| At present,the most common solar cells on the market are still dominated by crystalline silicon solar cells,but the cost of such crystalline silicon solar cells is relatively high and the utilization of light is low.In this project,black silicon is prepared by metal-assisted chemical etching to reduce the reflectivity of silicon surface to incident light and improve the utilization of light.Due to the unique size effect and quantum confinement effect of graphene quantum dots,the band gap is not zero,and the up-conversion luminescence property and efficient generation and collecting capacity of photo-generated carrier are obtained,it can be prepared in a black siliconbased graphene quantum dot/black silicon heterojunction solar cell.The solar cell utilizes graphene quantum dots as a P-junction instead of a portion of the silicon material of the original crystalline silicon solar cell,so that the structure is simple and the cost is reduced.A graphene quantum dot with a quantum yield of 0.34 can be prepared by hydrothermal method.The quantum dot has a small size,a lattice spacing of 0.24 nm and a thickness of 1.9 nm.The surface of the prepared graphene quantum dots contains a large number of functional groups,so it is easily soluble in water and which has unique fluorescence properties.Its fluorescence has excitation dependence,and it is cyan-green fluorescence under the illumination of 320 nm UV lamp,which has the unique property of up-conversion luminescence.In this project,a two-layer micro-nano structure was prepared on an N-type single crystal silicon wafer to prepare black silicon capable of achieving double-layer antireflection effect.The optimum preparation process can be obtained by analyzing the effect on nanostructures and reflectivity of the lack silicon by the composition of alkaline solution,Ag NO3 concentration,deposition time,H2O2 content in HF+H2O2 acid corrosion system and etching time.The optimum preparation process includes three steps.First,Pyramid structure will be obtained by treating N-type single crystal silicon wafer in a 2% Na OH solution at 85 ? water bath for 30 minites.Second,depositing the monocrystalline silicon wafer in a concentration of 0.03 M Ag NO3 solution for 15-20 seconds.Third,putting it into a 10%HF+10%H2O2 acid corrosion system to etched for 8 minites.Finally we obtained the black silicon which has very good absorbance,and the reflectance of incident light in the range of 200-1000 nm can reach 2% or less,what's more,it minimum reflectance of incident light can reach 1.8%.The graphene quantum dot aqueous solution is dropped onto the black silicon surface to be heated and dried to form a heterojunction with the black silicon surface,and the positive and negative electrodes are deposited on the heterojunction to prepare a photovoltaic device.The device has a simple structure and a simple preparation process.Due to the large difference between the graphene quantum dots and the silicon,the photo-generated electron-hole pairs can be separated,and the lowest electron nonoccupied orbital(LUMO)of the graphene quantum dots is lower than that of the silicon.Photo-generated electrons can only be transported to the n-Si by the Al cathode electrode and cannot be directly transferred to the Ag positive electrode.The graphene quantum dots mainly act as an electron blocking layer and hole transport in the photovoltaic device.The effects of different surface modification and different size distribution of graphene quantum dots and content on the photovoltaic performance of the device were investigated.The finally obtained graphene quantum dot/black silicon heterojunction photovoltaic device has a photoelectric conversion efficiency of 9.64% and an external quantum efficiency of 56.9%,which is maded in a dimensional distribution of 1-4 nm,and volume of 4 m L of graphene quantum dots with methylated modified black silicon. |
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