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

Highlights: • Heavy alkali treatment improves the open-circuit-voltage of Cu(In,Ga)Se₂ cells. • RbF is the most effective alkali in improving the open-circuit-voltage. • Wide bandgap Cu(In,Ga)Se₂ cells are limited by the interface recombination. -- 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 (V_OC) can be achieved. In spite of this improvement, the activation energy (E_A) of the treated samples remained smaller than the bandgap (E_g) and V_OC(t) transients under red light showed a negative slope. Hence, the wide gap CIGSe devices remain limited by recombination at the interface. However, the V_OC(t)-transient of treated and untreated samples with the same E_g show a different slope (d ΔV_OC(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 V_oc(t) slope, i.e. d ΔV_OC(t)/dt, with reference to the illumination dependent doping density (N_A,a) and to a possible Sodium exchange mechanisms.