Designing interfaces that function to facilitate charge injection in organic light-emitting diodes

Article abstract of DOI:10.1021/ja052170n

Layer-by-layer (LbL) assembly of triarylamine (TAA)-containing polymers has been applied for anode functionalizations in organic light-emitting diodes (OLEDs). Surface work function of the ITO electrodes was significantly altered with the functionalizatio

Full text of DOI:10.1021/ja052170n

Published on Web 08/02/2005
Designing Interfaces That Function to Facilitate Charge Injection in Organic
Light-Emitting Diodes
Shinji Kato
Kawamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba 285-0078, Japan
Received April 5, 2005; E-mail: kato@kicr.or.jp
Molecular engineering at an electrode surface is a challenging
subject to chemists in the field of organic microelectronics.1
Especially in organic light-emitting diodes (OLEDs), to tune the
electronic properties at an anode surface has been a crucial issue
because electronic profiles at the anode/organic interface strongly
affect electron/hole injection fluence and recombination of the
charge carriers that are central factors for providing efficient
OLEDs.2 In this communication, we wish to propose a new strategy
Figure 1. A basic concept of the present approach regarding an energetic
scheme at an anode/organic interface.
for designing interfaces that provide stepped electronic profiles
(Figure 1), which leads to remarkable facilitation of hole mobility
from the anodes to the organic layers.3
The basic concept of our approach depends on polymeric layer-
by-layer (LbL) assembly for anode modifications. This technique
is a nanoscale film-forming system of sequentially adsorbed
polymeric materials.4 We previously demonstrated that the poly-
meric LbL assembly was a useful tool for fabricating anode interface
layers that function to promote the interfacial cohesion in OLEDs.5
Basically, the most fascinating nature of the polymeric LbL
technique is that a precise control over component distribution can
be easily performed in the vertical direction from the substrate
surface. On the basis of this feature, we have attempted to extend
our original approach to construct stepped electronic profiles at
the anode/organic interface, by sequential deposition of systemati-
cally designed electroactive polymers in order of their redox
abilities.
Figure 2. Chemical structures of the polymers used in this study.
Figure 3. Schematic outline of the preparation of polymeric LbL multilayer
films: (i) deposition of a polymer; (ii) deposition of ethylenediamine; (iii)
thermally induced covalent bond formation.
A series of alternating copolymers of monomethyl maleate with
triarylamine (TAA)-functionalized vinyl monomers were employed
in this study. The chemical structures of the copolymers used (P1-
P4) are shown in Figure 2. These are regarded as dual-function-
alized copolymers consisting of two modules. The monomethyl
maleate unit allows the polymers to be LbL film-forming, where
the carboxylic acid group acts as an interlayer cross-linking moiety,
while the TAA-functionalized monomer unit is an electroactive
functionality capable of hole transport. These polymers possess
different TAA functionalities with regard to electron affinity of the
substituents (X) on the aryl rings. This allows for the tuning of the
redox potential of the polymers (Figure S1): E1/2(M+/M) vs Ag+/
Ag: P1, 0.57 V; P2, 0.75 V; P3, 0.87 V; P4, 0.96 V. This tuning
is important to optimize the energy differences between the highest
occupied molecular orbital (HOMO) of the anode interface layer
and the energy level of the adjacent materials, one is with the Fermi
level of the anode and the other the HOMO of the organic layer
(which is usually a hole transport layer) to control hole injection.
Figure 3 outlines the steps in the present film-preparation
approach. Pretreated substrates (ITO or quartz) having amine-
bearing surfaces were initially immersed in a 2-butanone solution
of the polymer (1 mM) at room temperature for 10 min and then
treated with a toluene solution of ethylenediamine (25 mM) at room
temperature for 5 min. The repetitive two-step process (denoted
hereafter as “cycle”) was used to laminate the polymers onto the
substrate surface, where a driving force of the polymer lamination
is an ionic or a hydrogen-bond interaction, between the carboxylic
acid group of the polymers and ethylenediamine. Finally, the
deposited polymer films were converted to covalently bound
counterparts via heat-induced condensation reactions between the
preassociated carboxy acid-amine functions.
That there was LbL fabrication of the polymers on quartz plates
was demonstrated by successive UV-vis electronic absorption
measurements (Figure S2). Efficiency of the stepwise film growth
was slightly different among the polymers, which increases in the
order P4 < P3 < P2 e P1 (Figure S3). On the basis of the linearity
of the absorbance change, the two-dimensional areas occupied per
the TAA units are estimated to be 0.44 (P1)-0.65 (P4) nm2 per
deposition cycle. These values appear to be rather small in
comparison with the size of a triphenylamine framework (0.74 nm2,
viewed as a cylinder), suggesting that the TAA units are densely
assembled in the polymer multilayer films. The thickness of these
films was found to be increased by 1.0-1.5 nm per deposition cycle,
based on X-ray reflectivity measurements (Figure S4).
Device performances employing the polymeric LbL films as
anode interface layers were investigated for conventional TPD/Alq
OLEDs. Figure 4 shows representative examples of current density,
luminance, and external quantum efficiency as a function of
operating voltage, for the devices with the LbL films prepared via
four deposition cycles of P1-P4. Functionalization with the LbL
films of P1 and P2 revealed large enhancement in current and
luminance levels in the OLEDs, compared to those with a bare
9
11538
J. AM. CHEM. SOC. 2005, 127, 11538-11539
10.1021/ja052170n CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 5. (A) Current density, (B) luminance, and (C) forward external
quantum efficiency as a function of operating voltage for OLED devices
(ITO/TPD/Alq/Al) with or without anode interface layers: (b) bare ITO;
(9) four deposition cycles with a P1-P1-P2-P2 sequence; (2) three
deposition cycles with a P1-P2-P3 sequence.
Figure 4. (A) Current density, (B) luminance, and (C) forward external
quantum efficiency as a function of operating voltage for OLED devices
(ITO/TPD/Alq/Al) with or without anode interface layers (four deposition
cycles): (b) bare ITO; (9) P1; (0) P2; (2) P3; (4) P4.
ITO anode. Furthermore, forward external quantum efficiency of
the device with the films of P1 and P2 reached 0.62 and 0.65%,
respectively, a large improvement over that of the bare ITO-based
device of 0.32%. On the other hand, only small effects were
observed on the OLED performances in the cases of the anode
interface layers of P3 and P4. Figure S5 shows plots of current
density (A) and luminance (B) versus deposition cycles of the
polymer films when applying a voltage of 20 V. There is a sharp
contrast in OLED performances between the devices with the films
of P1 and P2 and those with the films of P3 and P4.
To get the insight underlying the contrasting behavior of the
OLEDs, microstructural investigation for the ITO/TPD interface
was performed under thermal stress.6 Figure S6 shows SEM images
of the annealed TPD films (50 nm) on the bare or functionalized
ITO electrodes (four cycles of the P2 deposition). No significant
dewetting was observed for the TPD films in contact with the P2
films, unlike the case with the bare ITO electrodes. All the LbL
films prepared by using other polymers were as much effective as
the P2 films to prevent decohesion of the TPD films, regardless of
the number of deposition cycles. These results clearly indicate that
all of the polymer films equally enhance the ITO/TPD interfacial
stability.
On the other hand, the contrasting OLED characteristics can be
explained by different types of electronic profiles at the ITO/TPD
interfaces among the devices. By means of atmospheric photoelec-
tron spectroscopy, the bare ITO exhibited a surface work function
of 4.80 eV, but the electrodes functionalized with the polymer films
changed the values to 5.16, 5.36, 5.59, and 5.76 eV for P1, P2,
P3, and P4, respectively. The former two values are smaller than
the ionization potential (Ip) value of the vacuum-deposited TPD
film (5.44 eV), while the latter two are apparently opposite cases.
These results suggest that electronic profiles of the anode/organic
interfaces are crucial to OLED characteristics, where P1 and P2
are mediating the hole mobility, but P3 and P4 are blocking. Note
that, as shown in Figure S5, the values of current and luminance
sharply drop over six deposition cycles even with the P1 or P2
films, probably due to the partial insulating effect of relatively
thicker films.
intensities, which were greater than those of the devices with the
single polymer-component interface layers (compare to Figure 4).
These results strongly indicate that this heterodeposited polymer
film functions to enhance hole injection from the anode, through
the facilitation of energy level matching at the interfaces of the
ITO/(stepped interlayer)/TPD.
Finally, it is of crucial interest to note that the OLED performance
was also significantly improved by anode functionalization with
the heterodeposited film with a P1-P2-P3 sequence (Figure 5).
This is certainly an unexpected result since the polymer film of P3
blocks hole injection at the ITO/TPD interface (vide supra). An
interesting observation was provided from the surface work function
of 5.46 eV for the modified ITO with this heterodeposited film
(Figure S8B), which is significantly lower than that with the P3-
only film. This value is close to that of the TPD film. This finding
would be related to an inherent nature of the LbL technique that
offers a nanoscale blending system with polymeric interpenetration,7
thus leading to fabrication of a graded layer structure having a
graded electronic profile. Works are currently underway to further
investigate heterodeposited polymer films, along with optimization
of their effects on OLED characteristics. In this manner, the present
LbL technique offers a rational and versatile way to enhance OLED
performances, which is adjustable depending on the energetic
scheme of devices by rational design of polymer structure and layer
sequence.
Acknowledgment. The author thanks Mr. Hiromichi Arakawa
and Mr. Tomoaki Hara for precious technical information on the
fabrication and characterization of the OLED devices. The author
also thanks Mr. Hiroshi Maki for film thickness measurements.
Supporting Information Available: Experimental details, synthe-
ses and CVs of P1-P4, UV-vis spectra, X-ray reflectivity data, and
atmospheric photoelectron spectra of the LbL films, and SEM images
of TPD films on the modified electrodes. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
On the basis of the above-mentioned fundamental properties of
the polymer films, interface layers showing stepped electronic
profiles were fabricated at the anode surface. The film was initially
prepared by sequential deposition of each two-cycle process of P1
and P2. Figure S7A shows spectrophotometric changes for the LbL
films, revealing the successive increase of absorbance following
the deposition cycles. Of particular interest is that the functionalized
ITO electrode with this heterodeposited film exhibited a surface
work function of 5.37 eV, while that with the initial two-cycle film
of P1 showed a value of 5.14 eV (Figure S8A). It is clearly
suggested that the interface layer having a two-step energy level
was successfully constructed on the anode surface. As shown in
Figure 5, the TPD/Alq OLED device with this functionalized anode
exhibited appreciably reduced turn-on voltage and higher luminous
(1) Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L. J. Phys. Chem. B 2005,
109, 3966 and references therein.
(2) (a) Huang, Q.; Evmenenko, G.; Dutta, P.; Marks, T. J. J. Am. Chem. Soc.
2003, 125, 14704. (b) Hatton, R. A.; Willis, M. R.; Chesters, M. A.; Briggs,
D. J. Mater. Chem. 2003, 13, 722. (c) Lee, J.; Jung, B.-J.; Lee, J.-I.; Chu,
H. Y.; Do, L.-M.; Shim, H.-K. J. Mater. Chem. 2002, 12, 3494.
(3) For a polymer LED study based on the similar concept, see: Ho, P. K.
H.; Kim, J.-S.; Burroughes, J. H.; Becker, H.; Li, S. F. Y.; Brown, T. M.;
Cacialli, F.; Friend, R. H. Nature 2000, 404, 481.
(4) (a) Decher, G. Science 1997, 277, 1232. (b) Hammond, P. T. Curr. Opin.
Colloid Interface Sci. 2000, 4, 430 and references therein.
(5) Kato, S.; Pac, C. J. Phys. Chem. B 2004, 108, 19932.
(6) Cui, J.; Huang, Q.; Veinot, J. G. C.; Yan, H.; Wang, Q.; Hutchison, G.
R.; Richter, A. G.; Evmenenko, G.; Dutta, P.; Marks, T. J. Langmuir 2002,
18, 9958.
(7) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K.
Macromolecules 1998, 31, 8893.
JA052170N
9
J. AM. CHEM. SOC. VOL. 127, NO. 33, 2005 11539