Role of Fe Incorporation in Modulating the Structural, Optical, and Functional Behavior of ZnO‐Thin Films Using Green Synthesis

Developing environment‐friendly processes for producing semiconductor thin films is essential for the development of sustainable optoelectronic and energy harvesting technologies. In this study, films of Fe‐doped zinc oxide (ZnO) were synthesized for the first time using neem ( Azadirachta indica ) leaf extract as a reducing agent, spin coated, and annealed at 200°C and 500°C, for Fe concentrations of 0–20 at.%. Additionally, this method of synthesis avoids the use of toxic chemicals and is a cost and energy efficient alternative to conventional vacuum deposition methods. X‐ray diffraction (XRD) shows that all compositions contain wurtzite ZnO with (002) preferred orientation, and that Fe incorporation leads to a systematic decrease in the crystallite size, and an increase in internal stress, with a higher annealing temperature promoting grain growth and a partial relaxation of the strain. UV–vis spectroscopy shows that all films have high transparency in the visible range, and that Fe doping leads to a red shift of the absorption edge, due to defect states and associated microstrain which results in a narrowing of the band gap. Results of photoluminescence (PL) spectroscopy indicate that Fe doping suppresses the near band edge UV emission and enhances the broad visible deep‐level bands. This indicates that the density of defects, related to oxygen and cations, increases as a result of Fe doping, which is dependent on the concentration of the dopant and the annealing temperature. The optimized Fe:ZnO films containing intermediate amounts of Fe that were annealed at 500°C show that under mechanical stimulation, films were able to generate multiple volt outputs due to the appropriate balance between the piezoelectric response as a result of the orientation of the internal stresses along with the crystallographic defects. Such a result creates a clear structure–property–function relationship with neem‐assisted Fe‐doped ZnO‐thin films, confirming neem‐assisted Fe‐doped ZnO films are promising as eco‐friendly active layers in transparent, flexible nanogenerators and self‐powered sensors. Recent findings also reveal that eco‐friendly ZnO systems exhibit competitive photocatalytic and sensing performance, and continue to prove their technological importance.