The photoluminescence temperature dependence of aluminium tris (8-hydroxyquinoline) as a function of excitation energy
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hydroxyquinoline) as a function of excitation energy R.J. Curry and W.P. Gillin Department of Physics, Queen Mary and Westfield College, University of London London, El 4NS, United Kingdom.
Abstract The temperature dependence of the photoluminescence as a function of excitation energy has been investigated for aluminium tris(8-hydroxyquinoline) (AlQ). It was observed that the peak position of the photoluminescence is a function of both temperature and excitation energy. The observed photoluminescence spectra can be deconvolved into three peaks, with varying weighting factors, the positions of which do not change with temperature. Using these three peaks the measured photoluminescence data was then modeled using a rate-equation approach.
Introduction The group III chelate, A1Q, has been the subject of much interest since Tang and VanSlyke showed it possible, using AIQ as the emitting layer, to produce organic light emitting diodes (OLED's) with low turn on voltages and high efficiencies [1]. Although many other organic molecules have been developed for use in the fabrication of OLED's [2-3] AIQ is still commonly used as the emitting layer in many OLED's today. Despite the great interest in materials such as AIQ there has been relatively little characterisation, especially at temperatures other than 300 K, reported. Whilst the photoluminescence of an organic material such as A1Q is not always a good indication of the quantum yield of any electroluminescence, it can give an insight into the energy levels responsible for any emission. A systematic study of properties such as the photoluminescence is, therefore, a valuable tool. In the work presented here we have obtained the photoluminescence of AIQ as a function of both excitation energy and temperature. The resulting spectra can be deconvolved into three emission bands and have been modelled using a rate-equation approach. Some suggestions are made as to the identity of the three energy levels present in the photoluminescence spectra.
Experimental The A1Q was obtained from Alderich, with a stated purity of 99.995%, and was used without further purification. A small quantity of this material was placed into a cuvette inside a continuous flow cryostat. Photoluminescence was obtained by exciting the sample with the 363 nm, 457 nm, 465 nm, 476 nm and 488 nm lines from an argon ion laser. The photoluminescence was dispersed in a lm spectrometer using a 0.5 pm blazed grating and detected using a S-20 photomultiplier and conventional lock-in techniques. The resolution of the system was 0.8 nm and all spectra where corrected for the system response. Photoluminescence spectra where obtained for all the excitation wavelengths used between 25 K and 300 K. To obtain the electroluminescence spectra, OLED's where fabricated using indium tin oxide coated glass as the anode, onto which 400 A of N, N'-diphenyl-N,N'-bis(3-methylphenyl)421
Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000Materials Research Society
1,l'-biphenyl-4,4'-diamine (TPD) was deposited to form the hole transporti
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