Orateur
Simone Fratini
(Institut Néel)
Description
Because of the weakness of van der Waals inter-molecular bonds, organic semiconductors are commonly believed to stand in an intermediate region between the molecular limit and the extended band picture, calling for concepts that go beyond the standard paradigms of inorganic semiconductors. Experimental charge transport and optical studies of organic semiconductors over the past few years have indeed given contrasting indications on the nature of electronic carriers. The existence of low-mass quasiparticles and the apparently “band-like” temperature dependent electronic mobilities observed in the best crystalline systems are difficult to reconcile — within the framework of band theory alone — with absolute values of the mobility that hardly exceed few tens of cm2/V s. A number of photoemission studies of high-mobility organic semiconductors have recently appeared, providing evidence that even the most accurate ab initio band-structure calculations predict bandwidths that are significantly smaller than measured experimentally.
Based on a combined theoretical/experimental analysis of the photoemission spectra of pentacene, we demonstrate that this discrepancy stems from the interaction of electrons with the internal vibrations of the molecules. Unambiguous signatures are identified, showing that the nature of extended electronic states in organic materials is deeply modified by the electron-molecular vibration coupling. Such features should be general to broad classes of molecular systems including organic semiconductors, doped organic conductors and charge-transfer organic salts.
