We fabricate a series of CIGS absorber layers with Ga-contents x = Ga/(In + Ga) ranging from x = 0 (CuInSe2) to 1 (CuGaSe2) by single layer coevaporation. The open circuit voltages V-oc of the completed solar cells increase with increasing Ga-content but not proportional to the change of the band gap energy E-g of the CIGS-layers. In contrast to the behaviour at Ga-contents exceeding x = 0.3 the difference E-g - qV(oc) decreases from I = O to x = 0.3. thus, having a minimum at x approximate to 0.3. We determine defect concentrations in the absorber of these cells by admittance spectroscopy. These bulk defects also have a minimum concentration at x = 0.3. This low concentration of deep defects reduces recombination losses and thus. the difference of E-g - qV(oc). In addition, we fabricated absorber layers with a three stage process and an average Ga content x of approximately 0.3. The resulting solar cells have a lower defect concentration and a higher open circuit voltage than solar cells from single stage processes with x = 0.3. We find a correlation between volume defect concentrations and the difference E-g - qV(oc) suggesting that volume defects determine the open circuit voltage of CIGS solar cells in the whole composition range from CuInSe2 to CuGaSe2 and when using different absorber processes.