Decoupling Poly(3-alkylthiophenes)’ Backbone and Side-Chain Conformation by Selective Deuteration and Neutron Scattering

Abstract

Although considerable progress has been made to optimize the optoelectronic properties of conjugated polymers (CPs), the rational design of CPs with tailored physical properties for end-use applications remains a significant challenge. Specifically, experimental characterization of conjugated polymer backbone conformations remains underexplored due to limited techniques that are capable of distinguishing the backbone and side-chain structures at nanoscopic resolution. Thus, relating the electronically functional backbone conformation to the material’s macroscopic optoelectronic property is an ongoing challenge. Here, small-angle neutron scattering techniques (SANS) with contrast-variation (CV) experiments are employed on poly­(3-alkylthiophenes) (P3ATs) with both deuterated and protonated side chains in a mixture of protonated and deuterated solvents to decouple the backbone and side-chain scattering signals. We obtained the form factor of P3ATs’ backbone, side chains, and cross-scattering term by deconvoluting their respective scattering signals. Poly­(3-decylthiophene) shows a persistence length of 1.05 ± 0.1 nm for the conjugated polymer backbone and 2.10 ± 0.2 nm for the entire chain. The strong scattering signal from long and flexible alkyl side chains leads to a seemingly more rigid conjugated polymer, which is further revealed by coarse-grained molecular dynamics (CG-MD) simulations. This work offers a methodology to decouple the scattering contribution from the CPs’ backbone and side chains, thus elucidating the inherent conformation of the electronically active conjugated backbone, which provides guidance for the rational design of next-generation polymeric semiconductors.

Molecular Origin of Strain‐Induced Chain Alignment in PDPP‐Based Semiconducting Polymeric Thin Films

“…[ 30–32 ] For D–A polymers with large fused rings in the backbone, they are even harder to bend ( i.e ., l p (poly(diketopyrrolopyrrole‐co‐dithienylethene[3,2‐ b]thiophene) (PDPPDTT)) ≈ 8 nm). [ 33,34 ] Thus, the molecular picture of D–A polymer chains is more likely to be semi‐rigid rods that are hard to entangle with each other. [ 35–37 ] Under the thin‐film state, conjugated polymers typically display a complex 3D heterogeneous semicrystalline microstructure with three main characteristic domains: crystalline region, local aggregate, and amorphous region.…”

How rigid are conjugated non‐ladder and ladder polymers?

“…For example, by deuterating the sidechains, one can conceal the scattering signal from the sidechains using contrast variation neutron scattering (see Figure 3), so that the signals of the electronically functional main-chain rigidity can be directly measured without the interference from the sidechains. 55 For example, recently Cao et al demonstrated the strong scattering signal from long sidechains of sidechain deuterated P3HT leads to an "apparently increased" backbone rigidity for the polymer chain compared to its backbone (Figure 3). 55 It is worth note that care should be given when interpreting the solution scattering data to postulate the singlechain conformation.…”

“…55 For example, recently Cao et al demonstrated the strong scattering signal from long sidechains of sidechain deuterated P3HT leads to an "apparently increased" backbone rigidity for the polymer chain compared to its backbone (Figure 3). 55 It is worth note that care should be given when interpreting the solution scattering data to postulate the singlechain conformation. A prerequisite for characterizing the single-chain conformation of any conjugated polymer is that the polymer should be well dissolved to form an individual polymer coil in solution.…”

Control of aggregated structure of photovoltaic polymers for high‐efficiency solar cells

“…Small-angle neutron scattering (SANS) can be used to study the microstructure, especially size, shape and dynamics of their components. [114,117,170,171] Dadmun et al [164] using the SANS to characterize the morphology and aggregation of the PCPDTBT:PCBM blends in different solvent annealing conditions. Due to the significant differences between PCPDTBT and PCBM in scattering length density, the scattering profile of Figure 5G showed that the scattering strength of as-cast film (AC) is very weak, which suggests that the film is homogeneous and the phase separation is not obvious, while the scattering pattern of the blends with the solvent annealing by oDCB exhibit enhanced scattering, indicating a phase separation of the fullerene and polymer in the blend.…”

“…However, the characterization and control methods of polymer solution aggregation are still lacking. It is necessary to systematically study the aggregated structure of conjugated polymers in solution, [114,117,170,171] and then comprehensively establish the relationship between molecular structure, solution aggregated structure, film microstructure, and electrical properties. It provides further guidance for the design of highperformance conjugated polymers and the optimization of device performance.…”