Formation of a back contact by fire-through process of screen-printed silicon solar cells

We have investigated various mechanisms that participate in formation of a good, screen-printed, Si-Al contact on the back side of a crystalline-Si solar cell. We observed a rapid diffusion of Si into Al during the temperature ramp-up. The Si diffusion produces a graded composition that causes an Al-Si melt to initiate from the interface. The interface melt of eutectic composition can be used to promote a uniform, dimple-free Al melt. We have also investigated the kinetics of stratification of the back-contacts into various regions: P+, eutectic, and molten (but unconnected) Al particles. The properties of these regions are primarily dictated by the temperature ramp-down profile, and they strongly depend on incident light flux that heats the wafer, on thickness of Al, and on emissivity of the Al-Air interface. We will discuss methods to improve back-contact properties of screen-printed multi-crystalline Si solar cells.