20plµs - Modeling of Aluminum Alloying Processes for Silicon Solar Cells

The silicon solar cells currently dominating the PV market feature a metal contact made of alloyed aluminum on the backside. To further reduce the recombination losses of such backside contacts and thus to increase the cell efficiency, a better understanding of the formation and effect of these contact structures is important. The model for description of the Al alloying process developed at Fraunhofer ISE now allows the prediction of the electric quality of such contacts taking into account different influencing factors during their production. When transferring the model into the cell production, the process parameters then can be respectively adjusted for Al alloys to achieve best-possible contact formation.

In the current silicon solar cell production, the backside metal contact is produced by default using screen-printing of an aluminum-containing paste. Here, the aluminum is alloyed into the silicon surface in a short high-temperature firing step, and the contact is formed. A several microns deep Al-doped p+ region is generated in the silicon crystal, which is referred to as Al back surface field (BSF, Fig. 1). In the case of Al-BSF solar cells, such contacts are full-area. In the case of PERC (Passivated Emitter and Rear Cell) solar cells, they are local (dot/line-shaped). The alloying process model that was developed at Fraunhofer ISE is based on the binary Al-Si phase diagram and describes the mechanisms during aluminum alloying in silicon quantitatively. The composition of the Al-Si melt that is formed on the surface of the Si wafer during alloying, as well as Si recrystallization on this Si surface are modeled. Different influencing factors, such as the temperature during contact formation, the amount of paste applied, and additional doping substances such as boron in the paste are considered. Using the model, the doping profiles of the Al-B-doped p+ regions can be precisely calculated (Fig. 2). The structural setup and the electric quality of these contacts can be predicted. With that, a basis for the detailed optimization of Al alloyed contacts has been established that can contribute significantly to further performance increases of Al-BSF and PERC solar cells, which currently dominate the market worldwide.