Beneficial effects of Ga on solar cell performance, e.g. open circuit voltage (V-oc > 800 mV) in CuInS2 (CIS) thin film solar cells have been observed. These highly efficient cells were used for fundamental investigations on the recombination mechanism, it could be shown that the incorporation of small amounts of Ga does not change the recombination mechanism as such. CIS-based solar cells exhibit a change from tunnelling into interface states in the dark to a thermally activated process under illumination. However, V-oc extrapolates to a value below the bandgap energy (E-g) of the CIS. For the latter process a model is proposed in which dominant recombination occurs at the interface and V-oc is limited due to the non-ideal band alignment in CuInS2/CdS junctions.