Influence of substitution on the optical properties of functionalized pentacene monomers and crystals: Experiment and theory

Feasibility of p-Doped Molecular Crystals as Transparent Conductive Electrodes via Virtual Screening

Transparent conducting materials are an essential component of optoelectronic devices. It is proven difficult, however, to develop high-performance materials that combine the often-incompatible properties of transparency and conductivity, especially for p-type-doped materials. In this work, we have employed a large set of molecular semiconductors extracted from the Cambridge Structural Database to evaluate the likelihood of transparent conducting material technology based on p-type-doped molecular crystals. Candidates are identified imposing the condition of high highest occupied molecular orbital (HOMO) energy level (for the material to be easily dopable), high charge carrier mobility (for the material to display large conductivity when doped), and a high threshold for energy absorption (for the material to absorb radiation only in the ultraviolet). The latest condition is found to be the most stringent criterion in a virtual screening protocol on a database composed of structures with sufficiently wide two-dimensional (2D) electronic bands. Calculation of excited-state energy is shown to be essential as the HOMO-lowest unoccupied molecular orbital (LUMO) gap cannot be reliably used to predict the transparency of this material class. Molecular semiconductors with desirable mobility are transparent because they display either forbidden electronic transition(s) to the lower excited states or small exchange energy between the frontier orbitals. Both features are difficult to design but can be found in a good number of compounds through virtual screening.

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

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.