Conjugated polymers (CPs) hold promise for modern organic electronics due to their adaptability and cost-effectiveness. However, their molecular-scale characterization is still challenging. In this talk, I will show how the high-resolution imaging capabilities of scanning tunnelling microscopy (STM) can be used to analyse CPs and surpass conventional analytical limits. With a series of examples, I will demonstrate that this method allows molecule-by-molecule characterisation, revealing self-assembly, length distribution, sequence, and chemical structure of the polymers. These can be used to investigate microscale behaviour, comparing polymerization techniques, understanding side-chain effects, and fully characterise CPs where traditional methods fall short, due to aggregation or mass limitations.
The recently developed combination of vacuum electrospray deposition (ESD) and scanning tunnelling microscopy (STM) is here compared to commonly-used analytical techniques GPC and NMR. We study three DPP-based polymers, of which only two produce clearly-resolved NMR spectra. Our subnanometre-resolved images provide, in one single experiment, detailed information about complete mass distributions and exact polymer sequences, including the identification of polymerisation defects. We, therefore, show that ESD-STM represents a new powerful analytical tool, successfully benchmarked against NMR and GPC when these methods are viable, and that can be used to characterise with equal accuracy polymers that are inaccessible by traditional techniques.
The true structure of alternating conjugated polymers – the state-of-the-art materials for a number of organic electronics technologies – often deviates from the idealized picture but this gets relatively limited attention. Here, we quantify the amount of homocoupling defects resulting from Stille polymerization and shed new light on the actual distribution of these structural defects in a prototype polymer material. Further, when compared to a homocoupling-free variant, these defects hinder fullerene intercalation, with a clear implication on charge-transfer absorption. This demonstrates that molecular defects may (strongly) impact polymer and blend properties and calls for increased attention for defect-free materials.
Combining electrospray (ESD) and scanning tunnelling microscopy (STM) has shown in recent years to reliably grant access to individual polymer strands within self-assembled polymer monolayers with submolecular resolution. Here, this methodology is used to study the microstructural properties that influence intra- and inter-chain charge transport in the case of the amorphous polymer IDT-BT.
The analysis of STM images allows us to precisely determine backbone conformations and statistically infer torsional potential energies for IDT-BT, to quantify the size and relative orientation of nanoaggregates within the polymer monolayers, and to provide the exact geometrical configuration of short-contact crossing points between polymer backbones.
The recently developed combination of vacuum electrospray deposition (ESD) and scanning tunnelling microscopy (STM) is used to analyse pBTTT and its glycated side-chain counterparts, pgBTTT and p(g2T-TT). Our high-resolution images allow us to determine the conformation and assembly of the polymers with sub-monomer resolution. We identify clear differences in the tendency of assembling between the two types of polymers and trace them back to the different nature of their side-chain interactions. The generalisation of these findings from 2D monolayers to 3D thin films is proposed as a means to gain so far inaccessible information on the microstructure of conjugated polymers.
The combination of vacuum electrospray deposition (ESD) and scanning tunnelling microscopy (STM) is used to investigate the conformation and assembly of traditional p-type OFET materials (pBTTT and IDT-BT) and of novel fused electron-deficient rigid-rod polymers. High-resolution STM images clearly demonstrate that one of the main drivers for the polymer self-assembly is the maximization of alkyl side-chain interdigitation. Furthermore, a careful analysis of the STM data is used to establish the presence of synthetic defects in the polymer backbone, to ascertain their conformation, to infer torsional potential energies and to define the geometry of intermolecular couplings with sub-monomer resolution.
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