Steady-state analysis for contact barrier effects in metal/organic/metal structure using numerical bipolar transport simulation

We have studied contact barrier effects in ITO/MEH-PPV/Al and Au/MEH-PPV/Au structure using band-theory-based bipolar transport model that describes both injection limited and space charge limited current flow and the transition between them. Charge injection into the organic material occurs by thermionic emission and by tunneling. The model calculations show a good description of the measured I-V characteristics over a wide current range in both situations. In the ITO/MEH-PPV/Al structure with high contact barriers >0.3 eV, the model shows that net injected charge was relatively small and the carrier density and the electric field were nearly uniform. Thus, thermionic emission is the dominant mechanism at small bias in this regime because space charge effects were not important. However, in the symmetric Au/MEH-PPV/Au structure with very low contact barrier ∼0.1 eV, the current flow in the model is space charge limited and the electric field in the structure is highly non-uniform and parabolic-shaped energy profiles were observed. We also confirm that our bipolar model analysis is more physical than single-carrier model analysis in which the electric field in anode contact region has non-negligible negative value.