Theoretical study of the influence of salt doping in the functioning of OLEDs André Pereira,* a Helder M. C. Barbosa, a Helena M. G. Correia, a Luís Marques a and Marta M. D. Ramos a a Centre of Physics, Department of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal. Fax: +351 253 604 061; Tel: +351 253 604 330 * E-mail: andre@fisica.uminho.pt One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to dope the active organic semiconducting layer with inorganic salts, leading to the development of a hybrid organic/inorganic hetero-structure. However, it is hard to know from the experiments how each one of the electronic processes underlying the functioning of OLEDs are affected by the accumulation of inorganic ions of different sign at both organic/electrode interfaces. In order to assess these effects, we performed computer simulations by using a multi-scale model that combines quantum molecular dynamics calculations at atomistic scale with Monte Carlo calculations at mesoscopic scale. We focus our attention on the main differences obtained between doped and pristine organic layers, when bipolar charge injection occurs. Our results show a significant drop on the turn-on applied electric field while maintaining rapid response to the applied field as well as a clear increase in recombination rate and recombination efficiency far from the electrodes for the doped situation, which are responsible for the dramatic improvement of doped OLED performance found in the experiments. Introduction Organic light emitting diodes (OLEDs) using conjugated polymers as the active component are very attractive for their potential application in large area panel displays and general lighting. Low cost, easy fabrication and low operating voltages are some of the advantages of using these materials in the above applications. One of the parameters required to achieve high electroluminescence efficiency in organic light emitting diodes, is to have a balanced charge injection and transport throughout the active layer, which increases the probability of two charges of opposite sign recombine far from the electrodes and avoids quenching of radiative charge recombination by the electrodes 1 . Efficient charge injection depends on the zero-field barrier height at electrode/polymer interface (i.e. the energy mismatch between the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) of the organic and the anode/cathode Fermi Level). In general it is difficult to build OLEDs that present negligible zero-field barrier heights, due to the lack of appropriate electrodes, especially metallic cathodes. To solve this problem, one of the strategies used is to put a sub-monolayer of a thin alkali metal halide (i.e. LiF, LiBr, LiCl) 2-4 between the organic layer and the metallic cathode, and to use poly(ethylenedioxy) thiophene (PEDOT) doped with poly(styrene sulphonic acid) (PSS) 2,3 between the active polymer layer and the anode (usually indium tin oxide), that also works as a hole-transport layer. Although there is some controversy about the mechanism by which OLED efficiency is improved using these strategies 5 , several authors suggested that, for instance, LiF dissociate at the interface between the metallic cathode and poly(p-phenylene vinylene) (PPV), and lithium ions will diffuse through the polymer layer creating an hybrid ion doped region that enhances electron injection, whereas other authors suggest that the thin layer of polar metal halides between the metallic cathode creates a interfacial dipole, leading to strong band bending and thus to enhanced electron injection. Recently, another approach to increase OLED efficiency is to dope the polymer layer with small amounts of an inorganic salt, like LiBr 6 . The authors of this work suggest that when an external electrical field is applied, Li ions and Br ions present in the polymer film migrate to cathode and anode, respectively. The presence of a large concentration of these ions in the organic region close to the electrodes create hybrid doped charged zones that alter the electronic structure at organic/electrode interfaces, which leads to an increase in current density and light emission efficiency for significantly lower applied bias voltage as compared to pristine OLEDs. Although these experimental results clearly suggest the advantage in using inorganic salts to improve OLED performance, it is not clear how the presence of the inorganic ions affect the electronic processes of charge injection, transport and recombination, as well as, charge stored inside the active layer. The present work aims to clarify these issues by performing computational simulations of a single-layer OLED with a doped and pristine active layer of PPV, using a multi-scale model.