Microstructure evolution and crystal growth in Cu₂ZnSnS ₄ thin films formed by annealing colloidal nanocrystal coatings

A potentially low-cost approach for making copper zinc tin sulfide (CZTS) solar cells relies on annealing colloidal nanocrystal films cast on suitable substrates in a sulfur-containing environment to form thin films with micrometer-sized grains. The film microstructure and grain size affects the solar cell performance. We conducted a systematic study of the factors controlling crystal growth and microstructure development during annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 μm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The eventual coarsening of the CZTS nanocrystals reduces the driving force for abnormal crystal growth. Increasing the sulfur pressure by an order of magnitude to 500 Torr accelerates both normal and abnormal crystal growth though sufficient acceleration of the former eventually reduces the latter by reducing the driving force for abnormal grain growth. For example, at high temperatures (700 °C) and sulfur pressures (500 Torr) normal grains quickly grow to ∼500 nm which significantly reduces abnormal grain growth. The use of soda lime glass as the substrate, instead of quartz, accelerates normal grain growth. Normal grains grow to ∼500 nm at lower temperatures and sulfur pressures (i.e., 600 °C and 50 Torr) than those required to grow the same size grains on quartz (700 °C and 500 Torr).