Copolyfluorenes containing partially hydrolyzed phosphonate pendant groups: synthesis, characterization and application as electron injection layers for enhanced electroluminescence of PLEDs

Abstract

Balance in charge injection is crucial to improve the device performance of polymer light-emitting diodes (PLEDs); however, hole injection is much easier than electron injection in most PLEDs. We synthesized copolyfluorenes containing pendant phosphonate groups (PF-EP) or partially hydrolyzed phosphonate groups (PF-EPK) and successfully applied them as electron injection layers (EILs) in PLEDs to significantly enhance the device performance. Multilayer PLEDs [ITO/PEDOT:PSS/Superyellow (SY)/PF-EP + PF-EPK/Al] were readily fabricated using blends of PF-EPK and PF-EP as EILs by the solution process. The device with PF-EP doped with 2.5% PF-EPK as an EIL exhibited the best performance. The device performances were enhanced to 12 052 cd m⁻², 3.62 cd A⁻¹ and 1.38 lm W⁻¹, respectively, from 725 cd m⁻², 0.16 cd A⁻¹ and 0.05 lm W⁻¹ of a non-EIL device. The performance enhancement has been attributed to improved electron injection which has been confirmed by the increase in open-circuit voltage (V_oc) obtained from photovoltaic measurements. The results indicate that PF-EPK is a promising electron-injection material for optoelectronic devices with a high work function Al cathode.