In this work, a series of stain steel 15×15 cm2 CuInS2 solar cells with efficiencies close to the record one for this kind of devices, are analyzed. Through a careful and comprehensive study of the structural and electronic properties of the CuInS2 layer, we show that in a general fashion the strain originated by the thermal annealing affects the energy band splitting and reduces the short circuit current. Then, through an innovative combination of photoreflectance and Raman scattering analysis, we demonstrate that the presence of CuAu domains in the bulk layer of a CuInS2 is directly related with this strain reduction contributing to the improvement of short circuit current. We propose that the presence Cu-Au phase domains reduce the strain within the CuInS2 layer, and improve the quality of the CIS chalcopyrite crystals, leading to reduced carrier recombination while increasing carriers mobility. As a consequence we conclude that the presence of said domains improves the short circuit current in the studied devices