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.