Investigation of Cu(In,Ga)Se2 Solar Cell Performance Deviations in Nominally Equal Absorbers

Cu(In,Ga)Se2 (CIGSe) solar cells were fabricated independently by industrial scale co-evaporation in two separate production lines with the same nominal composition and thickness of the absorber film. Although the device properties were believed to be the same we observed substantial deviations of the respective values of the open circuit voltage (ΔVOC = 40 mV) and of the fill factor (ΔFF= 4%), whereas the short circuit current was essentially the same. We performed fundamental device analysis, space charge and defect spectroscopy, transient photoluminescence as well as in-depth profiling of the chemical gradients of the absorber films. Using the results from the experiments we set up a simulation baseline which allowed us to conclude that the apparent deviations can be related to the presence of deep recombination centers with different concentration within the CIGSe absorber as well as to variations of the band gap grading.