Study On Front Silver Paste Performance Of High Sheet Resistance Dense Gridline

Abstract:High sheet resistance emitter is the development trend of silicon solar cells.With respect to conventional sheet resistance,the surface recombination effect of silicon solar cell with high sheet resistance weakened while the short wavelength spectral response increased,which could contribute to the photovoltaic conversion efficiency promotion.However,the silver pastes for the front metallization of silicon solar cells only applied to conventional silicon solar cells. Therefore,it has a strategic meaning to investigate a type of localized front silver paste applicable to silicon solar cells with high sheet resistance.This thesis put an emphasis on the technical difficulties of front silver paste and mainly studied the effects of content of each component in organic vehicle on the the performance of front silver paste.The match between gridline electrode width and sheet resistance was also studied and the firing profile are optimized.By using liquid phosphorus source diffusion method,silicon wafers with various sheet resistance were made and the influences of sheet resistance on short circuit current,open circuit voltage,fill factor and photovoltaic conversion efficiency were researched.By using screen printing technique, the effects of gridline electrode width on short circuit current,open circuit voltage,fill factor and photovoltaic conversion efficiency were investigated.Influences of content of each component in organic vehicle on front silver paste volatility,screen printing performance,aspect ratio of gridline electrode, density of gridline electrode and photovoltaic conversion efficiency were studied.By using infrared fast firing technique,effects of peak firing temperature,belt speed and ramp-up rate on gridline electrode density as well as photovoltaic conversion performance were researched.The research results are mainly as following:(1) Sheet resistance has important effects on the photovoltaic conversion efficiency of multicrystalline silicon solar cells.With the increase of sheet resistance from65Ω,/□to90Ω/□,short circuit current and open circuit voltage increase respectively by4.85%and3.23%while fill factor decreases by4.15%.The photovoltaic conversion efficiency increases at first,then decreases,reaches up to17.33%at80Ω/□.(2) Fine gridline electrode width has significant influences on short circuit current and fill factor. With the increase of design width of silk screen from90μm to40μm,short circuit current decreases by1.98%while fill factor increases by5.15%.Photovoltaic conversion efficiency increases at first and then decreases.Silicon solar cells with three busbars has advantages over two busbars reflected in the increase of short circuit current and fill factor respectively by0.25%and0.25%. Photovoltaic conversion efficiency are promoted,reaching up to17.45%.(3) The ingredients of organic vehicle influence the performance of front silver paste.This thesis optimizes three different volatile solvents-terpineol,carbitol and carbitol acetate as mixed solvent and the ratio is60wt%,30wt%,10wt%.This mixed solvent hardly volatilizes at low temperature but volatilizes quickly at the drying temperature range.The optimal content for ethyl cellulose is6%.Under this condition,the viscosity of front silver paste is266Pa·S and the morphology of front electrode is good in high definition.The optimal content for castor oil is3wt%and the electrode aspect ratio can reach0.288at this condition.As for organic vehicle,the best content in front side silver paste is11%.The ingredient of organic vehicle is89.5wt%mixed solvent,6wt%ethyl cellulose,3wt%castor oil and1.5wt%other additives.Under this situation, the viscosity of front silver paste is268Pa·S,the electrode aspect ratio after screen printing and firing is high,all kinds of performance parameters are favorable and photovoltaic conversion efficiency can reach up to17.58%.(4) Firing profile can impact photovoltaic conversion performance materially.This thesis optimizes a suitable profile which is800℃peak firing temperature,660cm/min belt speed and65.7℃/s ramp-up rate.At this condition,the density of electrode after firing is good,all kinds of performance parameters are favorable and photovoltaic conversion efficiency is up to17.75%.