In-Depth Compositional Analysis of the Carbon-Rich Fine-Grain Layer in Solution-Processed CZTSSe Films Accessed by a Photonic Lift-Off Process

The existence of a fine-grain (FG) sub-layer between the top large-grain (LG) layer and the back contact is widely observed in kesterite absorbers prepared with organic solvents. In this paper, the distinguishing features of the lifted-off carbon-rich FG layer are investigated through direct analysis with a series of characterization techniques, including X-ray photoelectron spectroscopy (XPS), attenuated total reflectance, X-ray diffraction, and scanning electron microscopy. To access the FG layer for direct probing, a scalable and repeatable photonic lift-off method is developed for carrying out the separation of the kesterite absorber layer from the Mo-coated glass substrate. A very high light intensity of 4 kW cm-2 for a short interval of 1 ms is optimized by COMSOL simulations, and successful implementation is demonstrated. The XPS analysis has revealed significant carbon content at the exposed FG surface, which explains the hindrance of grain growth due to carbon abundance. The variations in cations and anions concentrations from FG layer leading into LG region are explored through argon ions (Ar+) assisted XPS depth profiling. The observed significant differences between the composition of FG and LG regions are speculated to negatively impact the performance of solar cells. The photonic lift-off (PLO) method is adopted to expose the rear side of a solution-processed kesterite absorber. Ar+ assisted X-ray photoelectron spectroscopy (XPS) depth profiling is used to investigate the variations in the elemental composition of fine-grain (FG) sub-layer and elemental trends from FG to large-grain (LG) are reported. Significant differences in elemental concentration are noticed between LG and FG.image