Cationic Conjugated Polyelectrolyte Electron Injection Layers: Effect of Halide Counterions

We examine the influence of halide counteranions on the efficiencies of solution-processed multilayer polymer light-emitting diodes (PLEDs) containing cationic conjugated polyelectrolyte (CPE) electron injection layers (EILs). The parent CPE used in these studies is poly[9,9-bis[6′-( N, N, N -trimethylammonium)hexyl]fluorene- alt - co -1,4-phenylene] bromide. Dialysis was used for exchanging counteranions, while X-ray photoelectron spectroscopy (XPS) provides a convenient technique for evaluating the final polymer composition. The luminous efficiencies of PLEDs with a poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) emissive layer decrease in the order F − > Cl − > Br − > I − . Oxidation of the halide counteranions is likely to occur at the MEH-PPV/CPE interface when the oxidation potential of the counteranion is aligned with the highest occupied molecular orbital of MEH-PPV, that is, Br − and I − . For these heavier halide counteranions, we find that pulsed bias measurements reduce ion migration to the MEH-PPV/CPE interface and result in an increase in the device efficiency. We propose that the oxidation potential of the counterion is a significant factor to consider when selecting CPEs as EILs.