Study On The Rheological And Ohmic Contact Properties Of Silver Front Side Metallization Paste For Crystalline Silicon Solar Cells

As one of the most important materials for fabricating crystalline silicon solar cells,silver front side metallization paste plays a great role in increasing cell efficiency and reducing the production cost and use-cost of solar cells.Moreover,achieving a better understanding of rheological and ohmic contact properties of silver front side metallization paste is of great significance in promoting the research and development of silver paste.The microstructure of silver front side metallization paste for crystalline silicon solar cells was investigated.Weak flocculation network of silver particles is produced by addition of free（non-adsorbing）EC polymer in paste due to depletion flocculation.Besides the interaction between thixotropic agent molecule chains,EC polymer also has a significant interaction with thixotropic agent through chain entanglements and thereby strengthens the network structure of silver paste by association with silver particles.Rotational and oscillatory measurements were conducted to study the effect of EC polymer,thixotropic agent,organic medium content and solvents on the rheological properties of silver paste.With the increase of EC polymer or thixotropic agent,the viscosity of silver paste increases but the shear-thinning degree of silver pastes keeps unchanged.The viscosity of silver paste increases with the decrease of organic medium at high shear rate while staying unchanged at relative low shear rate.With the increase of organic medium content,the properties of silver pastes were converted from rigidity to flexibility.The effect of solvent was studied by investigating the viscosity of polymer solutions and Ag dispersions.The effect of solvent on the viscosity of paste is mainly attributed to the difference of hydromechanics radius and configuration of polymer in solvents.The rheological properties were tailored by designing the composition of polymers,using a combination of controlled flocculating dispersing additive and thixotropic agent,and the addition of nano-sized carbon black.Particle interaction of silver paste can be enhanced by designing the composition of polymers while high shear-rate viscosity changed little and the balance between printing weight and paste spreading can be easily achieved.Through the combination use of controlled flocculating dispersing additive and thixotropic agent in silver paste,the network structure and its rebuilding rate is improved.The depletion flocculation induced by the addition of nano-sized carbon introduces a new attraction force between silver particles and strengthens the internal network structure of silver paste.The etching reaction properties of glass with Si and its dissolution behavior of Ag were studied.Ag⁺is the dominant form of silver dissolved in glass melts.The solubility of Ag₂O in weak acidity or basic glass with low softening temperature would increase with the increase of temperature,and thereby large-scale of nano-Ag colloids can be precipitated from Ag super-saturation molten glassy-phase during the cooling process due to the decrease of the solubility of Ag₂O.As a glass forming oxide,TeO₂ have a high solubility of Ag and the reaction of TeO₂ with Si is thermodynamically favored.The glass synthesized with TeO₂ is suitable for the situation of manufacturing contacts on high-resistance and shallow emmiter for crystalline silicon solar cellsThermal properties of Pb-Te-Bi-O and Pb-Te-Si-O system glasses and their etching reaction behavior with SiN_x and Si,and dissolution process of Ag were studied.The etching behavior of glass with SiN_x is highly related with viscosity-temperature properties but there is no direct correlation with the exact composition.When silver paste was sintered,high-viscosity glass flow to the surface of the cell and etch SiN_x ARC layer at relative high temperature and the time for etching through SiN_x ARC layer was longer.The amount of Ag fused into the glass has a great effect on the etching reaction rate of glass with Si and thereby the size of Ag crystallites grown into the Si emitter at at the position of inverted-pyramid etching pits.The etching behavior of interfacial glass with Si is also highly related with the viscosity-temperature properties of interfacial glass.The formation of a new low-softening-temperature glassy-phase,induced by the crystallization of glass,accelerates the dissolution of Ag at lower temperature.The effect of Pb-Te-O glass on ohmic contact properties was studied and current transport mechanisim in Ag/Si interface was analysised.Visocosity-temperature properties of glass has a great effect on the etching reaction with Si and the nucleating and growing process of Ag.Ag crystallites are not necessary for current conduction in front-side contact,suggesting“nano-Ag colloid assisted tunneling”plays a dominant role in cells.The densification process of silver paste has a great effect on the microstructure and electrical properties of Ag/Si contact by changing the flow behavior of glass melt to the surface of the cell,and thereby the subsequent etching reaction.Sintering process and contact formation process of PTO glass containing pastes were studied.The sintering process of pastes can be divided into three stages,the initial stage,intermediate stage and final stage,respectively.Only during the initial stage and intermediate stage of sintering,Ag-containing glass-phase is able to flow downward to the interface of Ag/Si through the continuous pore channels between Ag particles due to the great densification of Ag in the final stage of sintering.Contact formation process of silver paste was studied by observing the Ag contact and Si emitter interface.Ag precipitates can be observed in the interfacial glass-phase at low firing temperature.Ag crystallites embedded in Si emitter appear at relative high temperature,along with the decreased number of Ag precipitates located in the interfacial glass-phase.Lower viscosity and higher ion diffusion velocities of interfacial glass-phase will make Ag crystallites appear at lower temperature.The increased interfacial glass layer thickness and abnormal growth of Ag crystallites,together with the decreased number of Ag precipitates cause the increase of contact resistance with the increase of sintering temperature.The effects of liquidus temperature and content of glass frit,silver powder,and ZnO powder on the microstructure of Ag/Si interface were studied.The densification process of silver paste could be hindered by the glass with high liquidus temperature.Silver powder with high sintering activity could cause a rapid densification process of silver paste,promote the glass melts flow to the Ag/Si interface earlier and give a longer time for the etching reaction of glass with Si emitter.The more glass frit is added in silver paste,the easier glass melt will flow to the Ag/Si interface and the thicker interfacial glass layer is due to the he etching reaction.When sintered,ZnO added in silver paste could decrease the high-temperature viscosity of glass phase and promote the etching reaction with Si by fusing into glass melt as Zn²⁺.