Effects of selenium atmosphere on grain growth for CZTSe absorbers fabricated by selenization of as-sputtered precursors

Abstract

Pure kesterite Cu2ZnSnSe4 (CZTSe) absorbers with a large average grain size are vital for high-efficiency CZTSe solar cells, which are greatly influenced by the annealing atmosphere. Sputtered CZTSe precursors were annealed in the presence and absence of a selenium-containing atmosphere, and these materials were incorporated into solar cells. When annealed in an Ar atmosphere, the CZTSe absorbers were found to be composed of small grains with the presence of voids and secondary phases. When H2Se was added into the chamber at a temperature of 550 °C, the CZTSe grains increased in size and the presence of the voids and secondary phases reduced. When H2Se was added at a lower temperature (300 °C) earlier in the annealing process, the average grain size increased further and device efficiencies were higher. When H2Se was continuously added into the chamber during the whole selenization process, the average grain size increased to approximately 2 μm, the undesired secondary phases were not detected and the efficiency of the CZTSe devices reached a maximum of 6.78%. This work showed that exposing the CZTSe precursors to a selenium atmosphere during annealing increased the average grain size and reduced the presence of undesirable secondary phases, thus improving the performance of CZTSe absorbers and solar cells.