NSLS-II Seminar

Rubrene, a tetraphenyl derivative of tetracene, has demonstrated strong potential for organic electronic applications. The recent studies suggest that the field-induced hole mobility of rubrene single crystal is over 20 cm2/Vs at room temperature, which is among the highest in organic semiconductors and is comparable to that of poly-Si. Despite of the exciting achievements in the device application, the current understanding about the relationship between the molecular structure and the electronic properties is limited by several factors. In this talk, I will present, to our knowledge, the first band structure measurement of an bulk rubrene single crystal with angle-resolved photoemission spectroscopy (AR-PES). The high quality of the crystal surface was confirmed with atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Highly reproducible dispersive features were observed with nice symmetry about the Brillouin zone center and boundaries. The energy dispersion of the highest occupied molecular orbital (HOMO)-derived bands show strong anisotropic behavior in the a-b plane of the unit cell. The measured band structure is compared to the density functional theory (DFT) calculations. Furthermore, the energy level alignment at the interfaces between rubrene thin film and different metal substrates are studied with ultraviolet and x-ray photoemission spectroscopy (UPS and XPS). The charge injection barriers at these interfaces are discussed. Finally, the morphology of the thermally evaporated rubrene thin film is studied with AFM for various sample preparation conditions.