*2.2. Device Fabrication*

The cracked metal electrodes used in this work were prepared as schematically illustrated in Figure 1. The Ag thin films with a thickness of 30 nm were preferentially deposited on the deformable FEP substrates by thermal evaporation in vacuum (<1.0 × 10−<sup>6</sup> Torr). The samples were individually gripped in a rectangular frame for applying tensile forces, and then uniaxially and biaxially stretched to 120% and 140%, respectively (designated "UA120", "UA140", "BA120", and "BA140"; see also

Table 1). It should be noted that the maximum stretching ratio which was reliable without tearing or slipping of the sample in our experimental setup was 140%. To prevent an increase in electrical resistance, each cracked Ag film was transferred and welded onto another intact Ag film by means of cold-welding [44–47]. For the cold-welding process, the stretched Ag/FEP sample was brought into contact with the intact Ag film (with a 30-nm thickness) deposited on a glass substrate, and subsequently pressed with a pressure of 0.2 MPa for 90 s at room temperature. The FEP substrate was then easily peeled off from the sample without residue owing to a lower surface energy of FEP (see Figure 1c). All the transferred samples exhibited no significant change in high electrical conductivity, compared to a reference electrode (i.e., the intact Ag film with a 60-nm thickness deposited on the glass substrate). It was also confirmed that the cracked Ag films were neatly transferred onto the intact Ag films, using scanning electron microscopy (SEM).

**Figure 1.** (**a**) Schematic illustration of the preparation of cracked Ag electrode in a controlled manner, and the fabrication of device. (**b**) Chemical structure of (9-2-ethylhexyl)carbazole (EHCz). (**c**) Photographic images of the cracked Ag film after completing the cold-welding process. (**d**) Photographic image of the cracked Ag film inducing diffuse reflection.



X- and Y-axes are in-plane, which are perpendicular to each other.

Each prepared Ag electrode was covered with another glass substrate coated with indium-tin oxide (ITO) (20 Ω sq<sup>−</sup>1), and silica microsphere spacers (of 5-μm diameter) were used for a fixed gap distance between Ag and ITO. The gap between the two electrodes was then filled with EHCz by capillary action to complete the device structure (see Figure 2). The active area of each device was 1 × 1 cm2. It is worth noting that EHCz is highly viscous, which substantially hinders the infiltration into smaller gaps (in the submicron range). Even if EHCz well stays in the gap without leakage due to its high viscosity, the two substrates (i.e., lower and cover glasses) should be securely fixed to prevent slipping by the liquid.

**Figure 2.** (**a**) Configuration of device to investigate the charge injection properties via local electric field enhancement. (**b**) Photographic image of the device illustrated in (**a**), under 365 nm UV light. The optically excited EHCz material under UV irradiation clearly revealed its liquid boundary during the capillary action (indicated by yellow arrows). Red dashed boxes indicate the locations of silica microsphere spacers. (**c**) Optical microscopy image of a cross section of the device in (**b**). The gap between the two electrodes is indicated by red arrows.
