Light Effective Mass in the Widely-Dispersed Valence Band of Single Crystalline Rubrene Observed by High-Resolution Angl
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1197-D07-31
Light Effective Mass in the Widely-Dispersed Valence Band of Single Crystalline Rubrene Observed by High-Resolution Angle-Resolved Ultraviolet Photoelectron Spectroscopy Yasuo Nakayama1, Shin-ichi Machida2, Steffen Duhm2, Qian Xin2, Akihiro Funakoshi2, Naoki Ogawa2, Satoshi Kera2, Nobuo Ueno2, Yutaka Noguchi1,2, and Hisao Ishii1,2 1
Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan 2
Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ABSTRACT High-resolution angle resolved ultraviolet photoelectron spectroscopy measurements were conducted on rubrene single crystals assisted by a laser illumination resulting in successful relief of the sample charging. Significant dispersion of the valence band was clearly resolved. The band width W and the hole effective mass mh* were estimated to be 0.4 eV and 0.65m0, respectively, along the most conductive direction. The present results strongly suggest that the transport nature in rubrene single crystals should be described in the band transport of a delocalized charge carrier.
INTRODUCTION Despite recent rapid progress on the application side for the organic electronic devices, fundamentals of the carrier transportation in organic semiconducting materials are still far from satisfactory understanding. The single crystal (SC) of rubrene (5,6,11,12-tetraphenyltetracene) has been regarded as the most promising prototype to investigate the intrinsic transport nature in molecular solids, because it revealed the highest hole mobility of all organic semiconductors (μh = 40 cm2V-1s-1 [1]) reported so far. It has been proposed that the hole transporting characteristics in the rubrene SCs should be described in the band transport framework at least along the most conductive axis based on the temperature-dependence of the conductivity and Hall effect [2,3]. Within the band transport framework, however, whether a hole behaves as a small polaron with relatively heavy effective mass (> 2m0, where m0 is the free electron mass) [2,4] or as a “light quasi-particle” (< m0) [5] is still controversial. Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) is the most direct method to determine the electronic structures dominating the hole transport natures of matters. Nevertheless, successful ARUPS results on single crystals of rubrene or other organic semiconductors have not been published so far. The main reason of this situation should be ascribed to the sample charging problem. Because electric conductance of organic semiconductor crystals is generally not sufficiently high, photoemission results in charging of the sample via the capture of photogenerated holes within the crystals. In order to obtain reliable spectra on rubrene SCs, a release of the trapped “photo-holes” by an additional tactics (e.g. enhancement of photoconductivity through light illumination [6,7]) is indispensable.
In the present study, we conducted high-resolution ARUPS measurements on rubre
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