습식공정을 이용한 실리콘 나노구조체 제작 및 태양 에너지소자

Abstract

Solar energy has been considered as one of the most abundant and clean sources of renewable energy. However, solar powers currently provide only a small share of global energy production due to expensive cost of electricity generation. And huge efforts have been devoted in the last years to develop cost-effective silicon wafer technologies to cope with the dramatic price decrease for solar energy conversion. And nowadays fabrication of nanostructured Si for light trapping and conversion becomes fascinating and promising, since it opens a new sight to conventional solar devices. Tremendous charming works have been done in such a field all over the world, which moves the photovoltaic technologies further to serve people with supplying more and more renewable energy. And this thesis suggests several novel methods to get nanostructured Si, and attempts to investigate their photoelectrical performances, which suggests a fascinating way to reduce the cost of device production while maintaining conversion efficiency.

Here we demonstrate that a novel technique for the fabrication of mesoporous Si double layer by two-step electrochemical etching, which can be utilized for further Si-epitaxial growth and layer transfer. The first step was to apply a lower anodic current density for a certain period, self-organized mesoporous silicon with small pore diameter (≤ 4.5 nm) was formed. The second step, high anodic current density was applied to create the layer with high porosity beneath the first layer, enabling easy further detachment from the substrate after growing the epitaxial layer. Concerning the second step, a direct relationship between the anodic current density and etching time for optimum layer transfer was derived. Interestingly, we also found that the distribution of n+ mesopores generated by breakdown was stabilized by SCR effect, and the pore growth rate matched well with the Lemann’s formula based on n-type macroporous Si.

Fabrication of various nanostructures on a surface of a planar silicon wafer is a facile and popular way to increase a surface area for enhancing catalyzing reactions and/or light absorption. Metal-assisted electroless etching, usually employing silver (Ag) nanoparticles (NPs) as catalyst, is known to vertically pattern Si nanowires (SiNWs) from the surface of a planar substrate. SiNWs could be obtained via one- or two-step procedures of chemical etching, in which the Ag catalyst was formed from the wetting deposition using hydrofluoric acid (HF) with silver nitrate (AgNO3) solution or from the thermal evaporation of Ag pellets. Here, we present that the SiNWs with smoother sidewalls can be formed using silver oxide (AgO) nanoparticles (not conventional Ag metal NPs). The AgO-assisted chemical etching reveals a smoother sidewall morphology confirmed by TEM; moreover, the photoelectrochemical (PEC) performances were compared between these SiNWs and the conventional NWs fabricated by Ag-assisted electroless etching.results in a better PEC performance compared to the nanowires prepared by conventional methods. The improvement of 7.8% in photocurrent has been recorded compared to the Ag-assisted NWs, which also corresponded to the increase of 16.9% compared to a bare Si wafer. In addition, more anodic shift in the onset potential was observed with a good stability in comparison with the Ag-assisted NWs.

Usually, Au, Ag and Pt, such kinds of noble metals are used as the catalysts for chemical etching to fabricate kinds of nanostructures on Si wafer. But since these noble metals are rare and heavy elements in the earth, cost and environmental pollution problems would be encountered when such an etching method was introduced into industrial production; besides after etching the metal residues are detrimental impurities in silicon surface, which need to be removed clearly by kinds of treatments. There is few paper reported about Cu assisted chemical etching (CuEC) on Si because of its low anodic potential, bad stability in acids, and heavy contamination to Si. However, Cu is abundant compare to Ag, Au and Pt, cheaper in modern industry, and less toxic and friendly environments. In this work, we developed Cu as the catalyst to assist the electroless etching of Si wafer, and get a Si nanostructure with a reflectance under 5% in the visible wavelength. A tradeoff between Copper dissolution and redeposition is discovered, which affects the surface morphology during the etching process. In the end, photoelectrochemical cell tests were conducted on the nanostructure fabricated by Cu assisted chemical etching (CuEC), which shows a better performance compared to Ag assisted electroless etched nanostructrure.