Effect of Na on sulfurization growth of SnS thin films and solar cells using NaF/Sn-S precursor

•We investigate the role of Na treatment for SnS thin films and solar cells.•We reveal that the NaF diffuse into SnS and Na act as an acceptor.•NaF capping layer protects SnS from re-evaporation during sulfurization.•Forming large size grains and improving the Jsc of SnS-related solar cells by NaF.

The role of NaF treatment in the production of SnS thin films and solar cells was investigated. The grain size of SnS films containing Na was large (about 500 nm) compared to that of SnS films prepared by conventional sulfurization (typically 250 nm). The carrier concentration of SnS increased from 5 × 1016 to 2 × 1017 cm− 3 by Na diffusion into the SnS film, and Na acted as an acceptor. Moreover, the NaF layer exhibited a capping effect during high-temperature sulfurization at 500 °C and suppressed the re-evaporation of SnS. The short-current density tended to increase from 1.1 to 8.8 mA/cm2 with increasing thickness of the deposited NaF layer from 0 to 200 nm, because of the larger grain size and carrier concentration of SnS.