Interfacial physics in organic light emitting devices

Abstract

In this paper, we will present some studies of physics at the interfaces in the organic light emitting devices. The paper can be separated into two parts. First part is the manipulation of interfacial energy structures and electron transport properties of organic semiconductors. The second part is substitution and dopant dependence of electronic structures in organic thin films I will present an investigation of the energy structures and electrical doping mechanisms of the organic semiconductor surface through current-voltage (I-V) characteristics and photoemission spectroscopies. We found that both surface energy structures and transport properties can be manipulated with mix of LiF or Cs₂CO₃. The I-V characteristics show that the current efficiency is significantly improved with Cs₂CO₃ doped either at the surface or in the bulk Alq₃. As Cs₂CO₃ doping works efficiently with Al as well as other cathode metals, the interfacial chemistry and carrier injection mechanisms of such cathode structures are compared to that of the conventional LiF thin layers. To understand the mechanisms of the improvement on electron injection, the surface energy levels of metal and organic materials were measured with ultraviolet photoemission spectroscopy (UPS) and the interfacial chemistry was studied with X-ray photoemission spectroscopy (XPS). From UPS spectra, we found that a thin layer of Cs₂CO₃, as thin as 0.5 A, at the metal and organic ETL interface can bring the Fermi level of Alq₃ from mid-gap to less than 0.2 eV below the lowest unoccupied molecular orbital (LUMO), indicating that the Alq₃ film at the interface is heavily n-type doped with Cs₂CO₃ . The smaller gap between the Fermi level and LUMO with Cs₂CO₃ reduces the electron injection barrier. Strong dipole fields are also found at the surface, which also affects the electron injection considerably. The XPS data further show that Cs ions are dissociated at the interface as soon as Cs₂CO₃ is deposited on Alq₃. The result is different from the case of LiF, in which Al metal is needed for releasing Li ions. With co-evaporation of Cs₂CO₃ with Alq₃ in the bulk as n-doping ETL, the current efficiency can be further improved, which is presumably attributed to the enhancement of the electron transport in the Alq₃ films.