Study On The Key Technologies Of N-type High Efficiency Crystalline Silicon Solar Cells

With the development of the modern society,the demand for energy is growing to human being.The traditional fossil energy is increasingly exhausted.At the same time,it leads to environment problems by excessive exploited and improper used.It is necessary to make great effort to develop the renewable energy sources and resolve energy crisis,as well as to realize the sustainable development of the human being society.In the last ten years,solar photovoltaics?PV?is one of the fastest development of new energy industries due to the mature technologies and subsidy policies.The crystalline silicon solar cell is one of the important components of the solar PV,and is the dominant product in the solar PV market.At present,the p-type monocrystalline/multicrystalline silicon Al back surface field?Al-BSF?solar cells are the mainstream products because of the relatively simple manufacturing process and low cost.However,the light induced degradation?LID?from the p-type crystalline silicon?c-Si?solar cell has not been solved completely,it is also difficult to achieve the conversion efficiency up to 22%and so on.All of these factors will be the main obstacles in developing the p-type c-Si solar cells.Contrary to the p-type c-Si materials,n-type c-Si with higher minority carrier lifetime and higher tolerance to the commen metal?e.g.,Fe?is to be the preferred material to manufacture the high-efficiency c-Si solar cells.In recent years,n-type high-efficiency c-Si solar cell is greatly concerned by the PV industry.The conversion efficiencies of the HIT?Hetero-junction with Instrinsic Thin-layer?and IBC?Interdigitated Back Contact?solar cells are more than 22%.However,it is difficult to popularize the the two technologies because of the complicated processes and high costs.In this thesis,we investigated some high-efficiency and low-cost n-type c-Si,including introduction of the manufacturing processes,analysis of the photoelectric properties and further discussion of the feasibility of large-scale industrialization,and etc.The main detailed content of this thesis is divided into the following sections.Firstly,we have proposed thin Al₂O₃?up to 5 nm?with SiN_x:H capped?75 nm?films to passivate the B-doped p⁺emitter surfaces.Al₂O₃ with a wealth of negative charge mainly serves for the role of the field effect passivation,and the chemical passivation is mostly provided by the SiN_x:H capping layer.We have achieved the high efficiency of20.89%(V_oc=0.652 V,J_sc=40.44 mA/cm² and FF=79.25%)n-type bifacial c-Si solar cells?238.95 cm²?using boron tribromide?BBr₃?diffusion emitter and P ion-implanted BSF?to realize the front and back side doping easily and steadily,rather than the conventional complicated bifacial diffusion or co-diffusion?in conjunction with screen printed contacts.We have used a co-firing step to sinter the different metallization pastes and to form electrical contacts for the front p⁺emitter and rear n⁺BSF,i.e.,there is no need to open the front side dielectric passivation layer with laser or etching pastes due to the thin Al₂O₃.We have further shown through the PC1D modeling that the conversion efficiency of our n-type bifacial Si solar cells can be improved to be over 21.3%by taking a lighter doping concentration with a higher emitter sheet resistance of 100-110?/?.The present thin Al₂O₃ layer and its composite SiN_x:H structure,together with the simplified process from the traditional n-Pasha cell structure,have great advantages in the process of n-type bifacial c-Si solar cell industrialization,not only passivating the p⁺emitter surfaces effectively,but also compatible with the current industrial process at a low cost.Secondly,we have applied the Selective Emitter?SE?and Tunnel Oxide Passivated Contact?TOPCon?technologies to improve the conversion efficiency of the n-type silicon solar cell.We have employed the boron tribromide?BBr₃?diffusion to obtain the borosilicate glass?BSG?,and then the silicon front surface was locally laser doped by the the mothod of the laser scanning.The doping areas were corresponding with the screen printing pattern.The BSG layer was removed by the wet chemical etching and then the back furface was polished,which ensure the ultrathin oxide layer?SiO₂¹.5 nm?effective growtn on the back surface.Considering the compactness of the oxide layer,the ultrinthin oxide layer may obtained by the nitric acid oidaton method or high temperature thermal oxidation.After the ultrathin SiO₂ processing,the back side of the wafer were deposited poly-Si with P dopant.And then,the high temperature annealing process?890,910,930?and 10,20 min?may change the poly-Si into the amorphous silicon.Then,the TOPCon structure on the back side of the wafer,not only passivated the silicon wafer surface,but also selectively absorbed the charge carriers?released the electrons and blocked the holes?.At last,we have intergrated the SE and TOPCon technologies in the n-PERT structure and achieved the high efficiency of 21.16%(V_oc=0.665 V,J_sc=40.40 mA/cm² and FF=78.75%)after the bifacial passivated ARC and screem printing as well as the high temperature sintering processes.Finally,we have used a much simplified method to formed and integrate the three different doping concentration areas of the BJBC?Back-Junction Back-Contact?silicon solar cell,i.e.,based on one single high-temperature treatment?so-called co-diffusion?to form the n⁺FSF?Front Surface Field?and p⁺emitter,and the P ion-implantation and annealing oxidation to form the n⁺BSF after the laser patterning,together with the height difference between adjacent n⁺-and p⁺-doped areas on the back side?i.e.,there is no need to implement the gap process?.At the same time,we have obtained the average implied V_oc of 0.695 V of the BJBC silicon solar cell precursors by comparing the different drive-in times?1.5,2.0 and 2.5 hours?and annealing temperatures?870,900,930,960 and990 ^oC?for 30 min.We have employed the conventional SiN_x:H antireflection coating?ARC?to passivate the front and back surfaces in combination with the screen printing and a co-firing step to sinter the different metallization pastes and to form electrical contacts for the back p⁺emitter and n⁺BSF.The best produced BJBC silicon solar cell?4.04 cm²?has been independently confirmed with the efficiency of 22.20%(V_oc=0.660 V,J_sc=42.82 mA/cm² and FF=78.56%)by SERIS.We have further shown the potential of the same industrial processes for large size?6×6 cm²?BJBC silicon solar cells with the in-house measured efficiency up to 21.43%.The present simplified cell structure has great advantages in the low-cost BJBC silicon solar cell industrialization,and could be compatible with the existing production lines and processes.