Effects of Rubidium Fluoride and Potassium Fluoride Postdeposition Treatments on Cu(In,Ga)Se2 Thin Films and Solar Cell Performance

Postdeposition treatments (PDTs) with sodium fluoride (NaF) and potassium fluoride (KF) were introduced as a way to improve the efficiency of Cu­(In,Ga)­Se2 (CIGS) based solar cells. Here, we apply postdeposition treatments with rubidium fluoride (RbF) to low-temperature coevaporated CIGS absorbers after a first PDT with NaF and compare the effects of the addition of Rb and K on the solar cell performance and material properties of the CIGS films. KF and RbF PDTs lead to similar improvements in the open-circuit voltage (V oc) and fill factor (FF), while allowing a reduction of the thickness of the cadmium sulfide (CdS) buffer layer without loss in electronic performance. KF and RbF PDTs lead to comparable modifications of the morphology and composition of the CIGS films. After the PDT, K and Rb accumulate in a nanopatterned copper-poor secondary phase at the CIGS surface, while also diffusing within the CIGS layer and strongly reducing the concentration of lighter alkali element sodium. These findings corroborate theoretical calculations published by another group, which predicted the segregation of potassium indium selenide (KInSe2) and rubidium indium selenide (RbInSe2) at CIGS surfaces under the used PDT conditions.