Diffusion of sodium in single crystal CuInSe2

Abstract

The incorporation of sodium from sodium fluoride in single-crystal CuInSe2 (CIS) is investigated to provide insight into the intra-granular aspects of sodium incorporation in CIS-based thin films. Sodium was incorporated by evaporating NaF onto two CIS crystals of varying compositions and defect structures followed by heating under vacuum. Diffusion profiles show a near-surface reaction before a deeper diffusion zone which follows a complementary error function, confirming Na diffusion into the crystals. Transmission electron microscopy analysis indicates that dislocations do not control the diffusion process. The activation energy of diffusion is ∼0.7 eV for both crystals. This low activation energy suggests that Na diffusion occurs rapidly through the bulk at temperatures as low as 300 °C and helps explain the uniform Na concentration often observed in grain interiors of polycrystalline Cu(InGa)Se2 thin films.