Abstract
Current–voltage characteristics of single and hetero-layer light-emitting devices with an aromatic diamine (TPD) as hole transport material and tris-8-(hydroxyquinoline) aluminum (Alq 3) as electron transport material and emitter have been investigated over a wide temperature range and for various film thickness in order to identify the limiting mechanism: charge carrier transport or injection. From the observed thickness and temperature dependence, pure injection limitation can be ruled out as dominant mechanism. Instead, the voltage dependence of the current density can be well described by power laws j∝ V m+1 (with V corrected by the built-in potential) with temperature dependent exponents m ranging from 4 to 25. This can be interpreted in terms of space charge limited currents (SCLC) in Alq 3 with an exponential energetic distribution of traps where m is given by m= E t/ kT. A reasonable trap energy of 0.15–0.2 eV is obtained by a temperature dependent analysis of the I–V characteristics. However, the thickness dependence cannot be satisfactorily explained by the simple SCLC-model. This indicates that more sophisticated models are required.
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