High-Efficiency Ion-Exchange Doping of Conducting Polymers.

Ian E Jacobs,David Beljonne,Chen Chen,Seth R Marder,Christian B Nielsen,Lianglun Lai,Dion Tjhe,Simone Fratini,Xinglong Ren,Stephen Barlow,Henning Sirringhaus,Yuxuan Huang,Vincent Lemaur,Joseph Strzalka,Yue Lin,Martin Statz,Peter A Finn,Iain Mcculloch,Dimitrios Simatos,Gabriele D’avino ,William George Neal

doi:10.1002/adma.202102988

Abstract

Molecular doping-the use of redox-active small molecules as dopants for organic semiconductors-has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox-active character of these materials. A recent breakthrough wasa doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. Here, the equilibrium and kinetics of ion exchange doping in a model system, poly(2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene) (PBTTT) doped with FeCl3 and an ionic liquid, is studied, reaching conductivities in excess of 1000 S cm-1 and ion exchange efficiencies above 99%. Several factors that enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl3 , are demonstrated. In this high ion exchange efficiency regime, a simple connection between electrochemical doping and ion exchange is illustrated, and it is shown that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential.

Highlights

The simplest, most common approach to doping in semiconducting polymers, molecular doping[1,2,3,4] (Figure 1a) has several fundamental limitations
We find that the process is extremely efficient in acetonitrile because its high dielectric constant allows the ionic liquid cation to behave as a mere spectator ion
We have shown that the ion exchange process can essentially be understood as analogous to electrochemical doping wherein the redox potential of the dopant plays the role of the applied electrical voltage

Summary

Introduction

The simplest, most common approach to doping in semiconducting polymers, molecular doping[1,2,3,4] (Figure 1a) has several fundamental limitations. These arise from the requirement that the dopant molecule must perform two seemingly unrelated roles. Requiring a single chemical species to perform both these functions leads to several difficulties: 1) p-Type dopants are by definition strong electron acceptors (oxidizing agents), and quite reactive.[2,5,6] Because at equilibrium a small population of neutral dopants always exists, both redox.

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Conclusion