In this work, we investigated the electrical transport across the valence band injection barrier at the interface between the Cu(In,Ga)(S,Se)2 absorber layer and the Mo(Se,S)2 layer of the back contact of a chalcopyrite solar cell with respect to their different sulfur contents. For this purpose, samples and modules with three different [S][S]+[Se] (SSSe) ratios in the near vicinity of the interface were produced. The valence band offsets at this interface were determined by means of glow-discharge optical emission spectroscopy and ultraviolet photoelectron spectroscopy (UVPES). Measured current–voltage (IV) characteristics allowed to correlate the roll-over behavior related to the injection barrier with the SSSe composition. We discuss two contradictory findings in literature concerning the layer primarily responsible for the valence band edge offset and the injection barrier it creates, of which both findings could be confirmed in our experiments. We conclude that this could be explained by the sensitivity of the UVPES measurement to local differences in the Mo(Se,S)2 crystal properties. Furthermore, we employ a device simulation model to gain insight into the mechanisms at play at that interface, arriving at the following conclusions: (1) Thermionic emission by itself does not suffice to reproduce the measured IV data under the influence of said barrier. (2) The influence of Na doping on the hole density of the absorber narrows the injection barrier at this interface so that tunneling currents contribute considerably to the injected current, even for significantly large valence band edge offsets of 0.8eV. (3) Light soaking (LS) drastically reduces the roll-over effect, although the difference in bulk carrier concentration and its effect on IV behavior are negligible. Instead, the experimental IV data after LS could be reproduced by a higher interface acceptor trap occupation.