Get it white: color-tunable AC/DC OLEDs

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## ORIGINAL ARTICLE

# Get it white: color-tunable AC/DC OLEDs

Markus Fro¨bel 1, Tobias Schwab 1, Mona Kliem 1, Simone Hofmann 1, Karl Leo 1 and Malte C Gather 1,2

Organic light-emitting diodes (OLEDs) have gained considerable attention because of their use of inherently flexible materials and their
compatibility with facile roll-to-roll and printing processes. In addition to high efficiency, flexibility and transparency, reliable color
tunability of solid state light sources is a desirable feature in the lighting and display industry. Here, we demonstrate a device concept
for highly efficient organic light-emitting devices whose emission color can be easily adjusted from deep-blue through cold-white and
warm-white to saturated yellow. Our approach exploits the different polarities of the positive and negative half-cycles of an alternating
current (AC) driving signal to independently address a fluorescent blue emission unit and a phosphorescent yellow emission unit which
are vertically stacked on top of each other. The electrode design is optimized for simple fabrication and driving and allows for
two-terminal operation by a single source. The presented concept for color-tunable OLEDs is compatible with application
requirements and versatile in terms of emitter combinations.
Light: Science & Applications (2015) 4, e247; doi:10.1038/lsa.2015.20; published online 13 February 2015

Keywords: alternating current; color mixing; color tuning; vertical stacking; white organic light-emitting devices

INTRODUCTION
In recent years, organic light-emitting diodes (OLEDs) have evolved
into a mature technology and OLEDs are now used in various display
applications. OLEDs provide an internal charge-to-photon conversion efficiency of nearly 100% and deliver homogeneous emission over
large areas, making them promising candidates for new and innovative
lighting applications. 1 White OLEDs, in particular, offer great potential for energy-efficient general illumination: luminous efficacies of
more than 90 lm W 21, comparable to the best fluorescent tubes, have
already been reported. 2,3 Furthermore, OLED-based light sources can
be made mechanically flexible and transparent, offering new opportunities for architecture, visual art and decoration. 4 The reliable realtime tunability of the OLED emission color would impart further
momentum to OLED technology on its way to becoming a widespread
source of general illumination. Thus far, two different color-tuning
concepts have prevailed in the literature. One exploits voltagedependent changes in emission color and was demonstrated as early
as 1994 for OLEDs fabricated from polymer blends. 5 Voltage-dependent color shifts are the result of a variety of mechanisms, e.g., voltagedependent charge trapping, a spatial shift of the recombination zone, a
modified exciton distribution, or exciton quenching at high current
densities. 5–7 However, this approach has several drawbacks: not only
are the mechanisms that lead to voltage-dependent color-shifts difficult to control, but adjusting the driving voltage also unavoidably
results in a dramatic and undesired change in device brightness. The
second concept overcomes the disadvantages of the voltage-controlled
approach by using a stacked tandem OLED structure with two (or
more) independently addressable units emitting light of different colors. 8,9 In comparison with the first method, this approach provides

much greater control over the emission spectrum. However, to individually address each unit, an additional electrode must be added into
the device stack as a connection between two adjacent units. This
intermediate electrode must be transparent and therefore must be
made either of a thin metal layer (,15 nm) or from indium tin
oxide. 10 However, the deposition of indium tin oxide can be problematic due to sputter-induced damage to the organic material underneath, 11 whereas metal films absorb a significant amount of light and
introduce additional micro-cavity effects. 12–14 Hence, the device
development of highly efficient color-tunable OLEDs remains experimentally challenging: the color-tunable OLEDs that have been
reported thus far demonstrate relatively modest efficiencies (,10%
external quantum efficiency (EQE),,10 lm W 21 ), despite the use of
phosphorescent emitter systems. 15,16 Although these reports on vertical stacking of two independently controllable emission units have
demonstrated only moderate performance, we believe that an efficient
realization of such a system is a necessary and important step toward
RGB full-color devices in which three independent emission units are
stacked on top of each other. Such a vertically stacked RGB configuration is highly attractive for display applications as it allows for
greatly increased pixel densities and a close-to-optimal utilization of
the available display panel area for all colors.

In this work, we demonstrate highly efficient color-tunable white
OLEDs based on a combination of a fluorescent blue emitting unit and
a phosphorescent yellow emitting unit for the generation of white
light. In contrast to the majority of publications on this topic, our
devices are driven by an alternating current (AC) signal, which allows
us to reduce the required number of independently addressable electrodes from three to only two. 8,16–18 By using an ultrathin and highly

1Institute for Applied Photophysics (IAPP), Dresden University of Technology, D-01069 Dresden, Germany and 2School of Physics and Astronomy, University of St Andrews, St
Andrews KY16 9SS, UK
Correspondence: M Fro¨ bel, Institute for Applied Photophysics (IAPP), Dresden University of Technology, George-Ba¨hr-Str. 1, 01069 Dresden, Germany
E-mail: markus.froebel@iapp.de
Received 10 September 2014; revised 2 December 2014; accepted 2 December 2014; accepted article preview online 4 December 2014

M Frobel et al

transparent layer of highly conductive metal as the intermediate electrode, 19–21 our devices achieve efficiencies comparable to those of
state-of-the-art non-color-tunable white OLEDs with the additional
advantage of providing continuous color tuning at constant brightness from deep blue through cold-white and warm-white to saturated
yellow. Our device efficiencies of up to 36.8 lm W 21 at warm-white
color coordinates are—to the best of our knowledge—the highest
values achieved thus far for any freely color-tunable white OLED.
Furthermore, we compare the performance of three different emitters
within the blue emitting unit and provide general design rules to
achieve high luminous efficacies (LEs) in such devices. We refer to
our devices as AC-driven direct-current (DC) OLEDs, or AC/DC
OLEDs, to clearly distinguish them from capacitively coupled
OLEDs, which operate without direct charge injection from the electrodes and which are frequently referred to as ‘AC-OLEDs’. 22–24

MATERIALS AND METHODS
Our device architecture comprises a yellow phosphorescent pinOLED stacked on top of a blue fluorescent pin-OLED, as schematically
illustrated in Figure 1a. 25 Internally, our devices contain three electrodes (labeled E1, E2 and E3 in Figure 1a); however, E1 and E3 are
electrically connected inside the device and thus, are at the same
potential upon application of a voltage and can be addressed via a
single external contact. E2 is the independent counter-electrode. The
polarity between E1/E3 and E2 controls whether the blue- or the
yellow-emitting sub-unit of the device is active: when E1/E3 is placed
at a positive potential with respect to E2, we observe blue emission
because the blue sub-unit is biased in forward direction, whereas the
yellow sub-unit is biased in reverse direction and thus exhibits a negligible current density and no yellow emission. Upon a change in
polarity, i.e., the application of a negative voltage to E1/E3, the device
emits yellow light, as it is now the yellow-emitting sub-unit that is
biased in forward direction. This polarity-dependent emission is
depicted in Figure 1b. To study the influence of the blue-emitting
sub-unit on the performance and color tunability of our AC/DC
OLEDs, we investigated three different blue sub-units based on the
blue fluorescent emitters 4P-NPD (sample S1), PPIP (sample S2) and
MADN:TBPe (sample S3) (Figure 1c).

All layers were deposited in a UHV chamber at a base pressure of
approximately 10 28 mbar onto glass substrates coated with structured
tin-doped indium oxide, which served as a transparent bottom electrode. The thickness and deposition rates of the organic materials were
measured via quartz crystal monitoring. The evaporation rates varied
from 0.3 A ˚ s 21 for the emission layer to 1 A ˚ s 21 for the p- and n-type
layers. Doping was realized by co-evaporation of the matrix material
and the dopant.

As the p-type layers, we used 2,29,7,79-tetrakis-(N,N-di-methylphenylamino)-9,99-spirobifluorene (Spiro-TTB) doped with 4 wt-% 2,29(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ).
The n-type layers consist of 4,7-diphenyl-1,10-phenanthroline (BPhen)
doped with cesium (Cs). For the blue emission layer, either N,N’-di-1naphthalenyl-N,N’-diphenyl-[1,19:49,199:499,1999-quaterphenyl]-4,4999diamine (4P-NPD), 4,49-bis(1-phenyl-1H-phenanthro[9,10-d]phenanthroimidazol-2-yl)biphenyl (PPIP) or 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) doped with 1.5 wt-% 2,5,8,11-tetra-tertbutylperylene (TPBe) was used. 26 The yellow emission layer is a double-emission structure, fabricated from a combination of the primarily
hole-conducting 4,49,499-tris(N-carbazolyl)-triphenylamine (TCTA)
matrix and the electron-conducting 2,29,299-(1,3,5-phenylen)tris(1phenyl-1H-benzimidazol) (TPBi) matrix, both doped with 8 wt-%

Figure 1 (a) Schematic illustration of the architecture of the investigated colortunable AC/DC OLEDs with electrodes and wiring connected to an AC power
supply (VAC). Electrodes E1 and E3 are connected to each other and E2 is the
independent counter electrode, allowing for two-terminal device operation using
only one power supply. (b) Photographs showing an AC/DC OLED sample upon
application of a DC bias (blue emission), a DC bias with reversed polarity (yellow
emission), and an AC voltage with a frequency of 50 Hz (white emission). (c)
Chemical structures of 4P-NPD, PPIP and MADN : TBPe which were the three
blue emitter systems investigated in this study. AC, alternating current; DC,
direct-current; OLED, organic light-emitting diode.

of the phosphorescent yellow emitter bis(2-(9,9-dihexylfluorenyl)-1pyridine)(acetylacetonate)iridium(III) (Ir(dhfpy)2(acac)). Electronand hole-blocking layers (EBL/HBL) confine the charge carriers and
excitons to the emission layer. In the case of the yellow-emitting unit,
aluminum(III) bis(2-methyl-8-quninolinato)-4-phenylphenolate (BAlq2)
and 2,29,7,79-tetrakis-(N,N-diphenylamino)-9,99-spirobifluorene (SpiroTAD) were used as the HBL and EBL, respectively. The HBL/EBL
combinations were different for the different blue-emitting units.
In the case of 4P-NPD and PPIP, we used BPhen/Spiro-TAD
and BPhen/N,N’-di(naphthalen-1-yl)-N,N’-diphenyl-benzidine (alphaNPD), respectively. For MADN:TBPe, BAlq2 was used as the HBL and
alpha-NPD was used as the EBL. The highly transparent central multilayer electrode is a combination of 2 nm of gold (Au) and 9 nm of

E3 AI

BPhen:Cs

BAlq2

TPBi:lr(dhfpy)2(acac) (8 wt-%)
TCTA:lr(dhfpy)2(acac) (8 wt-%)
Spiro-TAD

Spiro-TTB:F6-TCNNQ (4 wt-%)

100 nm

55 nm

10 nm

12 nm
6 nm
10 nm

35 nm

9 nm
2 nm

30/40 nm

10 nm _V_ AC

Blue emitter

Ag
Au

BPhen:Cs

BPhen/BAlq2

Spiro-TAD/alpha-NPD

Spiro-TTB:F6-TCNNQ (4 wt-%)

ITO

Glass substrate

10 nm

30 nm

90 nm

DC DC (reversed polarity)

AC (50 Hz)

MADN:TBPe

(S3)

N N

4P-NPD PPIP
(S1) (S2)

silver (Ag). An aluminum (Al) layer of 100 nm thickness was used as a
highly reflective top electrode. E1 and E3 were processed using the
same shadow mask. As the organic layers do not completely cover
E1, the two electrodes have a direct electrical connection. In this
way, our device design allows for easy fabrication and the internal
connection between E1 and E3 requires no additional space on the
substrate. The active area of the device is 6.49 mm 2 . Prior to device
investigation, the samples were encapsulated in a nitrogen glovebox.

The current–voltage–luminance (j–V–L) characteristics of our
devices were measured using a source measure unit (SMU 2400;
Keithley Instruments, Cleveland, Ohio, USA) in combination with a
calibrated Si photo-diode. A calibrated spectrometer (CAS 140,
Instrument Systems, Munich, Germany) recorded the spectral radiance in forward direction. Angle-dependent measurements were used
to determine the EQE and LE of the blue and yellow units, respectively,
and were performed in a spectrogoniometer setup that included a
calibrated Ocean Optics USB4000 miniature spectrometer. The electrical characteristics of the device under AC driving were measured
using a high-precision digital power meter (WT1600; Yokogawa,
Tokyo, Japan). The WT1600 measures current, voltage and the phase
angle between the two quantities to provide correct values for the
dissipated power. The luminous flux under AC conditions was
obtained in a calibrated integration sphere (LMS-100, d5250 mm;
Labsphere, North Sutton, New Hampshire, USA).

RESULTS AND DISCUSSION
The electrode design of the AC/DC OLEDs allows independently
investigating the electrical and optical characteristics of the yellowand blue-emitting units under DC conditions by changing the polarity
of the applied voltage. Figure 2 presents the j–V–L characteristics for
each individual emission unit, as well as the EQE and LE of each unit.
For a negative bias, only the yellow unit is active and the choice of the
blue emitter should not influence the performance. Both the j–V characteristics and the V–L curves of the yellow unit are indeed identical
for all three samples (Figure 2a). As expected, the differences in efficiency between the individual samples are also within the range of the
sample-to-sample variation (Figure 2c). At a brightness of 1000 cd
m 22, we obtain EQEs of 15%–16% and LEs of 50–55 lm W 21 for
the yellow units in the various samples. For the three different blue
emitters, we observe very similar j–V behavior and only small differences in the V–L characteristics (Figure 2b). With regard to efficiency,
the AC/DC OLED with 4P-NPD—a deep-blue emitter—exhibits the

M Frobel et al

lowest LE of approximately 1.2 lm W 21 at 1000 cd m 22, whereas the
sky-blue emitter PPIP and the host:guest system MADN:TBPe yield
LEs of 2.3 lm W 21 and 4.4 lm W 21, respectively, as shown in
Figure 2d. The EQE values of the three different blue units range from
2.5% in the case of PPIP and over 3.1% in the case of 4P-NPD to
approximately 3.4% in the case of MADN:TBPe. The emission spectra
of the blue- and the yellow-emitting units of samples S1, S2 and S3 are
provided in Supplementary Fig. S1.

Upon application of an AC signal, the device alternately emits pulses
of blue and yellow light. When a sufficiently high frequency is used, the
human eye cannot resolve the separate emission from each unit, but
instead perceives a color equivalent to the integrated emission over
several operation cycles (cf. demonstration in Supplementary Movie
1 and Figure 1b). By modifying the pulse width and/or pulse height
ratios between the positive and negative half-cycles of the AC signal,
the relative contributions of blue and yellow emission can be adjusted,
thereby allowing for tuning of the emission color perceived by the
human eye (cf. demonstration in Supplementary Movie 2).

To study this color tuning in greater detail, we used a square-wave
signal with equal pulse lengths in both the positive and negative halfcycles and tuned the emission color by varying the ratio between the
voltages applied during the forward and backward pulses, as illustrated
in the top row of Figure 3. The emission spectra for the three samples,
S1, S2 and S3, are presented in the corresponding rows in Figure 3. For
the spectra shown in columns 1 and 5, the voltage applied during
either the negative or the positive half-cycle is zero, and pure emission
from either the blue or the yellow unit can be observed. Column 3
represents the symmetric situation in which the absolute voltage
applied during the forward and backward pulses is equal, leading to
mixed emission from both units (cf. Figure 1b). Intermediate nonsymmetric waveforms lead to a reduced contribution from one of the
emission units (columns 2 and 4). Figure 4 presents the corresponding
CIE coordinates for all emission spectra. The CIE coordinates associated with the different pulse forms are located along straight lines
starting in the blue region and ending in the yellow region of the color
space. The emission spectra of the pure yellow units are identical for
S1, S2 and S3. As a result, all three samples are characterized by the
same yellow color point in the CIE diagram while having different
origins in the blue CIE region because of the different blue emitter
systems used. As indicated by dotted lines in the CIE diagram in
Figure 4, all color points on the connecting lines between the beginning and end points can be reached by changing the ratio of the

a 10 2 b 10 2 c d

10 1

10 0

10 –1

10 –2

10 –3

10 –4

10 1

10 0

10 –1

10 –2

10 –3

10 –4

10 4

10 3

10 2

10 1

10 4

10 3

10 2

10 1

10 0

1 2 3 4

_V_ (V)

S1
S2
S3

0 0

10 1 10 2 10 3

_L_ (cd m –2 )

S1
S2
S3

10 0

–4 –3 –2 –1

_V_ (V)

S2
S3

0 0

10 0 10 1 10 2 10 3 10 4

_L_ (cd m –2 )

S1
S2
S3

Figure 2 Current density (j) and luminance (L) as functions of voltage (V) for the (a) yellow-emitting units and (b) blue-emitting units of samples S1, S2 and S3. The
corresponding EQE and LE values as functions of luminance are presented in (c) and (d) for the yellow and blue units, respectively. Data were acquired during DC
operation, neglecting leakage currents through the reverse-biased opposite unit of the device. DC, direct-current; EQE, external quantum efficiency; LE, luminous
efficacy.

M Frobel et al

0 10 20 30 40

_t_ (ms)

0 10 20 30 40

0 10 20 30 40 0 10 20 30 40 0 10 20 30 40 0 10 20 30 40
_t_ (ms) _t_ (ms) _t_ (ms) _t_ (ms)

–1

0.8
0.6
0.4
0.2

400 500 600 700 400 500 600

_λ_ (nm) _λ_ (nm)

400 500 600 700 400 500 600

_λ_ (nm) _λ_ (nm)

400 500 600 700 400 500 600

_λ_ (nm) _λ_ (nm)

400 500 600 700

_λ_ (nm)

400 500 600 700 400 500 600

_λ_ (nm) _λ_ (nm)

Figure 3 Emission spectra of S1, S2 and S3 when the five different 50 Hz square-wave signals depicted in the top row are applied. Columns 1 and 5 represent pure
emission from the blue or yellow unit, as either the negative or the positive half-cycle is zero. Column 3 represents mixed emission from both units when the samples are
driven by a symmetrical signal. Intermediate waveforms lead to a reduced contribution from one of the emission units (columns 2 and 4).

voltage pulse intensities and hence the fractions of blue and yellow
emission contributing to the resulting spectrum. The color-tuning
curves of S1 (4P-NPD) and S2 (PPIP) intersect the Planckian locus

0.9

0.8

0.7

0.6

0.5

0.4

520

540

4P-NPD (S1)

PPIP (S2)
MADN:TBPe (S3)
Planckian locus
Warm-white color point A
Cold-white color point E

560

580

600

620

0.3 E

0.2

0.1

480

460

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

CIE _x_

Figure 4 Color coordinates corresponding to the emission spectra presented in
Figure 3 for S1, S2 and S3.

(i.e., the line that describes radiation from black-body radiators at
various temperatures); for S1, this intersection is very close to the
warm-white color reference point A.

As the next step, we measured the LEs of S1, S2 and S3 at a brightness
of 1000 cd m 22 when the devices were tuned to warm-white color
coordinates. For this experiment, color tuning was performed by
adjusting the ‘on time’ of each unit, i.e., by varying the relative pulse
widths of the positive and negative half-cycles at equal and constant
pulse heights or driving voltages. Compared with the previously
demonstrated pulse height modulation, this approach allows for finer
tuning of the emission color, as small changes in the on time ratio
between the two units also lead to small shifts in emission color. By
contrast, small variations in the driving voltage result in large variations in luminance (because of the super-linear diode behavior of the
individual units) which lead to strong color shifts. For all three samples, Table 1 summarizes the LE, the CIE color coordinates, the color
rendering index (CRI) and the on time ratio between the blue and
yellow units that was required to reach the color point A. Samples S2
(PPIP) and S3 (MADN:TBPe) achieved LEs of 31.5 lm W 21 and
27.9 lm W 21, respectively.
Interestingly, the white AC/DC OLED with 4P-NPD achieved the
highest LE, although the LE of 4P-NPD is considerably lower than
those of the other two blue emitters (cf. Figure 2d). This observation is
explained by the considerable difference in LE between the blue and
yellow emitter systems (cf. Figure 2c and 2d), as the phosphorescent
yellow unit has a 40-fold higher LE than does the fluorescent blue unit.
It is therefore important to maximize the on time of the yellow emitter
and keep the contribution of the blue emitter to a minimum.
Considering the on time ratios of the three emitter systems under
investigation (cf. Table 1), we find that this is best achieved using

M Frobel et al

Table 1 CIE color coordinates, CRI, on time ratio of the blue and yellow units, and luminous efficacy for S1, S2 and S3

Sample Blue emitter CIE CRI On time (%) a blue unit/yellow unit Luminous efficacy b (lm W 21 )

S1 4P-NPD (0.44,0.45) 38 15/85 36.8

S2 PPIP (0.44,0.47) 36 20/80 31.5
S3 MADN : TBPe (0.41,0.44) 35 30/70 27.9

Abbreviation: CRI, color rendering index.
a 50 Hz AC signal, 2.93 Vrms, equal pulse heights for positive and negative half-cycles.
b At a brightness of 1000 cd m22.

the deep-blue emitter 4P-NPD, for which the on time of the blue unit
for warm-white emission is merely 15%. The more sky-blue emitters
PPIP and MADN:TBPe require increased on times of 20% and 30%,
respectively, resulting in reduced LEs compared with the 4P-NPD
sample. The obtained efficiency value of 36.8 lm W 21 is even higher
than the value of 32.6 lm W 21 that has recently been achieved for noncolor-tunable white OLEDs using the same emitter materials in a
triplet-harvesting configuration 27 and, to our knowledge, represents
the best efficiency achieved thus far for any freely color-tunable white
OLED. As previously mentioned, the high efficiency of AC/DC OLEDs
results predominantly from the high efficiency of the yellow unit,
which can operate close to its maximum internal quantum efficiency
because of the direct electrical excitation. By contrast, the internal
quantum efficiency of the yellow emitter in the triplet-harvesting
white OLED is limited to approximately 50% because of the moderate
exciton-harvesting efficiency. 27

The low CRI values for all three samples (cf. Table 1) result from the
combination of only two emitters (blue and yellow) to obtain white
light, which means that important contributions in the green and deepred spectral regions are missing. However, as the device design of AC/
DC OLEDs is highly flexible in terms of emitter combinations, this
problem of low CRI can be addressed quite easily, e.g., by modifying
the blue emission unit into a blue1red triplet-harvesting emission unit
and replacing the yellow phosphor with a green phosphorescent emitter. The presence of red, green and blue emitters would guarantee a high
CRI, and the combination of a highly efficient triplet-harvesting unit
(blue1red can be more efficient than blue1yellow because of the more
favorable relative triplet energy levels) and a phosphorescent green
emitter is expected to result in high overall efficiency.

The operational stability of AC/DC white OLEDs is comparable to
that of conventional DC-driven white OLEDs, and in preliminary
tests, we did not observe any adverse effects of driving these devices
with an AC signal instead of the commonly used DC bias.

CONCLUSIONS
In summary, we demonstrated highly efficient color-tunable AC/DC
OLEDs based on a combination of a blue-emitting unit and a yellowemitting unit. The fabrication and electrical driving of these devices
were simplified by reducing the number of independent electrodes to
two and using an AC voltage for color tuning. Color tuning can be
accomplished either by modifying the pulse height or by adjusting the
pulse width ratio between the positive and negative half-cycles of the
AC signal. The influence of the central electrode on the optical characteristics of the OLEDs was reduced by using a highly transparent
thin-film metal electrode. In combination with the deep-blue fluorescent emitter 4P-NPD, this leads to an impressive luminous efficacy
of 36.8 lm W 21 at warm-white color coordinates of (0.44,0.45) and at
brightness levels compatible with application requirements (1000 cd
m 22 ). All efficacy values reported here were obtained without any

outcoupling enhancement. By integrating well-established lightextraction techniques, a two- to three-fold improvement in luminous
efficacy can be achieved. 28

To demonstrate the versatility of our approach and to investigate
the influence of different blue emitters on the device performance,
measurements were performed using the sky-blue emitter systems
PPIP and MADN:TBPe. These measurements indicated that the use
of a deep-blue emitter such as 4P-NPD is beneficial for obtaining a
high luminous efficacy, as it allows the on time of the blue unit to be
reduced.

In future work, this concept could be extended to three pin-OLEDs
stacked on top of each other to provide red, green, and blue emission.
This configuration is particularly interesting for display applications,
in which a vertical stacking of the red, green and blue sub-pixels would
allow for higher pixel densities and an optimal fill factor in comparison with conventional side-by-side fabrication.

ACKNOWLEDGEMENTS
This work was funded by the European Social Fund and the Free State of Saxony
through the OrthoPhoto project. We kindly acknowledge the support from
NUDEV.

1 Sasabe H, Kido J. Development of high performance OLEDs for general lighting.
J Mater Chem C 2013; 1: 1699–1707.
2 Reineke S, Lindner F, Schwartz G, Seidler N, Walzer K et al. White organic lightemitting diodes with fluorescent tube efficiency. Nature 2009; 459: 234–238.
3 Rosenow TC, Furno M, Reineke S, Olthof S, Lu¨ssem B et al. Highly efficient white
organic light-emitting diodes based on fluorescent blue emitters. J Appl Phys 2010;
108: 113113.
4 OLED-Info.com. Beautiful OLED lighting designs at LG’s design contest. Available at
http://www.oled-info.com/beautiful-oled-lighting-designs-lgs-design-contest (accessed
August 2014).
5 Berggren M, Ingana¨s O, Gustafsson G, Rasmusson J, Andersson MR et al. Lightemitting diodes with variable colours from polymer blends. Nature 1994; 372:
444–446.
6 Yang Y, Pei Q. Voltage controlled two color light-emitting electrochemical cells. Appl
Phys Lett 1996; 68: 2708.
7 Gather MC, Alle R, Becker H, Meerholz K. On the origin of the color shift in whiteemitting OLEDs. Adv Mater 2007; 19: 4460–4465.
8 Burrows PE, Forrest SR, Sibley SP, Thompson ME. Color-tunable organic lightemitting devices. Appl Phys Lett 1996; 69: 2959.
9 Parthasarathy G, Gu G, Forrest SR. A full-color transparent metal-free stacked organic
light emitting device with simplified pixel biasing. Adv Mater 1999; 11: 907–910.
10 Shen Z, Burrows PE, Bulovic V, Forrest SR, Thompson ME. Three-color, tunable,
organic light-emitting devices. Science 1997; 276: 2009–2011.
11 Kim HK, Kim DG, Lee KS, Huh MS, Jeong SH et al. Plasma damage-free sputtering of
indium tin oxide cathode layers for top-emitting organic light-emitting diodes. Appl
Phys Lett 2005; 86: 183503.
12 Deppe DG, Lei C, Lin CC, Huffaker DL. Spontaneous emission from planar
microstructures. J Mod Opt 1994; 41: 325–344.
13 Becker H, Burns SE, Tessler N, Friend RH. Role of optical properties of metallic
mirrors in microcavity structures. J Appl Phys 1997; 81: 2825.
14 Zhang XW, Li J, Zhang L, Jiang XY, Haq K et al. Top-emitting organic light-emitting
device with high efficiency and low voltage using a silver–silver microcavity. Thin Solid
Films 2010; 518: 1756–1759.
15 Jiang Y, Lian J, Chen S, Kwok HS. Fabrication of color tunable light-emitting diodes by
an alignment free mask patterning method. Org Electr 2013; 14: 2001–2006.

M Frobel et al

16 Joo CW, Moon J, Han JH, Huh JW, Lee J et al. Color temperature tunable white lightemitting diodes. Org Electr 2014; 15: 189–195.
17 Gu G, Khalfin V, Forrest SR. High-efficiency, low-drive-voltage, semitransparent
stacked organic light-emitting device. Appl Phys Lett 1998; 73: 2399.
18 Liang CJ, Choy WCH. Tunable full-color emission of two-unit stacked organic lightemitting diodes with dual-metal intermediate electrode. J Organomet Chem 2009;
694: 2712–2716.
19 Schubert S, Meiss J, Mu¨ller-Meskamp L, Leo K. Improvement of transparent metal top
electrodes for organic solar cells by introducing a high surface energy seed layer. Adv
Energy Mater 2013; 3: 438–443.
20 Schwab T, Schubert S, Hofmann S, Fro¨bel M, Fuchs C et al. Highly efficient color
stable inverted white top-emitting OLEDs with ultra-thin wetting layer top electrodes.
Adv Opt Mater 2013; 1: 707–713.
21 Schwab T, Schubert S, Mu¨ller-Meskamp L, Leo K, Gather MC. Eliminating microcavity effects in white top-emitting OLEDs by ultra-thin metallic top electrodes. Adv
Opt Mater 2013; 1: 921–925.
22 Perumal A, Fro¨bel M, Gorantla S, Gemming T, Lu¨ssem B et al. Novel approach for
alternating current (AC)-driven organic light-emitting devices. Adv Funct Mater 2012;
22: 210–217.
23 Fro¨bel M, Perumal A, Schwab T, Gather MC, Lu¨ssem B et al. Enhancing the efficiency
of alternatingcurrent driven organic light-emitting devices by optimizing the operation
frequency. Org Electr 2013; 14: 809–813.

24 Fro¨bel M, Perumal A, Schwab T, Fuchs C, Leo K et al. White light emission from
alternating current organic light-emitting devices using high frequency colormixing. Phys Status Solidi A 2013; 210: 2439–2444.
25 Huang J, Pfeiffer M, Werner A, Blochwitz J, Leo K et al. Low-voltage organic
electroluminescent devices using pin structures. Appl Phys Lett 2002; 80: 139.
26 Kuo CJ, Li TY, Lien CC, Liu CH, Wu FI et al. Bis(phenanthroimidazolyl)biphenyl
derivatives as saturated blue emitters for electroluminescent devices. J Mater Chem
2009; 19: 1865–1871.
27 Hofmann S, Furno M, Lu¨ssem B, Leo K, Gather MC. Investigation of triplet harvesting
and outcoupling efficiency in highly efficient two-color hybrid white organic lightemitting diodes. Phys Status Solidi A 2013; 210: 1467–1475.
28 Saxena K, Jain V, Mehta DS. A review on the light extraction techniques in organic
electroluminescent devices. Opt Mater 2009; 32: 221–233.

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