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

机译：聚合物修饰阳极对有机LED性能影响的仿真研究

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摘要

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.
机译：从文献中众所周知，可以通过在（ITO）阳极和空穴传输层（HTL）之间引入高度（p +）掺杂的聚合物层来显着提高有机发光二极管（OLED）的性能。到目前为止，对于这种性能改进的起源，只提供了描述性的解释。因此，在本文中，对二极管电流以及相应的浓度和电势内部曲线进行了数值模拟。主要参数，阳极功函数决定空穴势垒，HTL和p〜+层的掺杂，层的厚度，迁移率和p〜+层的价带边缘（或电离能）已经变了。对于单HTL装置，当前特性已经表现出各种各样的特性，这些特性基于内部电势和浓度曲线进行了解释。规定了在阳极的p〜+层确实导致更高电流的条件。如果p〜+层由与HTL相同的材料制成，则如果阳极和HTL之间的势垒较大，则可能会适度增加。在这种情况下，由于掺杂而导致的迁移率的增加将导致电流的另外的（大致成比例的）增加。引入具有较低电离能的ap +杂层（价带边缘比HTL中的价带边缘高）会减小阳极的势垒，同时，在与HTL的异质结处确实会出现额外的势垒。两者都会在相反的方向上影响电流，因此在p〜+层和HTL之间存在价带偏移的最佳值。如果从一开始就在阳极和HTL之间具有相当低的势垒（或足够大的阳极功函数），则已经具有积累接触，而通过在阳极附近对HTL进行更高的掺杂不能显着改善积累接触，而通过使用对于p +层，其电离能低于HTL之一的材料。

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