Molecular Ionization Potential and Electron Affinity
The injection of the holes and electrons is given considerable attention to create optimized electronic devices in real world. The information about the organic device performance and its stability can be obtained from the parameters of ionization potential (IP) and electronic affinity (EA) that determine the charge injection through estimating the energy barrier for injection of hole and electron into molecule.
[65, 66]. The calculated IPs and EAs of compounds, both vertical (at the geometry of the neutral molecule) and adiabatic (optimized structure for both the neutral and charged molecules), are presented. More stable and hardly oxidized molecules are favoured for the practical applications that high IPs value provide these features to the molecules [67]. When the Au is used as an electrode, AB2 is predicted to has smaller injection energy for hole than AB1. The EA values of AB1 and AB2 are 1.56 and 2.13 eV, respectively (Table 6). For the devices if the EA value is high, it means that injection energy for electron will be small (commonly used metallic electrodes (3 eV)). From these EA values, we can see thatAB2 is better than AB1 for transporting electrons from both lowering the energy barrier for electron injection.
Besides that, the HOMO energy levels for both compounds are good aggrement with the work function of the gold electrode (-5.2 eV). Therefore, the injection of the hole from the gold to the organic semiconductor easily accompolished.