A protective layer that can be applied on a flat flexible transparent conductive film was prepared by combining silica sol and organic polymer. (3-Glycidyloxypropyl)trimethoxysilane (GPTMS) was used as a precursor for the silica sol, which hydrolyzed under moisture to form silanol groups and self-condensed to form a sol under acidic conditions. Therefore, the organic polymer used was poly(4-styrenesulfonic acid) (PSSA), which is acidic and water-soluble; thus, the silica precursor can form a sol and can cause chemical condensation with the silica sol under thermal conditions. However, as this protective layer was insulating, there was difficulty in conducting electricity to the lower portion through the upper contact. Therefore, a small amount of conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), was added to the protective layer to make the overcoating layer itself conductive, thereby enabling electrical conduction to the underlying conductive film. The network structure of the overcoating layer surface could block oxygen and moisture, thus improving chemical stability. Therefore, under high-temperature and high-humidity conditions for 500 h, the sheet resistance increased by 145% before overcoating but increased by 33% after the overcoating layer was formed with appropriate thickness. In addition, the bonding strength of the surface was further improved. Peel-off occurred after applying a pencil having hardness of 5B or more before the overcoating treatment; however, after the overcoating treatment, no damage was caused by a pencil having hardness of 5H or less. Consequently, the overcoated conductive film maintained flexibility and transparency; it also exhibited desirable electrical characteristics, improved chemical stability, and excellent scratch resistance.
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