**Fabrication of a Flexible Photodetector Based on a Liquid Eutectic Gallium Indium**

### **Peng Xiao** †**, Hyun-Jong Gwak** † **and Soonmin Seo \***

Department of Bionano Technology, Gachon University, Seongnam, Gyeonggi 13120, Korea; zhongpengxiao@gmail.com (P.X.); rhkrguswhd@naver.com (H.-J.G.)

**\*** Correspondence: soonmseo@gachon.ac.kr; Tel.: +82-31-750-8754

† P.X. and H.-J.G. contributed equally to this work.

Received: 27 October 2020; Accepted: 17 November 2020; Published: 18 November 2020

**Abstract:** A fluidic gallium-based liquid metal (LM) is an interesting material for producing flexible and stretchable electronics. A simple and reliable method developed to facilitate the fabrication of a photodetector based on an LM is presented. A large and thin conductive eutectic gallium indium (EGaIn) film can be fabricated with compressed EGaIn microdroplets. A solution of LM microdroplets can be synthesized by ultrasonication after mixing with EGaIn and ethanol and then dried on a PDMS substrate. In this study, a conductive LM film was obtained after pressing with another substrate. The film was sufficiently conductive and stretchable, and its electrical conductivity was 2.2 × 10<sup>6</sup> S/m. The thin film was patterned by a fiber laser marker, and the minimum line width of the pattern was approximately 20 μm. Using a sticky PDMS film, a Ga2O3 photo-responsive layer was exfoliated from the fabricated LM film. With the patterned LM electrode and the transparent photo-responsive film, a flexible photodetector was fabricated, which yielded photo-response-current ratios of 30.3%, 14.7%, and 16.1% under 254 nm ultraviolet, 365 nm ultraviolet, and visible light, respectively.

**Keywords:** eutectic gallium indium; EGaIn; liquid metal; gallium alloy; flexible photodetector; flexible electronics
