Carriers recombination in bilayer organic light-emitting diodes at high electric ®elds Shengyi Yang * , Zhenjia Wang, Zheng Xu, Yanbing Hou, Xurong Xu Laboratory of Materials for Information Storage and Display, Institute of Optoelectronics, Ministry of Railway Key Laboratory, Northern Jiaotong University, Beijing 100044, China Received 26 June 2000 Abstract Based on the Fowler±Nordheim tunneling theory at high electric ®elds, a model of carriers recombination in organic double-layer light-emitting diodes is presented in this paper. We provide the formula expressions of the carriers re- combination current density as function of factors such as the injection barrier, anodic electric ®eld and thickness ratio of cathodic layer to anodic layer, and discuss the in¯uence of these factors on the recombination current density and recombination eciency. Being in accord with the experiments very well, we ®nd it is reasonable to elucidate the controlling role of electric ®eld on recombination region. Ó 2001 Published by Elsevier Science B.V. Keywords: Charge carriers; Bilayer organic light-emitting diodes; High electric ®elds; Fowler±Nordheim tunneling 1. Introduction The operation of organic light-emitting diodes LEDs) [1±3] is based on the injection of both holes and electrons from two opposite electrodes, their migration in the bulk and recombination, and radiative decay of the excited states produced in the course of the mutual annihilation of a pair of charge carriers. The form of the current±voltage curve in conjunction with its temperature inde- pendence at high electric ®elds >10 6 Vcm 1 ) [4±6] argues in favor of tunneling injection for- mally described by the Fowler±Nordheim equa- tion. However, the electronic condition existing at the contact between a metal and non-crystalline organic solid are very dierent from those at a conventional metal±vacuum interface. In random organic conductors such as conjugated polymers or molecular materials, charge transporting states are localized states that derive from the highest occupied and lowest unoccupied molecular orbi- tals and form narrow transport zones in energy space [7]. The fact that jE curves, if plotted semi- logarithmically vs. inverse ®eld, reproduce as straight lines within the relevant ®eld range, and it indicates that the concept of tunneling through a triangular energy barrier is, nevertheless, a useful ®rst-order approach to describe the injection pro- cess. At lower ®elds, thermally assisted barrier crossing becomes increasingly important [6]. Chemical Physics 274 2001) 267±273 www.elsevier.com/locate/chemphys * Corresponding author. Tel./fax: +86-10-632-40605. E-mail address: y_shy@sina.com S. Yang). 0301-0104/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII:S0301-010401)00484-0