Electronic and Structural Evidences for Charge Transfer and Localization in Iodine-Doped Pentacene

Abstract

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.

Thermoelectric and bulk mobility measurements in pentacene thin films

“…Since we maintain a constant average sample temperature of 300 K, the decrease in carrier concentration is expected to be linear on a log-log plot of p versus t, dropping in proportion to 1 / t ␤ , where ␤ is related to the order of the dedoping chemical reaction. 23 This dependence is consistent with the approximately linear increase measured for the Seebeck coefficient versus log͑t͒, since S ϳ ln͑p͒ through Eq. ͑2͒.…”

Nano-structured calcium silicate hydrate functionalised with iodine

“…This is in contrast to the Ca 2+ ions for which a shift to more positive binding energies was observed. Furthermore, this was in contrast to the interactions in most complexes and reservoir compounds used in conjunction with iodine; no formation of tri-iodide or penta-iodide as reported in the literature for other iodine complexes has been found [19,20]. The lack of a detectable shift in the binding energy for iodine in the iodine-silicate composite sample could be attributed to the limited range of emitted photoelectron binding energies exhibited by this element in the presence of alkali metals [34].…”

“…Reservoir materials are based on the tendency of iodine to form charge transfer complexes with various organic molecules, the best known of which are complexes with starch [19]. For most complexes the formation of tri-iodide, I À 3 , and penta-iodide, I À 5 , ions is a requirement [19,20]. During studies aimed at the development of paint formulations with disinfectant properties, it was noted that iodine is readily absorbed by and released from nano-structured calcium silicate hydrate (NCaSilH) [21].…”

Surface and interface analysis of iodine‐doped pentacene structures for OTFTs

“…During this long time the so-called intermediate regime of diffusion should be obtained. [8] We found that after iodine diffusion for 48 h, the pentacene structure was completely changed. We confirmed this by XRD measurements where the XRD peak corresponding to the thin-film phase ( Fig.…”

“…A further reduction of the iodine concentration leads to still better structural film properties: After decreasing the time of iodine diffusion to 10 min (sample A10) we obtained the socalled intermediate regime of diffusion [8] which is characterized by the presence of the thin-film phase and also its intercalate phase in the layer. Both the measured XRD peaks corresponding to these phases are plotted in Fig.…”