A reinvestigation of the phase diagram of the Cu–In–Se system along the quasi-binary cut In₂Se₃–Cu₂Se reveals an existence range of the chalcopyrite α-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 α-phase, towards In- and Ga-rich compositions. We also investigate the interplay between phase segregation and junction formation in polycrystalline Cu(In, Ga)Se₂ films. Here, we attribute the band bending observed 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 different from those found for the bulk β-phase. We suggest that Cu-depletion is self-limited at the observed In/(In+Cu) surface composition of 0.75 because further Cu-depletion would require a structural transformation. Capacitance measurements reveal two types of junction metastabilities: one resulting from local defect relaxation, invoked to explain a light-induced increase of the open-circuit voltage of Cu(In, Ga)Se₂ solar cells, and one due to Cu-electromigration.