Effect of gravity on the microstructure of Al-Si alloy for rear-passivated solar cells

Abstract

We study the influence of the gravity field orientation on the microstructure of Al–Si forming alloy. Due to the difference between the diffusivity of aluminum and silicon, Kirkendall void formations are normally found at the back of rear-passivated solar cells instead of an eutectic layer. We show that the voids may partially be avoided by sintering the samples with the solid/liquid interface oriented opposite to the direction of the gravity field. A local back-surface-field is found underneath the voids using this approach. This phenomenon strongly applies to rear-passivated solar cells, which exhibited strong fill factor losses.

Optimization of local contact formation on screen-printed Al<inf>2</inf>O<inf>3</inf> passivated solar cells

“…The Si concentration in Al-Si melt at the opening sites plays a significant role in the BSF formation during the alloying process. Because only the excessive silicon (Si percentage well above the eutectic percentage p Si =12.6 wt%) could be rejected from the melt during the cooling down phase to form the epitaxial BSF layer [8,2,3,4]. Too much Si diffuses away from the opening sites in the sample with largepitch-pattern.…”

“…As is known that, during the alloying process, a higher volume of silicon atoms diffuse in the aluminum than aluminum atoms in the silicon. Consequently, Kirkendall voids form at the local contact instead of eutectic layer [2,3]. In the case of large-pitchcontact, silicon beneath the local opening, is not sufficient to saturate in the Al paste which is far away from the opening.…”

“…Some parameters, such as the composition of Al paste [4], firing condition [1], geometry of the rear contact opening [5], and effect of gravity [2] seem to affect the local contact formation substantially. Two common contact patterns, line contact and dot contact, are widely applied to the rear local contact of passivated emitter and rear solar cells.…”

Optimization of local contact formation on screen-printed Al2O3 passivated solar cells

“…Some parameters, such as the composition of AI paste [4], firing condition [1], geometry of the rear contact opening [5], and effect of gravity [2] seem to affect the local contact formation substantially. Two common contact patterns, line contact and dot contact, are widely applied to the rear local contact of passivated emitter and rear solar cells.…”

Influence of Al Particle Size and Firing Profile on Void Formation in Rear Local Contacts of Silicon Solar Cells

“…The addition of Si has already shown good results [8], [9]. In addition, the optimization of the contact spacing or firing with the Al paste on top was tested [10], [11]. Another approach is an adaption of the firing profile, like heating-up in several steps, to reduce the driving force of the Si diffusion due to the gradient of Si concentration on the Al/Si interface [12] or the broadening of the firing profile by reducing the belt speed with an additional lowering of the peak temperature, which also lowers the diffusion of Si [13].…”