Numerical Simulation of Organic Light-Emitting Diodes With Insulating Cathode Buffer Layer

In this paper, we present a numerical model for the quantitative simulation of organic light-emitting diodes (OLEDs) with insulating cathode buffer layer, in which Fowler-Nordheim and direct tunneling are the dominating injection mechanisms. In our approach, both tunneling mechanisms are covered by a simple compact model proposed by Schuegraf et al. We have found good agreement over more than seven orders of magnitude between simulated current and experimental data of a polymer OLED with sodium stearate buffer layer and aluminum cathode.

I. Introduction

This paper is a continuation of our former work [1], which we refer to as Part I. In Part I, we have studied a polymer organic light-emitting diode (OLED) using an ultrathin (2 nm) buffer layer of the insulating molecule sodium stearate (NaSt) [2] as an alternative to the widely used electron injector lithium fluoride (LiF) [3]–[6]. In that context, we have shown that the electron injection properties of the utilized NaSt/aluminum (Al) bilayer cathode can be described using a combination of the direct tunneling model [7] and the Fowler–Nordheim (FN) tunneling model [8]. For clarification, Fig. 1 shows a schematic energy level diagram of the OLED in the direct tunneling regime (left) and the FN tunneling regime (right), in which is the voltage across the NaSt buffer layer, is the elementary charge, and denotes the energy barrier height for electrons between the Fermi energy in the Al electrode and the NaSt conduction band edge. At low biases (), the energy barrier formed by the buffer layer is trapezoidal, so that the electrons tunnel through the whole energy barrier (direct tunneling). At higher biases (), the energy barrier is triangular-shaped, so that the electrons tunnel through only a part of the barrier (FN tunneling). Fig. 1.

Schematic energy level diagram of the investigated OLED with NaSt/Al bilayer cathode in the direct tunneling regime, (left) and the FN tunneling regime, (right).