Influence of the Ga-content on the bulk defect densities of Cu(In,Ga)Se2

Abstract We fabricate a series of CIGS absorber layers with Ga-contents x =Ga/(In+Ga) ranging from x =0 (CuInSe 2 ) to 1 (CuGaSe 2 ) 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 −q V oc decreases from x =0 to x ≈0.3, thus, having a minimum at x ≈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 −q V 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 −q V oc suggesting that volume defects determine the open circuit voltage of CIGS solar cells in the whole composition range from CuInSe 2 to CuGaSe 2 and when using different absorber processes.