We have investigated the doping mechanism of pentacene with iodine and its impact on the structure and on
the electronic properties of single crystals, powders, and thin films in a large range of iodine concentration
up to six iodine per pentacene (PEN) molecule (I/PEN = 6). Three regimes of doping have been identified.
In the low doping regime I/PEN < 0.05, the pristine pentacene structure of single crystals is maintained.
Electron spin resonance (ESR) evidences a Pauli susceptibility, that is, the characteristic fingerprint of
delocalized holes in the valence band of pentacene. In the intermediate doping regime (0.1 < I/PEN ≤ 2.0),
iodine diffuses between the (a,b) planes of the pentacene structure and forms an intercalate. Charge transfer
between iodine and pentacene is witnessed by both UV−vis and IR signatures of PEN+ cations and related
species, for example, cation dimers (PEN+)2 and typical Raman signatures of the I3
- and I5
- species. Spin
pairing of pentacene cation radicals is further supported by the observation of a thermally activated behavior
of the ESR spin susceptibility. In the heavy doping regime (2 < I/PEN ≤ 6), all traces of structural order
vanish, indicating that iodine penetrates within the (a,b) planes of the intercalate in a disordered manner,
forming an amorphous-like material. This high degree of disorder results in increased charge localization.
Most spin/charge species are ESR-silent and only a limited fraction (a few percents) exhibits a Curie-like
susceptibility. Because of disorder, the macroscopic conductivity of doped pentacene single crystals does not
exceed a few S/cm at 300 K.
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