Via hole conditioning in silicon heterojunction metal wrap through solar cells

A silicon heterojunction solar cell based on amorphous and crystalline silicon is combined with the metal wrap through technology. In this novel solar cell concept one critical process is the via hole conditioning. Raman measurements reveal that the amorphous silicon emitter layer hardly penetrates the via holes and that thereby the via surface is not fully covered. In the conventional process sequence with via hole formation prior to wet chemical cleaning, the effective carrier lifetime is reduced by about 50 % in the vicinity of the via hole. An improved process sequence is presented, which bases on via hole formation after the thin film depositions. In this sequence, the via hole formation process is crucial for the via surface passivation. The passivation remains poor when applying a 1064 nm laser process. However, very good surface passivation is achieved with a 532 nm laser process. The lifetime reduction was below 20 % at the via hole. The superior performance of the 532 nm laser process is correlated to a smoother via surface and enhanced via sidewall oxidation. Finally, large area SHJ-MWT solar cells based on the optimized via formation process are processed and analyzed.