Polyethylenimine-Supported rGO Conformal Coating over AgNWs to Fabricate Strongly Stable and Flexible Transparent Heaters

Recently, reduced graphene oxide (rGO)-overcoated AgNW films have emerged as high-quality flexible transparent electrodes (FTEs). Although the rGO overcoating provides stability against AgNW oxidation and adds some conductivity to the FTE, poor adhesion of rGO with AgNWs remains a challenge to fabricate a high-quality and ultrastable AgNW-based FTE. Due to the large flake size of rGO and its hydrophobic nature, it tends to aggregate when suspended in the water to make ink for coating, which inhibits the formation of a strong conformal coating around the surface of AgNWs. In order to achieve a strong conformal coating over the AgNWs and to increase the adhesion of rGO with AgNWs, a high-quality ink solution of rGO is desirable. In this work, an ink solution of probe-sonicated nanoscale rGO in water is prepared with the support of polyethylenimine (PEI). The electrostatic interactions between rGO and PEI resulted in a suspension that remained stable for several months. The rGO–PEI overcoating ensures uniform coverage of AgNWs, thus providing high conductivity and exceptional stability to the AgNW-based FTE. The PEI-supported rGO coating over the AgNW network delivered a FTE with a sheet resistance (R_s) of 18.4 ± 0.92 Ω/sq with 85% transmittance at 550 nm. The flexible transparent heater using FTE prepared on the PET substrate demonstrated excellent transparent heater properties and exhibited a temperature increase up to 160 °C in 60 s at an applied voltage of only 4 V, which is very suitable for antifogging applications on any curved object.