Molecularly "Engineered" Anode Adsorbates for Probing OLED Interfacial Structure-Charge Injection/Luminance Relationships: Large, Structure-Dependent Effects

Article abstract of DOI:10.1021/ja037174b

Molecule-scale structure effects at organic light-emitting diodes (OLED) anode?organic transport layer interfaces are probed via a self-assembly approach. A series of ITO anode-linked silyltriarylamine molecules differing in aryl group and linker density are synthesized for this purpose and used to probe the relationship between nanoscale interfacial chemical structure, charge injection and electroluminescence properties. Dramatic variations in hole injection magnitude and OLED performance can be correlated with the molecular structures and electrochemically derived heterogeneous electron-transfer rates of such triarylamine fragments, placed precisely at the anode?hole transport layer interface. Very bright and efficient (～70000 cd/m² and ～2.5% forward external quantum efficiency) OLEDs have thereby been fabricated. Copyright

Full text of DOI:10.1021/ja037174b

Published on Web 11/08/2003
Molecularly “Engineered” Anode Adsorbates for Probing OLED Interfacial
Structure-Charge Injection/Luminance Relationships: Large,
Structure-Dependent Effects
Qinglan Huang, Guennadi Evmenenko, Pulak Dutta, and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity,
EVanston, Illinois 60208-3113
Received July 9, 2003; E-mail: t-marks@northwestern.edu
Table 1. Characteristics of Anode Functionalization Layersa
Research on organic solid-state electroluminescence has bur-
geoned with the advent of practical and near-practical polymer and
small-molecule organic light-emitting diodes (OLEDs).1 The syn-
thesis and implementation of new bulk charge transporting and
emissive materials has afforded much fundamental information and
has resulted in greatly enhanced quantum efficiency, brightness,
and stability. Nevertheless, nanoscale interfacial phenomena which
are inextricably connected with charge injection,2 energy level
matching,3 physical decohesion and delamination,4 anode corrosion
and ion injection,5 interface dipoles,6 image forces,7 and exciton
quenching8 are poorly understood, and their precise control is of
great importance if electroluminescent response and durability are
to be truly optimized. We report here the implementation of a series
of probe molecules having incrementally varied structures and
surface linking characteristics, designed to form conformal, robust,
self-assembled monolayers2a,9 on OLED ITO anodes. It is seen that
molecular structure effects on OLED charge injection and response
characteristics can be very large.
TAA-Si1
TAA-Si3
TPD-Si2
TPD-Si4
λ
max (nm)
303
1.2
0.4
304
1.4
0.7
352
1.8
0.7
352
1.6c
1.3
thickness (nm)b
RMS roughness (nm)b
aq contact angle (deg)
E
90
87
90
90
p,a/Ep,c (V)d
1.180/
0.798
4.5
1.200/
0.734
4.2
1.160/
0.815
2.5f
1.130/
0.799
2.1f
coverage Γ
(×10-10 mol/cm2)e
∆Ep,1/2 (mV)g
IP (eV)h
340
460
5.8
350
6.1
440
a
b
Experimental details in Supporting Information. From X-ray reflec-
tivity of samples identically deposited on oxide-coated (111)Si. This
parameter is uncertain due to surface roughness. From cyclic voltammetry
(10 V/s). Estimated by CV (0.1 V/s). CV coverage consistent with XRR
data assuming two-electron process. 0.1 V/s scan rate. ∆Ep,1/2 > 90.6/n
mV, indicating redox site interactions, site heterogeneity, or both. From
c
d
e
f
g
h
UPS.
The chlorosilane-tethered series shown below was synthesized
Figure 1. Effect of SAM structure on hole injection for hole-only devices
having structures ITO/SAM/N,N-naphathyl-N,N′-phenyl-biphenyl-4,4′-di-
amine (NPB, 400 nm)/Au/Al.
and purified under rigorously anhydrous and anaerobic conditions.10
Self-limited anaerobic chemisorption of these compounds onto
smooth (∼2.5 nm RMS roughness), plasma-cleaned ITO surfaces
was carried out by immersing ITO substrates in 1.0 mM toluene
solutions,10 followed by rinsing, drying and curing. Adsorbate
characterization included AFM, aqueous contact angles, optical
spectroscopy, cyclic voltammetry, XPS, UPS, and X-ray reflectivity
(XRR), revealing formation of conformal, largely pinhole-free self-
assembled monolayers (SAMs) with sub-nanometer thickness
control and essentially identical aggregate surface energies, ioniza-
tion potentials, and coverages (Table 1).
The effect of SAM structure on ITO-organic interfacial hole
injection was first investigated by fabricating hole-only devices11
(Figure 1). Since the only difference in the four types of devices is
in SAM molecular structure, the results clearly reveal a significant
structure sensitivity of hole injection across the nano interfacial
region. For example, hole current densities at 25 V are ∼0.0004
A/cm2 (TAA-Si3) < ∼0.004 A/cm2 (TAA-Si1) < ∼0.01 A/cm2
(TPD-Si2) < ∼0.04 A/cm2 (TPD-Si4); hole injection fluences vary
by 1 to 2 orders of magnitude. The current densities are somewhat
lower than those in OLEDs studied below, principally due to the
thicker HTL (hole transport layer) deposited in the hole-only
devices. OLEDs were next fabricated to examine SAM structure
effects on EL response, which are also significant. Bare ITO and
phenylsilane SAM-coated ITO-based devices were also fabricated
for comparison. In Al cathode OLEDs, luminances at 20 mA/cm2
(a standard current density for device evaluation) are 200 cd/m2
(TPD-Si4) < 230 cd/m2 (TPD-Si2) < 400 cd/m2 (TAA-Si1) < 570
cd/m2 (TAA-Si3) (Figure 2). This order of current efficiency is
opposite to that of the hole currentdensities measured above and
can be understood in terms of electron injection-limited electron-
hole recombination events.12 Appreciable forward external quantum
efficiency (ηext) variations evidence large, anode-organic interface
effects on OLED charge recombination, currently an imprecisely
understood process.13 Compared to bare ITO-based devices, SAM-
induced OLED performance enhancement is observed, with the
modest phenylsilane SAM improvement mainly attributable to
improved ITO anode-HTL contact via surface energy matching.14
Comparison between phenyl and triarylamine silane SAMs indicates
that the latter result in lower anode-HTL hole injection barriers,
agreeing with lower turn-on and operating voltages at identical
9
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J. AM. CHEM. SOC. 2003, 125, 14704-14705
10.1021/ja037174b CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
seen previously,22 while TPD-Si4 adopts both “flat” and “upright”
orientations, yielding a rough surface. This can be correlated with
the greater charge transport capacity due to the smaller NAr3-
ITO anode spacing. Finally, differing intermolecular interactions
between triarylamine cores likely arise from the differing molecular
shapes and linker densities and should also affect interfacial charge
injection and transport.16,17
In conclusion, we present evidence for significant OLED anode-
organic interfacial molecular structure effects on hole injection and
EL properties and show that these correlate with heterogeneous
electron-transfer characteristics. Chemically tuning the interface
structure represents an effective approach to studying nanoscale
injection layers and yields OLEDs with high brightness (∼70 000
cd/m2), low turn-on voltages (∼4 V), and high current efficiencies
(∼8 cd/A).
Figure 2. Responses of OLEDs having structures ITO/(SAM)/NPB/tris-
(8-hydroxyquinolato)aluminum (AlQ): 1% diisoamylquinacridone (DIQA)/
Al. (A) Current density vs voltage. (B) Luminance vs voltage. (C) ηext vs
voltage.
Acknowledgment. We thank USDC and NSF through the
Northwestern MRSEC (DMR-0076097) for support. We thank Prof.
N. Armstrong and Dr. P. Lee (University of Arizona) for UPS
measurements, and Dr. B. Scott and Mr. J. Li for helpful
discussions.
Figure 3. Responses of OLEDs having structures ITO/(SAM)/NPB/AlQ:
1% DIQA/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Li/AgMg.
(A) Current density vs voltage. (B) Luminance vs voltage. (C) ηext vs
voltage.
Supporting Information Available: Synthesis of silane reagents,
SAM characterization, and device fabrication (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
luminance. In a second device configuration with enhanced electron
injection and a hole-blocking layer (Figure 3),15 the hole-electron
density imbalance is substantially alleviated, and more efficient
recombination is expected. This is indeed observed, with the
maximum luminance and ηext achieved by TPD-Si4-based OLEDs
(∼70 000 cd/m2 and 2.1%, respectively) nearly 1 order of magnitude
and 5 times greater, respectively, than in Figure 2. Note again the
poorer response of the phenylsilane-based device. Strong SAM
structure-OLED response correlations are again observed, with
the quantum efficiency ordering reflecting better recombination
balance for the superior hole injection SAMs. The light output of
the TPD-Si4-based OLED is ∼1.5 to 3 times brighter than that of
TPD-Si2 (∼50 000 cd/m2), TAA-Si1 (∼45 000 cd/m2), and TAA-
Si3 (∼23 000 cd/m2) at identical bias.
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