Microscopic Insights into Charge Formation and Energetics in N-Doped Organic Semiconductors

In the molecular doping of organic semiconductors (OSC), achieving efficient charge generation and managing the energetic cost for charge release from local molecular charge transfer complexes (CTCs) to the host matrix is of central importance. Experimentally tremendous progress has been made in this direction. However, the relation between OSC film structure on a nanoscopic level including different inter-molecular geometrical arrangements and the macroscopic properties of doped OSC films is usually only established quite indirectly. Explicit microscopic insights into the underlying doping mechanisms and resulting electronic structure are still scarce and mostly limited to the study of the individual molecular constituents or isolated bi-molecular dopant:host complexes. In the present study we investigate n-type doping of the frequently investigated OSC materials ZnPC and F8ZnPc and their mixtures which are n-doped with 2-Cyc- DMBI. We report significant electronic differences for complexes with nominally the same material composition but different geometrical structures. One specific important finding in this context is that complexes containing two adjacent dopant molecules show much reduced ionization energy values, leading to substantially reduced energy cost for charge release. Furthermore our results demonstrate that important trends towards macroscopic system behavior can already be obtained with increasing size and varying composition of the relatively small molecular dopant-host complexes considered, including systematic shifts in the Fermi level energies in the doped OSC.