The electrical and photophysical performances of axially-substituted naphthalene diimide-based small molecules as interface layer

To date, the number of publications on NDI-based small molecules is almost identical to the number of publications on NDI-based polymers. As opposed to the polymer examples, the design of NDI-based small molecules can be easily sub-categorized into axial (a) and core (c) substituted geometries, and have a practical use. For this purpose, in this study, initially, the symmetric and asymmetric axially-substituted-NDI-based small organic ligands (aNDIs 1A-1G) were synthesized as novel interface layers, and the values of atomic charges, geometry, dipole moment, and thermodynamical properties were calculated with DFT methods. Thus, initially, the theoric and experimental band gap values were calculated with DFT/TD-DFT and UV–Vis measurements, respectively. The theoretical band gap was found to be between 3.5 eV and 3.6 eV and the experimental band gap was found to be between 3.15 eV and 3.17 eV. Following synthesis and theoretical studies, the electrical and photophysical performances of naphthalene diimide-appended diode and photodiode devices (D1-D7) have been discussed with an analytical overview. For this, initially, the aNDIs, organic photodiodes, were fabricated with a thermal evaporation system, and the surface morphologies were examined with AFM for each interface layer. Due to the very small roughness values, about 1.8–2.1 nm, it was concluded that the thermal evaporation method is a suitable method for growing these structures. Following the fabrication of devices, the diode and photodiode parameters were investigated with current–voltage measurements at dark and different illumination conditions. The temperature-dependent performances of the fabricated devices were investigated at 100 K, 200 K, and 320 K. Additionally, the capacitance characteristics were investigated with the impedance spectroscopy method. As a result of all these studies, in addition to the literature for effective synthesis of symmetric and asymmetric aNDIs as simple structured photodiode candidates, besides the effective synthetic touches, the wide-range behavioral differences of these photodiode candidates of different natures have been investigated comparatively.