Electronic and structural evidences for charge transfer and localization in iodine-doped pentacene

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 less than or equal to 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 I-3(-) and I-5(-) 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 less than or equal to 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.