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 <= 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<sup>+</sup> cations and related species, for example, cation dimers (PEN<sup>+</sup>)<sub>2</sub> and typical Raman signatures of the I<sub>3</sub><sup>-</sup> and I<sub>5</sub><sup>-</sup> 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


Published in:
Journal of Physical Chemistry A, 108, 40, 8170-8179
Year:
2004
Keywords:
Note:
Inst. Charles Sadron, Strasbourg, France Copyright 2005, IEE 8248970 1089-5639 charge transfer charge localization iodine-doped pentacene doping mechanism electronic property single crystals powders thin films iodine concentration pentacene structure electron spin resonance Pauli susceptibility delocalized holes valence band intermediate doping regime intercalation UV-vis spectra IR spectra cations cation dimers Raman spectra spin pairing ESR spin susceptibility structural order Curie-like susceptibility conductivity 300 K
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 Record created 2007-04-03, last modified 2018-03-18


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