Improved hole injection in organic light emitting
devices by gold nanoparticles†
P. Justin Jesuraj and K. Jeganathan
*
We report the role of surface coverage of gold nanoparticles (AuNPs) at the interface of anode/hole
transport layer (HTL) in organic light emitting devices (OLEDs). AuNPs with an optimized coverage of
2.10% at the ITO/a-NPD interface increase the hole injection into the HTL which results in three orders
of magnitude enhancement in the current density in OLEDs. At low surface coverage, injected holes are
trapped at the interface. The zero field mobility resulting from the Poole–Frenkel equation is responsible
for the improved hole injection into the HTL layer. The turn-on voltage of OLEDs was efficaciously
lowered to 3 V with substantial increment in luminescence. The local field generated by AuNPs under
electrical biasing with sufficient coverage is ascribed to the improved characteristics in OLEDs.
1. Introduction
Organic light emitting devices (OLEDs) are fascinating
domain for energy efficient solid state lighting and at panel
display industries. The direction for achieving efficient
organic lighting has begun since the work pioneered by Tang
and Van Slyke.
1
The efficient charge transport at the interface
of anode/hole transport layer plays a crucial role in deter-
mining the OLED performance. The insertion of a buffer
layer namely hole injection layers (HIL) such as MoO
3
,
2
NiO,
3
CuPC,
4
graphene oxide
5,6
and a series of organic
compounds
7,8
at the interface improves the device efficiency
by reducing the injection barrier between the work function
of the anode to the highest occupied molecular orbital
(HOMO) of HTL.
9
Recently, metal nanoparticles have also
been employed at the anode/HTL interface to enhance the
device efficiency.
10
In general, metal nanoparticles are
utilized in OLEDs for plasmonic enhancement applications,
where the surface plasmons evolved at nanoparticle
boundary offers an effective energy transfer channel to an
excited organic molecule by increasing the emission rate of
photons without altering the charge transport characteristics
of the device.
11,12
Plasmonic enhancement would be effective
if the distance between nanoparticle and emissive dipole is
equal or less than 10 nm.
13
Further, it has been reported that
the presence of metal nanoparticles at the anode/HTL inter-
face increases the device performance,
14
where the thickness
of the HTL alone is more than 20 nm. Thus, the presence of
plasmonic nanoparticles on the anode may acts as hole
injection sites to offer better hole transport into the organic
frontier orbitals.
Surfactant functionalized various gold nanostructures have
already been utilized in solar cells,
15
organic memory devices
16
and OLEDs to achieve better performances in the devices.
However, the effects of nanoparticles density or coverage of
nanoparticles on anode has not been investigated towards the
enhancement of hole transport and luminescence in OLEDs. In
this report, we have extensively investigated the effect of surface
coverage of AuNPs at ITO/a-NPD interface for efficient hole
transport in both hole only devices (HODs) and OLEDs.
2. Experimental methods
2.1 Preparation of gold nanoparticles on anode
Chemically derived gold nanoparticles (AuNPs) have been
synthesized by reducing the gold(III) chloride trihydrate
(HAuCl
4
$xH
2
0) (0.25 mM) through tri-sodium citrate (1 wt%)
using Fren's method.
17
The surface plasmon absorption of gold
nanoparticles solution has been investigated using UV-visible
absorption spectroscopy. Atomic force microscopy (AFM) has
been employed to evaluate the size of AuNPs dispensed on
quartz substrate. To achieve distinct coverage of AuNPs, the as-
prepared AuNPs were washed with ultracentrifugation to
remove the un-reacted surfactant and diluted to vary the
concentration of nanoparticles in solution. Solutions with
various concentrations of AuNPs have been spin coated onto
pre-cleaned and oxygen plasma treated indium tin oxide (ITO)
anodes for subsequent device fabrication. The coverage
percentages of AuNPs on ITO were studied using AFM in non-
contact mode (Agilent 5500 model), eld emission scanning
electron microscopy (FESEM) (Carl Zeiss – Sigma model) and X-
ray photoelectron spectroscopy (XPS) (SpecsLab2 version-2.76-
r26050 – photon energy – Al K
a
1486.74 eV) techniques.
Centre for Nanoscience and Nanotechnology, School of Physics, Bharathidasan
University, Tiruchirappalli-620 024, India. E-mail: kjeganathan@yahoo.com; Fax:
+91-431-2497 045/+91-431-2407-020; Tel: +91-431-2407-057
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra09028d
Cite this: RSC Adv. , 2015, 5, 684
Received 21st August 2014
Accepted 21st November 2014
DOI: 10.1039/c4ra09028d
www.rsc.org/advances
684 | RSC Adv. , 2015, 5, 684–689 This journal is © The Royal Society of Chemistry 2015
RSC Advances
PAPER