Paper EM-MoM4
Interfacial Influences and Electron-Injection Mechanisms of Tris-(8-Hydroxyquinoline)-Aluminum Doped with Cesium-Derivatives In Organic Light Emitting Devices

Monday, October 15, 2007, 9:00 am, Room 612

The cesium-derivatives (Cs2CO3, CsF and CsNO3) have been investigated as a dopant in tris-(8-hydroxyquinoline)-aluminum (Alq3) or a thin electron injection layer in organic light emitting devices. Unlike low work function metal which would be evaporated from a complex deposition process, the cesium-derivatives have a very simple deposition process and are easy to handle. By using ultraviolet and x-ray photoemission spectroscopy, the properties of electronic structures and the interface chemistry are studied. The paper presents the investigation of interfacial interactions and electron-injection between cesium-derivatives and Alq3. According to our results, the Fermi level of Alq3 after doped with cesium-derivatives shifts inside the gap toward the lowest unoccupied molecular orbital (LUMO) as a result of the charge transfer from cesium atom to Alq3, showing that electron-injection ability would be improved as a result of strong n-type doping effect. It is noteworthy to emphasize that through the ultraviolet and x-ray photoemission spectroscopy measurement, Cs2CO3 does not decompose during evaporation with various evaporation rates and pressures. The relatively abrupt decreasing in vacuum level is found via UPS, which can be explained by charge exchanges and a strong dipole field at the interface with deposition of cesium-derivatives. Moreover, doping cesium-derivatives into Alq3 not only reduces the electron-injection barrier height, but also increases the carrier concentration for current conduction. We also demonstrate that the interfacial chemical reaction leads to the excellent electron injection efficiency.