Preparation Of Antireflective Structures For Silicon Solar Cells

New energy sources are needed to be developed due to the energy crisis, environmentpollution and other factors. Solar energy has the advantages of inexhaustion, non-pollution,etc, so it has been paid more attention in the past decades. The research of silicon solar cellsmainly focuses on reducing cost and conversion efficiency, and one of the effective methodsof improving photoelectric conversion efficiency of solar cells is to decrease the reflection ofincident sunlight onto the light-receiving surface. Recently, noble metal assisted chemicaletching to produce porous Si structure has been developed and paid increasing attentionbecause of its simple, low cost, controllable, and high effective process. In this paper, Agnanoparticles are deposited on the monocrystalline Si wafer and Si thin film surfaces by bothelectroless plating and magnetron sputtering, and antireflective structures are produced inetching solution composed of HF, H₂O₂and H₂O. The effects of HF, H₂O₂and their ratio onetching structures and properties are systematically researched by using field emission SEM,atomic force microscopy （AFM） and UV-vis spectrophotometry. The experimental results areobtained as follows:Ag particles are deposited on the surfaces of single crystalline Si wafers by bothelectroless plating and magnetron sputtering. It is found that the deposited Ag particle densityincreases with increasing deposition time, and the Ag particle density has a strong effect onthe etching morphology of Si wafer produced in HF-H₂O₂-H₂O solution. For a low density Agparticles covered Si, a disordered structure is easily formed; for a moderate density Agparticles covered Si, a vertical porous structure can be produced; for a Si wafer covered byhigh density Ag particles, a nanowire-like structure is obtained, while nanowire arrays can beproduced when the surface of Si wafer is covered by too high density Ag layer. The resultsabove indicate that different etching structures can be produced by changing the density of theAg particle layer on the Si surface.The effects of HF and H₂O₂contents on etching morphologies of monocrystalline Si surfaces are investigated. With H₂O₂content increases, the pore size of the porous structureincreases, and the etching rate increases first and then decreases. The etching rate is the fastestwhen the H₂O₂content is3ml in18ml etching solution. With increasing the HF content, thepore size decreases, and the etching rate increases first and then decreases, and it reaches thefastest when the HF content is7ml in22ml etching solution. With the increase of molar ratio,ρ=[HF]/（[HF]+[H₂O₂]）, the etching rate increases first and then decreases, and it is thefastest when ρ=74.74%. In this paper, the etched Si surface produced under the optimizedetching condition exhibits an average reflectivity as little as2.20%, showing that an excellentantireflection is provided.The effects of the etching time and concentration on etching morphologies on Si thinfilms are deeply researched. The experimental results show that the etching structures areporous. With the increase of etching time, the etching depth of the Si film increases, however,the pore number decreases. The resulting Si film surface etched for60s exhibits a reflectivityof2.22%in the spectral range of200-800nm. With increasing the content of HF and H₂O₂inthe etching solution, the etching rate of the silicon film increases, and the pore size increasesand the number of pores decreases. The optimized etching solution is HF: H₂O₂:H₂O=2.5:2.5:10. Etched in such a solution, the resulting structure on the silicon thin filmpresents a minimum reflectivity of1.73%, showing that an excellent antireflecting structurecan also be prepared on Si thin film.