Introduction of Na into Cu2SnS3 thin film for improvement of its photovoltaic performances

Impact of Na in Cu2SnS3 (CTS) films on their physical properties and photovoltaic performances is investigated. CTS films are fabricated by sulfurization of NaF/Cu/SnS2 stacked precursors. NaF precursor is used as primary Na source for Na introduction into the CTS films. It is demonstrated that Na content in the resulting CTS films is increased with enhancing thickness of NaF precursor, leading to the increase in Na/Cu atomic ratio but decrease in Cu/Sn atomic ratio. This suggests that the addition of NaF capping layer on the Cu/SnS2 stacked precursors suppresses evaporation of Sn-S compounds from the sample during sulfurization, thus forming Sn2S3 secondary. Therefore, with too thick NaF layer (greater than or equal to 75 nm), Sn2S3 secondary phase in bulk and near surface of the resulting CTS films, having the detrimental effect on cell performances, is severely formed. However, the CTS grain sizes are notably enlarged as the thickness of NaF precursor increases. As a result, conversion efficiency of CTS thin-film solar cell with anti-reflective layer is increased up to 4.8% with the optimized NaF thickness of 60 nm since the single-phase and large-grain CTS absorbing layer without Sn2S3 secondary phase near the surface is attained, thereby increasing open-circuit voltage and fill factor attributed to the decrease in the carrier recombination.