OLED matrix displays: technology and fundamentals

For more than a decade, considerable effort has been put into the development of light emitting devices based on evaporated layers of organic semiconductors. To date, the properties of matrix displays consisting of organic light emitting diodes (OLEDs) basically meet automotive and consumer product requirements. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. In contrast to polymer devices, OLEDs are processed in ultrahigh vacuum systems. The organic source materials are sublimated from effusion cells. Due to the sensitivity of organic thin films, device structuring by conventional etching techniques is not feasible and alternative structuring techniques were developed. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal-organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode set-ups. Further insights into the energy band alignment at organic heterointerfaces are gained by ultraviolet photoelectron spectroscopy (UPS). In addition to widely employed electrical (I-V, C-V) and optical (PI) measurements, thermally stimulated current (TSC) and luminescence (TSL) allow the characterization and a more detailed understanding of carrier traps and charge transport in organic devices. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.