金属氟化物对聚合物发光二极管发光性能的提高

Performance Improvement of Polymer Light-emitting Diodes with Metal Fluoride Cathode

在以聚合物发光材料MEH PPV为发光层的聚合物发光二极管(PLEDs)的金属阴极与聚合物发光层之间插入一层绝缘的金属氟化物,发光器件的发光性能有所提高,而且绝缘层的厚度会影响器件性能提高的最终效果。特别是具有LiF/Al的双层电极的发光器件,其发光特性与工作性能优良,但稳定性较差的Ba(Ca)/Al电极结构器件的发光特性具有可比性。初步分析表明绝缘层的插入造成了发光聚合物层与金属电极界面的能带发生弯曲,降低了发光器件中少数载流子电子的注入势垒,提高了发光器件中少数载流子电子的注入效率。从而最终导致了发光器件的开启电压、发光强度、外量子效率及电流效率等发光性能指标的显著提高。

The interface between metals and organic semiconductors plays a central role in the physics of polymer light-emitting diodes. In addition, the asymmetry in the work function of the two electrodes is responsible for the polymer light-emitting diodes rectifying behavior and governs the injection of carriers in polymer. In order to fabricate efficient devices and to minimize injection barriers, the low work function cathodes and high work function anodes are desirable. Ideal electrode materials have been difficult to obtain, however, an incorporation of injecting inter-layers at electrode/polymer interfaces has been used as an alternative route towards enhanced carrier injection. The performance of polymer light-emitting diodes with inserting metal fluoride insulating layer between the metal cathode and active polymer was studied. As for the polymer light-emitting diodes, optimization of charge injection lies in the dominant factors for substantive improvement performance. Since it is minority carriers dominate the recombination thus the radiation, their injection takes priority over that of majority carriers. Generally in semiconducting polymers, holes are majority carriers while electrons asminority, the carrier misbalance between electrons and holes further deteriorate in the fact that hole usually take possession of higher mobility and smaller injection barrier. To balance injection charge carriers and facilitate the electron injection, there are two commonly used stratagems in cathode fabrication. One is to employ low work function metals such as barium and calcium as cathode though they are susceptible to degradation upon water vapor and oxygen. The other choice is proposed to insert an insulating thin layer usually metal fluoride between polymer/ electrode interfaces to build a bilayer cathode. Proper inserting of insulating layer would lead to improvement in the overall working performance. But the thickness of the insulating layer is a fatal factor that influences the enhancement effect. As show in the experiment result, the LiF/Al double-layer-cathode devices are capariable with Ba (Ca)/Al electrode devices in luminescence performance. Band bending and injection potential decrease of minority carrier caused by the inlay of metal fluoride insulating layer, are proposed to be responsible for the enhanced electron injection and improved performance in polymer light emitting diodes.