Mapping the surface electronic landscape of solution-processed CuIn(S,Se)2 thin-films as a function of the Cu/In ratio

This study explores the influence of the Cu/In ratio, ranging from 0.80 to 1.10, on the surface composition, electronic landscape, and photovoltaic (PV) performance of solution-processed CuIn(S,Se)₂ (CISSe) thin films. X-ray fluorescence (XRF) confirmed that the Cu/In ratio in the CISSe films closely matches that of the precursor solution, while X-ray diffraction (XRD) indicated a consistent Se/(Se+S) ratio of 0.55 across all samples. In contrast, X-ray photoelectron spectroscopy (XPS) revealed that the surface Cu/In ratio is approximately 30% lower than the bulk value. CISSe devices fabricated in a standard substrate configuration (SLG/Mo/CISSe/CdS/i-ZnO/Al:ZnO) exhibited the highest power conversion efficiency (PCE) of 9.1% at a Cu/In ratio of 0.95. As the Cu content increased, PCE dropped sharply, accompanied by a reduction in band tailing (decrease in bulk disorder). For the first time, energy-filtered photoemission electron microscopy was used to reveal a direct correlation between PV performance and the evolution of the surface electronic landscape. The highest PCE corresponded to CISSe absorbers with a mean work function (WF) of 4.9 eV. Increasing the Cu/In ratio beyond 0.95 led to a significant decrease in mean WF. Notably, at a Cu/In ratio of 1.10, nanometer-sized domains with WF values as low as 3.9 eV emerged. These features are discussed in the context of the complex interplay between bulk and surface disorder and their impact on PV performance.