Investigation And Application Of Tunneling Silicon Oxide/metal Magnesium Electron-selective Collection Structure

N-type monocrystalline silicon cells have the advantages of higher minority carrier lifetime,lower light-induced degradation,better low-light response,and lower temperature coefficient.Besides,they display higher power output in working environments and show more space for improving the efficiency.According to the prediction of the international photovoltaic(PV)technology roadmap,the market share of N-type silicon cells will increase to about 35%by 2025.Since there is a substantial contact barrier between the N-type silicon and metals,to explore an Ohmic rear contact is critical for a high-efficiency N-type solar cell.The carrier-selective passivated contact structure including a carrier-selective collection functional layer and an ultra-thin passivation tunnelling layer can.realize Ohmic contact,and meanwhile,this structure achieves both the carrier-selective contact and surface passivation contact.Now the carrier-selective passivated contact has been a hot topic in PV research.The contact problem of N-type crystalline silicon cells has been solved with the application of electron-selective passivated contact structure.Recently,the power conversion efficiency(PCE)beyond 25%has been achieved by using two kinds of carrier-selective passivated-contact structures.The first one is the silicon heterojunction(SHJ)structure with the intrinsic a-Si:H/doped a-Si:H;and the second one is the tunnel oxide passivating contact(TOPCon)structure with the tunnel SiOx/doped poly-Si.Thus,it is of great value to develop the environmentally friendly,high-efficiency,and simple-structured N-type silicon cells technology.The application of low-work-function metal is simple and environmentally friendly,which displays excellent potential for high-efficiency N-type silicon solar cells.However,an excellent carrier-selective contact passivation structure must possess both excellent surface passivation and contact properties.In this work,a high-quality ultra-thin tunnelling silicon oxide layer was introduced to prepare a SiOx/Mg electron-selective passivated contact.The application of tunnelling oxide not only provides excellent surface passivation but also so avoids the Fermi level pinning effect.The properties of SiOx/Mg electron-selective passivated contact are studied.Also,this structure is integrated into two kinds of solar cells to examine its practical performance.The primary research contents are listed as follows:(1)A SiOx layer with a thickness of 1.2 nm to 1.8 nm was prepared by nitric acid bath with a post high-temperature annealing.The effects of the different bath temperature,the bath time and the post-annealing temperature on the thickness of SiOx layer were studied.The chemistry state of SiOx was analyzed by X-ray photoelectron spectroscopy(XPS).It was revealed that the Si4+proportion increases with the increment of the post-annealing temperature.Also,the effects of the SiOx annealed temperatures on the surface passivation and the contact properties of SiOx/Mg structures were investigated.It was found that the overall performance of the SiOx/Mg structure would be improved as the 7000C annealing,i.e.,the single-side saturated dark current density(JOe)is 115 fA·cm-2,the implied open circuit voltage(iVoc)is 661 mV,and the contact resistivity is 26 m?·cm2.(2)The Ag/PEDOT/N-Si/SiOx/Mg/Al-structured heterojunction solar cells were fabricated with the PEDOT:PSS coating as a hole-collection layer.In comparison with the A1 rear contact,the SiOx/Mg rear contact leads to significant improvement of the open-circuit voltage(Voc)and of the external quantum response(EQE)at the long wavelength.Finally,the hybrid solar cell with an efficiency of 15%,a Foc of 610 mV and a Jsc of 33.4 mA-cm2 is achieved by using the Siox/Mg rear contact.(3)N-type crystalline silicon solar cells with the Ag/Pd/Ti/SiNx/Al2O3/P+-Si/N-Si/SiOx/Mg/Al structure were fabricated to examine the practical performance of SiOx/Mg when used in Si solar cell.The fabrication of silicon cells was optimized and simplified by adjusting the preparation processes of the Al2O3/SiNx,the Ag/Pd/Ti/P+-Si and the Si/SiOx/Mg.Finally,a silicon cell with an efficiency of 16.7%is achieved.Furthermore,if the shading area and the series resistance could be reduced significantly,it is expected that a high-efficiency N-type crystalline silicon cell of>20.0%can be obtained with a high-quality SiOx/Mg passivated contact.