A facile and low-cost synthesis of Cu2ZnSn(SxSe1−x)4 nanocrystals with tunable composition and optical band gap

•CZTSSe nanocrystals are synthesized by a facile two-step process.•It involves a solvothermal reaction of CZTSe and a sulfurization post-annealing.•The Se2− is gradually replaced by the S2− when increasing the sulfurization time.•The optical band gap becomes large gradually.

Cu2ZnSn(SxSe1−x)4 nanocrystals have been successfully synthesized by a novel two-step route involving a solvothermal reaction of Cu2ZnSnSe4 and a sulfurization post-annealing. The Se2− is gradually replaced by the S2− to form the CZTSSe solid solutions with a tetragonal structure when increasing the sulfurization time. The A1 Raman modes of CZTSSe display a typical two-mode behavior, revealing that S composition plays an important role on tuning the vibrating modes. The optical band gap becomes large gradually from 0.91 to 1.30 eV with increasing S content from 0 to 0.76. CZTSSe nanocrystals derived from this method can be used as low-cost absorber layer for photovoltaic applications.