Morphological Effects in the Chemical and Photoluminescent Behavior of Aluminum Tris(8-Hydroxyquinoline) (Alq 3 )
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Rapid physical changes which limit OLED performance are slowly being controlled (for example, through the use of morphologically stable glasses[1O-12], substrate smoothing, or composite films[13]), particularly for the case of hole transporting materials.[14-16] Effects noticed at longer times, however, such as the gradual electroluminescence (EL) decay and accompanying increase in device resistance,[5,6] warrants investigation of typically slower physical aging[17] and chemical degradation[ 18,19] processes. Aluminum tris(8-hydroxyquinoline) (Alq3) has risen to a prominent position in the development of robust OLEDs due to its relative stability as an electron transporting and emitting material.[4,5] The glass transition temperature (Tg) of Alq3 is relatively high (at 172 'C), but crystallization may still occur below Tg at greater time scales. These processes are closely related to device stability since photophysica and electrical properties are affected by the molecular or supramolecular order. Furthermore, Alq3 absorbs moisture, and can hydrolyze significantly at elevated temperatures to form free 8-hydroxyquinoline (8-Hq) species.[20-22] The freed 8-Hq may then undergo further reactions to produce nonemissive species which can act as luminescence quenchers.[21] The observation of degradation reactions and byproducts in an actual device configuration (with thicknesses of 600-1200 A), has proven a formidable task,[18] since the amount of material that can be generated in an operating OLED is undetectable by most instruments. The volatile nature of 8-Hq, however, provides a measurable quantity: gas chromatography and mass spectroscopy (GC/MS) were utilized to investigate the degradation of Alq3.[22,23]. * To whom correspondance should be addressed. t Department of Chemistry, Wesleyan University, Middletown, CT 06459 521 Mat. Res. Soc. Symp. Proc. Vol. 488 @1998 Materials Research Society
EXPERIMENTAL Alq3 was synthesized according to the literature method and purified by sublimation [21 ]. Differential scanning calorimetric (DSC) measurements were conducted with a Perkin Elmer DSC-7, employing a 20 mL/min. flow of dry nitrogen as a purge gas for the sample and reference cells. The temperature and power ordinates of the DSC were calibrated with respect to the known melting point and heat of fusion of high purity indium. In a nitrogen dry box, about 10 mg of sublimed or annealed Alq3 was scraped from its substrate, packed, and compression sealed with a Perkin Elmer crimper press. The scan rate was 10 "C/min. For cooling, a two stage intercooler based on freon 502 and ethane 170 was used. X-ray diffraction (XRD) was performed on a Norelco/Phillips diffractometer using Cu K(X radiation (k=1.5418 A). An Alq3 film of several gtm was evaporated at 7-10 A/s on a glass slide for measurement of its XRD spectrum before and after annealing, followed by subtraction of the substrate background. The film was annealed at 200 "C in a vacuum oven (ix 10-5 Torr) equipped with a liquid nitrogen trap to prevent contamination by di
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