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.