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Phonon dispersio dash vertical5/15/2023 In this paper, we report on the ARUPS investigation of the band dispersion of rubrene single crystal. However, despite theoretical predictions and relevance for applications, a direct experimental evidence of band distortions in organic molecular semiconductors as mediated by the charge-phonon coupling has yet to be observed. Local change in the band curvature was similarly predicted to appear in the band dispersion of organic semiconductors, originating from the charge coupling with molecular vibrations 11. These effects may result, for example, in the enhancement of the carrier effective mass at the Fermi level, with consequent impact on the charge mobility 9. The kink position and the degree of the related band distortion reflect the energy of the phonons and the strength of their coupling with the charge carriers. In metals, the charge coupling with lattice phonons manifests in local change in the curvature of the band dispersion (i.e., kink structures), as directly revealed by a number of angle resolved ultraviolet photoemission spectroscopy (ARUPS) investigations 9. In this context, experimental studies on the impact of the charge-phonon coupling on the electronic properties of the organic molecular semiconductors may provide critical information (i.e., coupling strength, phonon energies, and so on) for refining the details of the various transport models and/or testing the validity of the corresponding predictions. More recently, the hole/electron coupling with intermolecular vibrations was also discussed as a possible origin of charge localization, as related to the continuous modulation of the spatial overlap between molecular wavefunctions 7– 10. Intense theoretical research is still on-going to include all these critical aspects in a fully consistent and accurate description of the charge carrier dynamics in organic molecular semiconductors 6– 8. localization processes 6– 8 both affected, in turn, by the details of the crystal structure and temperature 4, 6– 8. More realistically, the charge transport mechanism in molecular organic semiconductors is expected to result from the delicate interplay between charge spatial delocalization vs. As a matter of fact, the charge localization/delocalization phenomena in molecular organic semiconductors occur on comparable characteristic energy scale (~50–200 meV 4) which prevents the applicability of the above mentioned conventional limiting treatments 6. However, both the conventional band and hopping transport theories soon appeared inadequate for describing the measured charge mobility in a large number of organic materials 6. The latter mechanism is affected by the charge reorganization energy which depends on the hole/electron coupling with the intramolecular vibrations, as molecules generally undergoes structural relaxations upon receiving additional charges 4, 5. The former mechanism is dominated by the energy dispersion of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital derived bands which reflects the spatial overlap of the molecular wavefunctions 4. In this context, the charge transport in organic molecular semiconductors was described with respect to the limiting case of coherent band-like transport and incoherent hopping-like transport 4 corresponding to the extreme delocalization and localization of the charge carriers, respectively. This important issue was originally addressed by applying concepts and ideas developed in transport studies of inorganic semiconductors and metals. Despite the technological achievements, important aspects of the physics of these materials remain still elusive, as for example the exact nature of the charge transport mechanism. In recent years, the use of organic molecular semiconductors as base materials for optoelectronic applications marked a significant advancement in the technological field, resulting in an entirely new class of light, flexible and low-cost devices 1– 3.
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