simulations, which is determined
to be approximately 5 nm, was used in the optical model to
simulate the optical characteristics of the bipolar OLED
device.
FIGURE 7 (a) Simulated electric field and carrier density distributions and (b) recombination rate profile for the OLED with
EML thickness of 100 nm at an applied voltage of 15 V. Electrons are injected from the left-hand side (x ¼ 0) and holes are
injected from the right-hand side (x ¼ 105 nm).
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The optical performance of the bipolar OLED has been simulated using the optical models described in the text. The
simulated and measured results of the normalized radiation
pattern and the luminescent power as a function of the
viewing angle for the wavelength range 450–850 nm are
shown in Figures 8 and 9, respectively. The simulation
closely resembles the measured data, further supporting the
validity of the implemented electrical and optical models. As
is to be expected for a planar substrate, the results show
that light emitted at higher angles to the normal direction is
out-coupled less efficiently than light emitted at smaller
angles, resulting in a reduced spectral power density. However, the close agreement between measured and simulated
emission properties supports the simulated recombination
profile discussed above. Figure 10 compares simulated and
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FIGURE 9 Simulated (line) and measured (points) normalized
luminescent power as function of viewing angle (to the normal) for an OLED with 100 nm EML thickness.
measured luminance–voltage characteristics. Again, our simulation results are in good agreement with the experimental
data, verifying the validity and accuracy of the model and
the parameters used. Note that the simulation overestimates
the luminance at low bias. This trend is also seen when comparing the measured and simulated bipolar current in Figure
6. It is possible to reduce this discrepancy by allowing the
parameters describing injection and transport in the bipolar
device to derivate from the parameters deduced from fits to
the unipolar currents, but this procedure would introduce an
additional uncertainty. Also, the simulation reproduces well
the experimental data for relevant brightness levels (above
100 cd/m2).
The impact of the effective barrier heights for carrier injection at the anode and the cathode sides were investigated by
calculating the J(V) characteristics while varying the work
function of either the anode or the cathode with respect to
the reference device, with the work function of the other
contact kept at the reference values. The effective barrier for
hole injection was varied between 0.7 and 0 eV by setting
the work function of the anode to values ranging from 4.5
eV to 5.2 eV. Similarly, the influence of the effective barrier
for electron injection was investigated by varying the effec-
FIGURE 10 Simulated (line) and measured (points) luminance–
voltage characteristics of an OLED with 100 nm EML thickness.
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FIGURE 11 Influence of different hole or electron injection barriers on the current of a bipolar device with EML thickness of
100 nm. The barrier for the other contact was kept at the level
of the standard device. The normalized current density j15V
(normalized to the current of the reference device) is plotted
for the highest simulated voltage, i.e., 15 V.
tive work function of the cathode between 3.0 and 2.6 eV.
The obtained results are summarized in Figure 11, where
the current density at an applied voltage of 15 V is plotted
versus the effective barrier height for hole and electron
injection, respectively. As expected, the device current
increases significantly upon reduction of either barrier. However, reducing the hole-injection barrier causes a considerably stronger increase in the current and by that reduces the
bias needed to give a certain luminance.
To clarify the impact of the effective barrier for hole or electron injection on the whole device performance, the power
efficiency of the simulated devices was calculated at a luminance of 1000 cd/m2 and the results are shown in Figure
12. In accordance with the results shown previously, the
power efficiency increases upon reduction of either barrier.
An improvement of up to 50% on the device’s power
FIGURE 12 The power efficiency at a luminance of 1000 cd/m2
normalized to the efficiency of the reference device as a function of the effective barrier for hole or electron injection. The
barrier for the other contact was kept at the level of the standard device.
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efficiency can be achieved by reducing the effective barrier
for hole injection. A reduction of the electron injection barrier, on the other hand, increases the device efficiency only
by 15%.
In summary, the barrier for hole injection is found to be limiting the device efficiency, as the influence of this barrier is
much stronger on the current and efficiency when compared
to an equal change of the electron injection barrier. To
explore this behavior, the carrier density, the electric field,
and the recombination rate distributions were investigated
for different values of the injection barriers. The results of
these simulations indicate that reducing the injection barrier
for holes leads to a stronger increase of the hole density, the
electric field, and the recombination rate inside the device,
compared to a similar reduction of the effective barrier for
electron injection at the anode side.
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Research for supplying with the electron transporting polymer.
They also thank Anna Hayer and her colleagues at Merck KGaA
for fruitful discussions and supplying the phosphorescent dye,
the hole-injecting interlayer, sensing layer materials, and the filler. For the measurement of optical constants, authors are
grateful to Steve Albrecht (University of Potsdam). Authors
thank Norbert Koch (Humboldt-Universit€at zu Berlin) for supporting UPS measurements and data analysis, and Michael
Fl€ammich (Fraunhofer IOF Jena) for measurements of the spatial luminance distribution of the devices. This work was supported by the Bundesministerium für Bildung und Forschung
(BMBF project ‘‘NEMO’’, FKZ 13N10622).
REFERENCES AND NOTES
1 Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51,
913–915.
CONCLUSIONS
2 Gong, X.; Robinson, M. R.; Ostrowski, J. C.; Moses, D.;
Bazan, G. C.; Heeger, A. J. Adv. Mater. 2002, 14, 581–585.
In this article, we have successfully modeled the electrical
and optical properties of a new highly efficient material system for phosphorescent OLEDs. The good agreement of
simulated characteristics of the bipolar devices on the basis
of a set of parameters gained from simulations of electronand hole-only devices verified the validity and accuracy of
the models and parameters used. The simulation of the
investigated devices allows us to extract relevant information
such as position and extent of the recombination zone, carrier concentration profiles, and field distribution inside the
device, which is difficult to gain otherwise. The simulation
shows that the device performance is limited by the injection
of holes, which we assign to a large hole-injection barrier
between the PEDOT:PSS anode and HIL. As HIL serves as an
efficient electron blocking layer, the device current and with
this the device brightness at a given voltage is largely reduced
compared to an ideal ohmic hole injection. We propose that
the presence of the hole-injection barrier is the main reason
for the strong reduction of the power conversion efficiency at
higher bias. Also, due to the high barrier for hole injection, the
hole concentration in the EML is quite low and rather constant
throughout the layer. At the same time, the electron blocking
property of the HIL leads to a high electron concentration at
the EML/HIL interface. As a result, die recombination zone is
very narrow and located directly at the EML/HIL interface.
With this recombination zone as input for an optical device
model, the luminance characteristics of the device can be well
reproduced. Simulations with varying injection barriers show
that even a slight reduction of the hole-injection barrier would
improve the device efficiency by almost 50%, while a reduced
electron injection barrier has only little effect on device characteristics. Clearly, the information gained from these simulations
will not only allow us to develop new approaches for further
device optimization, but also serve as an important input to
understand device degradation.
3 Anthopoulos,T. D.; Markham, J. P. J.; Namdas, E. B.; Samuel,
D. W.; Lo, S. C.; Burn, P. L. Appl. Phys. Lett. 2003, 82,
4824–4826.
4 van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.;
€ ssel,
Langeveld, B. M. W.; Rothe, C.; Monkman, A.; Bach, I.; Sto
P.; Brunner, K. J. Am. Chem. Soc. 2004, 126, 7718–7727.
5 Yang, X. H.; Jaiser, F.; Klinger, S.; Neher, D. Appl. Phys. Lett.
2006, 88, 021107.
6 Wu, H.; Zhou, G.; Zou, J.; Ho, C. L.; Wong, W. Y.; Yang, W.;
Peng, J.; Cao,Y. Adv. Mater. 2009, 21, 4181–4184.
€ubler, T. K. Adv. Mater.
7 Yang, X.; Neher, D.; Hertel, D.; Da
2004, 16, 161–166.
8 Zhu, M.; Ye, T.; He, X.; Cao, X.; Zhong, C.; Ma, D.; Qin, J.;
Yang, C. J. Mater. Chem. 2011, 21, 9326–9331.
9 Zhang, Y.; Zuniga, C.; Kim, S. J.; Cai, D.; Barlow, S.; Salman,
S.; Coropceanu, V.; Bredas;J. L.; Kippelen, B.; Marder, S. Chem.
Mater. 2011, 23, 4002–4015.
10 Bagnich, S. A.; Unger, Th.; Jaiser, F.; Neher, D.; Thesen, M.
W.; Krueger, H. Appl. Phys. Lett. 2011, 110, 033724.
11 Kim, J. S.; Friend, R. H.; Grizzi, I.; Burroughes, J. H. Appl.
Phys. Lett. 2005, 87, 023506.
12 Choulis, S. A.; Choong, V. E.; Mathai, M. K.; So, F. Appl.
Phys. Lett. 2005, 87, 113503.
13 Yang, X. H.; Jaiser, F.; Stiller, B.; Neher, D.; Galbrecht, F.;
Scherf, U. Adv. Funct. Mater. 2006, 16, 2156–2162.
14 Choulis, S. A.; Choong, V. E.; Patwardhan, A.; Mathai, M. K.;
So, F. Adv. Funct. Mater. 2006, 16, 1075–1080.
15 Yang, X. H.; Wu, F. L.; Neher, D.; Chien, C. H.; Shu, C. F.
Chem. Mater. 2008, 20, 1629–1635.
16 Silvaco, TCAD. http://www.silvaco.com/, August, 2012.
17 Salert, B. Ch. D.; Krueger, H.; Bagnich, S. A.; Unger, Th.; Jaiser, F.; Al-Sa’di, M.; Neher, D. In Preparation.
18 Wang, B. Z.; Liu, J.; Wu, H. B.; Zhang, B.; Wen, S. S.; Yang,
W. Chin. Phys. B 2011, 20, 088502.
19 Shin, J.; Um, H. A.; Cho, M. J.; Lee, T. W.; Kim, K. H.; Jin, J.
I.; Kang, S.; Park, T.; Joo, S. H.; Yang, J. H.; Choi, D. H. J.
Polym. Sci. Part A: Polym. Chem. 2012, 50, 388–399.
ACKNOWLEDGMENTS
20 Zhu, M.; Li, Y.; Li, C. G.; Zhong, C.; Yang, C.; Wu, H.; Qin, J.;
Cao, Y. J. Mater. Chem. 2012, 22, 11128–11133.
Authors gratefully acknowledge Beatrice Salert and Hartmut
Krueger from Fraunhofer Institute of Applied Polymer
21 Bange, S.; Kuksov, A.; Neher, D. Appl. Phys. Lett. 2007, 91,
143516.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50, 1567–1576
1575
FULL PAPER
WWW.POLYMERPHYSICS.ORG
22 Peter, K. Optische Charakterisierung von Halbleiterschichten.
Diploma Thesis, University of Konstanz, Germany, 1993.
€mmich, M. Optical Characterization of OLED Emitter
23 Fla
Properties by Radiation Pattern Analyses. PhD Thesis, Frie€t Jena, Germany, 2011.
drich-Schiller-Universita
24 Blom, P. W. M.; de Jong, M. J. M.; Breedijk, S. Appl. Phys.
Lett. 1997, 71, 930–932.
25 Scott, J. C.; Karg, S.; Carter, S. A. J. Appl. Phys.1997, 82,
1454–1460.
26 Lee, C. C.; Chang, M. Y.; Huang, P. T.; Chen, Y. C.; Chang,
Y.; Liu, S. W. J. Appl. Phys. 2007, 101, 114501.
27 Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices;
Wiley: New York, 2007.
28 Selberherr, S. Analysis and Simulation of Semiconductor
Devices; Springer-Verlag: Wien, 1984.
29 Scott, J. C.; Malliaras, G. G. Chem. Phys. Lett. 1999, 299,
115–119.
32 Benisty, H.; Stanley, R.; Mayer, M. J. Opt. Soc. Am. A 1998,
15, 1192–1201.
30 Walker, A. B.; Kambili, A.; Martin, S. J. J. Phys.: Condens.
Matter 2002, 14, 9825–9876.
41 Tang, K. C.; Liu, K. L.; Chen, I. C. Chem. Phys. Lett. 2004,
386, 437–441.
€ ssler, H. Phys. Status Solidi B 1993, 175, 15–56.
42 Ba
31 Wang, Z. B.; Helander, M. G.; Xu, X. F.; Puzzo, D. P.;
Qiu, J.; Greiner, M. T.; Lu, Z. H. J. Appl. Phys. 2011, 109,
053107.
1576
JOURNAL OF
POLYMER SCIENCE
JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50, 1567–1576
33 Lukosz, W. J. Opt. Soc. Am. A 1979, 69, 1495–1503.
34 Lukosz, W.; Kunz, R. E. J. Opt. Soc. Am. A 1977, 12,
1607–1615.
35 Neyts, K. A. J. Opt. Soc. Am. A 1998, 15, 962–971.
36 Sapp, S.; Luebben, S.; Losovyj, Y. B.; Jeppson, P.; Schulz,
D. L.; Caruso, A. N. Appl. Phys. Lett. 2006, 88, 152107.
37 Li, J. H.; Huang, J.; Yang, Y. Appl. Phys. Lett. 2007, 90,
173505.
38 Koch, N.; Kahn, A.; Ghijsen, J.; Pireaux, J. J.; Schwartz, J.;
Johnson, R. L.; Elschner, A. Appl. Phys. Lett. 2003, 82, 70–72.
39 Wei, M. K.; Lin, C. W.; Yang, C. C.; Kiang, Y. W.; Lee, J. H.;
Lin, H. Y. Int. J. Mol. Sci. 2010, 11,1527–1545.
40 Reineke, S.; Walzer, K.; Leo, K. Phys. Rev. B 2007, 75,
125328.
43 Koch, N.; Vollmer, A.; Elschner, A. Appl. Phys. Lett. 2007,
90, 043512.