Control of solution wettability on fine-textured crystalline silicon surface to obtain high-quality passivation for solar cells

Abstract

We developed a suitable chemical cleaning procedure for a fine-textured crystalline silicon (c-Si) surface which was prepared by the microparticle-assisted texturing (MPAT) process. This cleaning is a key to obtaining high-quality surface passivation by catalytic chemical vapor deposition (Cat-CVD) of silicon nitride (SiNx)/amorphous silicon (a-Si) stacked layers. In the cleaning, hydrofluoric acid (HF) solutions were used for surface oxide removal. Due to the weak wettability of the solution on the c-Si surface, the oxide layer was not entirely removed, leading to a bad surface passivation. We controlled the wettability of the solutions by adding methanol with different mixing ratios. We named this technique as the methanol-assisted cleaning (MAC) process. As a result, the effective minority carrier lifetime (τeff) increased from a few milliseconds to ∼7.8 ms, corresponding to an extremely low surface recombination velocity (SRV) of ∼0.6 cm/s, when the mixing ratio is ∼50 vol. %. Note that τeff reaches almost a world-record value for textured c-Si passivated by Cat-CVD SiNx/a-Si. Since the HF solution is widely used in industry for cleaning the Si surface, the control of its wettability becomes a promising and fundamental application.