Effect of Spacer Length of Siloxane-Terminated Side Chains on Charge Transport in Isoindigo-Based Polymer Semiconductor Thin Films

A series of isoindigo-based conjugated polymers (PII2F-CmSi, m = 3–11) with alkyl siloxane-terminated side chains are prepared, in which the branching point is systematically “moved away” from the conjugated backbone by one carbon atom. To investigate the structure–property relationship, the polymer thin film is subsequently tested in top-contact field-effect transistors, and further characterized by both grazing incidence X-ray diffraction and atomic force microscopy. Hole mobilities over 1 cm2 V−1 s−1 is exhibited for all soluble PII2F-CmSi (m = 5–11) polymers, which is 10 times higher than the reference polymer with same polymer backbone. PII2F-C9Si shows the highest mobility of 4.8 cm2 V−1 s−1, even though PII2F-C11Si exhibits the smallest π–π stacking distance at 3.379 A. In specific, when the branching point is at, or beyond, the third carbon atoms, the contribution to charge transport arising from π–π stacking distance shortening becomes less significant. Other factors, such as thin-film microstructure, crystallinity, domain size, become more important in affecting the resulting device's charge transport.