Addition of Low-Molecular-Weight Batches Enhances Charge-Transport Properties of n-Type Polymer Semiconductors

The molecular weight (MW) and its distribution (MW-D) of conjugated polymers are important parameters to obtain desirable properties for organic field-effect transistors. Although the effects of high-MW chains on charge-carrier mobility are well-recognized, the role of low-MW chains has been overlooked. Herein, we investigate the effects of low-MW chains on the electrical properties of naphthalenediimide-based n-type polymers (PNDIs) having different electron-donating units (namely, thiophene, bithiophene, selenophene, and biselenophene). The MW-D of the PNDIs is systematically controlled by blending high- and low-MW batches of PNDIs. The presence of low amounts of low-MW chains universally enhances the field-effect electron mobility in the PNDI systems. We find that the addition of a small amount (3–10% by weight) of a low-MW polymer batch in a high-MW batch improves molecular ordering and reduces energetic disorder by optimizing the size and distribution of highly crystalline domains. As a result, the blend films exhibit ∼1.2–3.8 times higher mobility than the pristine high-MW PNDI films. This study provides new insight into the effects of MW-D on the electrical properties of polymer films and devices.