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Page 2 of 13 Shin et al. Soft Sci 2024;4:22 https://dx.doi.org/10.20517/ss.2024.03

INTRODUCTION
The field of skin-interfaced electronics has gained considerable attention in recent years, recognized for its
[1-5]
potential to revolutionize society and daily life . These devices offer groundbreaking applications,
including health monitoring , enhanced user experiences [10-12] , and human-machine interfaces [13-15] . Most
[6-9]
notably, the development of soft diodes is pivotal in advancing these technologies, owing to their crucial
role in rectification, signal clipping, switching, and various other functions within electronic systems [16,17] .
Particularly, soft diodes made entirely of soft electronic materials, also known as fully soft diodes, offer
significant advantages for seamlessly integrating electronics with the soft human body [18-20] , clearly
surpassing traditional diodes whose brittle and rigid nature results in a mechanical mismatch with soft
tissues. These advantages are especially notable when compared to devices that employ structural
engineering with non-soft elements, particularly in terms of simplifying the complexity of the device
fabrication processes such as deposition, lithography, and annealing.

Although there have been efforts to develop flexible diodes for wearable electronics [19,20] , these endeavors
have encountered significant challenges. Primarily, these challenges stem from the complex nature of the
manufacturing process and the inherent non-softness of materials, which fail to meet the specific
requirements of skin-interfaced electronic devices. To address these issues, significant advancements have
been reported, such as fabricating intrinsically stretchable polymer diodes [19,21] and developing fully rubbery
Schottky diodes . However, fabricating these devices involves either multiple layers, leading to
[22]
cumbersome manufacturing processes, or the laborious task of embedding silver nanowires into elastomers.
These complexities ultimately limit their scalability and cost-efficiency. Hence, there is an urgent need to
streamline the fabrication of fully soft diodes. Simplifying this process would not only increase their
availability but also enhance their effectiveness and usefulness in applications involving skin-interfaced
electronics.

Here, we have successfully developed a fully soft Schottky diode composed entirely of soft electronic
materials. This diode incorporates a eutectic liquid metal alloy, specifically gallium-indium, as the cathode, a
poly(3-hexylthiophene) nanofibril (P3HT-NF) and elastomer-based composite as the p-type soft
semiconductor, and soft poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the
anode. The strategic selection of these materials was based on their respective energy levels, which are
crucial for forming appropriate contacts. Specifically, a Schottky contact is formed at the interface between
the cathode and the semiconductor, and an Ohmic contact is established at the interface between the anode
and the semiconductor, thereby enabling the functional characteristics of diodes. Furthermore, all
components can be formed through a solution process, which allows a facile fabrication process of the
diode, thereby enhancing scalability and cost-efficiency. The fabricated fully soft Schottky diodes exhibit
impressive electrical characteristics, demonstrating a forward current density of 2.01 × 10 A/cm at 3 V and
3
2
a notable rectification ratio (RR) of 6.15 × 10 at ± 3 V. The devices retain their electrical performance even
4
when subjected to mechanical strains of up to 30%. Moreover, we have progressed in developing fully soft
rectifiers and logic gates using these fully soft Schottky diodes. Impressively, these components showcase
sufficient electrical characteristics even under a tensile strain of 30%. Expanding on this technology, we have
further demonstrated a skin-interfaced energy harvesting system, achieved by integrating a piezoelectric
nanogenerator (PENG) with a fully soft diode-based full-wave bridge rectifier. This system reliably offers a
self-powering process for skin-interfaced electronics that require direct current (DC) power. The
development of a fully soft diode, enabled by a simple process and device structure, along with integrated
circuits, has yielded encouraging results. This advancement potentially facilitates the future progression of
fully soft, skin-interfaced electronic systems.