Microstructure Evolution During Selenization of Cu2ZnSnS4 Colloidal Nanocrystal Coatings

Abstract

Annealing colloidal nanocrystal coatings in a selenium-containing environment to form polycrystalline thin films of the earth-abundant solar absorber copper zinc tin sulfoselenide (CZTSSe) is an attractive approach for making solar cells. We used a closed selenization system to investigate how coatings comprising copper zinc tin sulfide (CZTS) nanocrystals evolve into polycrystalline CZTSSe thin films and studied the effects of selenium vapor pressure, annealing temperature, and heating rate. These studies revealed two different types of microstructures and two different grain growth mechanisms depending on whether the CZTS nanocrystals are exposed to selenium vapor only or to both selenium vapor and liquid selenium. Coatings annealed in the presence of selenium vapor form a microstructure comprising micron-size CZTSSe grains on top of a nanocrystalline, carbon-rich, CZTSSe layer. The film microstructure is controlled by concurrent normal and abnormal grain growth, and the grain size distribution is bimod...