Abstract:

Accurate prediction of the energy levels (i.e. ionization potential and electronic affinity) of organic semiconductors is essential for understanding related mechanisms and for designing novel organic semiconductor materials. From a theoretical point of view, a major challenge arises from the lack of a reliable method that can provide not only qualitative but also quantitative predictions at an acceptable computational cost. In this study, we demonstrate an approach, combining the polarizable continuum model (PCM) and the optimally tuned range-separated (RS) functional method, which provides the ionization potentials (IPs), electron affinities (EAs), and polarization energies of a series of molecular semiconductors in good agreement with available experimental values. Importantly, this tuning method can enforce the negative frontier molecular orbital energies (-ε_HOMO, -ε_LUMO) that are very close to the corresponding IPs and EAs. The success of this tuning method can be further attributed to the fact that the tuned RS functional can provide a good balance for the description of electronic localization and delocalization effects according to various molecular systems or the same molecule in different phases (i.e. gas and solid). In comparison, other conventional functionals cannot give reliable predictions because the functionals themselves include too low (i.e. PBE) or too high (i.e. M06HF and non-tuned RS functionals) HF%. Therefore, we believe that this PCM-tuned approach represents an easily applicable and computationally efficient theoretical tool to study the energy levels of more complex organic electronic materials.

Key words: Organic semiconductor, Density functional theory, Optimally-tuned, Range-separated (RS) functional, Energy level