Effect of annealing in sulfur flux on Cu2ZnSnS4 formation by using molecular beam epitaxy system

Cu2ZnSnS4 (CZTS) thin films are highly suitable as the light-absorbing layer in solar cells. In this study, Cu, Zn, Sn, and S are co-evaporated on the Mo/soda-lime glass (SLG) surface at 320 °C and then annealed in sulfur flux at temperatures of 320-450 °C within the same molecular beam epitaxy system (one-stop process). The Raman spectroscopy of the coated surfaces shows that CZTS is formed with a secondary Cu2−xS phase, where the relative concentrations of the phases strongly depend on the annealing temperature. With an increase in the annealing temperature above around 400 °C, the fraction of CZTS decreases and that of Cu2−xS increases. The depth profile analysis shows that Cu segregates to the surface and Cu2−xS is formed on the film. The solar cells fabricated on these samples show that photovoltaic performance is limited by the surface Cu2−xS. The observed external quantum efficiency behavior is consistent with the degradation of carriers collected in the solar cell formed on the CZTS film owing to the presence of the secondary Cu2−xS phase.