geht´s zu den Apfel- und Kartoffeltags-Rezepten von den Landfrauen

Inverted topside-emitting organic light-emitting diodes
T. Dobbertin, D. Schneider, A. Kammoun, J. Meyer, O. Werner, M. Kr¨oger, T. Riedl,
E. Becker, Ch. Schildknecht, H.-H. Johannes, and W. Kowalsky
Institut f¨
ur Hochfrequenztechnik, Technische Universit¨at Braunschweig, Schleinitzstr. 22,
D-38106 Braunschweig, Germany
ABSTRACT
Top-emitting organic light-emitting diodes (OLEDS) for next-generation active-matrix OLED-displays (AMOLEDs) are discussed. The emission of light via the conductive transparent top-contact is considered necessary
in terms of integrating OLED-technology to standard Si-based driver circuitry. The inverted OLED conﬁguration
(IOLED) in particular allows for the incorporation of more powerful n-channel ﬁeld-eﬀect transistors preferentially used for driver backplanes in AM-OLED displays. To obtain low series resistance the overlying transparent
electrode was realized employing low-power radio-frequency magnetron sputter-deposition of indium-tin-oxide
(ITO). The devices introduce a two-step sputtering sequence to reduce damage incurred by the sputtering process
paired with the buﬀer and hole transporting material pentacene. Systematic optimization of the organic growth
sequence focused on device performance characterized by current and luminous eﬃciencies is conducted. Apart
from entirely small-molecule-based IOLED that yield 9.0 cd/A and 1.6 lm/W at 1.000 cd/m2 a new approach
involving highly conductive polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS) as anode buﬀers
is presented. Such hybrid IOLEDs show luminance of 1.000 cd/m2 around 10 V at eﬃciencies of 1.4 lm/W and
4.4 cd/A.
Keywords: organic light-emitting diode (OLED), inverted OLEDs, active-matrix displays, rf-magnetron sputtering, photoelectric measurements
1. INTRODUCTION
Since early reports on electroluminescence from multilayer thin ﬁlm devices comprising vacuum-sublimed small
molecules1 respectively spin-coated conjugated polymers2 substantial research has gone into improving device
eﬃciencies, color purity and lifetime. Incitement of this development is the potential use of organic light-emitting
diodes (OLEDs) in future commercial ﬂat-panel displays. At present two diﬀerent technologies for realizing
pixelated OLED-displays are subjected to intense research activity. OLED-displays driven in the passive-matrix
mode (PM-OLEDs) are based on conventional bottom-emitting OLEDs and are considered as promising approach
for small-sized, low-information content displays with moderate pixel counts. To accomplish the claim for largearea, high-resolution OLED-displays the active-matrix addressing scheme (AM-OLEDs) has been suggested
modifying inherently the requirements for the single pixel cell (Fig. 1).
Conventional organic light-emitting diodes (OLEDs) consist historically of a transparent conductive anode,
typically indium-tin-oxide (ITO)-coated glass, covered by an organic multilayer for selective carrier transport
to the emissive ﬁlm.1 The cathode contact is employed by evaporation of a low-work-function metal, such as
MgAg,3 or LiF/Al,4 forming an opaque electrode on top. Using this device architecture the emitted light is
coupled out through the bottom contact making “on-chip” OLED integration with silicon-based driver electronics
rather impossible. Especially for high-resolution full-color displays an active matrix addressing scheme involving
amorphous silicon (a-Si),5–8 polycrystalline silicon (p-Si)9–12 or complementary metal-oxide-silicon (CMOS)13, 14
technology is desirable. In this regard the need for high ﬁlling factors can be satisﬁed by a vertical integration of
OLED pixels deposited directly on top of the driver circuitry (Fig. 2). To overcome the limitation of substratesided emission it is necessary to develop a highly eﬃcient OLED structure that allows emission via the top
contact.
Following this guideline numerous concepts have been suggested by diﬀerent authors. A thin electron-injecting
Further author information: (Send correspondence to T. Dobbertin)
T. Dobbertin: E-mail: [email protected]
150
Organic Light-Emitting Materials and Devices VII, edited by Zakya H. Kafafi,
Paul A. Lane, Proceedings of SPIE Vol. 5214 (SPIE, Bellingham, WA, 2004)
0277-786X/04/$15 doi: 10.1117/12.505811
conventional OLED
inverted OLED
Emission
Ý
Metal
TCO/(Me)
ETL/EML
HTLs
HTLs
ETL/EML
Metal
arb. Substrate
TCO
transp. Substrate
ß
Emission
Figure 1. OLED concepts: Conventional bottom-emitting OLED (left) and inverted top-emitting OLEDs (right).
ﬁlm like MgAg,15 Ca or Al/LiF16 interposed between a conventional organic layer sequence and a thicker
overlying sputter-deposited ITO ﬁlm was demonstrated. To exclude intrinsic absorption losses of the interfacial
metal layers and for a further minimization of device reﬂectivity a new cathode comprising of a thin CuPc17 ﬁlm
underneath the ITO contact was presented. Adding an ultrathin Li layer in form of Li/CuPc18, 19 or Li/BCP20
bi-layers leads to increased eﬃciencies. Transparent OLEDs (TOLEDs) of this kind are promising approaches for
the use in head-up displays or full-color, stacked organic light-emitting devices. However, from the standpoint
of device integration it is advantageous to make use of n-channel ﬁeld-eﬀect transistors (FET) demanding for
an OLED structure featuring the bottom contact as cathode: namely an inverted OLED (IOLED) structure.
This contradicts the TOLED concept showing anodes as bottom contacts. Though being fundamental aspect in
successful preparation of active matrix OLED displays (AMOLEDs) there were only very few investigations on
inverted OLEDs.21, 22 Recently, a highly eﬃcient, phosphorescent23 and a low-voltage operating top-emitting
diode24 were presented having a thin semitransparent metallic contact on top. Although sputter-deposition on
organic semiconductors is regarded as crucial process in OLED fabrication a ﬁnal deposition of any transparent
conductive oxide is required to obtain low series resistance.
sub-pixel
T1
sub-pixel
T2
T1
T1
C
driving circuit
T2
T2
C
C
OLED
horizontal integration
OLED on top of circuit
vertical integration
Figure 2. Concepts for integration of active-matrix OLED-displays: Conventional bottom-emitting OLED (left) and
inverted top-emitting OLED (right).
Proc. of SPIE Vol. 5214
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OMBD1
OMBD2
Metalization 2
OMBD3
Metalization 1
transfer
system
load lock
LF-Test
Chamber
transfe r
rods
Sample Preparation
under N2
glove box
load lock
Figure 3. OMBD cluster tool.
2. TECHNOLOGY AND EXPERIMENTAL
2.1. Organic Molecular Beam Deposition
The preparation of the OLEDs apart from the ITO top contact was carried out in an OMBD cluster tool as
depicted in Fig. 3. For reproducible growth conditions and to avoid cross-contamination, the organic source materials are sublimated or evaporated from eﬀusion cells provided with mechanical shutters. The cell temperatures
vary computer-controlled from 100 to 230 ◦ C depending on the organic material. Low deposition rates of 0.1–
20 ˚
A/min determined via quartz crystal monitors yield smooth and homogeneous thin ﬁlms. Metalization was
done by thermal evaporation from resistively heated crucibles at a background pressure around 8 × 10−4 mTorr.
The organic growth chambers ensure a base pressure of 1.5 × 10−7 mTorr. All chambers are equipped with multiple rate monitors for a precise control of co-evaporated (doped) layers and rotating sample holders guaranteing
homogeneous thin ﬁlms. A linear transfer system operated at 4 × 10−5 mTorr allows for deposition of complex
multilayered structures without breaking the vacuum.
2.2. RF-Magnetron Sputtering
For deposition of the ITO anode the samples are transferred under inert conditions from the OMDB system
to the sputtering chamber (Leybold Z590). ITO ﬁlms have been made from 8-in. oxide target (90% In2 O3 /
SnO2 10%, 99.99% purity). Deposition was carried out at room temperature under pure Ar at 4 mTorr in a
two-step sputtering sequence. First a nominally 10 nm-thick ITO buﬀer layer was realized at very low power
level of 0.16 W/cm2 yielding rates of 2 nm/min. By this means the high-energy part of the sputtered specimen
and charged particles were suppressed.25 Additionally, intrinsic heating and UV stress was lowered. A short
overall time of exposure to detrimental eﬀects of ITO deposition was guaranteed by eventually raising the sputter
power to 0.56 W/cm2 giving rates of 8.5 nm/min to meet the integral anode thickness of 40–90 nm.
2.3. OLED Configuration and Materials
According to the conﬁguration of the emissive cell as an electrically inverted OLED (Fig. 1, right) the growth sequence starts with the evaporation of a contact metal followed by the organic stack and terminates with the transparent anode. Figure 4 displays the constituting layers and organic compounds. The six active areas obtained
in this layout are simply deﬁned by overlapping electrodes. In this study tris-(8-hydroxyquinolato)aluminum
(Alq3 ) was employed as electron-transport layer (ETL) and emission layer (EML), partly doped with the highly
ﬂuorescent dye N,N’ -diphenyl-quinacridone (Ph-QAD) at concentrations as low as 0.8–1 mol-%. The structure is
completed with a double hole-transport layer (HTL) 4,4-bis[N -(1-naphtyl)-N -phenyl-amino]-biphenyl (α-NPD)
and 4,4’,4”-tris(N-(1-naphtyl)-N-phenyl-amino)-triphenylamine (1-TNATA) and the buﬀer layer pentacene.
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Proc. of SPIE Vol. 5214
Pentacene
Ph-QAD
EL
O
1-TNATA
N
Alq
N
N
N
V
O
3
N
O
O Al
N
N
N
O
N
V
N
N
a -NPD
Figure 4. Structural layout of individual IOLED cells including constituting organic materials and alignment on
17×17 mm2 substrates.
Electro-optical characteristics were recorded under ambient conditions using source measure unit (Keithley
236) and calibrated Si photodiode (Advantest TQ 8210). Conversion to photometric quantities was done by
careful calibration with a luminance meter (Minolta LS-110). Device eﬃciencies were calculated assuming an
Lambertian emission pattern.26
3. OPTIMIZATION OF SMALL-MOLECULE INVERTED OLEDS
First, in order to reduce complexity of the devices, the electro-optical performance of IOLEDs based on neat Alq3
emission layers was investigated. Figure 5 depicts the properties of IOLED with diﬀerent contact metals and a
varied ETL/HTL interface. As expected, the use of Mg instead of Al cathodes leads to steeper J-V -L curves.
The incorporation of only 5 nm α-NPD that separates the starburst amine 1-TNATA from the Alq3 results in
an earlier onset of electroluminescence (EL) and increased current eﬃciencies. These eﬀects can qualitatively
be explained by the band scheme suggested in Fig. 6. The electron injection into Alq3 is facilitated by the low
(a)
102
(b)
1
10
0
10
Luminance (cd/m2)
150
225
103
Current Density (mA/cm2)
Luminance (cd/m2)
225
10-1
10-2
10-3
10-4
10-5
0
5
10
15
20
25
30
Voltage (V)
75
0
-5
0
5
10
15
Voltage (V)
20
25
30
Al cathode
Mg cathode
Mg cathode, a -NPD
150
75
0
0
5
10
15
20
25
Current Density (mA/cm2)
Figure 5. L-V characteristics (a) and L-J characteristics (b) of the following IOLED structures: 60 nm Alq3 /45 nm 1TNATA/50 nm pentacene either with Al od Mg cathode and 60 nm Alq3 /5 nm α-NPD/40 nm 1-TNATA/50 nm pentacene
utilizing a Mg cathode. All devices are capped with 40 nm ITO. Inset of (a): Corresponding J-V charts.
Proc. of SPIE Vol. 5214
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2.3
3.6
Al
4.0
3.1
Alq
a -NPD
Mg
3
1.9
2.8
1-TNATA
5.0
5.1
5.8
ITO
pentacene
5.5
Figure 6. Simpliﬁed sketch of the band structure of an eﬃcient invereted OLED visualizing possible energetic barriers at
metall/organic interfaces and interorganic junctions. Grey parts denote structural change according to the series presented
in Fig. 5.
work-function metal Mg providing more electrons in the recombination zone. Thus, the current eﬃciency is
increased indicated by faster growing L-J plot as displayed in Fig. 5 (b). The insertion of α-NPD establishes a
staircase-like evolution of the hole transport states in the diode such that hole emission into the luminescent layer
can occur at lower electric ﬁelds resp. reduced operation voltage. These observations agree very well to previous
studies on the energetics of a similar interface, formed by the low-ionization-potential (IP) starburst derivative
m-MTDATA (IP=5.1 eV) and Alq3 .27–29 Here, it is argued that an interface exciplex between the excited state
of Alq3 and the ground state of the HTL is constituted, which, in turn, can alter emission color and lowers
device eﬃciency. Adding a thin HTL with higher IPs such as TPD (IP=5.5 eV) or α-NPD positively aﬀects the
device eﬃciencies.27, 29 The IOLEDs employed 1-TNATA thin-ﬁlms due to the superior morphological stability
indicated by a high glass-transition temperature of 113 ◦ C.30
Former research in the ﬁeld of top-emitting OLEDs15–22 emphasized the incorporation of additional protective
buﬀer layers to prevent the OLEDs from damage inﬂicted by the sputtering process. Recently, we reported on
the use of pentacene as novel buﬀer and hole injecting/transport layer.31 Pentacene was chosen due to its
superior hole conducting properties32 and its chemical stability towards exposure to intense UV light, elevated
temperatures33 and water vapor.34
In order to ﬁnd an optimum value for the thickness of the buﬀer layer, pentacene was increased from 15 nm
via 31 nm to 50 nm. 1-TNATA was chosen to 38 nm. Figure 7 shows luminous and current eﬃciencies obtained
Thickness 1-TNATA (nm)
2.5
2.0
Glas
0.8
0.6
0.4
1.5
0.2
1.0
10
15
20
25
30
35
40
45
50
55
Thickness Pentacene Layer (nm)
Figure 7. Current and luminous eﬃciency obtained
for growing thickness of pentacene on top of 38 nm
1-TNATA/5 nm α-NPD/60 nm Alq3 at 150 cd/m2 .
Inset: Schematic of layer sequence.
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Proc. of SPIE Vol. 5214
3.5
Current efficiency (cd/A)
Ý
ITO
Pentacene
1-TNATA
a -NPD
Alq3
Mg/Au
Luminous efficiency (lm/W)
Current efficiency (cd/A)
Emission
58
48
38
28
18
8
-2
1.0
3.0
0.8
2.5
2.0
0.6
1.5
0.4
1.0
0.2
0.5
0.0
Luminous efficiency (lm/W)
1.0
3.0
0.0
30
40
50
60
70
80
90
Thickness Pentacene (nm)
Figure 8. Current and luminous eﬃciency obtained
at 150 cd/m2 for varying composition of 1-TNATA
and pentacene being 88 nm in sum deposited on top
of 5 nm α-NPD/60 nm Alq3 (solid lines are intended
as guide to the eye).
2000
7
1000
Current efficiency (cd/A)
Voltage (V)
3000
14
Luminance (cd/m2)
4000
10
10
1
1
(a)
0
20
40
60
(b)
0.1
0
100
0
80
Luminous efficiency (lm/W)
21
0
500
1000
1500
2000
2500
3000
0.1
3500
Luminance (cd/m2)
Current Density (mA/cm2)
Figure 9. U-J-L characteristics (a) and device eﬃciencies vs. luminance (b) of an optimized IOLED utilizing a 60 nm
Alq3 /5 nm α-NPD/45 nm 1-TNATA/43 nm pentacene organic stack deposited without breaking the vacuum.
by typical luminance for display applications of 150 cd/m2 . Evidently, the device performance is strongly coupled
to the thickness of pentacene yielding eﬃciencies of 2.7 cd/A and 0.5 lm/W at 50 nm. As a result of the good
hole-transport abilities of pentacene the J-V curves of the samples (not presented) are almost identical. Taking
the overall thickness of the thickest device the composition of 1-TNATA and pentacene was varied systematically
in a second series. It has to be emphasized that a vacuum-break is common to all devices in both series. In Fig. 8
the evolution of device eﬃciencies at 150 cd/m2 is depicted in the following manner. The pentacene thickness
was increased from 33 nm to 88 nm at the expense of the 1-TNATA ﬁlm maintaining an overall thickness of
153 nm. A broad maximum is clearly visible at the composition around 40 nm pentacene and 48 nm 1-TNATA
giving typical values of 3 cd/A and 0.5 lm/W for device eﬃciencies. The initial rise towards thicker pentacene
layers can be attributed to the growing impact of the protective buﬀer layer as seen in the ﬁrst series while
the ﬂattening is presumably caused by the optical losses in pentacene. In contrast to 1-TNATA that is nearly
transparent over the entire visible range pentacene exhibits distinct absorption bands in the emissive region of
Alq3 . Spectroscopic ellipsometry on a 100 nm-thick reference sample grown on Si was performed to clarify this
750
10
500
5
Current efficiency (cd/A)
Voltage (V)
1000
Luminance (cd/m2)
1250
15
10
1
1
250
(b)
(a)
0
0
4
8
12
Current Density (mA/cm2)
0
16
Luminous efficiency (lm/W)
10
1500
20
0.1
250
500
750
1000
1250
1500
Luminance (cd/m2)
Figure 10. U-J-L characteristics (a) and device eﬃciencies vs. luminance (b) of an optimized IOLED utilizing a 60 nm
Alq3 /5 nm α-NPD/45 nm 1-TNATA/43 nm pentacene organic stack deposited without breaking the vacuum.
Proc. of SPIE Vol. 5214
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300
U
(b)
top electrode
organic
semiconductor
substrate electrode
Current Densiyt (mA/cm2)
(a)
top
electrode
200
100
0
-100
substrate
electrode
-200
-300
-10
PEDOT
Pentacene
-5
0
5
10
15
Voltage (V)
Figure 11. Schematic of unipolar structures deﬁning polarity and contact names (a) and J-V characteristics (b) of
“hole-only” structures which are ITO/pentacene (100 nm)/ITO (90 nm) and ITO/PEDOT:PSS (85 nm)/ITO (90 nm).
issue. The complex optical parameters determined at the photoluminescence maximum of Alq3 at 520 nm are
n = 1.35 − j 0.13 for pentacene and n = 1.74 − j 0.006 for 1-TNATA thus assisting our assumption.
Eventually, an optimized layer sequence of 1-TNATA/pentacene with 45 nm and 43 nm, respectively, was
deposited without breaking process vacuum concerning the organic multilayer. The current density-voltage (J-V )
characteristics paired with luminance-voltage (L-V ) are presented in Fig. 9 (a) while related luminous eﬃciencyEL luminance and current eﬃciency-EL luminance are plotted in Fig. 9 (b). As expected device performance
was improved again to 3.9 cd/A and 0.7 lm/W at 1.500 cd/m2 for current and power eﬃciency respectively.
Moreover, the characteristics exhibit a fairly ﬂat behavior in the high luminance region. A maximum brightness
of 13.000 cd/m2 was observed at 25 V.
For further improvement of the eﬃciencies, the 60 nm-thick Alq3 layer was fragmented in equal shares to a
30 nm-thick ETL and a 30 nm-thick EML doped with approx. 0.8 mol% Ph-QAD.35 The splitting is derived
A.36 Additionally, for reduction of operation voltage and
from the diﬀusion length of holes in pure Alq3 of 350 ˚
adjusting the eﬃciencies the HTL 1-TNATA was reduced to 28 nm while other parameters remained unchanged
compared to the optically undoped IOLED. The electro-optical properties of this device are plotted in Fig. 10
displaying raised eﬃciencies of 9.0 cd/A and 1.6 lm/W at 1.000 cd/m2 . The operation voltage for the latter
brightness is 17.6 V, a level of 13.000 cd/m2 , also cited for the undoped emitter, is reached at 23.2 V.
4. ELECTRICAL CONTACTS
The eﬃciencies observed for the discussed IOLEDs , especially when the high electric ﬁeld in the emissive layer
is taken into account, indicate that Alq3 should not sustain substantial degradation from the sputtering process.
Facing the comparatively high operation voltage for the ensemble of organic materials, we infer that the charge
injection into the organic multilayer is most likely the key factor to improved device performance. To gain
further insight into the charge injection properties of both of the contacts, several sets of unipolar (“electrononly” resp. “hole-only”) test devices were fabricated. Polarity and counterelectrodes were chosen corresponding
to the IOLED contact sequence, that is an injective top contact for holes and an electron-injecting bottom
contact. The ITO top electrode was deposited according to the IOLED procedure. All devices were deposited
on oxygen-plasma activated ITO-coated glass carriers.
To study the hole injection properties of the “inverted” ITO/pentacene contact an unipolar structure consisting of ITO/100 nm pentacene/ITO was fabricated. A pronounced asymmetric behavior in the J-V characteristics
presented in Fig. 11 evidences reduced hole injection ability of the inverted contact. Apparently, this is due to
technological variations in contact formation and indicates that pentacene sustained damage due to the interaction with the sputtering plasma. A dramatic enhancement of the injection eﬃciency of the anode is observed
156
Proc. of SPIE Vol. 5214
F
DF
B
eU
B1
F
B2
+
U
Photocurrent (a.u.)
1E-3
Yb
Ca
1E-4
Sm
Al
1E-5
Ba
-1.0
-0.5
Mg
0.0
0.5
1.0
Voltage (V)
Al
Me
Figure 12. Internal ﬁeld as as result of
built-in potential diﬀerence and external
voltage for U = 0 (- -) and U > 0 (—).
Figure 13. Magnitude of photocurrent for a 10 nm
metal/100 nm Alq/150 nm Al structure.
by replacing pentacene with the highly conductive polymer polyethylene dioxythiophene-polystyrene sulfonate
(PEDOT:PSS). An ideal ohmic J-V plot independent of polarity is obtained.
It is commonly accepted that the injection eﬃciency of contacts depends to some extend on the order of
contact formation, e. g. the widely used Al/LiF composite cathode on Alq3 .38 For this reason other metals
with even lower work functions as the routinely used Mg (3.6 eV), such as the alkaline metals Ca (2.8 eV) and
Ba (2.7 eV) or the rare earth metals Sm (2.7 eV) and Yb (2.6 eV), were investigated. The device structure
is as follows: ITO-coated glass, 10 nm metal, 100 nm Alq3 and 150 nm Al. The substrate-sided contact is
semitransparent to allow illumination for determining the built-in voltage. A potential diﬀerence ∆ ΦB between
the barrier heights at the two metal/semiconductor interfaces leads to a built-in ﬁeld even when the two contacts
are shorted (12). Using an external voltage U the total internal ﬁeld can be compensated. The generation of
charge carriers by illumination of the organic layer with above-energy-gap light is leading to a photocurrent
scaling with the total ﬁeld in the structure. This built-in voltage has to be considered for evaluating and rating
of diﬀerent injective contact metals.
Figure 13 shows the magnitude of this photocurrent measured by lock-in technique as a function of the
applied external bias for diﬀerent metal combinations. The bias at the minimum of the photocurrent directly
corresponds to the diﬀerence in the barrier heights of the Al top electrode to the metal under investigation.
Only a small maximum diﬀerence of 0.33 eV in the barrier heights between Al and Sm as the material with the
highest and the smallest barrier could be detected. Obviously, by employing metals with lower work functions
than Mg, no signiﬁcant lowering of the injection barrier can be achieved. This result corresponds to previous
studies concerning overlying electrodes.37 The corresponding J-V characteristics plotted in Fig. 14, however,
are not conform to the energetic barriers that can be derived in relation to the Al top contact and favor Mg-based
cathodes. A simple correlation between barrier heights to the macroscopic current ﬂow in the high-ﬁeld regime
represented by the J-V plots is complicated by the marginal variations in ∆ ΦB and the reproducibility of devices
and measurements.
A fairly promising way to facilitate electron injection into an organic semiconductor is the intentional doping
with reactive metals such as Li, Sm or Sr by co-evaporation39 or sequential deposition of a thin metallic ﬁlm.42
In order to minimize non-radiative quenching of excitons in the bulk39 we suggest an interfacially doped electron
injection layer (EIL) adjacent to the metallic cathode. The thickness of the doped EIL was deduced from the
amount of Li stored in typically used ultrathin LiF intermediate layers (0.3–0.7 ˚
A) in Al/Alq3 based cathodes.
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300
200
100
Magnesium
Aluminium
Ytterbium
Samarium
Calcium
Barium
ITO
top
electrode
Current Density (mA/cm²)
Current Density (mA/cm2)
300
0
-100
substrate
electrode
-200
-300
-25 -20 -15 -10
-5
0
5
10
15
20
25
Voltage (V)
Figure 14. J-V characteristics of “electron-only”
structures as deﬁned in Fig.13. An ITO as cathode
was included to prove closed metallic coverage of the
ITO substrates.
top
electrode
Bphen/Li (1:1)
Bphen/Li (3:1)
Bphen/Li (5:1)
Mg
200
100
0
-100
substrate
electrode
-200
-300
-25
-20
-15
-10
-5
0
5
10
15
20
Voltage (V)
Figure 15. J-V characteristics of “electron-only”
structures having an interfacially doped EIL. The
structure data are: 5 nm Li-doped Bphen layers with
varying molecule to atom ratios (1:1, 3:1, 5:1), 5 nm
intrinsic Bphen and 90 nm Alq3 . Symmetric Alelectrodes were applied. A 100 nm thick Alq3 ﬁlm
sandwiched between an Al substrate and a Mg top
electrode serves as reference.
Here the improvement of electron injection is ascribed to unintentional doping of Alq3 with liberated Li.40, 41
Our estimations are based on the assumption of an optimum molecule to atom ratio of unity.39
Figure 15 assembles the J-V curves for varying doping levels of Li in 5 nm-thick 4,7-diphenyl-1,10’-phenanthroline (Bphen) ﬁlms realized by co-evaporation from separate sources. Additionally, a Mg-based cathode on an
intrinsic organic semiconductor is provided as reference. The doped EIL is followed by a nominally undoped
5 nm-thick Bphen layer that is supposed to work as hole-blocking layer (HBL) in a complete IOLED structure.
Clearly, the approach of interfacial doping results in signiﬁcantly diminished voltages at comparable current
density. For instance, with reference to the Mg-cathode a current density of 100 mA/cm2 is obtained 5.2 V
earlier in case of the highest doped EIL. In addition to that, the injection properties of the overlying Al contact
are successively improved as the doping level is raised. This phenomena can be attributed to Li diﬀusion through
the adjacent layers, thereby reducing the eﬀective length of the intrinsic organic semiconductor.38, 42
5. INVERTED OLEDS WITH DOPED CHARGE-INJECTION LAYERS
In order to assure enhanced current ﬂow as well as balanced carrier statistics in the recombination zone it is
desirable to introduce PEDOT:PSS as anode buﬀer paired with an interfacially doped EIL.
Moreover, the beneﬁts of incorporating highly conductive polymers, such as PEDOT:PSS or polyaniline, as
hole-injection and hole-transport layer in entirely polymer-based OLEDs44–46 and small-molecule based OLEDs
(SM-OLED)49, 51 have already been presented. Similarly, both device-architectures are reported to exhibit a
reduced operational voltage, higher luminance levels, improved or comparable eﬃciencies and a better longterm stability. These observations are, among others,45, 46, 52 predominantly attributed to the following, brieﬂy
outlined, mechanisms. First, the good ﬁlm-forming property of the intermediate polymeric layer provides reproducibly smooth and “spike-free” anodes resulting in a laterally homogenized current ﬂow49 thus inhibiting the
inﬂuence of local shorts.46, 53 In addition, the hole injection into the emissive region is enhanced due to ohmic
contacts49 and, possibly, a better matching of the electronic transport states. Similar eﬀects can be anticipated
by proper incorporating PEDOT:PSS to an top-emitting IOLED, referred to as hy-IOLED hereafter.
As stated above, thin ﬁlms of pentacene are able to withstand water treatment as the latter as a result of its
hydrophobic nature. This property makes it a good candidate to work as an in-situ deposited organic thin-ﬁlm
sealing for the underlying organic multilayer. In previous studies we could already demonstrate the feasibility of
158
Proc. of SPIE Vol. 5214
16
3
6
20000
6
8
Luminance (cd/m2)
Voltage (V)
12000
4
8000
4
2
4000
Current efficiency (cd/A)
16000
Luminance (cd/m2)
(a)
12
4
2
2
1
0
0
1
2
3
4
Voltage (V)
0
0
100
200
300
400
500
5
Luminous efficiency (lm/W)
(b)
6
0
600
0
0
5000
10000
15000
0
20000
Luminance (cd/m2)
Current Density (mA/cm2)
Figure 16. J-V-L characteristics (a) and device eﬃciencies vs. luminance (b) of a hybrid IOLED consisting of Al
cathodes with 5 nm Bphen:Li (2:1), 25 nm Bphen, 30 nm Alq:Ph-QAD (1.1 mol%), 5 nm α-NPD, 10 nm 1-TNATA,
20 nm pentacene, PEDOT:PSS and 90 nm ITO. Inset of (a): Onset of EL.
this approach.54 Consequently, thin ﬁlms of PEDOT:PSS (Baytron AI 4083) were deposited on an IOLED by
spin-coating from ﬁltered (0.45 µm) aqueous dispersion at 4000 rpm under ambient conditions. The process was
repeated 3–5 times until a visibly clear and homogenous ﬁlm has been constituted. By this means, employing
a non-optimized coating technique a fabricational yield of ∼ 70 % was achieved. Prior to the radio-frequency
magnetron deposition of ITO the samples were stored overnight in the sputter chamber for vacuum-assisted
desorption of residual water.
Figure 16 assembles the electro-optical characteristics of a hy-IOLED. The J-V characteristics document
typical space-charge limited current ﬂow without any superimposed artefact that might have found its origin in
local sorts due to the spin-coating process. Luminance values of 100 cd/m2 and 1.000 cd/m2 are reached at 7.6 V
resp. 9.9 V, the EL onset is oberserved at 3.5 V. The power eﬃciency peaks at 1.4 lm/W around 1.000 cd/m2
yielding a current eﬃciency of 4.4 cd/A. A maximum luminance of over 17.000 cd/m2 is evidence of a good
stability.
To estimate the eﬀects of potentially varying thicknesses of PEDOT:PSS a set of hole-only devices with
62, 85 and 110 nm thick polymeric layers was made. From the linear J-V curves a resistivity of only 51–
56 Ω was extracted. The transmissivity was determined to 92–96 % (PEDOT:PSS only) at the maximum of
electroluminescence so that the implication on the device performance should be negligible.
ACKNOWLEDGMENTS
The authors thank the Bundesministerium f¨
ur Bildung und Forschung (BMBF, 01 BK 918) for their generous
ﬁnancial support.
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