Phase segregation, Cu migration and junction formation in Cu(In, Ga)Se-2

A reinvestigation of the phase diagram of the Cu-In-Se system along the qua si-binary cut In2Se3-Cu2Se reveals an existence range of the chalcopyrite a lpha-phase that is much narrower than commonly accepted. The presence of 0. 1% of Na or replacement of In by Ga at the at.% level widens the existence range of the alpha-phase, towards In- and Ga-rich compositions. We also inv estigate the interplay between phase segregation and junction formation in polycrystalline Cu(In, Ga)Sen films. Here, we attribute the band bending ob served at bare surfaces of the films to a positively charged surface acting as a driving force for the formation of a Cu-poor surface defect layer via Cu-electromigration. The electrical properties of this defect layer are di fferent from those found for the bulk beta-phase. We suggest that Cu-deplet ion is self-limited at the observed In/(In+Cu) surface composition of 0.75 because further Cu-depletion would require a structural transformation. Cap acitance measurements reveal two types of junction metastabilities: one res ulting from local defect relaxation, invoked to explain a light-induced inc rease of the open-circuit voltage of Cu(In, Ga)Se-2 solar cells, and one du e to Cu-electromigration.