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Laser Assisted Nanomanufacturing with Solution Processed Nanoparticles for Low-cost Electronics and Photovoltaics

  • Author(s): Pan, Heng
  • Advisor(s): Grigoropoulos, Costas
  • et al.
Abstract

Nanomanufacturing is a term used to describe either the production of nanoscale materials, or to describe the manufacturing of parts `bottom up' from nanoscale material or `top down' in smallest steps for high precision. In recent years, nanomanufacturing is facing a new and broadened definition the essential themes lie on low cost, scalability, reliability and sustainability, besides the conventional requirements. The emphasis on this technology is towards realizing the low cost and large area manufacturing with control and benefits from nanoscale structures and nano enabled properties. The newly defined nanomanufacturing is expected to find extensive new applications in the large area electronics and renewable energy industries.

Nanomanufacturing based on solution processed nanoparticles is a promising approach towards this goal and is currently under intense research. This process significantly simplifies the manufacturing process and enables potential applications that are low temperature and extremely low cost. The solution deposition eliminates the need of multiple steps and vacuum deposition. It is capable of being integrated into the simple process, i.e. roll-to-roll manufacturing, to increase yield and process area. The suppressed melting temperature of nanoparticles also eliminates the need of high temperature processing. In solution processing, in general, nanoparticles prepared chemically with controlled stability and compositions are deposited in solution phase (e.g. printing) followed by annealing and/or nanopatterning steps for achieving the required spatial resolution and reasonable electrical and optical properties.

Laser processing, including annealing/sintering and ablation, of solution deposited nanoparticles is a new and interesting process that has barely been explored. Not only this process can take advantage of laser processing for low cost, low temperature device fabrication, but also a large amount of physics can be explored including laser-nanomaterial interaction, nanoscale heat/mass transport, defects / grain boundaries evolution, semiconductor device physics and photonics, etc. The present study examined various components in the low cost nanomanufacturing process, including laser annealing of semiconductor and metallic nanoparticles, optical characterization of sintering process and scalable nanopatterning techniques. The emphasis is on both demonstration of novel processes and working devices as well as fundamental experimental and numerical studies of transport phenomena involved in laser induced solid state and melt-mediated nanoparticle coalescence.

Firstly, to provide insight into the transport phenomena involved in laser induced non-melt and melt-mediated nanoparticle coalescence, experimental and numerical studies were performed. The Molecular Dynamics simulation was adopted to study the solid-state nanoparticle sintering by laser heating. The comparison between MD and macroscopic model reveals the unique mechanisms in sub-10nm nanoparticles sintering suggesting enhanced neck growth rate can be expected for nanoparticle with size below 10nm. The melt-mediated nanoparticle coalescence induced by ns laser is probed in-situ by a pump-and-probe technique. The probing results reveal several important characteristic times during laser annealing that will determine the morphology and crystalline structures of the laser processed film: characteristic coalescence time, cooling and resolidification times. The coalescence time compares favorably with MD simulation.

Secondly, the laser annealing of metal oxide nanoparticles for electronics and photovoltaics are demonstrated. Solution deposited ZnO nanoparticles can be transformed into active channel layer of thin film transistors (TFTs) by Excimer laser annealing (ELA). The device shows mobility ~0.1 cm2/V-s and current on/off ratios of more than 104. The properties of the laser annealed films were characterized by scanning electron microscopy, high-resolution TEM, DC conductance, and photoluminescence measurements. Furthermore, ELA is combined with spray deposition of TiO2 nanoparticle to fabricate dye sensitized solar cells (DSSCs) on glass and plastic substrates. All-laser-annealed TiO2 photoelectrodes achieve efficiency at ~3.8% with Ruthenium dyes.

Optical characterization of optical properties and film thickness by ellipsometry is also demonstrated for silver nanoparticle film sintering. The ellipsometric measurement is found to be robust in capturing the percolation transition, dielectric constant and film thickness evolutions. The non-contact optical measurement is also found to correlate well with DC measurement.

Finally, large-area optical near-field nanoprocessing, i.e. nanosintering and nanoablation, of solution deposited nanoparticle is demonstrated. The feature size down to ~150nm is obtained and annealing in furnace further reduces the feature size down to ~50nm. Nanoscale organic transistors are fabricated by nanosintered electrodes and solution deposited air-stable semiconductor polymer. Molecular Dynamics simulation is also employed to calculate the effective thermal conductivity of the nanoparticle film to facilitate thermal modeling of the nanoscale laser processing.

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