Cu2SnS3 is investigated as a potential solar absorber material, which is known to exist in several poly morphs. In the present work, the transition to the most efficient monoclinic polymorph is investigated and an explanation is given why this one seems to be the best for solar cells. Cubic, mixed polymorphs and monoclinic polymorphs are synthesised with increasing temperature. Photoluminescence spectra of the mixed and cubic polymorphs show mainly defect emissions below the band-gap energy, while the monoclinic modification shows only one sharp peak, which is attributed to the conduction to valence band transition. The monoclinic polymorph also grows in combination with a secondary phase containing sodium, NaxCuSnS3, with x between 0.5 and I. It exhibits a cubic (F43m) crystal structure like CuSn3.75S8 and a band-gap of around 1.6 eV. Devices with absorber layers of monoclinic Cu2SnS3, in which NaxCuSnS3 is either present or removed, gave similar power conversion efficiencies of above 1%. (C) 2016 Elsevier B.V. All rights reserved.