Improved lifetime and efficiency of organic light-emitting diodes for applications in displays

Organic semiconductors have been intensively studied over the past decades. The potential of this new class of materials for photonic and electronic device applications is demonstrated by successful fabrication of organic and organic-on-inorganic heterostructures for electroluminscent devices, photodetectors, and microwave diodes. The fabrication technology of organic semiconductor devices for photonic applications is discussed. In contrast to spin-on or dipping techniques for fabrication of polymeric films, organic compounds with low molecular weight are sublimated under ultra high vacuum (UHV) conditions. The organic molecular beam deposition (OMBD) technology employed allows the reproducible growth of complex layer sequences with a defined thickness of various organic semiconductors in combination with dielectric films, different metallizations, and indium-tin-oxide layers. Growth rates from 1 - 5 nm/min and substrate temperatures from 77 K to 350 K are used. Efficient organic light emitting diodes with bright emission in the blue (1-AZM-Hex), green (Alq 3 ) [tris(8-hydroxychinoline)-aluminum], and red (Eu-complexes, DCM) spectral region and with low operation voltages are presented. In general, an onset voltage of 2.7 V, efficiencies up to 7 lm/W and a luminance up to 2 10 5 cd/m 2 (cw, RT) are attained for N,N'-diphenyl-quinacridone-doped Alq 3 devices. An undoped device can be operated up to 5000h without any loss in brightness and just a small increase of the driving voltage of about 2V. Embedding emissive organic thin films with a narrow spectral characteristic into planar Fabry-Perot microcavities, a light intensity enhancement and a spatial redistribution of the emission is achieved. Research and development today expands its scope from the investigation of materials, device physics, and device optimization to peripheral technologies for flat-panel display applications, for instance micro patterning methods for the cathode of an OLED display.