How Much Rear Side Polishing Is Required? A Study on the Impact of Rear Side Polishing in PERC Solar Cells

In this work we study the impact of chemical polishing on the performance of mono-crystalline PERC cells. Starting from random pyramid textured surfaces, the rear side polishing step is performed by means of an inline single-side wet-etch tool. The influence of the pyramid smoothening on light trapping, surface recombination and contact formation is here studied separately. We conclude from our analysis that a moderate smoothening, corresponding to around 5-6 μm Si etch, is required in order to maximize the cell performance and that, on the other hand, a complete planarization of the rear surface can be detrimental, besides being cost un-effective. While the surface recombination generally benefits from a flat rear surface, light absorption and contact formation are more effective when roughness Ra is in the range between 300-500 nm. Contact formation is also influenced by the roughness because mass exchange between Al and Si takes place all over the rear side surface. On large area industrial PERC cells with Ag screen printed contacts we achieve a best cell efficiency of 20.1% with 6 μm of Si polishing removal and 19.9% with only 3 μm of Si removal. In the latter case the total Si consumption, including saw damage removal and texturing, is lower than 20 μm per cell.