Carrier Transport and Recombination Dynamics in Disordered Organic Light Emitting Diodes

Abstract

Organic light emitting diode (OLED) displays are forecast to be the promising display technology. They are thin, flexible, energy conserving, and suitable for large screen displays. For the developments of high-performance devices, high efficiency and good color purity are necessary. The emission wavelengths can be modified by blending dopants into the polymers light emitting diodes or by the incorporation of fluorescent dyes into the emissive layers for small molecule devices. The incorporation of fluorescent dyes into host materials has the advantages of efficient color tuning, good device efficiency, and narrow emission spectrum width [1-4]. In OLEDs, carriers are localized in molecules and charge transport is a hopping process [2]. Carrier mobility is determined by charge transport between neighboring hopping sites. The mobility usually shows the Poole-Frenkel characteristic [5]. By controlling the distance between hopping sites, carrier mobility can be adjusted [6]. At thermodynamic equilibrium, charge carriers mostly occupy the deep tail states of the density-of-states (DOS) distribution [7]. Carrier hopping occurs mostly via shallower states [8,9]. This shows that carrier density could affect mobility. Furthermore, dopants in OLEDs act as shallow trapping centers, which trap carriers and change the carrier density. Carrier trapping is the main emission mechanism in doped organic systems [10]. This also shows the dependence of the mobility on the dopant concentration. Although the efficiency of doped OLEDs has been improved, the carrier dynamics have not been well discussed [1-4]. To further improve the efficiency and lifetimes of OLEDs, the carrier transport as well as recombination dynamics of doped OLEDs should be well explored. In this study, the dependences of carrier transport behavior and luminescence mechanism on dopant concentration of OLEDs were studied. In the lightly-doped sample, higher carrier mobility and better device performance were observed. This shows that dopants create additional hopping sites and shorten the hopping distance. At a higher dopant concentration, dopants tend to aggregate and the aggregations degrade the device performance. In addition, the observed decay rates and luminescence efficiencies of the 5