High brightness deep blue/violet fluorescent polymer light-emitting diodes (PLEDs)

Abstract

Two new deep blue/violet emitting alternating co-polymers, comprising readily-available carbazole (C) and fluorene (F) monomer units, have been synthesised and shown to produce extremely bright solution-processed polymer light-emitting diodes (PLEDs) with the structure ITO/PEDOT:PSS/polymer/TPBi/LiF/Al. The para-conjugated polymer, CF1, gave PLED devices with external quantum efficiency (EQE) values of ηext,max 1.4%, Lmax of 565 cd m-2 with CIEx,y (0.16, 0.07). The EQE was raised to ηext,max 2.1%, after the addition of a TAPC hole injection layer. For the isomeric meta-conjugated polymer, CF2, values of ηext,max 0.35%, Lmax of 16 cd m-2 with CIEx,y (0.18, 0.12) were obtained. The λELmax was 409 nm for both the CF1 and CF2 devices. The CF1 devices also possess low turn-on and low operating voltages for devices of such high brightness. Moreover, the CF1 emission is very stable from 10 cd m-2 up to peak brightness, with only a negligible shift in CIE coordinates. The combination of a simple co-polymer structure synthesised using readily-available monomer units, and high brightness and good colour stability from a simple device architecture, makes CF1 suitable for a wide range of applications requiring deep blue/violet emission.