Flexible and transparent electrodes imprinted from Au nanowires: stability and ageing

Abstract

We study the stability of flexible transparent electrodes (FTEs) that were self-assembled from ultra-thin gold nanowires (AuNW) by direct nanoimprinting of inks with different particle concentrations (1 to 10 mg mL^-1). The resulting lines were less than 3 μm wide and contained bundles of AuNW with oleylamine (OAm) ligand shells. Small-angle X-ray scattering confirmed a concentration-independent bundle structure. Plasma sintering converted the wire assemblies into lines with a thin metal shell that contributes most to electrical conductivity and covers a hybrid core. We studied the relative change in sheet resistance and the morphology of the FTEs with time. The sheet resistance increased at all concentrations, but at different rates. The metal shell aged by de-wetting and pore formation. The hybrid core de-mixed and densified, which led to a partial collapse of the shell. Residual organics migrated through the shell via its pores. Lines formed at low concentration (c _Au = 2 to 3 mg mL^-1) contained less residual organics and aged slower than those formed at high c _Au ≥ 5 mg mL^-1. We passivated the conductive shell with thin, adsorbed layers of PEDOT:PSS and found that it decelerated degradation by slowing surface diffusion and hindering further rupture of the shell. Thick capping layers prevented degradation entirely and stopped pore formation.

Fig. 1. Relative change in sheet resistance (R_sh,t − R_sh,t0)R_sh,t0⁻¹ = ΔR_sh,tR⁻¹_sh,t0 after time t for electrodes imprinted at different ink concentrations c_Au. The results of repeated experiments and additional concentrations are shown in Fig. S1 in the ESI.

Fig. 2. Cross-sections and surfaces of lines imprinted at 2 mg mL⁻¹ before (A, B and J), immediately after (C, D and K), and one day after (E, F and L) plasma sintering. They show the core–shell structure after plasma sintering (C and D), the densifying (A, C and E) and coarsening (B, D and F) core, and the coarse surface morphology with small pores in the shell (from K to L). The illustrations in panels (G–I) illustrate the overall structure. Orange depicts gold wires or shells, green indicates oleylamine ligand molecules and their fragments (residual organics). The wires are arranged in hexagonal bundles (dotted black lines) in (G). They form porous shells in (H), and the bundles loose their order. The pores in (I) have gently grown by surface diffusion and the densifying core causing the shell to collapse.

Fig. 3. Cross-sections and surfaces of lines imprinted at 6 mg mL⁻¹ before (A, B and J), immediately after (C, D and K), and one day after (E, F and L) plasma sintering. They show the core–shell structure after plasma sintering (C and E), the densifying (A, C and E) and coarsening (B, D and F) core, and the coarse surface morphology with growing pores (from K to L). The illustrations in panels (G–I) illustrate the overall structure. Orange depicts gold wires or shells, green indicates oleylamine ligand molecules and fragments. The wires are arranged in hexagonal bundles (dotted black lines) in (G). They form porous shells in (H), and the bundles loose their order. The pores in (I) have grown prominently by surface diffusion and by the both de-mixing and densifying core causing the shell to collapse.

Fig. 4. (A–C): Relative change in sheet resistance (R_sh,t − R_sh,t0)R_sh,t0⁻¹ = ΔR_sh,tR_sh,t0⁻¹ after time t for electrodes imprinted at 6 mg mL⁻¹ with different coating finishes. (D–F): illustrations of the apparent microstructures.