Preparation Of Cu ₂ S Thin Films And Study On Thin Oxidation Of Single Crystal Wafer

This thesis includes two parts. In the first part, we have studied the preparation and characterization of copper sulphide (Cu2S) thin films. Cu2S is a new p-type semiconductor with a bulk band gap of1.2eV which has an excellent chemical and thermal stability. It is also a good photoelectric material. Therefore it has great potential applications for the thin film solar cells, nanoscale switches, photocatalysis, cold cathodes, and other related fields. Many methods have been developed to prepare Cu2S thin films, such as chemical vapor deposition, successive ionic layer adsorption and reaction, spray pyrolysis and hydrothermal method. However, these methods have some disadvantages and limitation, such as longer reaction time, higher pressure and temperature, more complex and high costs. Chemical bath deposition (CBD) becomes a suitable technology of choice, because of its low cost growth technique and capable of producing good quality thin film semiconductors over large area and at low temperature. The second part is the research on textured monocrystalline silicon for heterojunction with intrinsic thin-layer (HIT) solar cells. HIT solar cells become the fastest solar cells in photovoltaic market because of its higher efficiency and low cost. Textured monocrystalline silicon wafers can reduce the reflection and increase light absorption, improving the conversion efficiency of the HIT solar cells. The conventional texturization technologies are mostly adding isopropanol in mixed solution of sodium hydroxide and additives which prepared texturized surface with pyramid structure. However, the pyramidal size is nonuniform, reducing the antireflection effect. In order to further improve the light absorption of HIT solar cells, it is necessary to explore a method to form uniform pyramidal size and high quality textured structure. The following items are studied in this thesis:First, Cu2S thin films were deposited on glass by the simple chemical bath deposition technique. We studied the effect of deposition temperature on the structure, surface morphology and optical properties of Cu2S thin films. The structure and optical properties of the obtained Cu2S thin films were characterized by X-ray diffractometry (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrum (XPS), and UV-vis-NIR spectrophotometer. The XRD results reveal that the as-prepared films are monoclinic Cu2S and the best crystallinity is obtained for the Cu2S thin films deposited at40℃. The thickness of the Cu2S films also increases with increasing deposition temperature. However, the surface of films is relatively rough when the deposition temperature is lower or higher. The band gap of the as-prepared Cu2S films show the distinct blue shift compared with that of the bulk Cu2S.We first proposed and investigated a texturing technique for weak oxidizing monocrystalline silicon wafers. Monocrystalline silicon wafers are oxidized by hydrogen peroxide (H2O2) before texturization to form a thin oxide layer which can slow down the corrosion rate of silicon wafers in the texturing solution, and to get a textured surface with uniform pyramids. We investigated the effects of the weak oxidizing parameters such as the concentration of H2O2and weak oxidation time on the textured results of monocrystalline silicon wafers. The surface morphology of the silicon wafers is observed by scanning electron microscopy (SEM). Optical reflectance spectra measurements in the300-1100nm wavelength range are performed using an UV-vis-NIR spectrophotometer. Compared with traditional textured process, the reflectance and the textured surface morphology of monocrystalline silicon wafers which are oxidized were improved and enhanced significantly, and the textured surface owns an average reflectivity of9.89%. The results indicate that the optimal weak oxidizing parameter is performed in5%H2O2for8min. The textured surface obtained uniform pyramids and the lowest reflectivity. Besides, the HIT solar cells fabricated from the optimal weak oxidizing textured monocrystalline silicon wafers own a higher efficiency, which is improved to1.6%compared with the cells textured with the traditional process. The short-circuit current density and fill factor are also obviously improved.