We investigated industrially produced chalcopyrite solar cells based on the absorber modifications Cu(In,Ga)Se-2 and Cu(In,Ga)(S,Se)(2) in order to study the nature of the experimentally verified efficiency improvement, mainly caused by an increased open circuit voltage. We show that the introduction of sulfur during the absorber formation via rapid thermal processing leads to a substantial lowering of the surface doping concentration and widening of the space charge region (SCR). Temperature dependent diode analysis revealed a reduction of the SCR recombination in (Se,S) devices which would lead to a larger splitting of quasi-Fermi levels and hence to an increased open circuit voltage as compared to neat Cu(In,Ga)Se-2 devices. (C) 2011 Elsevier B.V. All rights reserved.