Formation Pathway of Wurtzite-like Cu₂ZnSnSe₄ Nanocrystals

Cu₂ZnSnSe₄ is a direct band gap semiconductor composed of earth-abundant elements, making it an attractive material for thin-film photovoltaic technologies. Cu₂ZnSnSe₄ crystallizes in the kesterite structure type as a bulk material, but it can also crystallize in a metastable wurtzite-like crystal structure when synthesized on the nanoscale. The wurtzite-like polymorph introduces unique and useful properties to Cu₂ZnSnSe₄ materials, including widely tunable band gaps and superior compositional flexibility as compared to kesterite Cu₂ZnSnSe₄. Here, we investigate the formation pathway of colloidally prepared wurtzite-like Cu₂ZnSnSe₄ nanocrystals. We show that this quaternary material forms through a chain of reactions, starting with binary Cu₃Se₂ nanocrystals that, due to both kinetic and thermodynamic reasons, preferentially react with tin to yield hexagonal copper tin selenide intermediates. These ternary intermediates then react with zinc to form the resulting wurtzite-like Cu₂ZnSnSe₄ nanocrystals. Based on this formation pathway, we suggest synthetic methods that may prevent the formation of unwanted impurity phases that are known to hamper the efficiency of Cu₂ZnSnSe₄-based optoelectronic devices.