Study On The Fabrication And Performance Of SiO₂ Nanosphere Assisted Diffusion And C-Si Solar Cells

As the core structure of the solar cells,p-n junction's quality can significantly influence the photoelectric conversion efficiency.In industrial production,the preparation of the n⁺layer usually adopts by POCl₃ diffusion and the p⁺layer adopts by BBr₃ diffusion.Yet both the chemicals are highly corrosive and cause serious consequence by improper operation.Phosphoric acid and boric acid as phosphorus and boron sources were proposed in this thesis because of its low risk characteristic.The manufacture process of solar cells was studied systematically,and the p-type solar cells were fabricated by optimized conditions.Firstly,phosphoric acid was spin coated on the surface of p-type silicon substrate to form n⁺emitter and p-n junction was fabricated by thermal diffusion.The performance of n⁺emitter was characterized by four-probe sheet-resistance test and electrochemical capacitance-voltage profiler?ECV?.The minority carrier lifetime of samples was measured before and after diffusion as well.In order to improve the uniformity,a new method by adding SiO₂ nanospheres into phosphoric acid solution was proposed.After diffusion,comparison was done between the one by phosphorus source only and that with mixed SiO₂ nanospheres.The results showed that the diffusion uniformity increased from 86.0%in sample without SiO₂ nanospheres to 96.5%in that with SiO₂ nanospheres,and the minority carrier lifetime also increased from 42?s to 56?s.Different SiO₂ nanospheres diameters with were used to explore the enhancement mechanism of diffusion uniformity by SEM test.Comprehensive analysis showed that the SiO₂ nanospheres with the diameter of 230 nm had a good effect on diffusion.It was found that with the increase of phosphoric acid concentration,the sheet resistance decreased gradually.In other words,the doping concentration was increased after diffusion.The junction depth showed a little change due to the different phosphoric acid concentration.Combining the simulation results of PC1D,by using a concentration of phosphoric acid of 65%,a diffusion temperature of 940?and the holding time of 40min,a high quality emitter could be obtained after diffusion.For the preparation of p⁺layer,the method by adding SiO₂ nanospheres into boric acid solution to make boron source was proposed.After spin-on doping?SOD?,the p⁺emitter was fabricated by thermal diffusion.By changing the concentration of boric acid solution,temperature and holding time for diffusion to explore the influence of the above factors on the diffusion process using four-probe,minority carrier lifetime and ECV test.Combined with the simulation results of PC1D,we conclude that when the boric acid concentration were 5 g/100mL,at 940?diffusion temperature and the holding time were 40 min,the doping concentration were 8×10¹⁸·cm^-3,junction depth were 0.28?m,the diffusion uniformity is 96.5%and the minority carrier lifetime is 3.39?s.The p/p⁺junction have a good performance.Through comprehensive analysis of the fabrication process of p-n junction and p/p+junction,the n⁺emitter was fabricated by spin coating 65%phosphoric acid mixed with SiO₂ nanospheres at 940?for 40 min diffusion,and then the p-type conventional solar cells were fabricated by industrialization process.In addition,the p-n junction and the p/p⁺junction were prepared by boron-phosphorus co-diffusion at 940?for 40 min and then the boron back filed solar cells were fabricated by industrialization process.The tests showed the diffusion mixed without SiO₂ nanospheres have a low efficiency and have a bad efficiency uniformity.The highest efficiency were 12.16%only.The electroluminescence?EL?results depicted that have a poor performance.The diffusion mixed with SiO₂nanospheres have a good performance and efficiency uniformity.The highest efficiency were 17.09%.The EL images of solar cells with SiO₂ diffusion have a uniform brightness.Compared with the conventional solar cells,the open-circuit voltage,short-circuit current and filling factor of the boron back-field solar cells was improved.The EL test shows that boron back-field solar cells have a higher brightness compared with the conventional solar cells.The maximum efficiency was 19.19%.