Abstract
A proper understanding on the charge mobility in organic materials is one of the key factors to realize highly functionalized organic semiconductor devices. So far, however, although a number of studies have proposed the carrier transport mechanism of rubrene single crystal to be band-like, there are disagreements between the results reported in these papers. Here, we show that the actual dispersion widths of the electronic bands formed by the highest occupied molecular orbital are much smaller than those reported in the literature, and that the disagreements originate from the diffraction effect of photoelectron and the vibrations of molecules. The present result indicates that the electronic bands would not be the main channel for hole mobility in case of rubrene single crystal and the necessity to consider a more complex picture like molecular vibrations mediated carrier transport. These findings open an avenue for a thorough insight on how to realize organic semiconductor devices with high carrier mobility.
Highlights
The state-of-the-art organic-technology, organic light emitting diode (OLED)[1,2,3,4], demonstrates the availability of organic molecules as materials for fascinating printable and flexible electronic devices[5]
The highest hole mobility estimated by angle-resolved photoelectron spectroscopy (ARPES) (29 cm2/Vs20), which should be the value of an ideal rubrene single” crystals (SCs) if carriers are 100% transport through the bands, is smaller than that obtained by transport measurement where SC with defects and disorder that may lower the mobility from the ideal value was used
We here notice that no visible light and/or continuous wave laser was irradiated on the sample during the ARPES measurements in the present study, though these lights were believed to be necessary to compensate the charging effect that occurs by the photoemission process and to reduce the photo-induced damage of rubrene molecules[19,20,21,22]
Summary
The state-of-the-art organic-technology, organic light emitting diode (OLED)[1,2,3,4], demonstrates the availability of organic molecules as materials for fascinating printable and flexible electronic devices[5]. Despite of the great experimental and theoretical efforts, there is still no complete consensus between the band dispersion and hole mobility, and not even between the experimentally and theoretical dispersions This means that the carrier transport mechanism in rubrene SC, which is an essential information for designing organic devices with high carrier mobility, is still an unresolved issue. Theoretical studies[23,24,25] reported the presence of two highest occupied molecular orbital (HOMO)-derived bands with strong anisotropy All these theoretical studies show the same dispersion behaviors, e.g., the downward dispersions from Γ to Y of the two bands and the degeneration of the two bands at Y, with small variations in the dispersion width from 420 meV23,25 to 520 meV24 and in the effective hole mass values from mh*/me = 0.6525 to 1.323 in the high mobility direction, the Γ-Y direction in reciprocal space (or the b direction in real space). We show the HOMO band dispersion of a high-quality rubrene SC obtained using high-resolution ARPES, explain the origin of the contradicted former results by taking the photoelectron diffraction (PED) effect into account, and discuss the mechanism of carrier transport in rubrene SC
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