UC Berkeley

Recently, flexible electronics have received the attention of many researchers. Among the various fascinating and actively investigated applications of flexible electronics, flexible optoelectronics, such as touchscreen panels (TSPs), organic light-emitting diodes, and organic thin-film transistors, is one of the most aggressively developing research fields. Flexible optoelectronics are often connected by flexible transparent conducting electrodes (FTCEs). FTCEs are considered indispensable components in optoelectronic devices. Researchers are looking for novel materials and processes that enable high transparency and low resistance. Indium tin oxide is the material most widely used in creating FTCEs, as it offers good optoelectronic performance (transmittance (T): > 80%; sheet resistance (Rs): ≈ 15 Ω sq-1) for flexible electronics. However, due to its several intrinsic drawbacks, including its brittleness, high cost of materials, slow rate of deposition, and large amounts of material waste, many researchers are investigating alternative materials and processes. In this regard, the laser sintering of metals or metal oxide nanoparticles and the patterning of ultra-thin metal film by pulsed laser ablation have attracted much attention due to the characteristics of nanoparticles and ultra-thin film as well as the low-cost facile fabrication capabilities of lasers.

First, I synthesized a nickel oxide (NiO) nanoparticles as a cheap substitute for silver nanoparticles and demonstrated successful laser reductive sintering of this system. High-resolution nickel (Ni) patterns were generated from NiO nanoparticle thin films by continuous-wave (CW) laser irradiation. By the laser reductive sintering of NiO nanoparticles, I developed a high-transmittance (T > 87%), electrically conductive panel for TSPs that is very robust to mechanical bending and adhesion. Furthermore, the mechanisms of photo-physical reductive sintering upon the irradiation of a CW laser on NiO nanoparticle thin films were studied by modulating the power density and illumination time of the laser. Once NiO nanoparticles are reduced to Ni by laser irradiation, they begin to coalesce, forming a conductive material. In-situ optical and electrical measurements were taken during the reductive sintering process to monitor the transient evolution of the process. Finally, low-cost Cu FTCEs were fabricated using 532 nm nanosecond laser ablation under ambient conditions. The fabricated Cu FTCEs have excellent optoelectronic characteristics (T = 83%; Rs = 17.48 Ω sq-1) and mechanical durability. The potential application of Cu FTCEs in flexible optoelectronic devices was successfully demonstrated by the fabrication of TSPs using Cu FTCEs. Furthermore, we employed diffractive optics for laser beam shaping. We successfully demonstrated the opto-electrical characteristics of the percolative regime by decreasing the areal density.