The Evolution of Intermolecular Energy Bands of Occupied and Unoccupied Molecular States in Organic Thin Films

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[ABSTRACT] In organic semiconductors, the hole and electron transport occurs through the intermolecular overlaps of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), respectively. A measure of such intermolecular electronic coupling is the transfer integral, which can experimentally be observed as energy level splittings or the width of the respective energy bands. Quantum chemistry textbooks describe how an energy level splits into two levels in molecular dimers, into three levels in trimers and evolves into an energy band in infinite systems, a process that has never been observed for the LUMO or beyond dimers for the HOMO. In this work, our new technique, low-energy inverse photoelectron spectroscopy, was applied to observe the subtle change of the spectral line shape of a LUMO-derived feature while we used ultraviolet photoelectron spectroscopy to investigate the occupied states. We show at first that tin-phthalocyanine molecules grow layer-by-layer in quasi one-dimensional stacks on graphite, and then discuss a characteristic and systematic broadening of the spectral line shapes of both HOMO and LUMO. The results are interpreted as energy-level splittings due to the intermolecular electronic couplings. Based on the Hückel approximation, we determined the transfer integrals for HOMO-1, HOMO, and LUMO to be ≤15 meV, (100 ± 10) meV, and (128 ± 10) meV, respectively.

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