Influence of Na in Cu2SnS3 film on its physical properties and photovoltaic performances

Cu2SnS3 (CTS) is the promising absorber of thin-film solar cell due to earth-abandant and non-toxic element. Influence of sodium (Na) on physical properties of CTS layers is investigated. In this work, CTS films with different Na concentrations are fabricated by sulfurizations of NaF/Cu/SnS2 and Cu/SnS2 stacked precursors on both soda-lime glass (SLG) and quartz substrates. The Na from NaF precursor layer and SLG substrate is introduced into CTS films. It is demonstrated that Na in forms of NaxCuSnS3 (x: 0.5-1.0) and Na2S is segregated in the large grain regions of CTS films. The crystal grains are significantly enlarged by Na addition into CTS films. As a result, the carrier lifetime, examined by time-resolved photoluminescence, is enhanced. From Raman scattering measurement, CTS films possess the peaks of monoclinic CTS structure. In addition, Cu2Sn3S7 and NaxCuSnS3 peaks are clearly observed in CTS films with high Na content. Finally, conversion efficiency of CTS thin-film solar cell is strongly enhanced from 0.61 (without Na addition) to 2.01% (with Na addition), mainly attributed to the increases in open-circuit voltage and fill factor through the decrease in carrier recombination.