Study Of Micro-structure On Mc-Si Surface Textued By Alkaline Solution

Multicrystalline Silicon (mc-Si) has wide applications in the field ofphotoelectric devices, whose performance is closely related to the surfacestructure of mc-Si material. Especially, during the production of solar cells,the texture of mc-Si surface has an important effect on the conversionefficiency of solar cell. In this dissertation, the textured structure of mc-Sisurface by alkaline solution is studied, the influence of additives onanisotropic property of alkaline solution in mc-Si surface is analyzed, andthereby it is discussed that how to reduce the surface reflectance of mc-Siwafers with alkaline texturing.Nowadays, the acidic corrosion method is commonly employed totexture mc-Si, owing to the isotropic property of acidic solution in mc-Sisurface, namely different crystal facets with the same reaction rate duringthe texturing process. Although pit-traps with low reflectivity can beobtained through this method, the acidic is harmful to the environment andhuman being. On the other hand, it is difficult to form pit-traps with lowreflectivity on mc-Si surface by using usual NaOH corrosion solution.For the problems above, the present paper shows a study on theimprovement of mc-Si corrosion with alkaline solution by adding variousadditives. Through the analysis of surface SEM images, it is found thatusual NaOH solution with a right additive can texture the small compactand crisscross silicon mountain-like chains with many long gorgesbetween them for the (100) crystal facet, while distorted triangles fullycover (111) crystal facet, and worm-like traps for (110) crystal facet. Ascompared with the usual NaOH solution, it can to some extent reduce thesurface reflectance to21.25%in the wavelength of400~900nm, in spite of that there are still some differences between different planes.Some further study has been taken on the variation of texturedmicro-structure of mc-Si surface, which corresponds to the amount of theadditive in alkaline solution, the texturing time and the reactiontemperature. Analyzing the SEM images, it is to be found that:1) thevariation in amount of additive added has influence on the texturedmicrostructures of mc-Si surface. Generally, if the amount is very little, itwill be hard to regulate the corrosive property of alkaline solution ondifferent crystallographic planes; in contrast, if too much additive is addedto the corrosion solution, it will be difficult to remove saw damage layerson mc-Si surface; only appropriate amount of additive is applied, one canget good textured surfaces with low reflectivity.2) the impact of thetexturing time: if the texturing time is too short, it will be very difficult toremove the mechanical damage layer in mc-Si surface; if the texturingtime is too long, the size of pit traps will be too large. So the right texturingtime is very important for the texture structure with low reflectance, andthen the experimental results prove that the variation range of texturingtime is from20min to30min.3) the effect of the texturing temperature onmc-Si surface microstructure: if the temperature is too low, it is verydifficult to completely remove the saw damage layer, and the leakagecurrent of solar cell is increased; if the temperature is too high, thehomogeneity of texture structure is very bad due to the excessvolatilization of ethanol in corrosion solution. Therefore, during thetexturing process, the temperature is controlled around80°C. Finally, thephotoelectric conversion efficiency of mc-Si solar cell and minority carrierlifetime of mc-Si textured are measured, and the measured result provesthat the conversion efficiency is improved to17.2%.