Effect of counterion tether length on stability, work function and application of a self-compensated, hole-doped triarylamine-alt-fluorene model polymer

Abstract

Self-compensated, hole-doped polymers with ultrahigh workfunction can provide ohmic hole contacts even for deep-ionization-energy semiconductors. The self-compensation is usually imposed by an anion tethered to a flexible –(CH₂)_n– chain, but the effect of tether length n is unknown. Here, using the mTFF triarylamine–fluorene as a model semiconductor, we have designed, synthesized and characterized a family of mTFF-CnSISC₂F₅ polymers, where C₂F₅SIS is the pentafluoroethanesulfonylimidosulfonyl anion, tethered to a Cn alkylene chain that is systematically varied between C2 and C6 in length. Solution doping yields self-compensated films with a work function of ca. 5.75 eV, which is ca. 0.15 eV higher than that obtained by film doping and higher than that of the corresponding hole-doped mTFF films counterbalanced by SbF₆⁻ anions. We attribute the higher work function to a frustrated packing of the counteranions about the holes. The ultrahigh work function is consistent with electroabsorption measurements and the ability of the films to inject holes into PFOP, a model deep-ionization-energy semiconductor, without bias pre-conditioning. While the tether length only weakly influences the work function, it strongly influences hygroscopicity, processability, and thermal stability of the hole-doped polymers. OPLS4 molecular dynamics simulations suggest that short tether lengths (C2 and C3) result in interchain charge compensation, but medium and long tether lengths (C4 and C6) result in mixed interchain/intrachain charge compensation. Overall, the C3 tether provides the best thermal and ambient stability. These results reveal new aspects regarding the role of tether length in self-compensated, charge-doped polymers.