Doppelinjektion und Elektrolumineszenz in dotierten Anthracenkristallen

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Abstract
The current-voltage dependence and the current dependence of the electroluminescence at different wavelengths have been investigated in pure and in tetracene doped anthracene crystals. The influence of traps is shown to be very significant. The current-voltage dependence is calculated by dividing the crystal in a hole injecting region (density of electrons negligible) and in a space-charge-free region. The latter arises by emptying the traps by recombination, its thickness is proportional to the applied voltage. The temporal decay of the electroluminescence was measured after the contacts were shorted. The decay shows a fast component which had the fluorescence spectrum characteristic of tetracene and a delayed component which had the fluorescence spectrum characteristic of anthracene. The delayed component is explained by assuming that each recombination, including free- trapped carrier recombination, has a high probability of yielding a triplet exciton. This will lead to delayed fluorescence by triplet-triplet annihilation. Analysis of the data shows free-trapped carrier recombination to be the dominant recombination mechanism. One gets, in agreement with the experiment, a maximum quantum efficiency of about 10%.