Enhanced Emission Using Thin Li-Halide Cathodic Interlayers for Improved Injection into Poly(p-phenylene vinylene) Derivative PLEDs

Abstract

In this study, the effects of thin Li-halide cathodic interlayers on electron injection were examined for electroluminescent layers of polymer light-emitting diodes ͑PLEDs͒. An order of magnitude increase in current density is observed as Li-halide salts are varied down the group VII column of the periodic table. When considering luminance, devices with a LiCl interlayer were 2.3ϫ greater than those with LiF, whereas devices with LiBr were 2.8ϫ greater, while concurrently lowering the turn-on voltage. This resulting enhanced current density and subsequent luminance could be due to a lowered work-function difference at the cathode created by either the Li-halides dissociation-induced doping of the polymer surface or an interfacial dipole of ionic Li-halide compound, leading to band bending. © 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2961823͔ All rights reserved. One of the important rate-limiting steps for high-efficiency polymer light-emitting diodes ͑PLEDs͒ is charge injection into the polymer semiconductor. It has been known that the interface, and subsequent potential barrier, between the active light-emitting conjugated polymer layer and the metal cathode plays an important role in controlling charge injection across this interface.1 Because most light-emitting conjugated polymers are good hole transporters but poor electron transporters, electron injection becomes a critical issue for balanced current components. It has been shown by numerous groups that the insertion of a thin compound interlayer, such as alkaline earth metal fluorides ͑i.e., LiF and CsF͒, between the Al cathode and the electroluminescent polymer improves PLED device performance by enhancing electron injection.2-4 The physical mechanism behind this improved performance still remains a topic of some speculation.5-7 However, two competing mechanisms have been proposed: a metal-halide dissociation introducing a localized surface doping effect, thus creating a low work-function contact at the interface and an interfacial dipole created by the polar metal halides, leading to strong localized band bending. 8,9 Previous work has principally investigated fluoride-based chemistries, and this study extends that work by examining other halide-based chemistries ͑LiF, LiCl, and LiBr͒, which are considered salts. PLEDs using poly͓2-methoxy-5-͑2Ј-ethyl-hexyloxy͒-1, 4-phenylenevinylene͔ ͑MEH-PPV͒ with various thin metal-halide interlayers between the electroluminescent layer and Al cathode were fabricated and tested. This study examined the effects of metal halides on the electron injection into the electroluminescent layers of PLEDs from their current density-voltage ͑J-V͒ and luminancevoltage ͑L-V͒ characteristics and electroluminescence ͑EL͒ emission spectra of the devices.Patterning of the indium-tin-oxide ͑ITO͒ coatings was performed to define eight finger-shaped transparent anodes using conventional photolithography and HCl-based wet etching. The ITO film thickness and sheet resistance were ϳ1500 Å and 10 ⍀ cm, respectively. After photolith...

Comparative study of using different alkali metal alkylcarboxylates as electron injection materials in OLEDs

“…3 After that, many materials that could help to inject electrons into organic materials have been reported successively, and these compounds are mainly alkali metal salts. According to the anion of these salts, they can be classified alkali metal halide (such as NaF, 4 KF, 5 RbF, 6 CsF, 7 LiCl, 8 NaCl, 9,10 KCl, 11 RbCl, 12 CsCl 13 ), alkali metal carbonate (such as Li 2 CO 3 , 14 Na 2 CO 3 , 15 Rb 2 CO 3 , 16 Cs 2 CO 3 , [17][18][19][20] ), alkali metal nitrides (such as Li 3 N, 22 ), alkali metal (Li + , Na + , K + , Rb + , Cs + ) acetates, [24][25][26] organic alkali metal complexes (such as Liq (8-hydroxyquinolinato lithium), 27,28 Naq (8-hydroxyquinolinolato sodium), 29 Csq (cesium quinoline-8-oxide), 30 and so on. It is reported that alkali metal halide complexes can react with hot Al atoms impinging on them during the vapor deposition.…”

Theoretical study of the influence of salt doping in the functioning of OLEDs

“…To solve this problem, one of the strategies used is to put a sub-monolayer of a thin alkali metal halide (i.e. LiF, LiBr, LiCl) [2][3][4] between the organic layer and the metallic cathode, and to use poly(ethylenedioxy) thiophene (PEDOT) doped with poly(styrene sulphonic acid) (PSS) 2,3 between the active polymer layer and the anode (usually indium tin oxide), that also works as a hole-transport layer. Although there is some controversy about the mechanism by which OLED efficiency is improved using these strategies 5 , several authors suggested that, for instance, LiF dissociate at the interface between the metallic cathode and poly(p-phenylene vinylene) (PPV), and lithium ions will diffuse through the polymer layer creating an hybrid ion doped region that enhances electron injection, whereas other authors suggest that the thin layer of polar metal halides between the metallic cathode creates a interfacial dipole, leading to strong band bending and thus to enhanced electron injection.…”