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Jeon et al. Soft Sci. 2025, 5, 1 https://dx.doi.org/10.20517/ss.2024.35 Page 21 of 39

While each technique offers distinct advantages, they also have drawbacks. PLD requires high temperatures
and long processing times, while ALD has a slow deposition rate and is limited in scalability for large areas.
Spin-coating suffers from material wastage and difficulties in achieving uniform coatings on large or
curvature substrates. Spray-coating faces issues with high film roughness and long processing times. Inkjet
printing, though precise, has lower throughput and limited ink choices due to viscosity and clogging
concerns. Despite these challenges, each of these deposition methods offers unique advantages and specific
applications, playing a crucial role in fabricating high-performance flexible MO TFTs and enabling the
development of advanced flexible electronic devices with improved electrical properties and mechanical
flexibility. Table 6 is a comprehensive overview of deposition techniques, the materials deposited, and the
electrical properties of materials.

DEVICE STRUCTURE
Flexible MO TFTs affect repeated bending stress. Repeated bending stress in MO TFTs creates oxygen
vacancies, generating carriers and causing a negative V shift, increased saturation mobility, and higher
TH
drain current. This occurs because strain-induced atomic distance expansion reduces the energy band gap
(ΔE) between bonding and antibonding orbitals, leading to more electrons in the antibonding state. It
increases electron density in the conduction band and MO conductivity which affect V negatively [191,192] .
TH
Strain-induced oxygen vacancies also increase electron density, further contributing to the negative V
TH
shift. Therefore, extensive research is being conducted on structures that are less affected by mechanical
stress to reduce these negative impacts. The structure of flexible MO TFTs can be varied, generally falling
into three categories: island structure TFTs, junctionless TFTs, and electrode and channel layer architecture.

Island structure TFTs, as the name suggests, are fabricated on substrates featuring either an island structure
or a flat substrate with islands created. In this architecture, the MO TFTs are fabricated on isolated islands
in the substrate as presented in Figure 7. This structure offers several advantages, including reduced leakage
current and enhanced device reliability. Island structure TFTs are commonly implemented in flexible
displays, where high performance and uniformity are essential.

Junctionless TFTs are characterized by the absence of a boundary between the source-to-drain electrode
and the channel layer. Unlike traditional TFTs, which form source-to-drain and channel junctions,
junctionless TFTs utilize a single semiconductor material for both the channel and the electrodes. This
simplifies the fabrication process and reduces manufacturing costs. In junctionless TFTs, there is no overlap
region between the source/drain (S/D) and gate electrodes. Consequently, parasitic capacitances are not
present, making them suitable for high-speed and low-power applications such as integrated circuits and
sensors.

Flexible TFTs employing electrode and channel layer architecture release flexible stress by distributing
mechanical stress within the devices. The electrode and channel layer architecture serves to distribute
mechanical stress, preventing the deformation or failure of the active semiconductor layer during bending
or stretching. By mitigating mechanical stress, TFTs utilizing an electrode architecture exhibit enhanced
mechanical reliability and durability. Island structure TFTs offer improved performance and reliability but
may require more complex fabrication processes. Junctionless TFTs simplify device fabrication and reduce
costs but may exhibit lower carrier mobility compared to traditional TFTs. TFTs utilizing electrode
architecture enhance mechanical reliability but may introduce additional processing steps and electrode
conductivity issues depending on the electrode architecture. In this part, the configurations, examples of
implementation, advantages, limitations, and potential uses of each structure of flexible MO TFTs are
reviewed and discussed in detail.