산화아연 나노선 기반의 태양에너지 소자 합성 및 최적화 연구

Abstract

Although human beings has been growing steadily owing to fossil energy, much attention has been directed to the development of alternative sources of energy in order to overcome energy crisis and the problem of greenhouse gas exhaustion. The solar power device is not only environmentally friendly but also energy resources disproportionately problem is not serious, unlike fossil fuels and other alternative energy. The solar power device can be roughly divided by into two types. The first one is photovoltaic cell, which convert sunlight directly into electrical energy. The other one is the photoelectrochemical cell, which convert sunlight into the chemical fuels. Both device are consist of semiconductor materials and produce electron-hole excitons absorbing the sunlight. In case of photoelectrochemical cell, electron-hole excitons produce chemical fuel through oxidation-reduction reaction, and this principle has been applied to decompose the water in the hydrogen production technology. The nanoparticle, which can utilize visible spectrum region and a cheaper materials, have been a lot of research in progress for the development of efficient photoelectrode. Especially, CdSe/CdS/ZnO nanowire photoelectrode allows utilizing the whole visible spectrum, and 1-dimensional ZnO nanowire provide direct electron pathway. However, it has a limit of efficiency because of a lack of understanding of the surface state of CdSe nanoparticles. In chapter 2, cadmium selenide (CdSe) quantum dots (QDs) was prepared by chemical bath deposition and the formation of an amorphous selenium oxide (SeO2) layer on the surface of CdSe QDs was suppressed through synthesis condition control. The precursor ratio for the chemical bath deposition (CBD) was optimized to minimize the SeO2 layer growth and develop high-efficiency CdSe QDs for quantum-dot-sensitized solar cell (QDSSC) applications. The morphologies, optical and electrical properties of the CdSe QDs were investigated and the growth mechanism was also proposed for the CdSe and SeO2 layer formation. Furthermore, electrochemical impedance spectroscopy results indicated that the SeO2 layer reduced the recombination resistance of photogenerated electron-hole pairs, thus degrading the cell efficiency. Therefore, a quantum-dot-sensitized solar cell based on an optimized CdSe/CdS/ZnO nanowires (NWs) photoanode, polysulfide electrolyte and Au counter electrode can enhance the power conversion efficiency (PEC) to 4.8 % under AM 1.5 G one-sun illumination. In chapter 3, photoanodes prepared using CuInS2/CdS/ZnO nanowires were fabricated by a solution-based process for constructing a photo-driven hydrogen generation system. For efficient light harvesting and photoexcited charge collecting, ZnO nanowire (NW) photoanode arrays were co-sensitized with CdS and CuInS2 (CIS). A CdS layer was deposited on the ZnO NW via successive ion layer adsorption and reaction (SILAR), and the CIS layer was prepared by depositing a molecular precursor solution onto the CdS/ZnO NW. The generated anodic photocurrent was increased with the subsequent deposition of the CIS and CdS layers. Ultraviolet photoelectron spectroscopy analysis revealed cascade type-II band alignments for the CIS/CdS/ZnO NW photoanodes, which enabled efficient electron collection. Our heterostructure photoelectrode has generated a greatly improved photocurrent density of 13.8 mA/cm2 at 0.3 V vs. SCE under 1 sun illumination. In chapter 4, we developed an environmental friendly and cost effective solution approach for the synthesis of a Cu2ZnSnS4(CZTS)/CdS/ZnO heterostructure nanowires (NWs) and their application as photoelectrode for photoelectrochemical (PEC) hydrogen production. The CZTS sensitizing layer was prepared by spin-coating using a methanol based precursor solution, CdS and ZnO NW were also synthesized by solution processes, SILAR (successive ion adsorption and reaction) and hydrothermal growth method, respectively. The CZTS/CdS/ZnO NW had a cascade band-structure for efficient charge transfer with broad photoresponse from UV to near IR region and generated higher photocurrent than CdS/ZnO NW or ZnO NW. Furthermore, for higher light harvesting, we designed an innovative photoelectrode, which has a stainless steel mesh (SSM)-supported structure. Since the SSM anode has the porous and stackable property, sheets of mesh can be stacked on FTO based electrode to expand photoelectrode active area. This unique stacked mesh photoelectrode structure proves improvement of the overall light conversion efficiency of PEC through an enhancement of light absorption.