Photonic crystal slabs in flexible
organic light-emitting diodes
Arfat Pradana and Martina Gerken*
Institute of Electrical and Information Engineering, Christian-Albrechts-Universität zu Kiel,
Kaiserstr. 2, D-24143 Kiel, Germany
*Corresponding author: mge@tf.uni‑kiel.de
Received August 6, 2014; revised November 12, 2014; accepted November 25, 2014;
posted January 7, 2015 (Doc. ID 220337); published March 17, 2015
Photonic crystal slabs integrated into organic light-emitting diodes (OLEDs) allow for the extraction of waveguide
modes and thus an increase in OLED efficiency. We fabricated linear Bragg gratings with a 460-nm period on
flexible polycarbonate substrates using UV nanoimprint lithography. A hybrid organic–inorganic nanoimprint
resist is used that serves also as a high refractive index layer. OLEDs composed of a poly(3,4-ethylenedioxythio-
phene) polystyrene sulfonate (PEDOT:PSS) polymer anode, an organic emission layer [poly(p-phenylene vinyl-
ene) (PPV)-derivative “Super Yellow”], and a metal cathode (LiF/Al) are deposited onto the flexible grating
substrates. The effects of photonic crystal slab deformation in a flexible OLED are studied in theory and experi-
ment. The substrate deformation is modeled using the finite-element method. The influence of the change in the
grating period and the waveguide thickness under bending are investigated. The change in the grating period is
found to be the dominant effect. At an emission angle of 20° a change in the resonance wavelength of 1.2% is
predicted for a strain of 1.3% perpendicular to the grating grooves. This value is verified experimentally by ana-
lyzing electroluminescence and photoluminescence properties of the fabricated grating OLEDs. © 2015 Chinese
Laser Press
OCIS codes: (050.2770) Gratings; (160.5298) Photonic crystals; (230.3670) Light-emitting diodes; (160.4890)
Organic materials; (160.5470) Polymers; (250.5230) Photoluminescence.
http://dx.doi.org/10.1364/PRJ.3.000032
1. INTRODUCTION
Organic light-emitting diode (OLED) technology has pro-
gressed rapidly over the last decade with OLED flat panel dis-
plays already available on the market [
1]. As OLEDs may be
processed on a variety of substrates they are particularly
promising for flexible light-emitting devices that may be
rolled up or applied to shaped elements. Flexible top-emitting
OLEDs on steel foil have been fabricated [
2]. Flexible indium
tin oxide (ITO) anodes [
3], flexible OLED operation lifetime
[
4], contrast [5,6], and performance under bending conditions
[
7] have been studied. Light emission from polymer light-emit-
ting electrochemical cells was demonstrated for strains as
large as 120% [
8]. In the context of enhanced light extraction
efficiency, antireflection nanopillars in flexible OLEDs have
been investigated [
9]. Periodic nanostructures have been em-
ployed for extraction of OLED waveguide modes in standard
rigid OLEDs [
1,10,11]. They are a means for increasing OLED
efficiency as well as for tailoring the angular and spectral
emission characteristics. We demonstrated an ITO-free OLED
design on a nanostructured substrate employing a polymer
anode [
12]. We also reported on a composite TiO
2
nanopar-
ticle–polymer nanoimprint resist suitable for nanoimprint
lithography [
13,14]. We showed waveguide mode extraction
for ITO-free OLEDs fabricated on this composite resist
material imprinted with a periodic nanostructure [
13].
Furthermore, we demonstrated flexible ITO-free OLEDs on
polycarbonate substrates with an integrated periodic nano-
structure employing the composite TiO
2
nanoparticle–
polymer nanoimprint resist [
14]. Here, we investigate the
effect of the photonic crystal slab deformation in flexible
OLEDs in theory and experiment. Figure
1(a) shows a device
schematic. Each substrate has four OLEDs on it. Controlled
bending of the substrate is achieved using adjustable screws
as seen in Fig.
1(b). The effect of device bending is measured
in electroluminescence (EL) and photoluminescence (PL) ex-
periments and compared to theoretical predictions.
This paper is structured as follows. Section
2 presents the
theory of waveguide mode extraction for deformed photonic
crystal slabs on bent substrates. Both the elongation of the
grating period and the reduction of the OLED waveguide stack
thickness are considered. The theoretically expected reso-
nance emission angle is calculated for an emission wavelength
of 550 nm. Section
3 describes the fabrication of the nano-
structured flexible OLEDs. Section
4 demonstrates EL of
the nanostructured flexible OLEDs and compares the I–V
curves with and without bending. In Section
5 PL experiments
are presented demonstrating the change in the resonance
emission wavelength with deformation. Conclusions are
drawn in Section
6.
2. THEORY OF PHOTONIC CRYSTAL SLAB
DEFORMATION
The average refractive index of the OLED layer stack is higher
than the refractive index of the substrate. Thus, part of the
light is emitted into waveguide modes in the OLED layer stack
[
1]. This light is not emitted to the outside and reduces OLED
efficiency. The integration of periodic grating structures
(photonic crystal slabs) allows for extracting these waveguide
32 Photon. Res. / Vol. 3, No. 2 / April 2015 A. Pradana and M. Gerken
2327-9125/15/020032-06 © 2015 Chinese Laser Press