Page 65 - Read Online

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

of the PENG was measured using a Tektronix TBS 2000B digital oscilloscope.

RESULTS AND DISCUSSION
Fully soft Schottky diode and soft electrode
Figure 1A shows a skin-interfaced 5 × 5 array of fully soft Schottky diodes, consisting of EGaIn, P3HT-NFs/
PDMS, PEDOT:PSS, and SEBS, serving as the cathode, semiconductor, anode, and substrate, respectively.
Since the device is constructed entirely from soft components, it can be conformally mounted on the
human body. Furthermore [Figure 1B], it exhibits excellent mechanical durability under various
deformations. Besides the fully soft nature of the diode, a key advantage of our devices is their simple
fabrication, based entirely on solution processes such as spin-casting and doctor blading, as schematically
illustrated in Figure 1C. This approach offers significant potential for enhanced scalability and low-cost
manufacturing. To achieve a sufficiently low surface energy of PEDOT:PSS for uniform electrode formation
on the hydrophobic SEBS surface, a surfactant, tergitol [Supplementary Figure 2A], was introduced in
PEDOT:PSS [27,28] . As the weight ratio of tergitol increases, the contact angle of the PEDOT:PSS solution
decreases [Figure 1D and Supplementary Figure 2B]. This demonstrates the reduction in surface energy of
the solution due to the addition of tergitol. Supplementary Figure 3 clearly shows the excellent coating
quality of the PEDOT:PSS film achieved with this addition. Furthermore, PEG was selected as a plasticizer
for the PEDOT:PSS-based soft electrode, which is utilized as the cathode in the fully soft diode. PEDOT:PSS
has high tensile strength which results in low elongation at break, but adding the plasticizer lowers the
tensile strength and allows higher steretchability [29,30] . Indeed, PEDOT:PSS without PEG demonstrates a
dramatic increase in resistance under mechanical strain. In contrast, the PEG addition enables stable
resistance under such strain, particularly when the added amount exceeds 8 w/w. This is evidenced by the
normalized resistance change in response to mechanical strain [Figure 1E]. While such severe resistance
change results from the formation of microcracks in the PEDOT:PSS film without PEG, the film containing
PEG shows no obvious physical damage [Figure 1F]. It is noted that the electrical properties of PEDOT:PSS
could be further improved by applying additional post-treatment processes to remove the excess insulating
[31]
phase . The optimized soft PEDOT:PSS electrode exhibits saturated resistance hysteresis after the second
strain cycle [Supplementary Figure 4]. This suggests that the film is excellent for use as a soft electrode in
fully soft diodes.

Characteristics of fully soft Schottky diode
Figure 2A shows the energy band diagram of the associated electronic materials, which is critical for
operating the fully soft device as a Schottky diode. While a sufficient Ohmic contact is formed between the
PEDOT:PSS anode and P3HT, the large energy barrier between the highest occupied molecular orbital
(HOMO) level of P3HT and the Fermi level of EGaIn creates the Schottky contact between the
semiconductor and the cathode electrode. This allows for suitable device operation as a Schottky diode. As
illustrated in Figure 2B, the junction between P3HT and EGaIn causes energy band bending under thermal
equilibrium, ensuring a constant Fermi level throughout the system. In this case, the recombination of
electrons from EGaIn with holes in P3HT generates a depletion region, inducing a large energy barrier. This
barrier is expressed by the built-in potential barrier (V ), which impedes charge carrier transport through
bi
the junction. With a positive potential (forward bias, V) applied to EGaIn relative to P3HT, the barrier is
f
reduced (V - V), facilitating charge transport from the HOMO of P3HT to the Fermi level of the metal,
f
bi
enabling electric current flow. Conversely, under reverse bias (V ; negative potential to EGaIn relative to
r
P3HT), an increased energy barrier (V + V ) at the junction hinders charge transport, thus suppressing
bi
r
current flow. Consequently, the fully soft Schottky diodes exhibit typical diode characteristics [Figure 2C].