Influence of heavy alkali post deposition treatment on wide gap Cu(In,Ga)Se2

Abstract

The effect of Potassium, Rubidium, and Cesium post deposition treatment (PDT) on wide bandgap Cu(In,Ga)Se2 (CIGSe) absorbers has been investigated. The results show that the efficiency of the cells can be improved by the alkali treatment and a higher value of the open-circuit-voltage (VOC) can be achieved. In spite of this improvement, the activation energy (EA) of the treated samples remained smaller than the bandgap (Eg) and VOC(t) transients under red light showed a negative slope. Hence, the wide gap CIGSe devices remain limited by recombination at the interface. However, the VOC(t)-transient of treated and untreated samples with the same Eg show a different slope (d ΔVOC(t)/dt). CIGSe samples treated with heavy alkalis (RbF- and CsF-PDT) illustrate a smaller slope in comparison to no-PDT and KF-PDT samples. In this contribution, we discuss the Voc(t) slope, i.e. d ΔVOC(t)/dt, with reference to the illumination dependent doping density (NA,a) and to a possible Sodium exchange mechanisms.