Contact optimization in polymer light-emitting diodes

Abstract

To fully exploit the properties of light-emitting polymers (LEPs) in electroluminescent applications, it is of paramount importance to develop efficient electrical contacts. An ideal electrode is highly conductive, stable, provides a low barrier to carrier injection, and does not degrade the LEP upon contact. It is difficult to find a single homogeneous material that satisfies all of these requirements. Hence, contact optimization has often required the development of multilayer structures. In particular, indium tin oxide covered by a film of poly(ethylene-dioxythiophene):poly(styrene sulfonic acid) {ITO/PEDOT:PSS} has become a favorite combination for the transparent anode, and heterostructures of LiF and CsF with metals (Al and Ca) have proven to be efficient electron-injecting contacts. Here we review our progress in the understanding of the operation of light-emitting diodes incorporating such contacts, in particular by gauging the materials' energy-level lineup via electroabsorption measurements. Among the series of LEDs investigated, using a high-energy-gap blue polyfuorene polymer, CsF/Ca/Al and LiF/Ca/Al electrodes lead to the best improvements in electron injection. The most promising performance for applications, where a high luminance (∼1600 cd/m2 at 5 V) is also accompanied by a high maximum efficiency (∼3 1m/W), was obtained with LiF/Ca/Al cathodes and ITO/PEDOT:PSS anodes.