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.