By treating the rear surface of solar cells with (NH 4) 2 S solution for 10 min, the dangling bonds on the p -Si surface are passivated by S with the formation of Si–S bonds evidenced by XPS. Consequently, the τ eff of p -Si is improved from 13.48 to 41.45 μs, confirming the enhanced chemical passivation endowed by sulfurization treatment.
Eventually, by employing sulfurization in hole-selective contacts, remarkable efficiencies of 19.85% and 22.01% are attained for NiO x - and MoO x -based passivating contact c -Si solar cells, respectively. Our work highlights a promising sulfurization strategy to enhance surface passivation and hole selectivity for dopant-free c -Si solar cells.
By employing the sulfurization strategy in hole-selective contact, we manage to achieve champion efficiencies of 19.85% and 22.01%, both of which are, to the best of our knowledge, the highest efficiencies reported so far for dopant-free passivating contact c -Si solar cells employing NiO x and MoO x as HTLs without a -Si:H, respectively.
Consequently, after the sulfurized rear surface was stored in the air for 3 days followed by Ag deposition, the solar cell exhibited worse photovoltaic performance than the one with the as-sulfurized rear surface (Figure S3).
By sulfurizing p -Si, the Voc of the solar cell is enhanced to 649.58 mV along with an improved PCE of 21.35%, confirming the beneficial role of sulfurization in ameliorating the selectivity of the passivating contact.
As an optimization of surface passivation in solar cells, an additional Al 2 O 3 film was deposited through ALD with a substrate temperature of 50°C after sulfurization, where one ALD cycle consists of 0.1 s trimethylaluminum (TMA; Al (CH 3) 3) pulse, 15 s N 2 (30 sccm) purge, 0.05 s H 2 O pulse, and 15 s N 2 purge.