Revealing the Role of Polydispersity in Multilevel Assembly Structures and Its Correlation with the Mechanical and Electrical Properties of IDTBT Thin Films
Molecular weight optimization is crucial for high-performance stretchable conjugated polymer films. However, an in-depth understanding of molecular weight distribution on solution assembly, film microstructures, and electrical/mechanical properties of conjugated polymers is lacking. Herein, a model conjugated polymer, poly(indacenodithiophene-co-benzothiadiazole) (IDTBT), with a similar weight-average molecular weight but different polydispersity indexes (PDIs) of 3.2, 2.4, and 1.6 is investigated. The low-PDI polymer, containing a high content of homogeneous long chains, facilitates sufficient interchain aggregation caused by the enhanced chain entanglement and prolonged aggregation dynamics, which creates a low-crystallinity film containing long-chain well-connected aggregates and chain entanglement networks. Consequently, the charge mobility increases from 2.1 to 3.1 cm2 V–1 s–1 as PDI decreases from 3.2 to 1.6. During stretching, the polymer chains align more effectively along the strain direction in the low-PDI film, which creates more dynamic sliding sites and short-range aggregates to dissipate the strain energy. Thus, the low-PDI polymer film exhibits a high charge mobility of 1.0 ± 0.1 cm2 V–1 s–1 at 100% strain and 0.9 ± 0.1 cm2 V–1 s–1 after 100 cycles of stretching–releasing at 25% strain, which significantly outperforms the high-PDI film. This work demonstrates the significance of polydispersity optimization for developing mechanically robust polymer semiconductor films in stretchable electronics.
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