Simulation study of the influence of polymer modified anodes on organic LED performance

It is well known from the literature that the performance of organic light-emitting diodes (OLEDs) can be improved significantly by introducing a highly (p + ) doped polymer layer between the (ITO) anode and the hole transport layer (HTL). Until now for the origin of such improved performance, only descriptive explanations have been presented. Therefore, in this paper numerical simulations of the diode currents and of the corresponding internal profiles of concentrations and potentials are presented. The main parameters, anode work function determining the hole barrier, doping of the HTL and of the p + -layer, thickness of the layers, mobility, and the valence band edge (or the ionization energy) of the p + -layer have been varied. Already for the mono-HTL device the current characteristics exhibits a large variety of peculiarities which are explained on the basis of the internal potential and concentration profiles. Conditions are formulated under which a p + -layer at the anode does lead to higher currents. If the p + -layer is made from the same material as the HTL, a moderate increase is possible if the barrier between the anode and the HTL is large. In this case, an increase of the mobility due to the doping will lead to an additional (roughly proportional) increase of the current. Introduction of a p + -hetero-layer with a lower ionization energy (the valence band edge higher than in the HTL) reduces the barrier at the anode and at the same time, an additional barrier does occur at the hetero-junction to the HTL. Both influence the current in opposite directions and hence there will exist an optimum for the valence band offset between the p + -layer and the HTL. If one has from the beginning a rather low barrier between the anode and the HTL (or sufficiently large anode work function) one has already an accumulation contact which cannot be significantly improved by higher doping of the HTL near the anode and becomes even worse by using for the p + -layer a material with an ionization energy lower than that one of the HTL.