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 b...