New aspects of phase segregation and junction formation in CuInSe₂

A reinvestigation of the phase diagram on the quasi-binary Cu₂Se-In₂Se₃ cut reveals a much narrower existence range of the chalcopyrite α-phase as compared to previous phase diagrams towards In- and Ga-rich compositions. The presence of Na or replacement of In by Ga inhibits defect ordering and widens the existence range of the α-phase. Capacitance measurements reveal two types of junction metastabilities in Cu(In,Ga)Se₂ solar cells. An increase of the capacitance after electron injection into the space charge region most likely relates to a local defect reaction. A change of the spatial distribution of charges after annealing under applied bias points to Cu-electromigration. We suggest that the potential determined by a positive surface charge and Cu-migration assist the formation of the Cu-poor (`ODC') surface. This mechanism also causes distinct differences in the properties of the Cu-poor surface and the bulk β-phase.