Structural, spectroscopic and electrical investigations of novel organic thin films bearing push-pull azo – Phenol dye for UV photodetection applications

Abstract

In the current study, novel thin films of a phenol-based push-pull azo dye, 2-acetyl-4-(4-chloro-phenylazo) phenol (ACAP), with tunable optical and electronic properties were designed, synthesized and characterized for UV photodetection applications. The crystalline structure and morphological features of the thermally evaporated ACAP thin films are investigated. The fabricated thin films exhibit an amorphous-like structure with low-intensity crystalline regions of average crystallite size of about 29.51 nm and a smooth surface with nanostructured sheets formation. The optical transmittance, reflectance, and absorption of ACAP thin films are measured in the spectral range UV–vis-NIR. A significant high UV absorption extending from 190 nm to 385 nm is observed with semi-transparency nature in the visible region. Furthermore, a good agreement is obtained between the estimated value of the direct energy gap that is obtained experimentally (3.62 eV) and that calculated from the theoretical DFT approach (3.74 eV). The dispersion behavior is analyzed in terms of the single oscillator model and is employed to estimate the dispersion parameters. Finally, an organic/inorganic heterojunction device based on Au/ACAP/n-Si/Al for UV photodetection is successfully fabricated. The current–voltage relations of the manufactured photodetector showed significant stability and sensitivity to the incident UV illumination. The fabricated UV photodetector exhibits responsivity ~25.7 mA/W, specific detectivity ~2 × 109 Jones, efficiency ~16.74%, a fast and reproducible ON/OFF switching behavior with 480 ms and 218 ms rise and fall time, respectively.