Study on the sintering and contact formation process of silver front side metallization pastes for crystalline silicon solar cells

Abstract

In the paper, sintering process and contact formation process of PTO glass containing pastes were studied by interrupting the firings at different temperatures. Microstructures of contact fingers fabricated using pastes with PTO glass by interrupted firings were obtained by FIB/SEM technology and the sintering process of pastes was 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. The great disharmony between the flow process of glass-phase and Ag densification would cause much glass trapped in densified Ag and thereby non-uniform etching of Si wafers.Contact formation process of pastes 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.