Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.

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 GaF 3 , 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 In 2 Se 3 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 Cu x Se 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.

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