The Effect Of Etching Surface Morphology On Light-trapping Efficiency Of Multi-crystalline Silicon Wafers

Recently,diamond wire sawing?DWS?technique has been considered as a replacement of multi-wire slurry sawing?MWSS?due to the low loss rate of high-purity silicon,higher slicing speed and shallower saw marks.The acid etching is a texturing process for MWSS multi-crystalline silicon?mc-Si?wafers.With its simple process and low cost,it is favored by the majority of manufacturers.However,as-cut Si wafer morphology makes a big difference to the evolution of the wafer surface structure during the wet chemical etching.The morphology of wafer surface obtained by DWS technique was different with that by MWSS technique,which resulted different wet chemical etching behaviors during texturing process.The morphology of DWS wafers don't match with the traditional texturing technique.The conventional acid recipes have failed to reduce the reflectivity of DWS wafers to a satisfactory level.Many treatments,such as reactive ion etching,reaction vapor etching and metal-assisted chemical etching were proposed to decrease the reflectivity of mc-Si to meet the need of photovoltaic industry.Among these texturing techniques,wet-chemical etching technique was widely used due to the low cost and simple process.This work draws the following conclusions by studying the acid etching of DWS mc-Si wafers:?1?The parallel saw marks on the DWS mc-Si wafers will influence the texturing process during acid etching.On the textured surface,the oval pits were appeared along the saw marks;the value of the diameter in the perpendicular direction for most pits is greater than that of the parallel direction,for 75%of the oval pits the ratio of them are approximately1:1-1:3.?2?Glacial acetic acid can increase the density of etching pits on texturing wafer;sodium nitrite will increase the reflectivity of silicon wafers after texturing;the corrosion inhibition effect of phosphoric acid in high concentration areas is very obvious and can reduce etching pore size;the corrosion inhibition effect of polyethylene glycol is very obvious,which can change the surface morphology;Dodecyl benzene sulfonic acid with a mass fraction of1%as an additive can reduce the reflectivity to 18.82%.?3?A Nano-scale oval-pit Si wafer surface was obtained after wet etching method with addition of 1000ppmw NaNO₂,100ppmw PEG-PVA and 10ppmw DBSA.The reflectivity of which is 20.10%.Compared to the textured wafers without additives the reflectivity was reduced by 47.50%and the removal rate was reduced by 23.42%.The photovoltaic conversion efficiency of the cell is 18.52%.?4?The H/D values in parallel direction are much smaller than that in perpendicular direction for the same sample.In the meantime,the anisotropy of H/D causes the difference of Rave values detected by different ways.When the light is incident from the parallel direction,the R_ave is 22-27%larger than that when the light is incident from the perpendicular direction.Moreover,the results also reveal that R_ave vary inversely with increasing H/D.?5?When the thin grid line of Ag electrode is parallel to the saw marks,the energy conversion efficiency is 0.6-0.8%higher than that when the thin grid line of Ag electrode is perpendicular to the saw marks for the same sample.This may attribute to that the contact between the wafers and the Ag electrodes in parallel direction is closer than in perpendicular.