**4. Conclusions**

In summary, Cs+ ions were adsorbed into IGZO film by CsHCO3 solution immersion method at low temperature, and had the benefit of improving IGZO TFTs performance. Four kinds of CsHCO3 concentrations were selected to optimize the transfer property and stability of Cs-IGZO TFTs. According to the electrical measurements of Cs-IGZO TFTs, it was found that the Cs-IGZO TFT with CsHCO3 concentration of 2% in water solution had the optimized electrical properties, including high μ*FE* of 18.7 cm<sup>2</sup> V−<sup>1</sup> s<sup>−</sup>1, small threshold voltage shifts of 1.3 V, and low OFF current of 0.8 × 10−<sup>10</sup> A. These superior performances of optimized Cs-IGZO TFT were attributed to the change of surface structure and oxygen vacancy concentrations of IGZO film, by appropriate Cs+ ions adsorbed into IGZO film. Compared with the traditional a-IGZO TFTs fabricated through annealing process, the optimized Cs-IGZO TFT had superior mobility and comparable device stability, which might be applicable to future flexible electronics.

**Author Contributions:** Conceptualization, H.Z., C.L. and X.Z.; methodology, H.Z.; software, Y.W.; validation, H.Z., R.W. and Y.W.; formal analysis, H.Z.; investigation, H.Z.; resources, X.Z.; data curation, H.Z.; writing—original draft preparation, H.Z.; writing—review and editing, Y.W. and X.Z.; visualization, R.W.; supervision, R.W.; project administration, X.Z.; funding acquisition, X.Z. and Y.W.

**Funding:** The authors would like to acknowledge the financial support provided by National Natural Science Foundation of China (61771382, 51807156), China Postdoctoral Science Foundation (2017M623174), and Projects of International Cooperation and Exchanges Shaanxi Province (2018KW-034).

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