**5. Conclusions**

This work describes a new type of flexible photodetector based on a liquid gallium alloy. A simple and reliable method was introduced to fabricate a flexible and transparent photodetector based on LMs. The photodetector was fabricated with a material, EGaIn. Both a conductive electrode and a photo-responsive layer could be obtained from the material and fabricated on PDMS substrates. The fabrication process of a conductive film based on LM microdroplets is an efficient method to fabricate a large-area (5 × 5 cm), flexible, and stretchable LM film. The laser ablation method was also used to fabricate flexible and stretchable electrodes, and the width of the patterned electrodes could be controlled at a level of 20 μm. A photo-responsive layer (~8.7 nm) was exfoliated with an incompletely-cured PDMS by peeling off from the surface of the oxidized LM film. Finally, the photodetector could be made by combining the patterned electrodes and the photo-responsive film. It shows 30.3%, 14.7%, and 16.1% of the photo-response–current ratio under wavelengths of 254 nm and 365 nm in the ultraviolet band, and ordinary visible light, respectively.

The key contribution of this method is that a photosensitive device was fabricated with one material. The semiconducting active layer was exfoliated from conductive materials, and both layers were used in the same device. The laser ablation method shows high performance of controllable, accurate, and efficient patterning. It is expected that the results with LM and various techniques in this work will contribute to advances in the fields of flexible and stretchable sensors.

**Author Contributions:** Conceptualization, P.X. and S.S.; methodology, P.X. and H.-J.G.; formal analysis, P.X. and H.-J.G., S.S.; investigation, P.X. and H.-J.G.; data curation, P.X. and H.-J.G.; writing—original draft preparation, P.X. and H.-J.G.; writing—review and editing, S.S.; visualization, P.X. and H.-J.G., S.S.; supervision, S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2018R1D1A1B07041253) and by the Gachon University research fund of 2020(GCU-202004460001).

**Conflicts of Interest:** The authors declare no conflict of interest.
