  Sulphurization processes in Cu(In,Ga)Se$_2$ thin-film solar cells has been
intensively studied in the last decade as a viable alternative to the existing
Ga-grading. The main advantage of using S grading is that by substituting Se
with S we will achieve not only an upshift of the conduction-band minimum as
done by employing Ga grading, but also a downshift of the valence-band maximum.
Several existing studies stipulate that S is very often inserted in too high
concentrations into Cu(In,Ga)Se$_2$ absorber by sulphurization resulting in a
deteriorated device performance instead of the expected beneficial effect.
However, we demonstrate here that the intense sulphurization process when
accompanied by Ga-grading leads to improved electrical properties of the
buffer/absorber heterojunction. More exactly, this double grading at the
absorber surface leads to strong reduction of the p-doping and hence to a
change in the band diagram. This work also proves that the intense
sulphurization process is accompanied by strong structural and chemical
changes, i.e., by the formation of a S-rich CuIn(S,Se)$_2$ compound at the
absorber surface. Finally, all these experimental findings were complemented by
ab-initio calculations of the conduction-band and valence-band offsets between
absorber and buffer obtained by using density functional theory. Hence, the
present work opens up new possibilities for synthesizing Cu(In,Ga)(Se,S)2 solar
cells with superior cell performance when using an intense sulphurization
process.
