Investigation of the effect of potassium on Cu(In,Ga)Se2 layers and solar cells

We investigate the influence of potassium (K) on the Cu(In,Ga)Se2 (CIGS) growth kinetics on alkali-free alumina substrates and the electrical parameters of the CIGS solar cell by intentional K doping of the CIGS layer by a KF-precursor layer and KF-post deposition treatment (PDT). Secondary ion mass spectroscopy measurements revealed that K can be incorporated into the CIGS layer by both processes. The CIGS composition of the KF-precursor sample shows a stronger [Ga]/([Ga] + [In]) (GGI) profile. By analysing the samples with scanning electron microscopy we observed smaller CIGS grains for the KF-precursor sample compared to the K-free reference and KF-PDT sample. jV-measurements of the KF-PDT and the KF-precursor sample show an increase in the cell efficiency η from 10.7% to 13.6% and 13.7%, respectively, compared to the K-free reference sample. The external quantum efficiency measurements of the KF-precursor sample show an increased absorption in the infrared region. Capacitance-voltage measurements reveal an increase in the net doping concentration of both samples treated with K. We assume that the enhancement is caused by passivation of grain boundaries and donor-like defects by K, as previously demonstrated for Na.