Atomic Layer Deposition of Cu₂SnS₃Thin Films: Effects of Composition and Heat Treatment on Phase Transformation

Herein is presented the first report on the atomic layer deposition (ALD) of ternary Cu2SnS3 (CTS) thin films within a reasonably wide temperature window of 150-190 °C using a supercycle growth strategy. The use of rationally designed deposition schemes that involved matching the diffusion length of cations to the sublayer thickness in each supercycle resulted in homogeneous monoclinic CTS films. The optoelectronic quality of the films was manifested by the presence of a double absorption edge, which is scarcely observed with other deposition techniques. Further characterization of the films showed that excursions from ideal stoichiometry minimally impacted optical properties, whereas electrical properties were significantly impacted, with hole concentration varying by orders of magnitude. On the other hand, postdeposition heat treatments initially aimed at reducing recombination-active grain boundaries strongly affected both optical and electrical properties. This was identified to be the result of cation disorder induced during heat treatment, which triggered a progressive phase transformation from monoclinic to cubic CTS. The first-order effect of this transformation was a decrease in photoabsorptive ability and the creation of intra-band-gap states leading to electronic disorder. In addition, the heat treatment resulted in notable alterations in hole concentrations. From the perspective of solar cell performance, the results suggest that deviation from stoichiometry and the formation of secondary phases in near stoichiometric CTS films will strongly affect fill factors, while open-circuit voltage (Voc) and short-circuit current (Jsc) are less affected. Conversely, cation disorder associated with phase transformation during heat treatment will have a more direct impact on Voc and Jsc. Last, the photovoltaic viability of the ALD CTS films was demonstrated, with the best cell obtained after heat treatment yielding a power conversion efficiency of 1.75%, which although encouraging represented a compromise between degraded bulk optoelectronic quality and reduced recombination-active grain boundaries.