Cationic Conjugated Polyelectrolyte Electron Injection Layers: Effect of Halide Counterions

Abstract

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.

Fluorene Based Conjugated Polyelectrolyte/Silica Nanocomposites: Charge‐Mediated Phase Aggregation at the Organic–Inorganic Interface

“…Materials : PFP‐NR3 and PBS‐PFP, with molecular weights M n ≈ 3500 and 6500 g mol −1 (from gel permeation chromatography),24 respectively, were synthesized as previously reported 10, 43, 44. TEOS and n‐dodecylpentaoxyethylene glycol ether (C 12 E 5 ) were purchased from Aldrich.…”

“…This may be particularly important for photovoltaic devices as it may offer a means of minimising the charge carrier diffusion distance between donor and acceptor components. It has recently been observed that the efficiency of cationic CPEs as charge‐transport layers in polymer light‐emitting diodes is strongly influenced by the charge‐compensating counterion present 42, 43. The next step is a systematic study of the interaction of different charged terminal groups at the inorganic‐organic interface to determine if any structure‐activity correlation can be identified.…”

Molecular design of interfacial layers based on conjugated polythiophenes for polymer and hybrid solar cells

“…Charge‐compensating counterions should be taken into consideration in the molecular design of CPEs since their properties (size, hydrophobicity, mobility, redox potential, charge density etc.) enable fine‐tuning of the optoelectronic properties both in solution and in the solid state . Bazan and colleagues have notably reported that the solid‐state photoluminescence properties of a cationic CPE based on a poly((9,9‐dihexylfluorene)‐ alt ‐(4,7‐(2′,1′,3′‐benzothiadiazole))) backbone ( PFBT-X ) (Scheme ) can be manipulated by changing the size of the counterion: larger anions lead to increased interchain distance and thereby increased emission quantum yields .…”

“…enable fine-tuning of the optoelectronic properties both in solution and in the solid state. 90 -93 Bazan and colleagues have notably reported that the solid-state photoluminescence properties of a cationic CPE based on a poly((9,9dihexylfluorene)-alt-(4,7-(2 ′ ,1 ′ ,3 ′ -benzothiadiazole))) backbone (PFBT-X) (Scheme 4) can be manipulated by changing the size of the counterion: larger anions lead to increased interchain distance and thereby increased emission quantum yields. 94 The same authors also showed that replacing the bromide counterion with the larger tetrakis(1-imidazolyl)borate (BIm 4 − ) anion in a cationic narrow bandgap CPE (Scheme 4) derived from the poly((2,6-(4,4bis(2-ethylhexyl)cyclopenta-[2,1-b;3,4-b ′ ]-dithiophene))-alt-(4,7-(2 ′ ,1 ′ ,3 ′ -benzothiadiazole))) (PCPDTBT) framework (Scheme 3) not only modified the HOMO and LUMO energy levels, but also its semiconducting character (p-type for Br − and n-type for BIm 4 − ).…”

Poly(9,9‐dioctylfluorene)‐Based Conjugated Polyelectrolyte: Extended π‐Electron Conjugation Induced by Complexation with a Surfactant Zwitterion

“…This result is important in view of the application in optoelectronics devices where the formation of an ionic junction, created by the presence of a CPE injection layer, is needed to achieve a better charge injection 37–39. We noticed that the choice of ZW as a complexing agent eliminates the eventual formation of salty inclusions formed by free salts, which would further hinder the formation of ordered structure and also the efficient charge transport in the CPE.…”