Microstructures of front-side Ag contact of crystalline Si solar cells fired at temperatures from below to above optimal were systematically investigated using advanced electron microscopy. Ag pastes studied included commercial pastes and an experimental paste containing nano-sized metallic Zn additive. Microstructures of optimally fired cells determined from cross-sectional and top-view images were found to be consistent with a primary tunneling mechanism for current flow (a “nano-Ag colloids assisted tunneling” model) due to the lack of Ag crystallites connecting the silver conductor line to the silicon emitter. We mapped the evolution of the interfacial microstructures across the firing temperature window and correlated it with cell performance. Our result sheds light on the relative importance of the two current transport models as the peak firing temperature was swept from below to above optimal.

We investigated the quality of the n+-p diffused junction beneath the front-side Ag contact of multicrystalline Si solar cells by characterizing the uniformities of electrostatic potential and doping concentration across the junction using the atomic force microscopy-based electrical imaging techniques of scanning Kelvin probe force microscopy and scanning capacitance microscopy. We found that Ag screen-printing metallization fired at the over-fire temperature significantly degrades the junction uniformity beneath the Ag contact grid, whereas metallization at the optimal- and under-fire temperatures does not cause degradation. Ag crystallites with widely distributed sizes were found at the Ag-grid/emitter-Si interface of the over-fired cell, which is associated with the junction damage beneath the Ag grid. Large crystallites protrude into Si deeper than the junction depth. However, the junction was not broken down; instead, it was reformed on the entire front of the crystallite/Si interface. We propose a mechanism of junction-quality degradation, based on emitter Si melting at the temperature around the Ag-Si eutectic point during firing, and subsequent re-crystallization with incorporation of Ag and other impurities and with formation of crystallographic defects during quenching. The effect of this junction damage on solar cell performance is discussed.