ELSEVIER Synthetic Metals76 ( 1996) 77-83 Injection-controlled and volume-controlled electroluminescence in organic light-emitting diodes J. Kalinowski a,b , P. Di Marco a,N. Camaioni a, V. Fattori a,W. Stampor b, J. Duff’ a Istituto di Fotochimica e Radiazioni di Alta Energia de1 CNR, via P. Gobetti 101, 40129 Bologna, Italy b Department of Molecular Physics, Technical Universiry of Gdarisk, ul. G. Narutowicza llN2, 80-952 Gdan’sk, Poland ‘Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ont., LSK 2L1, Canada Abstract Two types of electroluminescence (EL), injection-controlled (IC) with T,,> T=,and volume-controlled (VC) with ;T~ < TV, in organic light-emitting diodes (LEDs) are discussed using the notions of recombination (T& and transit (TV) times of charge carriers. The results of experimental and theoretical studies presented show that the EL intensity ( QEp,,) is, in general, a nonlinear function of the current density (j) in an EL diode. Often it can be approximated by a power-type function @EL ajn, with the power, n, dependent on LED structure, the nature of light-emitting material, injection and transport mechanisms of the charge carriers. In the LEDs with carrier mobilities independent of electric field 12 n < 2 for both ICEL and VCEL operation modes. In the ICEL, n = 1+ w,/w,, where w1 and o2 are quantities characterizing various charge injection mechanisms into the EL sample. Field-dependent mobility and charge carrier trapping lead @EL to be a sublinear or supralinear function of j. Selected examples of experimental &(j) relationships on single-layer ITO/Alq,/Mg/Ag (%hydroxyquinoline aluminium complex, Alq,), double-layer ITO/TPD/Alq,/Mg/Ag (aromatic diamine, TPD), ITO/QAC/Alq,/Mg/Ag (quinacridone, QAC) , and triple-layer ITO/QAC/Alq,/PBP/Mg/Ag (perylene bisimide pigment, PBP) are presented briefly to illustrate various types of operation modes of organic LEDs. Keywords: Electroluminescence; Diodes 1. Introduction The demonstration that organic materials can serve as car- rier-transporting and electroluminescent layers in high-quan- tum efficiency light-emitting diodes (LEDs) has imposed a strong demand of a comprehensive understanding of physical mechanisms underlying the LED operation. Because prop- erties of emitting states determine to a large extent electro- luminescence (EL) efficiency, there is now much discussion about the nature of elementary excitations in organic solids of use or potential use in LEDs [ l-l 11. Less attention has been paid to the brightness-device current relationship, although the supply and transport of charge carriers are pre- requisites for the successful operation of organic EL devices. The variety of dependences of EL intensity (&) on the current flowing through the EL cell 0’) is substantially un- limited. This statement is borne out experimentally by the fact that EL intensities have been observed to increase line- arly, supralinearly and sublinearly with increasing current. On the analytic side, charge injection and generation of emit- ting states seem to account for the variety of observations. Some attempts to explain the G&G) behaviour have already 0379-6779/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved been made as, for example, the treatment of the high-field injection and recombination traffic of charge carriers into trapping states, leading to a supralinear behaviour of $, withj [ 12-141. In the present paper we discuss the problem in a more versatilemannerincluding various conditions of injection and recombination of charge carriers. The injection-controlled (IC) and volume-controlled (VC) EL are defined, based on the relation between recombination and charge-carrier transit time. It appears that these two LED operation modes lead to different behaviour of @EL versus j. The role of injection mechanisms is critically examined. The theoretical consid- erations are illustrated by selected examples of single-, dou- ble- and multi-layer EL structures based on commonly used emitting material, %hydroxyquinoline aluminium complex (Ah,). 2. Theoretical considerations Under steady-state conditions the EL yield, (PEG, at a distance, x, from the transparent electrode is expressed by