Effects of a pre-annealing treatment (PAT) on Cu2ZnSn(S,Se)4 thin films prepared by rapid thermal processing (RTP) selenization

In this study, CZTSSe precursors (Se/Cu/SnS/ZnS) were deposited by sputtering and evaporation and then annealed without added chalcogen substances in an argon-filled atmospheric chamber using rapid thermal processing (RTP). A low-temperature pre-annealing treatment (PAT) was applied to induce a pre-reaction between Se and Cu/SnS/ZnS. It was found that the PAT leads to the formation of CuSe and Cu2−xSe and enhances Se incorporation during RTP chalcogenization. Scanning electron microscopy (SEM), X-ray diffractometry, Raman analysis and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) were applied to investigate the differences between the absorber with the PAT and the one without the PAT. Better morphologies on the surface and in the cross section were obtained for the CZTSSe absorber annealed using the PAT compared with the absorber annealed without the PAT. Both absorbers showed nearly pure kesterite phases in their upper regions, as confirmed by Raman analysis. However, STEM-EDS maps revealed that when the absorber was prepared without the PAT, Zn- and S-rich secondary phases and voids were easily formed near the back contact. The electrical characteristics and efficiencies of the CZTSSe thin films showed drastic changes; the CZTSSe solar cell without the PAT showed no diode response, but the cell with the PAT showed an efficiency of 6.77% with an open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) of 376 mV, 31.39 mA/cm2 and 57%, respectively.