External electric field treatment for improving the morphology and electrical performance of P(NDI2OD-T2)

Abstract

Charge transfer characteristics of conjugated polymers are dominantly influenced by the morphology; therefore, pre- or post-treatment processes are essential for reducing the disorder in terms of intra- and inter-molecular structures. Herein, we propose an effective strategy known as external electric field (EEF) treatment to improve the crystallinity and alignment of conjugated polymers. We apply EEF treatment to primary blade-coated P(NDI2OD-T2) films of an n-type semiconducting polymer. The EEF treatment applied in the direction normal to the substrate resulted in an increase in chain linearity and crystal size in bothout-of-plane and in-plane directions. Consequently, by applying an EEF-treated conjugated polymer to the active layer in organic field-effect transistors, we successfully improve the average electron mobility from 1.69 to 2.14 cm² V⁻¹ s⁻¹ (max. 3.33 cm² V⁻¹ s⁻¹) at an EEF intensity of 0.75 kV cm⁻¹. Moreover, the correlation between the molecular morphology and EEF has been elucidated through various analyses, including molecular energy calculations and microscopic, optical, and electrical investigations. The findings demonstrate that this simple EEF treatment process can enhance polymer chain linearity, alignment and the size of crystals leading to greatly improved OFET device performance.