原子層沉積技術與奈米結構於功能性薄膜之應用: 太陽能電池之吸光與載子傳輸層、透明導電薄膜及封裝

Abstract

Applications of atomic layer deposition (ALD) and nanostructures in functional thin films were studied to address the key issues of finding alternative energy and reducing carbon emission. In addition to the device performance, the compatibility of techniques with cost-effective mass production is also the heart of matter. In the study, we focused on the development of common functional films in optoelectronics, including charge transporting layer and absorbing layer of photovoltaics, transparent conductive film (TCF) and gas barrier, to solve the essential problems of performance, stability and high cost in green energy industry. Thin film fabrication can be divided into two categories: vacuum process and solution process. Considering the compatibility with the flexible plastic substrates and roll-to-roll process, we chose ALD and solution process from nanostructure dispersion for their low process temperature among other techniques. In addition, both of them possess unique characteristics, rendering the as-fabricated films with multifunctionality. In the part of photovoltaics, we deposited hole transporting layer (HTL) with ALD and tried to fabricate novel absorbing layer with solution process from silicon nanoparticles dispersion. Through properly adjusting process parameters, we successfully developed nickel oxide (NiO) process able to control film thickness with great precision. The power conversion efficiency (PCE) achieved with an ultrathin NiO film (4 nm) was comparable to that of devices with the commonly-used but instable organic HTL, (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). In the attempt to develop absorbing layer, stable Si nanoparticles dispersion was successfully fabricated with conjugated linkage between particles and functional groups. From the analysis of optical properties, we suggested the linkage had the capability of assisting charge transport between nanoparticles and functional groups, which might benefit the application in solar harvesting. Solution-processed Si film with electrical properties similar to that of amorphous Si film deposited by vacuum process was obtained from Si nanoparticles dispersion. In the field of TCF, the resistivity of hafnium-doped zinc oxide (Hf:ZnO), a good transparent conductive gas barrier process developed in our laboratory, was further improved to 0.00045 Ω/cm by an additional 2 s exposure after doping. To further improve the conductivity and flexibility, we tried to insert a metal layer in Hf:ZnO by a low temperature copper process. Although a relatively good conductivity close to the value of bulk Cu was obtained in a 17 nm Cu film at process temperature as low as 120 °C, the reproducibility was poor due to the lack of a self-limiting growth mechanism. In the last part of the dissertation, the feasibility of blending monomer with graphene oxide (GO) for fabricating multifunctional gas barrier was verified. Monomers with ligands which formed strong interaction with GO, e.g. phenyl or hydroxyl groups, were found to disperse GO well. Among the studied monomers, (Hydroxyethyl)methacrylate (HEMA) possessed low viscosity, able to obtain better sonication efficiency and therefore well-exfoliated GO sheets. In addition, the dispersion could accommodate GO concentration high enough to form liquid crystal (LC) without becoming too viscous. At 3 wt% GO in HEMA, the dispersion appeared milky, indicating certain degree of self-orientation of GO. The orientation was beneficial for controlling GO arrangement during film formation, increasing the length of gas permeation path and therefore gas barrier property. Besides, GO/HEMA dispersion showed great storage and thermal stability, which were compatible with industrial production and post reduction of GO for fabricating multifunctional gas barrier film.