Low Electron Carrier Concentration Near the p-n Junction Interface: A Fundamental Factor Limiting Short-Circuit Current of Cu(In,Ga)Se2 Solar Cells

A spike-like conduction-band offset (CBO) in heterojunction solar cells is shown to limit the short-circuit current density (J(SC)) dramatically when the spike height is large. It is widely believed that the spike-like CBO produces a potential barrier, which resists the photogenerated carriers flowing through the junction interface and thus decreases J(SC). However, our device simulation studies on Cu(In,Ga)Se-2 (CIGS) solar cells reveal that a large spike-like CBO causes an extremely low electron carrier concentration in the near-interface region of the buffer layer, which is the major factor that limits the carrier transport and thus J(SC). If the near-interface electron concentration is increased, J(SC) can be increased despite the fact that the large spike-like CBO and potential barrier are still present. These results indicate that the near-interface electron concentration is the fundamental factor limiting the J(SC), more fundamental than the potential barrier. Therefore, not only the commonly adopted band-alignment engineering, but also various other methods, for example, choosing buffer materials with suitable effective density of states, introducing favorable interface defects, or increasing the doping level, can be adopted for improving the current collection in heterojunction solar cells. Therefore, J(SC) can always be increased even when the large spike-like CBO is inevitable.