| The cost of Solar photovoltaic system producing electricity is about 2 times as much as conventional energy.Lower cost and higher performance via optimized solder process could lead the photovoltaic technology into larger application.By firing aluminum paste on the rear side of multicrystalline silicon solar cell,it forms back surface field（BSF）,which also reduces the surface recombination velocities,acts as a good reflector and improves the response to infrared.The inert film of Al2O3 is difficult to remove,which prevents the solder from wetting Al BSF and ribbons.Then Ag electrode is made on the rear side in industry.It locally prevents the formation of Al BSF,leading to a decrease in open-circuit voltage.So the direct bonding to Al BSF could avoid the use of Ag electrode,improve photovoltaic performance and reduce the cost.In this study,ultrasonic-assisted soldering was developed to bond Al BSF and Al ribbon with Sn-based solder.Mechanism of removing Al2O3 on Al BSF,microstructural evolution of layers and interfaces under different ultrasonic action time and its influence to electric conductivity and mechanical property was analyzed.Finally the photovoltaic performance of Al BSF versus Ag electrode was studied and proven.While coating Sn-3.5Ag onto Al BSF under ultrasonic-assisted soldering,the solder filled into the gaps between Al spheres,removed the oxide films,dissolved and wetted Al BSF.Over time,the oxide film was removed thoroughly.Al spheres were surrounded by liquid solder and the oxide film was broken with notches,liquid solder dissolved the Al particles.The longer ultrasonic was applied,the more Al was dissolved,as Ag2Al,Ag3Al2 andα-Al gradually generated.When it came to 10s,Al BSF dissolved into solder layer totally.The joint resistance reduced at first and then increased as the ultrasonic action time got longer,which was lowest at about 6 s as 1.26 mΩ.It was found that the joint resistance was affected by the Al paste residual layer and Al doped p+layer.When ultrasonic was applied shortly,the oxide film contributed the most part of the resistance.When oxide film was removed,Al BSF was wetted and dissolved,the resistance of Al doped p+layer increased,leading to the increase of joint resistance.As ultrasonic action was applied longer,more Al BSF was diffused,the peel force increased obviously,which could reach as 1.44 N/mm under 6s,and reached as high as2.06 N/mm under 10s.The photovoltaic performance was lower when bonding Al ribbon and Al BSF under 2s of ultrasonic,while the performance around 6s was higher and stable.When it was under 6s of ultrasonic and tested under 88.0 mW/cm2 of irradiation,fill factor（FF）was 0.71343 and photoelectric conversion efficiency was 19.909%,while the cell with Ag electrode joint performed FF as 0.59822 and efficiency as 16.045%. |
