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Impact of rear side roughness on optical and electrical properties of a high-efficiency solar cell
The performance of PERC solar cells benefits from a smooth rear surface due to a reduced surface recombination and increased light trapping. State of the art surface roughness is defined solely by the polishing silicon removal. However the initial texture height can vary, therefore there is no universal definition of roughness. In this work roughness parameters from an earlier study will be used to determine the rear surface roughness of mono-crystalline industrial p-type solar cells . The effects of rear side roughness on the passivation quality and thus minority carrier lifetime will be discussed. Additionally the influence of rear side roughness on reflection and therefore light trapping will be investigated. Furthermore solar cells with PERC concept are presented. Up to now used roughness parameters describe vertical structures and thus represent polishing results but they are not sensitive to horizontal structure expansion. Effective lifetime of polished samples first increases with decreasing roughness. For further polishing values stagnate or decrease. Implied VOC of saw damage etched surface is higher compared to polished surface although they show similar vertical roughness, hence there are more parameters influencing passivation quality than vertical roughness. According to roughness measurements nano-roughness, valleys and horizontal structure expansion seem to affect passivation quality in addition to vertical roughness. Decreasing roughness increases reflection at 1200 nm and thus increases light trapping. As the highest benefit in light trapping appears with decreasing structure height, vertical structure expansion seems to affect back reflection less than horizontal structures. Solar cell results cannot confirm the results of implied VOC and reflection measurements but prove the benefit of smooth rear sides. For polished polished rear sides the gain in efficiency is decreasing with increasing silicon removal.