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.