Ambient atmosphere-processable, printable Cu electrodes for flexible device applications: structural welding on a millisecond timescale of surface oxide-free Cu nanoparticles

Abstract

Recently, various functional devices based on printing technologies have been of paramount interest, owing to their characteristic processing advantages along with excellent device performance. In particular, printable metallic electrodes have drawn attention in a variety of optoelectronic applications; however, research into printable metallic nanoparticles has been limited mainly to the case of an environmentally stable Ag phase. Despite its earth-abundance and highly conductive nature, the Cu phase, to date, has not been exploited as an ambient atmosphere-processable, printable material due to its critical oxidation problem in air. In this study, we demonstrate a facile route for generating highly conductive, flexible Cu electrodes in air by introducing the well-optimized photonic sintering at a time frame of 10(-3) s, at which the photon energy, rather than conventional thermal energy, is instantly provided. It is elucidated here how the surface oxide-free, printed Cu particulate films undergo chemical structural/microstructural evolution depending on the instantly irradiated photon energy, and a successful demonstration is provided of large-area, flexible, printed Cu conductors on various substrates, including polyimide (PI), polyethersulfone (PES), polyethylene terephthalate (PET), and paper. The applicability of the resulting printed Cu electrodes is evaluated via implementation into both flexible capacitor devices and indium-gallium-zinc oxide (IGZO) flexible thin-film transistors.

Newly Designed Cu/Cu10Sn3 Core/Shell Nanoparticles for Liquid Phase-Photonic Sintered Copper Electrodes: Large-Area, Low-Cost Transparent Flexible Electronics

“…In a previous study, we demonstrated that oleic acid was capable of acting as a primary surface capping molecule that could stabilize the surface atoms of Cu nanoparticle. 12,25 We believe that oleic acid-involved chemical stabilization is also valid for 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 12 conventional thermal sintering, the efficient role of a low melting-point phase as a flux has been demonstrated widely for various ceramics, for enhancing densification rates, achieving accelerated grain growth, or producing specific grain boundary properties. [26][27][28][29][30] The requisites for a flux phase applicable to Cu nanoparticles are as follows: the possibility of a facile wetchemical synthesis reaction, spatially uniform distribution inside particulate films, interfacial compatibility at a heterogeneous junction between the flux and Cu phase, a low melting point, an electrically conductive nature, and earth-abundance/cost-effectiveness of elements.…”

“…The initial increment of electrical resistance was due to the release of structural stress accumulated inside instantly-converted films, which is easily observable in photo-sintered metallic films. 12,13,20 As shown in Figure S8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 The crucial role of a low melting-point Cu 10 Sn 3 phase is further clarified in a large area process using continuous photonic sintering with a moving stage, as shown in Figure 4. All the Cu and Cu-Cu 10 Sn 3 films were formed on polymeric substrates by continuous spray deposition.…”

“…12,13,15,20,21 It has been reported that the undesirable oxidation behavior in Cu nanoparticles is restrictied effectively in a kinetically-controlled manner during a photonic sintering process in air. 12 With Cu nanoparticles, the resistivity of ~7 µΩ·cm evolved above the optical dose of 1.9 J/cm 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 and 12.2 µΩ·cm at energy dose conditions of 0.97 and 1.1 J/cm 2 . Interestingly, even under a low energy dose of 0.97 J/cm 2 (where the electrically active electrodes were not obtainable from Cu nanoparticles), the Cu-based electrodes were generated with a resistivity value acceptable in electronics applications.…”

“…[22][23][24] The sub-peaks positioned at 932.9 and 934.1 eV were associated with the Cu 2 O and CuO phases, respectively. 12,16,17 The semi-quantitative areal fraction of the oxide phases was 0.16. Taking into consideration the measuring depth in the XPS analysis, we speculated that a surface oxide layer existed with a negligible volume in the Cu/Cu 10 Sn 3 nanoparticles.…”

The secret of Cool Plasma Sintering for low-temperature bulk formation from copper nanoparticles

“…This point excels compared with photosintering [4], where the shapes of original grains remain and voids between the grains are observable. Once a grain starts to swallow adjacent grains and grow, voids easily occur where the swallowed grains once were.…”