Study On The Performance Of The Porous Silicon Prepared By Electrochemical Etching

Porous silicon is a sponge-like porous structural one which is prepared on thesurface of silicon wafer by electrochemical anodic etching. It can be used as anantireflective coating of the solar cells to reduce the loss of the incident light andimprove the light conversion efficiency because of the unique microstructure andantireflective characteristics. This technique is comparable to conventional surfacetexture and deposited antireflective coating, so it has important value on science toconduct study of porous silicon.At the beginning, present thesis expounds the research status, current preparationmethods and the different understanding on the formation theory of the porous siliconused as an antireflection coating for silicon solar cells. In the experimental research, acomparative study was conducted firstly between the different electrochemical etchingmethods to prepare porous silicon. Their process characteristics and technical factorswere explored respectively. The effects of various process parameters on themorphology and performance of the porous silicon prepared by electrochemicaletching were investigated. The experimental results were analyzed and discussed indetail combined with scanning electron microscopy （SEM）, UV-visible-near-infraredspectroscopy and electrochemical tests. The experimental results are showed below:Comparative experiments showed that the surface morphology of the poroussilicon prepared by double-cell electrochemical etching was more uniform than theporous silicon prepared by single-cell electrochemical etching, the pore for formerwas denser and the surface reflectivity was lower.Under the selected technical parameters the porous silicon prepared on n-typemonocrystalline silicon by double-cell electrochemical etching had uniform surfacemorphology, dense pore and low surface reflectivity. Using HF/C₂H₅OH/HNO₃asetching solution with a ratio of1:2:1and the temperature of25degrees centigrade,through changing the current density and the etching time, we can come to thefollowing rule: As the current density increased, the diameter of aperture increasedgradually, the hole spacing decreased gradually, the porosity increased in thebeginning and then decreased rapidly, the reflectivity decreased in the beginning andthen increased rapidly; As the etching time increased, the thickness of the porous layerincreased gradually, the small branch pore generated on the hole wall was more and more, the porosity in the beginning increased and then reduced, the reflectivity in thebeginning reduced and then increased. The porous silicon etched with the currentdensity of30mA/cm²and etching time of90min showed the lowest reflectivity, theaverage reflectivity was5.6%within the measured wavelength range of400¹100nm.Under the selected technical parameters the porous silicon prepared on p-typemonocrystalline silicon by double-cell electrochemical etching also had uniformsurface morphology, dense pore and low surface reflectivity, but the averagereflectivity was slightly higher. Using HF/C₂H₅OH/HNO₃as etching solution with aratio of1:2:1and the temperature of25degrees centigrade, through changing thecurrent density and the etching time, we can come to the following rule: as the currentdensity increased, the diameter of aperture increased gradually, the thickness of theporous layer increased in the beginning and then decreased, the porosity increased inthe beginning and then decreased sharply, the reflectivity decreased in the beginningand then increased sharply; as the etching time increased, the thickness of the porouslayer increased in the beginning and then thined because of excessive corrosion, theporosity increased in the beginning and then decreased, the reflectivity decreased inthe beginning and then increased. The porous silicon etched with the current densityof20mA/cm²and etching time of90min showed the lowest reflectivity, the averagereflectivity was8.3%within the measured wavelength range of400¹100nm.Prepared of porous silicon on p-type polycrystalline silicon, the etching solutionwas HF/C₂H₅OH with a ratio of1:1and the temperature of25degrees centigrade.Investigated the influence of the current density and the etching time on thepreparation of the porous silicon, the results showed that: a crack-like suedemorphology was formed in that etching solutionï¼›with the increase of the currentdensity, the width of the cracks gradually increased, the depth of the cracks was alsogradually increased, the porosity increased gradually, the reflectivity decreased in thebeginning and then increased; with the increase of the etching time, the crackgradually widened and deepened, the porosity gradually increased and the reflectivitydecreased in the beginning and then increased. The porous silicon etched with thecurrent density of5mA/cm²and etching time of60min showed the lowest reflectivity,the average reflectivity was16.2%within the measured wavelength range of400¹100nm. An orthogonal experiment was carried out with the etching time, currentdensity and the solution ratio as the factors in the etching solution ofHF/H2O/C₂H₅OH. The optimum process in that etching solution was got: the etching time of45min, the current density of10mA/cm²and the solution ratio of1:1:3. Theporous silicon etched in that optimum process showed the lowest reflectivity, theaverage reflectivity was12.7%within the measured wavelength range of400¹100nm.The experimental results showed that preparation of an antireflective coating onthe surface of the silicon wafer by double-cell electrochemical etching with selectedtechnical parameters was a simple, low cost, good effect and environment-friendlymethod.