Performance of Injection-Limited Polymer Light-Emitting Diodes
- PDF / 252,406 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 87 Downloads / 213 Views
P8.6.1
Performance of Injection-Limited Polymer Light-Emitting Diodes Paul W.M. Blom1, Teunis van Woudenbergh1 and Hans Huiberts2 1 Materials Science Center and DPI, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 2 Philips Research Laboratories, Professor Holstlaan 4, 5656 AA Eindhoven, The Netherlands ABSTRACT. The electro-optical characteristics of a polymer light emitting diode (PLED) with a strongly reduced hole injection have been investigated. The device consists of a poly-p-phenylene vinylene semiconductor with a Ag hole injecting contact, which has an injection barrier of about 1 eV. It is observed that the light and current density of such an injection-limited PLED strongly exceed the expected device characteristics. Numerical calculations of the injection-limited PLED show that the enhanced performance can be explained by a very high electric field at the hole injecting contact, due to trapped electrons. INTRODUCTION Directly after the discovery of polymer light emitting diodes (PLEDs) [1], charge injection has been recognized as an important process for the performance of a PLED [2]. However, the mechanisms of charge injection into conjugated polymers are poorly understood, compared with the knowledge of inorganic semiconductors. Contrary to inorganic semiconductors like Si, the charge transport in conjugated polymers is determined by tightly bound charge carriers on transporting sites that are subject to disorder both energetically and spatially [3]. Consequently, the nature of the charge injection from a metallic contact into the conjugated polymer will differ notably from mechanisms like thermionic emission and Fowler-Nordheim tunneling. These mechamisms are normally used to describe injection in crystalline semiconductors with welldefined transport levels of the delocalized charge carriers. To account for the nature of transport in conjugated polymers, a model based on thermally assisted tunneling of carriers from the contact into localized states of the polymer has been formulated [4]. This model has been further investigated by including energetic disorder and the image force effect in Monte Carlo (MC) simulations [5,6]. These simulations indicate that in conjugated polymers an increase of J with V is due to the field dependence of the mobility and to an additional increase of the carrier density at the contact caused by the image force [6]. Moreover, analytical treatment investigates explicitly the injection process by a first jump from the contact level into a random hopping system, followed by either a diffusive escape from the interface or a backflow to the electrode [7]. This approach has been confirmed by MC simulation that shows that the primary injection event is essential and determines the temperature- and field dependence of the injection process [8]. Recently, it has been demonstrated that the hole injection from a Ag blocking contact into poly-p-phenylene vinylene (PPV) is excellently described by the hopping based injection model [9]. It has been shown that
Data Loading...
