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The present results demonstrate that electron mobility in high-mobility organic semiconductors is indeed limited by polaron formation. This model quantitatively reproduces not only the transfer integrals, but also the temperature-dependent HOMO and LUMO bandwidths, and the hole and electron mobilities. To reproduce this bandwidth reduction, we propose an improved (partially dressed) polaron model that accounts for the electron–intramolecular vibrational interaction with frequency-dependent coupling constants based on Debye relaxation. The derived transfer integrals and bandwidths from the LUMO are substantially smaller than those predicted by density functional theory calculations. Here we employ angle-resolved low-energy inverse photoelectron spectroscopy to reveal the LUMO band structure of pentacene, a prototypical high-mobility organic semiconductor. Despite the discovery of the valence (the highest occupied molecular orbital (HOMO)) band structure in the 1990s, the conduction band (the lowest unoccupied molecular orbital (LUMO)) has not been experimentally observed. The energy band structure provides crucial information on charge transport behaviour in organic semiconductors, such as effective mass, transfer integrals and electron–phonon coupling.
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