The Effect of Alkyl Spacers on the Mixed Ionic‐Electronic Conduction Properties of N‐Type Polymers

Iuliana P Maria,Sahika Inal,Ruiheng Wu,Aniruddha Basu,Bryan D Paulsen,Iain Mcculloch,Thomas D Anthopoulos,Alexander Giovannitti,Achilleas Savva,Jonathan Rivnay,David Ohayon,Rawad Hallani,Weiyuan Du

doi:10.1002/adfm.202008718

Abstract

AbstractConjugated polymers with mixed ionic and electronic transport are essential for developing the complexity and function of electrochemical devices. Current n‐type materials have a narrow scope and low performance compared with their p‐type counterparts, requiring new molecular design strategies. This work presents two naphthalene diimide‐bithiophene (NDI‐T2) copolymers functionalized with hybrid alkyl‐glycol side chains, where the naphthalene diimide unit is segregated from the ethylene glycol (EG) units within the side chain by an alkyl spacer. Introduction of hydrophobic propyl and hexyl spacers is investigated as a strategy to minimize detrimental swelling close to the conjugated backbone and balance the mixed conduction properties of n‐type materials in aqueous electrolytes. It is found that both polymers functionalized with alkyl spacers outperform their analogue bearing EG‐only side chains in organic electrochemical transistors (OECTs). The presence of the alkyl spacers also leads to remarkable stability in OECTs, with no decrease in the ON current after 2 h of operation. Through this versatile side chain modification, this work provides a greater understanding of the structure‐property relationships required for n‐type OECT materials operating in aqueous media.

Highlights

Conjugated polymers that allow simultaneous transport of voltage regime,(VG), and the threshold the transconductance is gvoivletangeby(Vg mT).=IWn LtdheμCsa*.With both ionic and electronic elements captured, the μC*ionic and electronic charge carriers have experienced a surge term is the key parameter for evaluating the mixed conductionDr B
Monomers C3-gNDI and C6-gNDI were prepared from 2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic dianhydride using a one-pot procedure as previously reported,[18] which allowed both functionalization with the alkyl spacer at the imide position and extension with the ethylene glycol (EG) unit via an ester linkage in one step. 2,6-Dibromonaphthalene-1,4,5,8tetracarboxylic dianhydride was treated in turn with 3-amino1-propanol and 6-amino-1-propanol, followed by an excess of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid to afford monomers C3-gNDI and C6-gNDI, respectively as detailed in the Supporting Information
The more stable current response of p(C3-gNDI-gT2) and p(C6-gNDI-gT2) compared with p(gNDIgT2) is most likely attributed to their lower extent of swelling during charging, as determined by EQCM-D. These results demonstrate that optimization of the length of the alkyl spacer within the side chains is a successful strategy for improving the long-term operational stability of OECT materials

Summary

Introduction

(VG), and the threshold the transconductance is gvoivletangeby(Vg mT).=IWn LtdheμCsa* (tVuTra−tVioGn). The polymers with the hydrophobic alkyl unit adjacent to the backbone showed improved longrange ordering and electron mobility in OFETs.[35] For polymers designed for aqueous-based OECTs, this may prevent encroachment of ions at the location of polaron delocalization along the backbone, avoiding charge trapping and potentially increasing the charge carrier mobility. Motivated to explore this design strategy in electrochemical devices, a series of NDI-T2 copolymers functionalized with hybrid alkyl-glycol chains, attached via an alkyl spacer, were investigated. Our findings show that the introduction of alkyl spacers can be used to limit detrimental swelling in the aqueous environment, improve the performance and long-term stability of the n-type polymer during operation in OECTs

Results and Discussion

Conclusions

Experimental Section

Data Availability Statement