Effects of KF and RbF post deposition treatments on the growth of the CdS buffer layer on CIGS thin films - a comparative study

In this contribution we are analyzing and comparing the impact of two different alkali-fluorine post deposition treatments (KF and RbF) on the growth of chemical-bath-deposited CdS buffer layers on Cu(In, Ga)Se-2 absorber layers for thin film solar cells. By combining Raman scattering, scanning electron microscopy, current-voltage analysis and measurements of the internal quantum efficiency we provide a comprehensive picture of this issue on the material and device level. We find that both PDTs lead to a better CdS-coverage of the surface of the CIGS, which leads to an improved junction quality at early growth stages compared to untreated devices. Furthermore the growth rate of the CdS is enhanced on KF-treated absorber layers while it is decreased on those treated with RbF (compared to the reference). This leads to a more stable behavior of RbF-treated devices after longer duration of the CdS deposition, while the KF-treated devices suffer from reduced fill factor and open circuit voltage. Furthermore we show that not only both PDTs but also the growth of the CdS lead to a reduction of the amount of the so called ordered defect compound, which is initially present at the surface of our absorber layers. This behavior indicates either the formation of Cd-Cu -anti-sites or of a secondary phase at the interface.