Nitrogen-mediated phase transformation in RF-sputtered CuInSe2

• RF-Sputtered CuInSe 2 thin films were prepared in the presence of nitrogen. • At low nitrogen flows the CuInSe 2 chalcopyrite structure is slightly disrupted. • At low nitrogen flows the thin filmś bandgap can be tuned from 0.96 eV to 1.3 eV. • Large nitrogen flows facilitate the dissociation of CuInSe 2 into CuSe and InSe. • CuInSe 2 :N with a 1.3 eV bandgap may be used for substituting CuInGaSe 2 in solar cells. Pure and nitrogen‐incorporated CuInSe 2 (CIS) thin films were fabricated by a reactive sputtering technique under controlled nitrogen flow rates ranging from 0 to 1.5 sccm. Systematic investigation using X-ray diffraction (XRD) confirmed that deposited films retained a chalcopyrite‐type structure, dominated by the (112) plane, with secondary phases (e.g., CuSe, InSe) emerging at high nitrogen flows. Topographical analyses through scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that moderate nitrogen incorporation yielded dense, smooth, and compact CIS thin films, while higher flows (>0.3 sccm) led to an increasingly rough surface and the formation of additional binary compounds. Compositional studies by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) verified the incorporation of nitrogen into the CIS lattice and identified concomitant changes in stoichiometry. Optical measurements demonstrated a noticeable bandgap shift from approximately 0.96 eV for pristine CIS to 1.3 eV assisted with 0.1 sccm nitrogen, highlighting the potential for bandgap engineering. However, an excess of nitrogen flow (>0.1 sccm) facilitated the partial decomposition of CIS, raising the effective bandgap above 1.6 eV and underscoring the delicate balance required to maintain chalcopyrite integrity. These findings confirm that appropriate nitrogen doping conditions can enhance both the optical and structural properties of CIS films, offering new avenues for optimizing photovoltaic performance in solar cell applications. The study provides insight into how controlled nitrogen doping can potentially fine-tune the bandgap, while highlighting that excessive nitrogen promotes detrimental phase separation.