KF post deposition treatment in co-evaporated Cu(In,Ga)Se2 thin film solar cells: Beneficial or detrimental effect induced by the absorber characteristics

Recent breakthroughs in Cu(In,Ga)Se-2 (CIGS) thin film solar cell energy conversion efficiency are related to the application of a potassium fluoride post-deposition treatment (KF-PDT) to the completed absorber. Using X-ray photoelectron spectroscopy and Raman scattering, we compare CIGS layers prior and after the KF-PDT in the case of a deterioration and an improvement of the solar cells photovoltaic performance. The purpose is to study and model the modification of the surface in both cases and address some of the required characteristics of the absorber, grown on soda lime glass by 3-stage process, in order to take advantage of the treatment. We show that, in both cases, KF-PDT induces the formation of GaF3, which is removed during the subsequent chemical bath deposition of CdS, explaining the Ga depleted absorber surface, already reported in literature. However, the presence or not of an ordered defect compound (ODC), correlated with the third stage duration during the CIGS growth, is shown to be crucial in the modifications of the surface induced by the treatment. When an ODC is present prior the treatment, KF-PDT leads to the formation of a surface layer of In2Se3 containing K, and the photovoltaic performance of completed solar cells are improved. When no ODC is present prior KF-PDT, no trace of K is found at the absorber surface after the treatment, copper (Cu) segregates into detrimental CuxSe phases, high amount of elemental Se is formed, and the photovoltaic performance are lowered. The role of the ODC during the KF-PDT is finally discussed.