Rubrene electronic structure, interface energy level alignment, and growth dynamics

Abstract

We have investigated the electronic structure, interface formation, and thin film growth dynamics of rubrene using ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), angle-resolved photoemission spectroscopy (ARPES), and atomic force microscopy (AFM). From UPS and IPES we obtained an injection gap of 2.67 eV, which is derived from the onset separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and a transport gap of 3.98 eV, which is derived from the peak separation. The ARPES results indicate that the HOMO band dispersion along Γ-X is 0.25 eV, and the electron effective mass is 1.3 times of the free electron one. These values can produce an estimate of the hole mobility of 15 Vs/cm². The investigation of the electronic structure of the interfaces between rubrene and various metals, such as Au, Ag, Al, and Ca, shows that the Fermi level shifts linearly within the band gap as a function of metal workfunction, until it is been pinned at the LUMO by a low workfunction metal like Ca. The growth morphology dependence on the film thickness, deposition rate, and substrate temperature will also be discussed.