Crystalline Characterisitics of Epitaxial Cu(In,Ga)Se2 Layers on GaAs (001) Substrates

Epitaxial Cu(In,Ga)Se2 (CIGS) solar cells with different Cu compositions were produced. In the CIGS layers with Cu-rich composition, scanning transmission electron microscopy revealed deep voids and there was a high density of voids between the CIGS layers and the GaAs substrates. A distinctive layer structure was also observed between the CdS and CIGS layers with KF postdeposition treatments and the layer may have been composed of KInSe2 compounds. The highest efficiency of 20.2% was achieved using Cu-poor CIGS layers. For the CIGS solar cells with near-stoichiometric Cu composition, the net accepter concentrations were quite low. Therefore, Cu-poor CIGS layers were favorable for creating high-efficiency CIGS solar cells.