| Efficiency improvement of solar cell and cost reduction are always important for large scale commercialization of PV industry.In order to improve cell efficiency,a lot of work has been done on optimization of solar cell structure,as well as process optimization including anti-reflection coating and passivation improvement.SiNx,SiOxNyand AlOx films have been widely used for crystalline silicon solar industry,however,it is not adequate to use just one material to realize perfect anti-reflection and passivation on PERC cells.Furthermore,each manufacturing process steps of solar cell fabrication are correlated to each other,which makes it compulsory to investigate the correlation of each process steps,consider the process matching in order to achieve commercial competiveness.Considering the issues above mentioned,the target of this thesis is to increase the photovoltaic conversion efficiency of solar cells,which is compatible for mass production by processing multi-layer SiNx film on front side,and fabricating AlOx/SiOxNy/SiNx stacked layers on rear side using PEC VD technology in order to reduce the reflection of sunlight and improve passivation.Based on theoretical analysis and simulation,high quality multi-layer SiNx film and AlOx/SiOxNy/SiNx stacked layers were fabricated with optimized deposition parameters.Integration effect was considered during design and processing of these films in order to match with silicon wafer characteristic and whole cell process.Finally,the cell efficiency was increased and production implementation was done for PV industry.Key investigations and results are summarized as below:First,the development history and commercial status of crystalline silicon solar cell was summarized,as well as the basic introduction of plasma and the application of plasma in crystalline silicon solar cell industry;physical fundamentals,principal of photovoltaic and industrial technology of solar cells were introduced which is the base of this research work.A method of SiNx fabrication using plasma enhanced chemical vapor deposition(PECVD)was introduced,the effect of plasma pretreatment before SiNx deposition and plasma treatment parameter between 2 SiNx layers to wafer lifetime and p-n junction were investigated.Based on these study,effect of plasma treatment to electrical performance of different p-n junction multi-crystalline solar cells is investigated.It was found that,highest cell efficiency of 18.325%achieved on 90 ohm/square sheet resistance after plasma treatment.Different phosphorus doping profiles of p-n junction were realized by variation of gas flow.It was found that higher cell efficiency with 0.13%absolute gains can be achieved by implementation of plasma treatment on low surface concentration and deep p-n junctions.However,wider distribution of cell efficiency and open-circuit voltage(Voc)were observed for such junction,which may be due to the mismatch of low surface concentration and deep junction to different wafer quality.Three different quality grade wafers were classified using in-line Photoluminescence tool according to wafer lifetime,impurity ratio and dislocation density.The correlation between p-n junction characteric after plasma treatment and wafer quality was investigated.It was found that lower surface phosphorus concentration had better blue light response,however it had worse gettering effect.Due to this reason,0.08%absolute efficiency gain was achieved on high lifetime wafers,however there was 0.44%absolute efficiency loss on bad quality wafers,and there was no effect to high dislocation density wafers.For high impurity wafers,0.28%absolute efficiency gain was achieved by optimization of low temperature annealing in order to have a better gettering effect,and lower light induced degradation(LID)was also observed.With the implementation of PERC technology to mass production,front surface antireflection and surface passivation are more and more important.With the help of OPAL2 software simulation,3 layers and 4 layers SiNx film were realized by variation of plasma and gas parameters,and cells were fabricated with these film structures.Simulation showed that with the increasing from 2 layers SiNx to 4 layers SiNx,light absorption was reduced gradually,thus the light in the silicon bulk was increased from 95.6%to 97.5%.Corresponding experiments showed that short-circuit current gain of 27mA and 31mA on 3 layers and 4 layers SiNx solar cells respectively.This was well aligned with simulation result,more current was generated due to more light reached to silicon bulk.However,the inner layer SiNx of double layer structure had higher refractive index which had better passivation effect and thus higher Voc.Finally,the experiment results showed that 4 layers SiNx solar cells achieved 22.15%cell efficiency,which was absolultely 0.09%higher than that of double layers.Calculation of economic showed that 4.54 million yuan/year profit for a capacity of 1GW cell mass production.An investigation of rear side stacked layer is done based on the results analysis of front side.The SiOxNy layer was introduced between AlOx and SiNx,thus a high quality stacked layer of AlOx/SiOxNy/SiNx was formed.Experiment showed that SiOxNy could effectively improve stacked film uniformity and as well as in increase of rear side reflection.With the increase of rear side stacked film layer thickness from 120.5nm to 186.2nm,film uniformity got better,and the reflectivity was increased from 19.1%to 25.3%,which enabled more current generated due to more long wavelength light could be reflected back to Si bulk at the rear side of the solar cell.Experiment result showed that stacked layers of AlOx/SiOxNy/SiNx could effectively increase reflection and improve passivation effect,PERC cell efficiency as high as 22.42%was achieved with absolutely efficiency gain of 0.06%compared with AlOx/SiNx stacked layer.Calculation of economic showed 2.68 million yuan/year profit for a capacity of 1GW cell mass production. |
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