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We have used numerical methods to calculate the current and the efficiency of single-layer organic light emitting diodes, taking into account field-dependent mobilities, diffusion, and thermionic injection at the metal/organic interfaces. It is shown that when the barrier for charge injection from the metal to the organic is lower than 0.3 eV, the contact is, for practical purposes Ohmic, supplying the bulk of the organic layer with a space charge limited current. Determination of the carrier mobility by fitting experimental current–voltage data is discussed and the voltage dependence of the current and the efficiency are analyzed. The recombination profile is shown to evolve with voltage, shifting towards the electrode which injects the carrier with the lower mobility. The numerical results are compared with experimental data from poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) based devices. © 1999 American Institute of Physics.