As the world’s demand for energy grows, the search for cost competitive and earth abundant photovoltaic (PV) materials is becoming increasingly important. One promising material system for earth abundant PV is the Cu-Sn-S family, in which several ternary compounds, including Cu4SnS4, Cu4Sn7S16, and Cu2SnS3, have been suggested as PV absorbers in prior literature. Our work focuses on the Cu2SnS3 phase, which in our previous studies has been shown to avoid the numerous problems associated with Cu4SnS4 and Cu4Sn7S16 compounds. Despite the relatively narrow band gap of Cu2SnS3, we believe it to be the most promising absorber material in the Cu-Sn-S family (Appl. Phys. Lett., 103, 253902 (2013)). In this study, we used combinatorial sputtering to deposit cubic Cu2SnS3 films with a spatial gradient in Cu:Sn ratio across each sample library. Experimentally, we found that the cubic Cu2SnS3 has a large phase width, in contrast to previous reports that show Cu2SnS3 as a line compound on the Cu2S-SnS2 pseudobinary phase diagram. We are able to synthetically control the S chemical potential during film growth, which allowed us to synthesize combinatorial Cu2SnS3 libraries under high, intermediate, and low S chemical potentials. We find different phase progressions in each library as a result of the changing Cu:Sn ratio across each sample. These phase progressions can then be mapped onto the chemical potential phase space as lines of constant S chemical potential (Chem. Mater., 2014, 26(17) 4951-4959). By establishing synthetic techniques to access the entire range of Cu2SnS3 on the chemical potential phase space, we were then able to explore how the chemical potentials of the constituent elements affected the electrical properties of the material. We found that hole concentrations varied between 1018 cm-3 – 1021 cm-3, depending on the chemical potentials of Cu and S. The lowest carrier concentrations (desirable for PV applications) were achieved at low Cu and S chemical potentials, when Cu2SnS3 is in equilibrium with SnS. Using this understanding, we performed post-deposition anneals in an SnS atmosphere, which resulted in further reductions in the carrier concentration. The project “Rapid Development of Earth-Abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

Control of doping in Cu2SnS3, a novel photovoltaic absorber, through defects, alloying, and annealing

249th National Meeting (2015)

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