Study On The Performances Of Aluminum Back Surface Field Of PERC Solar Cells

Solar cells are semiconductor devices that can directly convert light energy into electric energy.Among them,crystalline silicon solar cells have the advantages of high conversion efficiency and good long-term reliability,and have always been dominant in the photovoltaic market.PERC?Passivated Emitter and Rear Cell?solar cell is an improved technology in the structure of conventional crystalline silicon solar cells.Due to the passivation and local contact structure on the back side,the recombination rate of the carrier on the back surface of the cell is significantly reduced,which greatly improves the cell performance.And it is one of the recent research hotspots.The improvement of PERC solar cell structure not only improves the performance,but also puts forward special requirements for aluminum paste.The formation mechanism of the local aluminum back surface field is also quite different from that of conventional solar cells.In this paper,the influence of glass frits in aluminum paste on the back surface field performance of PERC solar cell was studied,and the firing mechanism and related phenomena of aluminum back surface field of PERC solar cell were analyzed.The paper studied the thermal properties of V,Pb and Bi glass frits and the performance of PERC cells prepared by three aluminum pastes.The V glass frit has a higher transition temperature Tg,a wider melting temperature range?Tm-Tg?,a higher cell efficiency,and better adhesion and boiling characteristics of back surface field.The Tg of Pb-B-Si glass frit is about 450°C,there is no obvious melting temperature range,the cell efficiency is low,and the boiling characteristics are poor.The melting temperature range of Bi-B-Zn glass frit is wide,but The Tg is low,and at around 370°C,the fabricated cell is less efficient and the adhesion properties is generally.The Tg of the V glass frit is higher than that of the other two glass frits,and its melting temperature range is wide,which can effectively promote the formation of Al-Si local contact during the firing process without excessive damage to the passivation layer,and has a high open circuit voltage and conversion efficiency.In this paper,the influence of the ratio of V-B-Si and the content of Ba on the performance of Al back surface field in PERC cells was studied.When the ratio of formed oxide is w?V₂O₅?:w?B₂O₃?:w?SiO₂?=25:10:15,the comprehensive performance of the cells is the best,the efficiency is 19.21%,and the appearance performance of the back surface filed is better.Through the comprehensive differential thermal analysis of glass frits,it was found that the cell efficiency first increased and then decreased with the increase of Tg,and the efficiency is best at Tg=500°C.The content of BaO will affect the Al back surface filed characteristics of the cells.The BaO content is higher,the square resistance and appearance characteristics of Al back surface field of cells are better,while the content of BaO is lower,the square resistance and appearance characteristics will become worse.The paper also studied the effect of the mixing of V glass frit and Pb glass frit on the performance of the cells.The performance of the cells prepared by the aluminum paste with mixed glass frit is linear with the content of a single glass frit.The effect of mixed glass frit on the performance and appearance of the cell is directly related to the content of the glass frit.By means of scanning electron microscopy?SEM?,X-ray energy dispersive spectrometer?EDS?and other analytical methods,the microstructure and element distribution of the local Al back surface field and Al film of PERC cells were analyzed.It was found that in the firing process,Si elements at the local contact position of the Al back surface field of PERC cells would conduct longitudinal and transverse diffusion to the Al layer.The Al layer near the contact hole contained more Si and formed a dark area?Si?on the surface of the Al back surface field.The width of the dark area was proportional to the size of the local contact hole.The square resistance and adhesion characteristics of the Al back surface field are proportional to the width of the dark area.The wider the width of the dark area is,the wider the area of Al-Si alloy formed after Si diffuses into the Al layer in the firing process,and the better the performance of the Al back surface field will be.There will be voids in the local contact position,and the formation of voids occurs during the cooling stage of the firing process,which is caused by the Al-Si melt shrinking back to the Al layer at the Al-Si contact position.There are three kinds of shrinkage of Al-Si melt:non-shrinkage,partial shrinkage and complete shrinkage,and finally leads to local contact forming three local Al back surface field states of complete filling,partial filling and no filling.