Enhanced light extraction efficiency in organic light-emitting diode with randomly dispersed nanopattern

Abstract

An optical scattering layer composed of randomly dispersed nanopatterns (RDNPs) was introduced in an organic light-emitting diode (OLED) to increase the out-coupling efficiency. An RDNP was fabricated by direct printing on a glass substrate. Owing to its low haze and high transmittance, the RDNP acted as a light extraction layer in the OLED. The RDNP OLEDs showed higher current density and luminance than the reference devices at the same voltage. The current and power efficiencies of the RDNP OLED increased by 25% and 34%, respectively, without electrical degradation. Furthermore, the RDNP devices achieved an external quantum efficiency of 27.5% at 1 mA / cm 2 .

Full text

Enhanced light extraction efficiency in organic
light-emitting diode with randomly dispersed
nanopattern
YANG DOO KIM,1,†KYUNG-HOON HAN,2,†YOUNG HOON SUNG,1JUNG-BUM KIM,2HAK-JONG CHOI,1
HEON LEE,1,3 AND JANG-JOO KIM2,4
1Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul 136-713, South Korea
2Department of Materials Science and Engineering and the Center for Organic Light Emitting Diodes, Seoul National University,
599 Gwanangno, Gwanak-gu, Seoul 151-744, South Korea
3e-mail: heonlee@korea.ac.kr
4e-mail: jjkim@snu.ac.kr
Received 6 August 2015; revised 1 September 2015; accepted 1 September 2015; posted 16 November 2015 (Doc. ID 246919);
published 14 December 2015
An optical scattering layer composed of randomly dispersed
nanopatterns (RDNPs) was introduced in an organic light-
emitting diode (OLED) to increase the out-coupling effi-
ciency. An RDNP was fabricated by direct printing on a glass
substrate. Owing to its low haze and high transmittance, the
RDNP acted as a light extraction layer in the OLED. The
RDNP OLEDs showed higher current density and luminance
than the reference devices at the same voltage. The current
and power efficiencies of the RDNP OLED increased by
25% and 34%, respectively, without electrical degradation.
Furthermore, the RDNP devices achieved an external quan-
tum efficiency of 27.5% at 1 mA∕cm2.© 2015 Optical
Society of America
OCIS codes: (230.0250) Optoelectronics; (290.0290) Scattering;
(300.2140) Emission.
http://dx.doi.org/10.1364/OL.40.005838
Owing to the fast response, high color quality, applicability
to flexible substrates, and potentially low price, organic light-
emitting diodes (OLEDs) are expected to find application in
displays and solid state lighting sources. However, the out-cou-
pling efficiency of OLEDs is still below 30% [1–3] because of
the waveguide effect at the glass/ITO and air/glass interfaces,
surface plasmon polariton (SPP) loss at the organic/metal inter-
face, and internal absorption of the materials. The light extrac-
tion efficiency still needs to be improved for OLEDs to be
competitive in the industry. Therefore, various light extraction
methods have been developed; according to the position of the
light extraction structure, they can be classified as external light
extraction methods for substrate-based techniques, and internal
light extraction methods for techniques based on waveguides
and SPP modes. External extraction methods entail microlens
arrays [4,5], roughened surfaces [6,7], luminaires [8], etc., and
generate a blurring effect on the display panel because of the
presence of thick substrates [9]. Internal extraction methods
involve the use of low index layers [10], photonic crystals [11],
high refractive index substrates [12,13], low index grids [14],
Bragg gratings [15–17], randomly dispersed nanopillar arrays
[18,19], nanoparticles with thin electrodes [20], and moth-eye
structures [21]; however, they remain inadequate because
of largely confined lights, material reliability, availability for
commercial displays, etc.
In addition to the light extraction efficiency, image quality is
an important display parameter. The pixel blur and radiation
pattern must be considered when selecting the light extraction
method. To achieve a uniform radiation pattern and clear pixels,
randomly distributed nanostructures with low haze have been
used as the light extraction layer [15–19]. Among the various
methods, the use of a Bragg grating, which provides the SPP
modes with additional momentum, is convenient to extract the
SPP modes. However, the periodicity of the Bragg grating in-
volves a trade-off relation with the extraction efficiency and a
spectral shift with the incidence angle. In this Letter, a fully ran-
domized structure was used as a light extraction structure which
is fulfilling a phase matching condition [22]:
βksin θΔkx:(1)
We expect that the fully randomized structure will have a
slightly lower extraction efficiency than the periodic pattern’s,
but will extract without spectral shift. The surface protrusion
pattern of a fluorine-doped tin oxide (FTO) thin film grown by
chemical vapor deposition was utilized as the light extraction
structure [23]. This random structure was expected to extract
the SPP modes to some extent and have the same spectral dis-
tribution at different viewing angles.
Furthermore, in this Letter, the surface pattern of FTO,
which consists of randomly dispersed nanorods, was replaced
with a hydrogen silsesquioxane (HSQ) replica pattern because
of the absorption and refractive index of FTO. HSQ has a
very low extinction coefficient at visible wavelengths and a
similar refractive index to glass, which provides a proper light
5838 Vol. 40, No. 24 / December 15 2015 / Optics Letters Letter
0146-9592/15/245838-04$15/0$15.00 © 2015 Optical Society of America