Organic electroluminescent devices

In recent years, considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. By the successful fabrication of single devices and matrix displays, very favorable properties of organic light emitting diodes (OLEDs) are demonstrated, which make OLEDs promising candidates for active pixels in flat-panel displays. The layer sequences of low-molecular-weight molecules building an OLED are sublimated under ultrahigh vacuum (UHV) conditions in an organic molecular beam deposition system (OMBD). Efficient OLEDs with bright emission in the blue, green and red spectral regions exhibiting low operation voltages and a micro-structured 5×7-matrix display are presented. To explain the operation of an OLED, five physical processes are identified: carrier injection at the contacts, carrier transport to the recombination site, recombination while forming an exciton, exciton diffusion and decay. If an OLED is embedded in a planar Fabry-Perot microcavity, a spatial redistribution and a slight enhancement of the integrated emission intensity is obtained. Finally, energy level determination by ultraviolet photoelectron spectroscopy (UPS) and measurements at model devices utilizing doped sensor layers are briefly discussed.