*3.2. Evaluation of Liquid Inks*

Simple liquid dispersions containing the copper compounds were prepared. To increase the reduction property of the copper compounds, liquid dispersions containing ethylene glycol as a reduction compound were also prepared. Films of the inks were prepared by dropping the ink onto a cover glass, and IPL sintering was performed at 12.45 and 16.60 J cm−<sup>2</sup> with pulse widths of 1500 and 2000 μs, respectively; for both irradiation conditions, the applied voltage, irradiation period, and number of pulses were 2.3 kV, 1000 ms, and 1, respectively. Table 2 shows the copper conversion ratios, as estimated from the integral strength obtained by means of X-ray diffraction (Figure S2), and sheet resistances of the films made from the liquid dispersions.

In the absence of ethylene glycol, copper nitride film irradiated at 12.45 and 16.60 J cm−<sup>2</sup> had copper conversion ratios of 0.91 and 0.88, respectively. However, in the presence of ethylene glycol, copper nitride film irradiated at 12.45 and 16.60 J cm−<sup>2</sup> had copper conversion ratios of 0.99 and 0.96, respectively. No conversion to copper was observed for the copper(I) oxide and copper(II) oxide films, irrespective of the absence or presence of ethylene glycol.

Sheet resistance after IPL sintering was measured by means of a four-point probe method in the presence or absence of ethylene glycol (Table 2 and Figure 3). The sheet resistances for the copper(I) oxide films were beyond the maximum limit of quantitation of the instrument (>9.999 × 107 <sup>Ω</sup>). Similarly, sheet resistances for the copper(II) oxide films could not be obtained because the films broke apart after sintering (Figure S3).


**Table 2.** Conversion ratio and sheet resistance obtained with different irradiation energy densities and vehicles.

**Figure 3.** Sheet resistance of films containing copper nitride (Cu3N) and copper (Cu) after intense pulsed light sintering. Vehicle, ethanol (EtOH) with or without ethylene glycol (EG).

For the copper nitride films, higher sheet resistances were obtained in the absence of ethylene glycol (4.52 × 106 and 2.37 × 100 <sup>Ω</sup> sq−<sup>1</sup> at 12.45 and 16.60 J cm<sup>−</sup>2, respectively) than in the presence of ethylene glycol (1.34 × 100 and 6.95 × <sup>10</sup>−<sup>1</sup> <sup>Ω</sup> sq−<sup>1</sup> at 12.45 and 16.60 J cm<sup>−</sup>2, respectively). However, the reason for the particularly high sheet resistance obtained for the film not containing ethylene glycol and irradiated at 12.45 J cm−<sup>2</sup> was that the conductive path in the film was broken by the probes during measurement, because the mechanical strength of the film was weak; visual inspection after measurement of the sheet resistance revealed that the film was broken. Compared with the sheet resistances of the films containing the copper nanoparticles, the sheet resistances of the copper nitride films were comparable or one order of magnitude higher.

Visual and scanning electron microscopy (Figure 4) inspection revealed that the surface of the film containing ethylene glycol became coarse after IPL irradiation at 16.60 J cm−2. In addition, the surface was found to contain hollow particles of copper with a particle size of 5 μm or more. The coarse particle size and failure of necking to form between particles are likely the reasons for the weakness of the film. We propose the following process for the formation of the hollow copper particles (Figure S4): Copper nitride particles are chemically changed to copper particles by the thermal energy produced by IPL irradiation; these particles rapidly fuse with nearby particles because of the decreased surface energy of the particles, and copper plate-like films form and then become round owing to surface tension. This would suggest that it is necessary to avoid rapid heating by IPL irradiation [16] and rapid cooling of the particles to prevent the generation of hollow particles. Specifically, it relaxes the heating rate to transfer the thermal energy, which is rapidly generated by IPL irradiation, to organic compound. As a result, we conclude that the condition of the film could be improved because the intense reaction is inhibited.

**Figure 4.** Appearance and scanning election microscopy images of a film containing ethylene glycol (**a**,**c**) before and (**b**,**d**) after intense pulsed light irradiation at 16.60 J cm<sup>−</sup>2.
