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Flexible alkyl side chain in conjugate polymers (CPs) improves the solubility and promotes solution processability, in addition, it affects interchain packing and charge mobilities. Despite the well-known charge mobility and morphology correlation for these semi-crystalline polymers, there is a lack of fundamental understanding of the impact of side chain on their crystallization kinetics. In the present work, isothermal crystallization of five poly(3-alkylthiophene-2,5-diyl) (P3ATs) with different side-chain structures were systematically investigated. To suppress the extremely fast crystallization and trap the sample into amorphous glass, an advanced fast scanning chip calorimetry technique, which is able to quench the sample with few to tens thousands of K/s, was applied. Results show that the crystallization of P3ATs was greatly inhibited after incorporation of branched side chains, as indicated by a dramatic up to six orders of magnitude decrease in the crystallization rate. The suppressed crystallization of P3ATs were correlated with an increased π–π stacking distance due to unfavorable side-chain steric interaction. This work provides a pathway to use side-chain engineering to control the crystallization behavior for CPs, thus to control device performance. 

Abstract Donor–acceptor (D–A) type semiconducting polymers have shown great potential for the application of deformable and stretchable electronics in recent decades. However, due to their heterogeneous structure with rigid backbones and long solubilizing side chains, the fundamental understanding of their molecular picture upon mechanical deformation still lacks investigation. Here, the molecular orientation of diketopyrrolopyrrole (DPP)‐based D–A polymer thin films is probed under tensile deformation via both experimental measurements and molecular modeling. The detailed morphological analysis demonstrates highly aligned polymer crystallites upon deformation, while the degree of backbone alignment is limited within the crystalline domain. Besides, the aromatic ring on polymer backbones rotates parallel to the strain direction despite the relatively low overall chain anisotropy. The effect of side‐chain length on the DPP chain alignment is observed to be less noticeable. These observations are distinct from traditional linear‐chain semicrystalline polymers like polyethylene due to distinct characteristics of backbone/side‐chain combination and the crystallographic characteristics in DPP polymers. Furthermore, a stable and isotropic charge carrier mobility is obtained from fabricated organic field‐effect transistors. This study deconvolutes the alignment of different components within the thin‐film microstructure and highlights that crystallite rotation and chain slippage are the primary deformation mechanisms for semiconducting polymers.