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

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. (C) 2001 Elsevier Science B.V. All rights reserved.