Potassium versus Sodium in Cu(In,Ga)Se2—Similarities and Differences in the Electrical Characteristics of Solar Cells and Thin Films after NaF or KF Postdeposition Treatment

Abstract

The improvement of CIGSe solar cells efficiency due to potassium fluoride postdeposition treatment (KF‐PDT) is often interpreted as being caused by changes in the interface region. The impact of KF‐PDT on bulk properties of the CIGSe absorbers is investigated by measuring hole densities in the solar cells and conductivities of corresponding thin films and is then analyzed together with similar data for NaF‐treated absorbers and thin films. It is found that both alkali metals change the electrical properties of the whole CIGSe layer and not only the absorber surface. By measuring electrical conductivity and doping level in thermal equilibrium (relaxed) and metastable state (after light soaking) as well as their evolution with the amount of Na and K after postdeposition treatment (PDT) at various temperatures, one can observe effects that can only be explained by the influence of alkali metals on grain boundary properties. In contrast to Na, low concentration of potassium leads to a decrease in conductivity. Persistent photoconductivity exhibits a linear relationship with the relaxed conductivity in both NaF‐ and KF‐treated samples. These effects directly show that electrical transport in CIGSe is limited by potential barriers at the grain boundaries, which are impacted by the alkali treatment.