"Ambient Processed Two-Step SILAR-Deposited NiCo2S4\/ZnS Heterojunction for UV\u2013Visible Photodetection on Flexible Paper Substrat"

"Photodetectorsplayamajorroleinopticalcommunications, medical imaging, and environmental monitoring because of their high sensitivity, fast response times, and ability to detect light at different wavelengths. However, the development of photodetectors that com bine a broad spectral range, cost-effective fabrication, and excellent mechanical flexibility continues to be a challenge. This work reports a NiCo2S4\/ZnS heterojunction photodetector fabricated using the Suc cessive Ionic Layer Adsorption and Reaction (SILAR) technique on a f lexible cellulose paper substrate. UV-visible spectroscopy measures the absorbance anddeterminesthedirect bandgap of ZnSandNiCo2S4 to be 3.6 eV and 2 eV, respectively. Structural features of the fabricated heterojunction are characterized using X-ray diffraction (XRD), confirm ing the crystalline nature of both materials. Morphological features are analyzed using scanning electron microscopy (SEM), providing evidence of heterojunction formation. The combination of ZnS, a wide-bandgap semiconductor, and NiCo2S4, a transition metal sulfide with excellent electrical conductivity, enables improved photoresponse. The NiCo2S4\/ZnS heterojunction shows responsivities of 54 A\/W (UV) and 39 A\/W (visible), with response times of 6.4 s and 2.2 s, respectively. The observed photoresponse is due to the strong UV absorption of ZnS and the visible absorption of NiCo2S4. A Fermi level difference induces charge transfer at the heterojunction, creating a built-in electric field that enhances carrier separation and responsivity. The device exhibits excellent mechanical durability, withstanding over 1000 bending cycles, making it suitable for wearable sensors, environmental monitoring, and optical communication."