High Mobility Preservation of Near Amorphous Conjugated Polymers in the Stretched States Enabled by Biaxially-Extended Conjugated Side-Chain Design

Abstract

Polymers with an abundant amorphous domain should facilitate energy dissipation upon stretching, making near amorphous π-conjugated polymers have immense potential in realizing intrinsically stretchable field-effect transistor (FET) devices. In this study, high mobility preservation under the stretched state is attempted by replacing typical alkyl-monothienyl (T-R) on a benzo­[1,2-b:4,5-b’]­dithiophene-difluorobenzothiadiazole backbone with three other biaxially extended side-chains, including alkyl-dithienyl (2T-R), branching alkyl-trithienyl (3T-R), and alkyl-benzotrithienyl (B3T-R) groups. Despite showing near amorphous features, the semi-2D BDT-based polymers with bulkier biaxially extended side chains (PBDT-2T, PBDT-3T, and PBDT-B3T) still present comparable mobility to the reference semicrystalline polymer (PBDT-T). Although these four polymers yield comparable mobility, they show distinctly different mobility retention in the stretched state. From the study of their mobility-stretchability relationship, the interdigitating and/or entanglement of these biaxially extended conjugated side chains are shown to play a nontrivial role in the resultant mechanical robustness against the stretching force. Owing to the proper spatial mobility and geometry, the branched 3T-R side chain possesses a more intense interdigitating and/or entanglement capability than the linear 2T-R one and the fused B3T-R one, providing better mechanical strength under stretched states. Meanwhile, it maintains sufficient interchain connectivity for intermittent interchain hopping to compensate for the 1D charge transport along the backbone, ensuring good charge transport even in the stretched state. As a result, the printed PBDT-3T film is demonstrated to deliver a high mobility retention of 73% at a 60% strain exerted parallel to the charge-transporting direction and a very stable mobility retention of 88% after 1000 stretching-releasing cycles at a 60% strain, being one of the best stretchable near amorphous conjugated polymers reported thus far. Our result underlines the effectiveness of using biaxially extended conjugated side chains to realize high-performance stretchable polymers.

Mechanical Fatigue Resistance of Polydiketopyrrolo‐Pyrrole‐Dithienylthieno[3,2‐b]thiophene‐Based Flexible Field‐Effect Transistors

“…[ 24 ] Despite their amorphous nature, benzodithiophene‐based polymers with a biaxially‐extended side chain design also reveal high flexibility and a good charge carrier transport during deformation. [ 25 ] It was shown that polymers with abundant amorphous domains facilitate energy dissipation during stretching maintaining the charge carrier transport. The interdigitating and/or entanglement of these biaxially extended side chains are responsible for the mechanical robustness upon elongation.…”

A GIWAXS study of crystallization in annealed conjugated polymers presenting technological interest for organic solar cell applications

“…Thus, a possible explanation is an inhibition of the interdigitation effect detected at longer annealing times due to chain confinement in thin solid films. Indeed, several authors have explained how the interdigitation effect occurs in alkyl chains of conjugated polymers [20,21,26,35,36], reducing the volume of the crystalline arranges and presenting differences in FWHM between regioregular and regiorandom polymers. One of the studies [26] even points out that the methyl chain end on P3HT exhibits the closest packing, as in n-alkane crystallization, and causes a structural mismatch due to the difference between their packing density and the areal density of the attached ends.…”

Strain-insensitive naphthalene-diimide-based conjugated polymers through sequential regularity control

“…To study the mobilitystretchability property of the PNDI2T film, a film transfer technique was applied according to the method reported in the literatures. 26,27 Briefly, a transferred/stretched film was prepared in a nitrogen-filled glove box by transferring the PNDI2T film onto an elastomeric poly(dimethylsiloxane) (PDMS) slab, and then the PNDI2T film supported by the PDMS slab was stretched and transferred back to the wafer substrate. Note that the film transfer process was conducted at 70 1C.…”

“…20 To improve the mobility-stretchability properties of n-type CPs without sacrificing their thermal and mechanical integrity, structural engineering should be used to maintain strength while improving the fracture strain of CPs. 21 The feasible approaches to structural engineering can be divided into three categories: (1) random copolymerization with dimensionally similar monomers, 22,23 (2) adoption of rigid fused-ring backbones for a near-amorphous structure, 24,25 and (3) introduction of conjugated side groups on the side chains to promote chain aggregation 26,27 or the combination of a fused-ring backbone and conjugated side groups. 28 Most of the methods capable of improving mobility-stretchability properties are applicable mainly to p-type CPs and are seldom reported in n-type CPs owing to the instability of carrier transport and limited types of backbones.…”