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Page 2 of 39 Jeon et al. Soft Sci. 2025, 5, 1 https://dx.doi.org/10.20517/ss.2024.35

display and integrated circuit applications for next-generation flexible and stretchable electronics are introduced
and discussed. Finally, the review concludes with an outlook on the potential achievements and prospects of MO
TFTs in the development of next-generation flexible display technologies.

Keywords: Flexible electronics, metal oxide, thin-film transistors, high-k dielectric, wearable devices, flexible
devices

INTRODUCTION
Thin-film transistors (TFTs) are three-terminal semiconductor devices with a thin-film structure that act as
switches in electronic circuits, controlling the flow of current and facilitating the operation of various
electronic components. TFTs are currently the dominant technology for in-pixel switches and drivers in
flat-panel displays, performing a critical function in active matrix (AM) displays, such as liquid crystal
displays (LCDs) and active-matrix organic light-emitting diodes (AMOLEDs) . Advances in TFT
[1-4]
technology have aimed to reduce the carbon footprint while overcoming the limitations of conventional
silicon-based transistors, such as limited form factor, difficulty in large-area processing, and high
manufacturing costs . As a result, TFTs have attracted significant interest for use in foldable, flexible and
[5-8]
stretchable displays and other electronic products, potentially replacing traditional silicon complementary-
metal-oxide-semiconductor (CMOS) circuits. TFTs are developed using various materials, including
amorphous silicon (a-Si), polycrystalline silicon (poly-Si), metal oxides (MOs), and organic
semiconductors [1,5,9,10] . Among these, MO semiconductors are particularly promising due to their advantages
such as low cost, low process complexity and temperature, large area scalability, and higher carrier mobility
compared to a-Si and organic materials [11-14] . In addition, MO TFTs offer greater resistance to mechanical
stress than low-temperature polysilicon (LTPS) devices, making them ideal for next-generation flexible and
stretchable electronics [12,15-17] . These advantages of MO TFTs have facilitated their successful
commercialization in the display market, particularly through indium gallium zinc oxide (IGZO) TFT-
based backplanes, which have driven significant growth and innovation in high-resolution organic light-
emitting diode (OLED) panels since 2013 [10,18,19] . With their transparency, flexibility, and high electron
mobility, these MO TFTs have proven highly suitable for next-generation electronic devices, playing a
crucial role in advancing flexible and transparent displays.

Flexible displays offer characteristics such as thinness, lightweight design, and durability, which allow them
to be fabricated on curvilinear surfaces, enabling shape transformation. The history of flexible displays dates
back to early innovations such as Polymer Vision’s Readius in 2006, which used rollable e-paper, and
Nokia’s Morph concept in 2008, which envisioned a bendable and interactive mobile display. The
development of thin, flexible OLED displays further expanded these possibilities. During the 2013
Consumer Electronics Show, Samsung introduced its “Youm” concept, which showcased flexible display
prototypes, including a smartphone that folded outward into a tablet-sized display. The first commercial
application of this technology appeared in the Galaxy Note Edge, with its curved OLED screen. In 2018,
Royole released the first foldable smartphone, the Flexpai, followed by Samsung’s Galaxy Fold in 2019,
featuring an inward-folding Infinity Flex display. That same year, Huawei and Xiaomi also introduced their
folding smartphones, signaling the growing maturity of flexible display technology and its impact on the
smartphone market . This wave of foldable devices underscored the maturation of flexible display
[20]
technology and its potential for transforming the smartphone market. Since then, flexible displays,
particularly flexible AMOLEDs, have achieved commercial success, driven by the popularization of high-
resolution TVs, smartphones, and smart devices. Market research projected that shipments of flexible
AMOLED displays would reach 335.7 million units by 2020, accounting for 52.0% of total AMOLED panel
[21]
shipments, underscoring the growing market demand and technological advancements in this area .