Page 68 - Read Online

Shin et al. Soft Sci 2024;4:22 https://dx.doi.org/10.20517/ss.2024.03 Page 7 of 13

due to its construction from inherently soft materials, a carefully designed structure that evenly distributes
strain, and minimized stress concentrations, ensuring intact layer adhesion and consistent electrical
performance. These features make the fully soft Schottky diodes suitable for future wearable electronics,
which are designed to withstand various forms of mechanical deformation.

Fully soft bridge rectifier
The primary function of a diode as a rectifier is converting alternating current (AC) input into DC output.
This function is essential in power supply units, battery chargers, transformers, and numerous consumer
[32]
electronics . Considering these requirements, we have developed a fully soft bridge rectifier for full-wave
rectification using four fully soft Schottky diodes interfaced by PEDOT:PSS-based soft electrodes [Figure 3A
and B]. Figure 3C shows the circuit diagram of this rectifier. Its rectification characteristics, under an input
voltage (V ) of ±10 V at a frequency of 500 Hz, show a reliably rectified output voltage (V ) [Figure 3D].
out
in
The voltage drop in the fully soft bridge rectifier is primarily attributed to the forward voltage of the diodes,
[33]
which occurs when two diodes conduct simultaneously . Additional voltage drops are caused by the
internal resistance of the diodes and by higher load currents . Figure 3E and F plots the V of the devices
[34]
out
against varying V amplitudes and frequencies, demonstrating the reliable performance of the rectifier
in
across a wide range of V . Detailed characteristics are provided in Supplementary Figure 7.
in
The fully soft bridge rectifier was further characterized under various tensile strains to evaluate its
mechanical stability. Figure 3G shows the device under mechanical strain, exhibiting excellent deformability
without physical damage. Figure 3H presents the rectification results at ± 10 V input and 500 Hz frequency
under mechanical strains of 0%, 30%, and 0% (released). The V values were maintained without significant
out
degradation, highlighting the robustness of the device under mechanical strain. Figure 3I and
Supplementary Figure 8 show V corresponding to different input frequencies at a constant V of ±10 V
out
in
under 0, 30, and 0% (released) mechanical strains. These results confirm that the fully soft bridge rectifier
operates effectively under various electrical conditions and can withstand mechanical strain up to 30%,
making it suitable for diverse applications in wearable and skin-interfaced electronics.

Fully soft OR and AND logic gates
The logic gates are crucial components in various electronic circuit systems, providing Boolean functions
essential for digital systems [35,36] . Thus, we further demonstrated the functionality of fully soft logic gates,
specifically OR and AND gates, based on fully soft Schottky diodes. The functional verification of the logic
gates was conducted by applying biases of 0 and 3 V to the input voltages (V and V ) of the diodes,
in, B
in, A
representing logic input states of 0 and 1, respectively. Figure 4A and B illustrates the schematic and circuit
diagram of the fully soft OR gate, respectively. Notably, the device exhibits exceptional mechanical
endurance [Figure 4C]. The electrical characteristics of the fully soft OR gate were evaluated under
mechanical strains of 0, 30, and 0% upon release [Figure 4D-F]. These results confirmed that a logic output
state of 0 was consistently observed when both V and V were set to a logic input state of 0. In contrast,
in, A
in, B
applying a logic input state of 1 to either one or both diodes resulted in a logic output state of 1, regardless
of mechanical strain. Additionally, Figure 4G and H presents the schematic illustration and circuit diagram
of the fully soft AND gate. The optical images of the devices, both with and without mechanical strains
[Figure 4I], highlight their soft and deformable nature. The electrical operation of the AND gate is
presented in Figure 4J, demonstrating that the logic output state is 1 only when both logic input states are 1.
This device maintained its logic gate functionality even under a 30% mechanical strain and after the strain
was released [Figure 4K and L]. The results from these fully soft logic gates, including their mechanical
stability in standard logic operations, are summarized in truth tables provided in Supplementary Figure 9.
These comprehensive analyses underscore the robust and reliable functionality of these fully soft logic gates,
even under variable mechanical stresses.