Abstract-In this study, the contact formation process of Al containing Ag screen-printing pastes to BBr 3 -based B emitters on Si is investigated. Therefore, a detailed scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy study of top-view and cross-sectional samples was conducted.

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In the past years, the contact formation of Ag screen-printing pastes to n + emitters has been profoundly investigated and at least in parts explained. However, p + emitters cannot be contacted well with standard Ag pastes. It has been shown that adding Al to Ag screen printing pastes leads to lower contact resistances. Therefore different mechanisms must play a role in the contact formation process. The role of Al and the exact mechanism of contact formation of these Al containing Ag screen-printing pastes have not been well understood up to now. A drawback of Al containing pastes is that metal spikes growing into the Si wafer can be deep enough to corrupt the space charge region and contact the base thus shunting the pn-junction. A better understanding of the contact formation process is necessary to enable the development of improved screen-printing pastes with a reduced probability of shunting. In this work the influence of differently structured Si surfaces on the contact formation to a BBr 3 based boron emitter is investigated. The Ag/Al contact spots that grow into the Si surface show the same surface structure as the surrounding Si. It is concluded, that the SiN x :H layer acts as a mould of the Si surface for the growth of the contact spots. The presented observations are then explained by a recently introduced model for the contact formation of Al containing Ag screenprinting pastes to p + emitters through a SiN x :H layer.

Abstract-For contacting boron emitters by screen-printing metal pastes, up to now, it has been necessary to add a small amount of Al to the Ag paste to facilitate a reasonable contact resistivity. With the addition of Al to the Ag paste, deep Ag/Al spikes appear, which can be deep enough to penetrate the emitter and, therefore, affect the emitter and space charge region, and, finally, affect the performance of the solar cell. In this paper, a transmission electron microscopy (TEM) analysis of these Ag/Al spikes is presented. The crystalline nature of the Ag/Al spikes is revealed for different surface structures of the crystalline Si wafer and different Al contents in the screen-printing paste. This result is confirmed by X-ray diffraction measurements of etched-back contacts. Additionally, TEM energy-dispersive X-ray spectroscopy facilitates the examination of the Si-rich inclusions found in the Ag/Al spikes. They prove to be multicrystalline Si precipitates with at least 99 at% Si. The observations help to understand the contact formation process of Al containing Ag screen-printing pastes and support the previously presented model. Index Terms-Ag/Al, boron emitter, crystallinity, screenprinting, transmission electron microscopy (TEM), X-ray diffraction (XRD).

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