Application Of Black Silicon Structure In Solar Cells

Crystalline silicon solar cells have always been the mainstream products in the photovoltaic market.Monocrystalline silicon and polycrystalline silicon solar cells have developed competitively.With the technology improvement,the cell efficiency has also been promoted year by year.In recent years,polycrystalline silicon solar cells have received great attention due to their cost advantages.Black silicon structure have long been used in solar cells due to its superior light trapping properties,but have not been widely popularized due to some technical and cost reasons.Until the introduction of the diamond-wire-sawn technology for polycrystalline silicon wafers,the black silicon technology has been adopted by photovoltaic researchers once again to replace the traditional texturing method to improve cell efficiency because such diamond-wire-sawn wafers could not be finely textured by traditional acid-texturing technology.At present,black silicon technology has been widely recognized as the best way to develop polycrystalline silicon solar cells.In this thesis,nano-submicron black silicon structure has been successfully prepared by copper?Cu?metal catalyzed etching method on polycrystalline silicon and monocrystalline silicon wafers.The lowest average reflectance of the hole-shaped black silicon structure on polycrystalline silicon wafers can reach 10%or less?in the spectral range of 300¹100 nm?.In combination with the corresponding post-treatment process,a smooth chamfered cone-shaped anti-reflection structure is obtained,which can match the following cells process and reduce the recombination loss.Finally,the flow-through verification on the solar cells production line shows that the black silicon technology can not only reduce the reflection of the silicon wafer surface,but also effectively improve the efficiency of the solar cells.Therefore,such black silicon technology is expected to replace the conventional acid texture process.The inverted pyramid structure obtained on the monocrystalline silicon wafer can further reduce the surface reflectivity compared with the industrial alkali-texture pyramid structure.Also,the processing steps are quite simple and suitable for industrial production.The main findings of the thesis are summarized as follows:1.We develope a complete two-step Cu metal catalyzed etching process for polycrystalline silicon wafers,including three main processing steps,namely etching,post-treatment and metal removal.We systematically investigate the effects of the change of HF and H₂O₂ content in the etchant and etching time on the black silicon structures and their reflectivities.2.Such black silicon structure is applied to the polycrystalline silicon solar cell and the effect of the post-treatment process is studied.Although the optical loss is reduced through using the black silicon structure,the cell efficiency cannot be improved due to the increase in electrical loss.This is because the particular structure characteristics of such black silicon structure are hard to match with the subsequent cell processes.Therefore,we use HF/HNO₃/H₃PO₄ post-treatment process to modify the black silicon structure.As the post-processing time increases,the size of the pores of the structure increases gradually,the depth decreases,finally the surface becomes smooth,the surface reflectivity of however gradually increases.Finally,the 120 s post-treatment time was optimized with a balance between anti-reflection and cell efficiency improvement.A 156.75×156.75 mm² large-area DWS multi-crystalline black silicon solar cell with an efficiency of 18.88%is successfully prepared,which is 0.4%higher than that of the conventional acid texture DWS multi-crystalline silicon solar cell in production line.3.We fabricate inverted pyramid structure on monocrystalline silicon wafer using a one-step Cu catalytic etching method and systematically investigate the effects of the change of HF content and the ratio of HF/H₂O₂ and etching time on the morphology and reflectivity of the inverted pyramid structure.Compared with the reflectivity of about 12%of the pyramid structure on a conventional monocrystalline silicon wafer,we have prepared an inverted pyramid structure to reduce the surface average reflection of the monocrystalline silicon wafer to about 6%.By applying such inverted pyramid structure to conventional monocrystalline silicon cells,the efficiency is equivalent to that of conventional monocrystalline silicon cells.In addition,the reaction time of the inverted pyramid preparation process developed in this thesis can be adjusted to be about 5 min,which means that our process can satisfy the monocrystalline silicon cell process without reducing the cells yield.