Temperature dependent device characteristics of organic light-emitting devices

Current-voltage characteristics of single and hetero-layer light-emitting d evices with an aromatic diamine (TPD) as hole transport material and tris-8 -(hydroxyquinoline) aluminum (Alq(3)) as electron transport material and em itter 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 depende nce, pure injection limitation can be ruled out as dominant mechanism. Inst ead, the voltage dependence of the current density can be well described by power laws j proportional to Vm + 1 (with V corrected by the built-in pote ntial) with temperature dependent exponents m ranging from 4 to 25. This ca n 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 temperat ure dependent analysis of the I-V characteristics. However, the thickness d ependence cannot be satisfactorily explained by the simple SCLC-model. This indicates that more sophisticated models are required. (C) 2000 Elsevier S cience S.A. All rights reserved.