Study On The Lead-free Silver Electrodes For The Front Side Metallization Of Crystalline Silicon Solar Cells

Photovoltaic solar power technology has been widely used,crystalline silicon solar cells accounted for more than 90%of solar cells,crystalline silicon solar cell metallization is the key to ensuring battery performance.At present,the silver paste is basically used as lead paste,its performance is excellent,but lead as heavy metal has a greater harm on the environment,the development of environmentally friendly lead-free paste has great practical significance.In this paper,the metallization principle of crystalline silicon solar cells is analyzed,and the Ag-Si contact model and current conduction model are discussed.The influence of lead-free silver paste on the performance of the cells was studied by the design of glass frits,the preparation of front-side silver paste and the performance analysis of the cells.The thermal properties and cells properties of V-Bi,Ge-Bi,Te-Bi glass frits were studied.The transition temperature?T_g?of V-Bi glass frits is lower than 300?;the T_g of Ge-Bi glass frits is higher than 420?;the T_g of Te-Bi glass frits is about 390?,the thermal characteristics of the glass frits is suitable for the sintering process of cells,the conversion efficiency of cells is higher.The paper designed 5 factors,4 levels of orthogonal experiments(L₁₆?4⁵?),the basic recipe and composition of Te-Bi glass were determined.When the content of TeO₂ was45wt.%,the content of Bi₂O₃ was 36wt.%,the content of alkali metal oxide L₂O was2wt.%,The intermediate oxide MO content was 12wt.%,the glass performance is better,the polysilicon solar cell efficiency is 16.87%,cell efficiency of lead glass frits G0 is17.07%at the same process of production,it means that the cell performance is basically close to the level of lead glass frits.The glass frits has a T_g of 379.07?,and the corresponding front contact silver paste exhibits an endothermic reaction at 612.8?and has a large weight loss?-0.16%?,which contributes to the corrosion of the antireflective film and the formation of silver microcrystalline.And front contact silver electrode of the glass frits has a large number of silver microcrystalline,the size is less than 0.3?m,the distribution is more uniform,it is conducive to the formation of Ag-Si conductive channel,reduce the series resistance of cells,improve the conversion efficiency of cells.The rare earth element oxide,transitional element oxide and the fluoride was doped in the Te-Bi glass frits,when the glass recipe was 43.6wt.%TeO₂-34.8wt.%Bi₂O₃-17.7wt.%RO-2wt.%L₂O-1.9wt.%GeO₂-2wt.%LiF,The sample has a better performance,the cell efficiency increased by 0.14%than the formulation with no additions.It means that the Te-Bi glass frits performance can be improved by doping of some of the rare earth oxide or fluoride.In this paper,the thermal characteristics of glass frits,front contact silver paste and the microstructure of front contact silver electrode were analyzed by means of scanning electron microscopy?SEM?,differential thermal analysis and XRD analysis.The physical and chemical reactions of the front contact silver paste and the silicon emission junction were determined by differential thermal analysis?TGA-DSC?during the sintering process.It was found that the properties of the front contact silver paste were greatly affected by the thermal properties of the glass frits.Te-Bi glass T_g is about 390?,it is more able to match the cells sintering process.The microstructures of Ag-Si interface were analyzed by electron microscopy of SEM.Te-Bi glass formed dense and uniform silver microcrystals at Ag-Si interface during sintering,it is easy to realize ohmic contact of electrodes.In this paper,the sintering characteristics of the front contact silver paste were studied.The lead-containing paste could be sintered in a wide temperature window??40??,the sintered temperature window of Te-Bi glass is relatively narrow??30??.The effect of Te-Bi glass frits on the polycrystal silicon wafer is better than that of the monocrystal silicon wafer,the adaptability and performance stability of the Te-Bi glass frits need to be improved.