The Study Of Passivation Mechanism Of Double-sided TOPCon Structure And Its Application In High Efficiency Solar Cells

The carrier selective passivation contact has been proved to be an important way to improve the efficiency of solar cells,where TOPCon is the widely studed structure because of the high efficiency potential.In TOPCon solar cells,an ultra-thin layer of silicon oxide plus heavily phosphorus doped polysilicon is used as the electron selective contact or heavily boron doped polysilicon is as the hole selective contact.Poly-Si/SiO_x structure passivation contact solar cells are fabricated using the high temperature process,which is compatible with the PERC or PERL technology.The record solar cell efficiency has reached 25.7%using the n⁺-poly Si/SiO_x as the back contact without IBC.The poly-Si/SiO_x passivation technology is expected to be integrated into the mass production line soon.Although the cell efficiency improvement with the ultrathin Si O_x/poly-Si has gained momentum,several fundamental principles have not been understood clearly,including the passivation mechanism namely the chemical or field effect passivation,and the fundamental reasoning for the difference between n⁺-poly-Si and p⁺-poly-Si in terms of passivation quality.For the passivation mechanism,one can qualitatively say that the chemical passivation terminates the interface defects to reduce thr interface recombination and the electric field passivation repels the minority carriers away from the defective interface region,but no quantative assessment has been made on each component in a real passivation contact.All of these issues have not been well studied in the literature.The objective of this paper is to investigate the passivation mechanisms by the ultrathin SiO_x/poly-Si on c-Si wafer,where the poly-Si was made with three types of undoped,P-doped and B-doped precusors,while the c-Si includes both n-type and p-type wafers.Because the undoped poly-Si has no intentional doping and the passivation would result purely from the chemical passivation by reducing the interface defects,while the doped poly-Si causes a band bending and therefore an electric field effect passivation is expected,which could be positive or negative depending on the combination of the types of poly-Si and c-Si wafer.Additionally,the crystalline structure in the poly-Si and the dopant diffusion through the SiO_x and penetration into the c-Si wafer were investigated to explore other factors affecting the passivation quality.Based on the study of passivation mechanism,the passivation quality of poly-Si/SiO_x structure is further improved by optimizing the fabrication process.Three types of silicon oxide are made with high temperature nitric acid oxidation,plasma-assisted N2O oxidation and thermal oxidation.Two types of TCO layer are used for completing the double-side TOPCon solar cells,which are the ITO prepared by magnetron sputtering and AZO prepared by ALD.Finally,the solar cells with an effective area of 4 cm² were prepared on 4×4 cm² n-type c-Si substrate.With the AZO as the top contact,the efficiencies of the solar cells with HNO₃-Si O_x,N₂O-SiO_x and T-SiO_x are 15.54%,17.41%and 13.83%,respectively.The cell efficiency with the ITO top contact is slightly lower than with the AZO,with the value of 14.06%,14.89%and 16.03%from the cells with the HNO₃-SiO_x,N₂O-SiO_x and T-SiO_x respectively.Because of the better performance of the N2O-Si O_x,combining the AZO,we did further device optimization on this structure and achieved the highest conversion efficiency of 19.26%.