Observation of the Temperature Dependence of the Dynamics of Photoexcited States in Pristine Tris(8-Hydroxyquinoline) Al
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ABSTRACT We have investigated the temperature dependence of the dynamics of radiative excited states in pristine thin films of tris(8-hydroxyquinoline) aluminum (Alq3). By measuring the transient photoluminescence (PL) response with subnanosecond resolution, our results revealed an increase in the radiative excited state lifetime and fluorescence quantum yield with decreasing temperature from 300K to 77K. At low temperature we observed a decrease in the bimolecular recombination rate constant, singlet exciton diffusion coefficient and diffusion length. A singlet exciton trapping model is used to explain these results. INTRODUCTION Since the first observation of its efficient luminescence1 , tris(8-hydroxyquinoline) aluminum (Alq3) has been widely used as the emitting layer in organic light emitting diodes (OLED's).2 ' 3 In fact, it has become the prototypical material among numerous organic electroluminescent systems. The luminescence from this material originates from the radiative decay of singlet excitons formed by photoexcitation, or in the case of electroluminescence (EL), by charge carrier injection. Prior to decay, diffusion and interaction of these excitons with trapping sites can occur and result in a reduction in the PL quantum efficiency or EL quantum yield of Alq 3 . Nevertheless, very few publications have been reported regarding the temperature dependence on trapping of singlet excitons. In previous work, Abe et al.4 observed an increase in the PL and EL intensity at low temperatures in Alq 3 based devices. They associated this increase with a reduction in the nonradiative transition probability typical of molecular crystals at low temperatures.5 However, no analysis on the dependence of temperature on the excited state lifetime was performed. An investigation into the nature of trapping sites in Alq 3 has been reported by Burrows et al.6 where they suggested that charge transport in Alq3 based OLED's is limited by a large density of traps. They determined that a distribution of trapping sites located at 0.15 eV below the LUMO level exists in Alq3 and the EL observed is 695 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
produced by the recombination of trapped electrons with holes injected by the hole transport layer. However, they do not address the changes we observed in the behavior of singlet excited states at various temperatures. In this letter we have performed an investigation on the temperature dependence of the time resolved luminescence of Alq3. We have observed for both low temperature and low excitation, an increase in the singlet excited state lifetime which we associate with a reduction in singlet exciton trapping. In addition, we will demonstrate that for low temperature and high excitation, the mutual annihilation of singlet excitons is lowered. For both low and high excitation, an increase in the PL quantum efficiency was observed with decreasing temperature. EXPERIMENT The metal chelate compound Alch was synthesized at the Eastman Kodak Co. and purified by t
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