Spectator cation size effect on the work function and stability of self-compensated hole-doped polymers

Abstract

The ‘free’ ions needed to balance the ‘fixed’, covalently bonded ions in a conventional polyelectrolyte are often thought to play little role beyond charge balancing, processability and morphology control. In self-compensated, hole-doped polymers, free cations, together with mobile holes on the polymer backbone, balance the covalently tethered counter-anions. We show here that these ‘spectator’ cations not only influence the ionization energy (and work function) of the polymer, but also, perhaps surprisingly, determine the stability of the ultrahigh-workfunction state. We attribute these two effects to the dependence on the spectator cation size of surface ionic layering, and coulombic stabilization of the anion sites, respectively. The smaller cations provide larger coulombic stabilization of the anionic site, raising the energetic barrier for hole transfer to the anion or its hydration water, thereby blocking de-doping of the polymer backbone. We demonstrate these effects in an important model of a high-ionization-energy triarylamine–fluorene copolymer TFB with trifluoromethanesulfonylimidosulfonyl as a tethered counter-anion, and a family of spherical monovalent cations as spectators (Li⁺, Na⁺, Cs⁺, NMe₄⁺ and NEt₄⁺). Using Li⁺ as a spectator cation, we further demonstrate ultrahigh-workfunction hole injection layers for TFB semiconductor with markedly enhanced stability to ambient processing and baking. This work leads to a new design rule for stable, self-compensated, hole-doped polymer systems with ultrahigh work function.