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 (cAu = 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 T̄400–800 in the range from 400 nm to 800 nm increased by up to ≈ 3%, while the sheet resistance Rsh 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 cAu < 15 mg mL−1) or stability (cAu ≥ 15 mg mL−1). The plating of Au from the hybrid core improved the FTEs' Figure of Merit FOM = T̄400–800·Rsh−1 by up to ≈ 5.8 times and explains the stable value of ≈ 3.3%·Ωsq−1 reached after 7 days of ageing at cAu = 30 mg mL−1.


Comparing optical transmittance and electric conductance of different grids
We determined the sheet resistance R sh as resistance R of a quadratic electrode section with edge length l = 15 000 µm, used Ag paste for contacting and miniature crocodile clamps for two-point probe measurements. The square area contained 750 conducting lines. Its anisotropic sheet resistance was measured parallel to the lines, with G being the conductance of the 750 lines in parallel.
To restrict the influence of R sh data points at the lowest concentration c Au (they were off-the-charts) when fitting the trade-off between sheet resistance and optical transmittance, the relation above was used for fitting R −1 sh = G vs. T 400−800 nm instead of R sh vs. T 400−800 nm . Thus, the fits in Fig. S1B correspond to the inverse of the linear fits in Fig. S1A. A B Figure S1: Trade-off between (A) conductance G = R −1 sh and optical transmittance T 400−800 as well as (B) between sheet resistance R sh and optical transmittance T 400−800 for electrodes right after plasma sintering (initial state) and fourteen days later (aged state). The data points for c Au = 3 mg/mL have been omitted in B due to off-the-charts high R sh . | The graphs show averaged values from three measurements each, the corresponding standard deviation as well as a fit (dashed lines). Light colours indicate lower c Au .

Quantification of electrode ageing
We tracked the sheet resistance of AuNP electrodes imprinted at ink concentrations ranging from 3 mg/mL to 30 mg/mL on PET foil over time to investigate their ageing behaviour.
Nanoimprinting at 3 mg/mL resulted in structures near the percolation threshold, exhibiting initial sheet resistances which were too high for FTEs.   Figure S2: Relative change in sheet resistance (R sh, t − R sh, t 0 ) · R −1 sh, t 0 = ∆R sh, t · R −1 sh, t 0 with time t for three electrodes each, nanoimprinted at the respectively same concentration c Au .
In addition, we measured the optical transmittance spectra of the FTEs immediately after plasma sintering and 14 days later for all concentrations.   3.2 Core center-to-centre distance and sphere arrangement after imprinting Small-Angle X-ray Scattering (SAXS) was employed to study the spheres' structural arrangement within lines nanoimprinted at c Au = 3 mg/mL and 30 mg/mL. The single peak position q * suggests an amorphous sphere arrangement. Based on the Ehrenfest relation, 1,2 the core center-to-center distance for spheres with a short range order (amorphous arrangement) corresponds to:  Figure S6: Radially integrated SAXS curves of electrodes nanoimprinted at 30 mg/mL (highest concentration) and 3 mg/mL (lowest concentration) on PET as well as of the bare PET as reference (the Kapton ® peak is due to the Kapton ® window separating sample and evacuated scattering path | scattering curves are shifted for improved visibility).

Organic content
The organic content of the synthesized and twice-purified Au nanospheres was determined with Thermogravimetric Analysis (TGA) and amounted to 16.98 wt% (see Fig. S7). This corresponds to a volume fraction of: