Consolidation and performance gains in plasma-sintered printed nanoelectrodes

We report on the unusual, advantageous ageing of flexible transparent electrodes (FTEs) that were self-assembled from oleylamine-capped gold nanospheres (AuNPs) by direct nanoimprinting of inks with different particle concentrations (c_Au = 3 mg mL^-1 to 30 mg mL^-1). The resulting lines were less than 2.5 μm wide and consisted of disordered particle assemblies. Small-Angle X-ray Scattering confirmed that particle packing did not change with ink concentration. Plasma sintering converted the printed structures into lines with a thin, electrically conductive metal shell and a less conductive hybrid core. We studied the opto-electronic performance directly after plasma sintering and after fourteen days of storage at 22 °C and 55% rH in the dark. The mean optical transmittance T̄_400-800 in the range from 400 nm to 800 nm increased by up to ≈ 3%, while the sheet resistance R_sh strongly decreased by up to ≈ 82% at all concentrations. We correlated the changes with morphological changes visible in scanning and transmission electron microscopy and identified two sequential ageing stages: (I) post-plasma relaxation effects in and consolidation of the shell, and (II) particle re-organization, de-mixing, coarsening, and densification of the core with plating of Au from the core onto the shell, followed by solid-state de-wetting (ink concentrations c_Au < 15 mg mL^-1) or stability (c_Au ≥ 15 mg mL^-1). The plating of Au from the hybrid core improved the FTEs' Figure of Merit FOM = T̄_400-800·R_sh^-1 by up to ≈ 5.8 times and explains the stable value of ≈ 3.3%·Ω_sq^-1 reached after 7 days of ageing at c_Au = 30 mg mL^-1.