Unraveling the Unconventional Order of a High-Mobility Indacenodithiophene-Benzothiadiazole Copolymer

TL;DR

This study uses advanced microscopy to reveal that a high-mobility D-A copolymer has substantial local order and nanocrystallites despite lacking long-range crystallinity, challenging traditional assumptions about structure and mobility.

Contribution

It demonstrates the nanoscale and mesoscale organization of a high-mobility D-A copolymer using low-dose TEM, revealing local order and nanocrystallites.

Findings

High charge mobility exceeds 1 cm²V⁻¹s⁻¹.

Presence of nanocrystallites with aligned chains across the film.

Significant short- and medium-range order with specific grain boundary orientations.

Abstract

A new class of donor-acceptor (D-A) copolymers found to produce high charge carrier mobilities competitive with amorphous silicon ($> 1 cm^{2}V^{-1}s^{-1}$) exhibits the puzzling microstructure of substantial local order, however lacking long-range order and crystallinity previously deemed necessary for achieving high mobility. Here, we demonstrate the application of low-dose transmission electron microscopy to image and quantify the nanoscale and mesoscale organization of an archetypal D-A copolymer across areas comparable to electronic devices (~ $9 {\mu}m^{2}$). The local structure is spatially resolved by mapping the backbone (001) spacing reflection, revealing nanocrystallites of aligned polymer chains over nearly the entire film. Analysis of the nanoscale structure of its ordered domains suggests significant short- and medium-range order and preferential grain boundary orientations. Moreover, we provide insights into the rich, interconnected mesoscale organization of this new family of D-A copolymers by analysis of the local orientational spatial autocorrelations.