Why Annealing Processes Affect the Optical Properties of Alq 3 Films and OLEDs G. Baldacchini a , T. Baldacchini b , P. Chiacchiaretta a , R.B. Pode c , M.A. Vincenti a , and Q.-M. Wang d a ENEA, Frascati Research Center, Department FIM, 00044 Frascati, Roma, Italy b Newport Corporation, Technology and Application Center, Irvine, CA 92606, USA c Department of Physics, Kyung Hee University, Seoul 130-701, South Korea d Department of Bioscience and Biotechnology, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan Organic light emitting diodes, OLEDs, have been so much improved in the last two decades that they are being currently used in commercial applications for displays and lighting. Among many improving technologies, thermal treatments affect greatly their performances. In case the small molecule Alq 3 is the active material, it was never understood why and how the temperature could have played such an important role. Now, we have discovered that such molecules tend to aggregate themselves in morphological states which are very much sensible to annealing processes, with the result of improved emission efficiency and lifetime of Alq 3 thin films. Moreover, the latter ones are described by a four components model, FCM, which has been validated by degradation experiments during 6 years, a world record in its own. Introduction In 1987 Tang and Van Slyke devised a new organic light emitting diode, OLED, which was able to emit efficiently in the green (1). It was made of a few tiny layers of different inorganic and organic materials, and the one acting as electron transport and light emitting layer contained molecules of tris(8-hydroxyquinoline)aluminum, Alq 3 . Within two decades, it was so much improved (2) that it has been and is being currently used in commercial applications (3,4). Among the many approaches utilized to improve the OLEDs, annealing processes have played a central role since the beginning. Already in 1993 it was found that particular annealing procedures resulted in more efficient and stable devices (5). Later on, more systematic studies found that annealing improves the performance of devices, by decreasing the operation voltage and increasing the energy conversion efficiency (6). At the same time, better efficiency and lifetime enhancement of OLEDs containing Alq 3 were obtained by performing post annealing treatments and the results published by several groups (7-13). It was also found that thermal treatments of thin films of Alq 3 affect both photoluminescence, PL, and lifetime (14-23), which in a few cases were accurately measured during long time degradation processes, over 1,000 h (19,20). However, in spite of the previous and other results, which were of paramount importance especially for applicative purposes, the basic motivations behind such evident improvements remained substantially unknown, although it was clear that the morphology of the molecular film was playing an important role. Indeed, relatively few studies were devoted then to the correlation between molecular structure and optical ECS Transactions, 16 (31) 3-9 (2009) 10.1149/1.3112631 ©The Electrochemical Society 3 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 201.242.72.78 Downloaded on 2014-10-14 to IP