The Fabrication And Optimization Of Femtosecond-laser Etched Black Silicon Solar Cells

Solar energy is a kind of inexhaustible green power source, the use of solar cell is the most effective way to utilize sun power. However, conventional silicon solar cell has a relatively low conversion efficiency of about 18%, mainly due to silicon is a kind of indirect-band material, more energy is needed for carriers move to conduction band of silicon than that of direct-band material like GsAs, causing great power loss. But for semiconductor material, silicon is relatively cheap. Up to 2014, silicon solar cells occupy more than 90% of market share, so further improve the conversion efficiency of silicon solar cells is of great significance in promoting the development of photovoltaic industry.There are mainly two obstacles preventing silicon solar cells to achieve a higher efficiency: Firstly, untreated silicon surface is highly reflective, sun light reflected from silicon can’t be transfer into electricity, which results in power loss; Secondly, silicon is a narrow band-gap material, in which near-infrared light can’t be absorbed. In this work, we mainly concentrate in reducing surface reflectance on silicon surface.In1998, "black silicon" was invented. The material has unique light absorbing characters which can absorb more than 90% of visible as well as near-infrared light. The unique character of black silicon greatly make up for the weakness in light trapping of conventional silicon material, which show great development prospect in solar cell application. However, there are plenty of laser-induced defects on black silicon, as a result, solar cells made by black silicon without surface recombination control always result in low conversion efficiency.To further improve the conversion efficiency of black silicon solar cells, in this work, we mainly concentrate in two aspects: Firstly, we focus on fine surface treatment of removing laser induced defects on black silicon surface; secondly, a fabrication procedure which is suitable for black silicon solar cells is designed. In the research of removing laser induced defects, three different ways(ICP etching, anisotropic-etching, isotropic-etching) are investigated, we find that the isotropic-etching provide the best result; then we work on fine optimization of anisotropic-etching parameters by investigate minority carrier life time inside black silicon as a function of etching time, a novel mixed surface morphology of mixed upright pyramid/inverted pyramid mixed texture(MT) is obtained, which show excellent light trapping ability over conventional pyramid texture(PT), and laser induced defects is mostly removed. To obtain the best cell fabrication parameters, we work on laser fluence optimization, surface passivation, and an aluminum back surface field is added. Eventually, we use optimized procedure to fabricate actual black silicon solar cells and investigate its light trapping ability, I-V characters, and quantum efficiency. Results show that the etching procedure improve open circuit and internal quantum efficiency of black silicon solar cells significantly; the MT morphology induced a higher short circuit current density of black silicon solar cells than that of PT cells. Eventually, the black silicon solar cells fabricated in this work show an improved efficiency of 15.6% from a previous value of 14.2%, which also show higher efficiency than PT cells(fabricated at the same time with black silicon solar cells as reference) which efficiency of 15.3%.