Accurate control and characterization of Cu depletion layer for highly efficient Cu(In,Ga)Se2 solar cells

The unintentional formation of a Cu depletion layer (CDL) on the surface of Cu(In,Ga)Se-2 was observed in 1993, and CDLs have been expected to improve the performance of Cu(In,Ga)Se-2 solar cells. However, methods of controlling CDLs have hitherto been unavailable. For the first time, we succeeded in controlling a uniform CDL on a Cu(In,Ga)Se-2 surface by introducing irradiation with Se into the typical three-stage growth process of Cu(In,Ga)Se-2. We discuss the characterization and effects on device performance of CDLs. A uniform, smooth CDL with a thickness of 200 nm was formed at a Se irradiation time of 5 minutes, whereas the CDL formed at an irradiation time of 10 minutes was rough and non-uniform. A maximum efficiency of 19.8% was achieved at an irradiation time of 5 minutes via the formation of a complete CDL. This simple, unique method may help to maximize efficiency of Cu(In,Ga)Se-2 solar cells.