Research On Monocrystalline Black Silicon Microstructure And Its Application In Silicon Solar Cells

Over the past decades,crystalline silicon(c-Si)solar cells have occupied the main share of the photovoltaic(PV)market.As the main theme of the PV industry,is not only the main theme of the development of the c-Si solar cell industry,cost reduction and efficiency improvement are also the driving forces of solar cell technology innovation.As the first production process in c-Si solar cells,the texturing process usually uses a chemical etching method to form a textured structure on the silicon surface to enhance the light trapping effect and improve the light absorption of the solar cells,which is the basis for improving the conversion efficiency of the solar cells.In recent years,due to the rapid development of texturing additives,the alkaline texturing process for monocrystalline silicon(mono-Si)wafers has been greatly improved.At present,mixed alkaline solutions composed of alkali(KOH or NaOH)and fast-texturing(FT)additives are mainly used in the PV industry to form random-pyramid structured textures with sizes of 1-3μm.The reaction temperature is generally approximately 80℃,the texturing period is approximately 7 min,and the bath lifetime is up to approximately 400 runs.It is still a hot research topic to further improve the texturing process for mono-Si solar cells in the PV industry.In the past few years,our group has developed a metal catalytic chemical etching(MCCE)process to apply it for submicron texturing for multicrystalline silicon(mc-Si)solar cells and realized industrial application,promoting the progress of mc-Si solar cell technology.In this dissertation,the preparation of monocrystalline black silicon microstructures and its application in mono-Si Passivated emitter and rear cell(PERC)solar cells are studied.The main achievements in this research are summarized as follows:Firstly,nanowire-and porous Si-structured black silicon structures were fabricated on polished surfaces and conventional micron-pyramidal textured surfaces of mono-Si wafers by the MCCE method.The effects of different Ag+concentrations and p(the ratio of HF concentration to the sum of HF and H2O2 concentrations)values on the morphology and light-trapping property of black silicon structures were studied.The results show that the density of black silicon nanostructures on the polished surfaces and the micron-pyramidal textured surfaces of mono-Si wafers increases with increasing Ag+concentration.On the surfaces of the polished mono-Si wafers,silicon nanowire structures with a depth of approximately 6 μm are prepared when the Ag+concentration is 1 ×10-3 mol/L and p is 85%,and the lowest reflectance is 4.3%.On the surfaces of micron-pyramidal textured wafers,the depth of black silicon structures increases with increasing p value,but the etching degree of the pyramid spires also increases.When the Ag+concentration is 5x 10-3 mol/L and p value is 80%,the surfaces of the composite structures constructed by micron-pyramidal and black silicon porous Si achieve the lowest reflectance of 2.6%.Secondly,the pyramidal textures of mono-Si wafers were prepared based on the nano wire-and nanopore-structured black silicon structures.The black silicon structures under different preparation conditions and their influences on the formation of subsequent pyramid structures were studied.The results show that when the Ag+concentration is 1×103 mol/L,silicon nanowires with a depth of 1-2 μm are prepared by MCCE etching for 150 s,and then,a mature pyramid texture can be obtained by placing the structures in alkaline texturing solution for 300 s.When the Ag+concentration is 3×10-4 mol/L,nanopore structures with a depth of 200～1000 nm are prepared by MCCE etching for 180 s,and then,the mature pyramid texture can be obtained by placing the structures in alkaline texturing solution for 180 s;that is,an ultrafast texturing(UFT)process for mono-Si wafers is realized.The weighted average reflectance of the pyramidal texture prepared by using the UFT process is 11.8%in the wavelength range of 300～1100 nm.Compared with the pyramidal texture prepared by using the conventional FT process(the weighted average reflectance is 12.5%),the light-trapping property is slightly improved.Finally,the electrical properties of mono-Si PERC solar cells fabricated by using the UFT process and the conventional FT process were compared and studied.The results show that the solar cells with small pyramidal textures prepared by using the UFT process have better light-trapping ability and passivation effect.We have achieved an average efficiency of 22.88%and a highest efficiency of 23.02%for mono-Si PERC solar cells on an industrial pilot line,surpassing those of the reference counterparts fabricated by the conventional FT process(average efficiency is 22.73%,highest efficiency is 22.83%).In addition,it is estimated that the total chemical cost of the new UFT process is comparable to that of the conventional FT process.