Low-onset organic blue light emitting devices obtained by better interfacecontrol

We demonstrate that highly efficient single-layer light emitting devices (L ED) can be realized by better control in the device production process, esp ecially regarding the interface between the active material and the metal c athode. The device cross-section was investigated using Auger depth profili ng and spectroscopy. Following this approach, LEDs with para-sexiphenyl (op tical gap: 3.1 eV) as active medium and aluminum as cathode were realized, which emit blue light at a bias of only 3.5 V, when the aluminum deposition rate is drastically reduced, namely from 10 to below 1 Angstrom/s. We find that the lower deposition rate of Al results in a decreased width of the i nterfacial region, where carbon, aluminum, and oxygen are intermixed. At th e same time the relative oxygen concentration at the cathode interface is i ncreased. However, the presence of oxygen does not lead to the formation of oxidized aluminum species, as verified by the Al local mixing model (LMM) Auger signal. (C) 1999 American Institute of Physics. [S0003-6951(99)00820- 7].