Insight into electronic structure and the impact of annealing temperature on the optical properties of rubrene thin films for optoelectronic devices

Rubrene thin films were deposited using thermal evaporation, followed by a detailed investigation of their structural and optical properties. The structural characteristics of rubrene thin films before and after annealing were investigated using FTIR and XRD techniques. The FTIR spectra of rubrene in both powder and thin film forms showed similar patterns, indicating the chemical stability of rubrene after the evaporation process. Computational analysis using the DMOL3 program was employed to determine vibrational frequencies, HOMO and LUMO energies, and global reactivity descriptors of rubrene. XRD analysis revealed the polycrystalline orthorhombic phase of rubrene powder and the amorphous nature of rubrene thin films. The optical constants of pristine and annealed rubrene thin films were obtained utilizing a spectrophotometer. The electronic transitions were identified as indirect, with onset bandgaps measured at 1.90 +/- 0.02 eV, and direct transition, with onset band gaps measured at 2.20 +/- 0.01 eV. Following the annealing process, a minor decrease was observed in the indirect energy gap, whereas the direct energy gap remained unchanged. Numerous dispersion parameters, such as the energy of the dispersion (Ed) and the single-oscillator (E0), the dielectric constant of the lattice (epsilon L), and the dielectric constant at infinite frequency (epsilon infinity) of the rubrene films before and after annealing were determined and discussed in the light of the single-oscillator (Wemple-Didomenico) model. These findings suggest the potential of the produced rubrene thin films for application in optoelectronic devices.