Thin Ag Precursor Layer-Assisted Co-Evaporation Process for Low-Temperature Growth of Cu(In,Ga)Se-2 Thin Film

Abstract

Achieving favorable band profile in low-temperature-grown Cu(In,Ga)-Se-2 thin films has been challenging due to the lack of thermal diffusion. Here, by employing a thin Ag precursor layer, we demonstrate a simple co-evaporation process that can effectively control the Ga depth profile in CIGS films at low temperature. By tuning the Ag precursor thickness (similar to 20 nm), typical V-shaped Ga gradient in the copper indium gallium diselenide (GIGS) film could be substantially mitigated along with increased grain sizes, which improved the overall solar cell performance. Structural and compositional analysis suggests that formation of liquid Ag-Se channels along the grain boundaries facilitates Ga diffusion and CIGS recrystallization at low temperatures. Formation of a fine columnar grain structure in the first evaporation stage was beneficial for subsequent Ga diffusion and grain coarsening. Compared to the modified co-evaporation process where the Ga evaporation profile has been directly tuned, the Ag precursor approach offers route for absorber engineering and is potentially more applicable for roll-to-roll fabrication system.