Assessing the Impact of Alkali Post-Deposition Treatment on the Surface Properties of High-Efficiency Kesterite Absorbers.

The performance of kesterite solar cells has steadily improved over the past five years, driven by advances in molecular precursor composition and processing. Inspired by their success in high-efficiency chalcopyrite devices, alkali-salt post-deposition treatments (PDTs) have been proposed to further enhance the opto-electronic properties of kesterite thin films. In this work, we investigate how light (LiF, NaF) and heavy (RbF, CsF) alkali fluoride PDTs influence the surface composition and electronic properties of solution-processed (Ag,Cu)2ZnSn(S,Se)4 (ACZTSSe). Reference devices with the structure SLG/Mo/ACZTSSe/CdS/ITO/Ag, in which the absorber is not subjected to PDT, achieved efficiencies of up to 12.20%. While LiF and NaF PDTs feature an efficiency drop to values close to 11%, RbF and CsF PDTs cause substantial degradation, particularly in fill factor. To elucidate the origin of these effects, we probe the absorber using energy-filtered photoemission electron microscopy (EF-PEEM), X-ray photoelectron spectroscopy (XPS), and angle-resolved XPS (ARXPS). Our measurements reveal, for the first time, that the thermal treatment associated with PDT, even in the absence of alkali fluorides, promotes surface elemental disorder. This disorder manifests as a pronounced broadening of the work-function distribution and results in a 200 mV loss in VOC. Furthermore, PDT induces significant changes in both the surface composition and electronic landscape of ACZTSSe, with markedly different responses observed for light and heavy alkali elements.