The Study Of Black Silicon Solar Cells

High efficiency and low cost are very important for the solar cells widely applications and even substitution the conventional energy. Silicon solar cells take over most of the photovoltaic market with its advantages of both high efficiency and low cost. In the past decade years, a lot of high efficiency structures of crystalline silicon solar cell have been studied and developed, which pushed the efficiency to a rather high level. However, the cost is also high and it blocks the large-scale applications in solar cells. A new material, black silicon, which is expected to be high efficiency and low cost, provides a new solution for reducing the cost of silicon solar cells.Black silicon can nearly absorb all the light from ultroviolet to infrared with ultro-low reflectance, which leads to that black silicon solar cells can use more of sun light than crystalline silicon solar cells and increases conversion efficiency. Although the black silicon has ultra-low surface reflectance, the conversion efficiency of black silicon solar cells can not be improved significantly as our expected because of the enhanced surface recombination and Auger recombination in the nanostructured silicon. In this thesis, the black monocrystalline silicon solar cells were fabricated, and the carrier recombination mechanisms were studied. We successfully fabricated high efficient black silicon solar cell by suppressing the emitter recombination and optimizing the fabrication processes of black solar cells. The summary of this thesis is listed as follows:(1) Large area (156 × 156 mm2) black silicon with nanowires (SiNWs) on the pyramidal surface was prepared by silver-nanoparticle-assisted etching. The mechanism of formation of silicon SiNWs can be understood as a self-assembled Ag-induced selective etching process based on localized microscopic electrochemical cell. After the growth of SiNWs, the reflectance of black silicon reduces to 1.86%from 11.62%. The reflectance depends on the length of SiNWs. In order to improve the uniformity of SiNW arrays and reduce the reflectance, the process of the etching was adjusted:the processes deposition of Ag nanoparticles and silicon etching were carried out by two steps. KOH solution was applied to modulate the structure of SiNWs, which broadens the range of the size of SiNWs.(2) The black silicon solar cells were successfully produced by standard process. The conversion of the cells is far less than the ordinary monocrystalline silicon solar cells as references. We investigated carrier recombination mechanisms and electrical performance of black silicon solar cells by a series of measurements, including minority carrier lifetime as a function of carrier concentration, doping concentration profile, I-V characteristics and IQE. The surface recombination and Auger recombination at and near the emitter area suppress the carrier lifetime and concentration efficiency of black silicon solar cells.(3) A series of passivation methods were employed to suppress the carrier recombination of black silicon solar cells. First of all, we used the method of high temperature thermal oxidation to passivate black silicon solar cells, and investigated the carrier lifetime and electrical performance as a function of oxidation temperature. The second, high pressure thermal oxidation was used as passivation method to reduce the thermal defect density which was usually induced by high temperature oxidation. At the last, the black silicon solar cell with SiO2/SiNx:H stacks passivation film was prepared, which exhibits a good efficiency of 17.1%. The investigation of IQE suggests that the SiO2/SiNx:H stacks films decrease the Auger recombination through reducing the surface doping concentration and surface state density of the Si/SiO2 interface, and SiNx:H layer suppresses the SRH recombination in the black silicon solar cell, which yields the best electrical performance of b-Si solar cells.(4) We optimized the processes of the black silicon cells fabrication and enhanced the conversion efficiency. Firstly, we optimized the diffusion process to balance the low surface doping concentration and low contact resistance. Secondly, we modulate the SiNWs size to find the best balance between the reflectance and carrier recombination. Thirdly, we increased the electrode fingers density, and in the meantime decrease the electrode covered area and series resistance, which can decrease the carrier transport distance in high recombination area of emitter area. Fourthly, we optimized the black silicon etching process. Through all the above optimization of fabrication processes, the conversion of black silicon solar cells increased to 16.78%. Finally, in order to reduce the recombination at and near the surface in SiNWs while maintaining good light trapping properties, we propose here an effective method of etching by TMAH after the process of in-diffusion. Through the process of TMAH etching, surface recombination and Auger recombination at and near the surface of b-Si solar cells were dramatically suppressed, and 17.33%efficient black silicon solar cell without antireflection coatings was achieved, and 19.05%efficient black silicon solar cell with SiO2/SiNx:H stacks passivation films was achieved, which is the highest conversion efficiency for p-type black silicon solar cell up to now.