Industrial Cleaning Sequences for Al2O3-passivated PERC Solar Cells

In this paper, we investigate different industrial applicable cleaning sequences on test wafers and PERC solar cells in comparison to a laboratory type RCA clean. The cleaning sequences pSC1, HF/HCl, HF/O3 and HF/O3 show lifetimes between 1 ms and 2 ms which is comparable to a laboratory type RCA clean corresponding to a surface recombination velocity Spass below 15 cm/s. The pSC1, HF/HCl clean achieves lifetimes around 1 ms, whereas the PSG-etch shows poor cleaning quality with lifetimes around 500 μs. Reference PERC cells using a rear protection layer before texturing and diffusion demonstrate efficiencies up to 20.4% for the cleaning sequence pSC1, HF/HCl prior to passivation which is comparable to the RCA clean. The HF/O3 cleans result in lower PERC efficiencies up to 20.0% mainly due to a lower Fill Factor which is likely caused by etching of the emitter and hence increased contact resistance. Investigations of polished test wafers show that the cleaning sequences pSC1, HF/HCl, HF-Dip and pSC1, HF/HCl, HF/O3 are able to sufficiently remove porous silicon from the front side and simultaneously allowing excellent rear surface passivation. A first batch of PERC solar cell results with polished rear surface post texturing and POCl3 diffusion achieves efficiencies of up to 20.7% when applying an RCA clean. However, the pSC1, HF/HCl and pSC1 HF/O3 still exhibit significantly lower efficiencies since in this batch the porous silicon of the emitter was not yet sufficiently removed, which is subject to further optimization.