Secondary compound formation revealed by transmission electron microscopy at the Cu₂ZnSnS₄/Mo interface

One promising candidate considered for solar cell absorber layers is Cu₂ZnSnS₄ (CZTS). Transmission electron microscopy (TEM) investigations of such solar cells to date are scarce. We present microanalysis results on our fully processed CZTS solar cells based on absorber layers deposited by reactive sputtering of a precursor layer followed by a short anneal. The initially small grain size for precursor layers increases rapidly due to annealing, typically spanning the entire absorber layer thickness. Energy dispersive X-ray spectroscopy in a TEM clearly reveals the formation of secondary compounds containing Zn-, Cu- or Sn-sulfides located at the Mo/CZTS back contact interface after annealing. Simultaneously a MoS₂ layer is formed at the back contact. The extent to which secondary compounds and MoS₂ form scales with annealing time, indicating that Mo is not stable when in contact with CZTS. Understanding the chemical reactions at the back contact is considered to be essential to limit the secondary phase formation during annealing. - Highlights: ► We investigate the Mo/Cu₂ZnSnS₄ back contact before and after precursor annealing. ► The precursor is initially uniform in composition and grew in small columnar grains. ► Annealing causes formation of secondary compounds and MoS₂ at the back contact. ► This indicates decomposition of Cu₂ZnSnS₄ when in contact with Mo. ► Transmission electron microscopy is capable to characterize the secondary compounds