Investigation of aluminum-alloyed local contacts for rear surface-passivated silicon solar cells

Abstract

The surface passivation of rear aluminum-alloyed p+ emitters is highly beneficial to increase the efficiency of back-junction n-type silicon solar cells, thus however demanding the application of locally defined emitter contacts. The formation of the rear contact by full-area screen-printing and alloying of Al-pastes on the locally opened contact points in the passivation layers exhibits two main problems: (i) increase of the contact depth leading to an enlargement of the contact area and (ii) low Al-p+ emitter thicknesses underneath the point contacts, both implying the danger of emitter shunts. In this study we therefore focus on controlling the formation and structural properties of contact points by systematically modifying the composition of the rear Al paste. By examining the contact geometry over a broad range of pitches, we demonstrate that the contact point depth and the Al-p+ emitter thickness in the contact region are directly linked to the percentage of Si that is dissolved into the Al-Si melt during alloying. For conventional Al pastes, the Si percentage in the melt was calculated to be far too low, so that we provided additional Si by manually adding Si powder to the Al paste. Thus, we could significantly reduce the contact depth and significantly enlarge the Al-p+ thickness in the point contacts, respectively. A first quantitative evaluation of the electrical properties was carried out, showing that the saturation current density is decreased by increasing the Si content of the paste, very likely due to the decreased contact area and improved electron shielding. In summary, we demonstrate that the local rear contact formation by alloying of full-area screen-printed Al pastes can be considerably improved by intentionally adding Si to the paste. The results of this investigation are highly interesting for applications to both n-type and p-type Si solar cells with passivated rear side.