Enabling close backbone stacking in near-amorphous n-type polymer semiconductors via side-chain engineering

Abstract

Side chains of semi-crystalline donor–acceptor (D–A) type conjugated polymers (CPs) have played a crucial role in the regulation of microstructure and semiconducting transport properties. However, the effects of the side chains on near-amorphous copolymers have not been investigated. Herein, three alkyl side chains with different branching points away from the conjugated backbone were introduced into the copolymer composed of electron-deficient double B ← N bridged bipyridine (BNBP) unit and benzobisthiazole (BBTz) unit, i.e., PBN27-C2, PBN27-C4 and PBN27-C5. Side chains have significant influences on the aggregation behavior and film morphology. These polymers exhibited different aggregation behaviors in both the solutions and films, which were rarely observed for CPs. It is found that the crystallinity is gradually increased in the order: PBN27-C4 < PBN27-C2 < PBN27-C5, studied with X-ray diffraction. However, the π–π stacking distances were monotonically decreased after moving out the branching point. Among them, PBN27-C5 has the closest π–π stacking distance, as small as 3.41 Å, and the highest electron mobility of 0.3 cm² V⁻¹ s⁻¹, obtained from the organic field-effect transistor (OFET) devices. In addition, these OFET devices exhibited ideal n-type transport behaviors with excellent uniformity. Our research results reported here could reveal that shortening π–π contact is more important than improving crystallinity when aiming to enhance the mobility of these rigid near-amorphous polymer semiconductors.