Bendability optimization of flexible optical nanoelectronics via neutral axis engineering
Sangmin Lee
1
Jang-Yeon Kwon
2
Daesung Yoon
0
Handong Cho
1
Jinho You
0
Yong Tae Kang
0
Dukhyun Choi
0
Woonbong Hwang
1
0
Department of Mechanical Engineering, College of Engineering, Kyung Hee University
, 1 Seocheon-dong, Giheung-gu, Yongin-si 446-701,
Republic of Korea
1
Department of Mechanical Engineering, Pohang University of Science and Technology
, San 31, Hyoja, Namgu, Pohang, Gyungbuk 790-784,
Republic of Korea
2
School of Integrated Technology, Yonsei University
, 162-1, Songdo- dong Yeonsu-gu, Incheon 406-840,
Republic of Korea
The enhancement of bendability of flexible nanoelectronics is critically important to realize future portable and wearable nanoelectronics for personal and military purposes. Because there is an enormous variety of materials and structures that are used for flexible nanoelectronic devices, a governing design rule for optimizing the bendability of these nanodevices is required. In this article, we suggest a design rule to optimize the bendability of flexible nanoelectronics through neutral axis (NA) engineering. In flexible optical nanoelectronics, transparent electrodes such as indium tin oxide (ITO) are usually the most fragile under an external load because of their brittleness. Therefore, we representatively focus on the bendability of ITO which has been widely used as transparent electrodes, and the NA is controlled by employing a buffer layer on the ITO layer. First, we independently investigate the effect of the thickness and elastic modulus of a buffer layer on the bendability of an ITO film. Then, we develop a design rule for the bendability optimization of flexible optical nanoelectronics. Because NA is determined by considering both the thickness and elastic modulus of a buffer layer, the design rule is conceived to be applicable regardless of the material and thickness that are used for the buffer layer. Finally, our design rule is applied to optimize the bendability of an organic solar cell, which allows the bending radius to reach about 1 mm. Our design rule is thus expected to provide a great strategy to enhance the bending performance of a variety of flexible nanoelectronics.
-
Background
There has been rapid development in the field of flexible
optical nanoelectronics such as organic solar cells (OSCs)
and organic light-emitting diodes for future portable and
wearable electronic nanodevices, which have potential
personal and military applications [1-10]. These optical
nanodevices basically require an optically transparent
window to absorb or emit light. Indium tin oxide (ITO) thin
films have been widely used as transparent electrodes for
such optical nanoelectronics because of their high visible
transparency, chemical stability, and excellent adhesion to
a substrate [11-13]. However, despite its advantages, ITO
is still difficult to apply to flexible optical nanodevices
without damaging the electronic functionality under an
external bending load because of its brittleness.
Researchers are thus trying to find substitutes for ITO such as
carbon nanotube, graphene, and aluminum-doped zinc oxide
(AZO) [10,12-14]. However, with these alternatives, it is
still difficult not only to successfully achieve a high-quality
and low-cost production that is as good as ITO with high
transparency (higher than 90 %) and low electric
resistance (less than 10 ), but also to successfully increase
bendability due to their brittleness which is common with
ITO. Thus, it is critically necessary to develop innovative
ideas and solutions to enhance mechanical stability of
ITO under external bending loads.
To improve the bendability of flexible nanoelectronics, a
buffer layer has been adopted [3,6,9,15]. Researchers have
reported that the mechanical bendability of electronic
nanodevices can be increased by using a buffer layer above
or below the ITO layer. However, they did not suggest an
optimized design rule that considers both the thickness
and elastic modulus of the buffer material. Because
various buffer layers could be used to increase the thermal,
chemical, and mechanical stabilities of flexible electronic
nanodevices, a governing design rule is crucially needed to
optimize the bendability of these flexible nanodevices
regardless of the buffer layers that are chosen. In this article,
we report a design rule for the bendability optimization of
flexible optical nanoelectronics through controlling the
neutral axis (NA). If we place the fragile layer such as ITO
in a nanodevice at the NA position, the bending stress and
strain in the layer are greatly reduced, thus enhancing the
bendability of the device. Therefore, we first investigate
the behavior of the NA position and the effect on the
device bendability by independently considering the elastic
modulus and thickness of a buffer layer on the ITO.
Because the elastic modulus and thickness of a buffer layer
influenced each other when determining the NA, we
should consider these parameters together. Therefore, we
develop a de (...truncated)