High Efficient Black Silicon Solar Cells Achieved By Ag/Cu Assisted Chemical Etching Process

In current PV industry,the crystalline silicon solar cells take up more than 90% market shares.Due to their lower cost and easy to slice,solar cells made from multicrystalline silicon(mc-Si)have become the hot researching topics of PV researchers and PV enterprises.But the high surface reflectance of mc-Si(average 24% after acid texture)causes a great loss of incident light that restricts the improvement of the conversion efficiency of solar cells.Black silicon is one kind of materials which can nearly absorb all the light from ultraviolet to near-infrared that has an excellent application prospect in PV industry.But at this moment,the black silicon is still mainly remained in lab.How to make this technology into industrial application is the main research contents of this thesis.We first used metal assisted chemical etching(MACE)method with dual metal elements system containing AgNO3,Cu(NO3)2,HF and H2O2 to prepare as-etched black silicon structures with excellent light trapping properties on mc-Si at room temperature.A trace amount of AgNO3 was used to accelerate the etching speed of Cu that makes sure the process can be realized at room temperature.Compared to other methods,this Ag/Cu MACE method is more cheaper and easier to realize industrial production.The effects of different etching parameters on the properties of black silicon were studied.Also,we analyzed the corrosion mechanism of Ag and Cu dual elements.The research results showed that the concentration of HF as well as H2O2,molar ratio of Ag+ and Cu2+(Ag+:Cu2+)and the etching time presented very important effects on the morphology and surface reflectance of black silicon.Finally,under HF of 1.8M,H2O2 of 0.4M,Ag/Cu of 1:5 and etching time of 180 s,we obtained an as-etched black silicon sample with a lowest surface reflectance of 4.74% in the wavelength range of 400-900 nm.But the conversion efficiency of solar cells doesn't increase with the reduction of the surface reflectance.The reason is that the heavy defects on the as-etched black silicon surface will have a bad effect on the quality of p-n junction as well as the passivation process.So we proposed a novel technology named nano-structure rebuilding(NSR)to enlarge the as-etched nanopores and controllably prepare invert pyramid structures.Furthermore,we use atomic layer deposition(ALD)method to prepare passivation layer of Al2O3.The effects of different NSR treatment parameters on the properties of invert pyramid samples were studied.Also,we analyzed the formation mechanism of the invert pyramid structures.The research results showed that the surface reflectance,minority carrier lifetime and the size of the invert pyramids all increased with the increase of the NSRtreatment time as well as the temperature.It was found that the surface structures transformed from square holes to invert pyramids and the depth of the as-etched black silicon structures had remarkable influence on the final size of the invert pyramids.Finally,we successfully prepared different size of invert pyramid structures from 100 nm to 900 nm.After ALD treatment,the invert pyramid sample with size of 600 nm showed excellent properties of reflectance and passivation effect.The minority carrier lifetime was 37.82?s,and the surface reflectance was 8.87% in the wavelength range of 400-900 nm after passivation.The mc-Si solar cells with different surface structures including invert pyramid structures,asetched nanopores structures and acid textured structures were made at production line.The solar cell with invert pyramid structures of 600 nm edge length showed an optimal properties: the Voc,Isc and FF were 631 mV,8.962 A and 79.95%,respectively,and the average conversion efficiency was 18.58%.Also we researched the texture property of our methods on diamond wire sawn(DWS)mc-Si.The research results showed that the saw marks on DWS mc-Si surface were nearly disappeared and uniform invert pyramid were formed after Ag/Cu MACE method and NSR treatment.We obtained the optimal DWS mc-Si solar cell with the same size of 600 nm invert pyramid sample,with Voc,Isc and FF at 632 mV,8.980 A and 80.18%,respectively,and the average conversion efficiency was18.71% which is higher than that of conventional multi wire slurry sawn(MWSS)mc-Si solar cell.We believe our method will provide an effective technological way to realize mass production of DWS mc-Si solar cells.