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...