Site-selective fabrication of patterned transparent copper mesh on flexible substrates at mild temperature for green, low cost electronics

A high-performance transparent conductor which is inexpensive, flexible and low-temperature deposited is essential for fabrication of a variety of printed electronic devices. Recently, copper nanomaterials have received increasing attention as a good candidate due to its low cost and plentiful supply. However, there are three crucial issues need to be addressed: (1) strong adhesion between printed copper nanomaterials and the flexible substrates, (2) anti-oxidation of copper nanomaterials in air and (3) the relative lower sintering temperature below the T_g of common polymer foils (e.g. PET). Here, a new approach based on the combination of mussel-inspired surface modification and patterning technology was developed to fabricate the transparent copper mesh. The approach involves (i) deposition of poly(dopamine) (PDA), an excellent stable adhesive on the flexible substrate in the aqueous dopamine solution at room temperature for several minutes, (ii) development of the PDA patterns via a simple and rapid photolithography or microcontact printing technique, and (iii) green electroless plating of copper only on the exposed PDA regions, the chemi-sorption sites of metal ions, without any catalyst at mild temperature. This approach enables an alternative to carbon nanotubes, graphene and silver nanowires based flexible transparent conductors and indium tin oxide based brittle transparent conductors. The micro structure, resistivity, transparency and peel strength of copper mesh were examined by scanning electron microscopy, four-point probe technique, UV-vis, 90° peel test. As a result, the smooth copper pattern exhibited uniform thickness, strong adhesion to substrate and low sheet resistance, which was still stable even stored in air for several months.