Prof. Tsung Lung Li, Department of Electrophysics, Institute of Optoelectronics and Solid-state Electronics, National Chia-Yi University, Taiwan, China

Title: Photoemission Spectroscopy of Alkali-Doped Rubrene Thin Films: A First-Princi ples Study

Abstract: Organic semiconductive materials have been the subject of intensive research due to their advantages on low-cost, material flexibility, ease of processing, and large-scale production. Rubre ne (5, 6, 11, 12-tetraphenyltetracene, C42H28) is one of the most studied organic semiconductive materials for its high electron mobility. For device applications, rubrene is frequently doped with alkali metal to enhance device performance. Alkali-doped rubrene thin films had been experimentally studied with photoemission spectroscopy (PES) of synchrotron radiations. In this work, the PES spectra of alkali-doped rubrene thin films are simulated with density functional theory (DFT), where the alkali-rubrene complexes formed in the deposition process are computationally approximated by an isolated molecule. The validity of this approach is justified by good agreement of the simulated energy density-of-states (EDOS) spectra with experiments reported in the literature. It is found that the PES spectra of rubrene thin films deposited by heavy alkali metal (cesium, rubidium, and potassium) at the alkali/rubrene stoichiometric ratio of 1:1 exhibit essentially identical qualitative characteristics. This uniformity of alkali-rubrene electronic structures persists in total as well as decomposed emergy density-of-states spectra. Detailed analysis on the geometric and electronic structures of the alkali-rubrene complexes suggests that the uniformity of PES spectra of alkali-rubrene fundamentally emerges from the weak dependence of electronic structures on alkali adsorption sites, and is strengthened by the structural similarity of minimum-energy alkali-rubrene complexes for the case of heavy alkali.

Keywords: Alkali-doped rubrene, Alkali-rubrene complexes, Organic semiconductors, Organic thin films , Photoemission spectroscopy, Density functional theory