Cyano substituted stilbene-based polymers for OFETs: From p-type to n-type

Abstract

The performance of n-type semiconductor materials, typically the charge transport mobility, greatly fall behind compared to those in the p-type ones, which limiting the development of the semiconductor applications. Thus, the molecular design for n-type semiconductor is both crucial and challenging. In this work, three polymers containing isoindigo and stilbene units with varying numbers of cyano-substituted were designed and applied as the semiconductor for the organic field-effect transistors (OFETs). The introduction of cyano groups transforms the stilbene from electron-rich to electron-deficient. As a consequence, the polymers transition from a donor-acceptor type (P0) to an acceptor-acceptor type (P1, P2). With an increasing number of the cyano groups, both of the HOMO and LUMO energy levels of the polymers were decreased, with P1 and P2 exhibiting particularly low LUMO energy levels (<−4.15 eV) and ultra-low HOMO levels (<−5.7 eV). Therefore, P0 exhibited p-type semiconductor behavior with the hole transport mobility of 0.40 cm² V⁻¹ s⁻¹, while cyano-substituted P1 and P2 displayed n-type characteristic with 0.62 cm² V⁻¹ s⁻¹ (P1), and 0.83 cm² V⁻¹ s⁻¹ (P2), respectively. In addition, the aggregation and molecular packing of the polymers in the thin film state improved with the increasing number of cyano units in the single repeat units, which facilitated interchain charge transport. Thus, P2 exhibited the highest charge transport mobility among the three polymers. This work demonstrates that introducing cyano units into the polymer backbone to synthesize an acceptor-acceptor (A-A) type polymer is a simple and effective strategy for developing n-type semiconductor materials. In addition, incorporating cyano units into the polymer backbone not only increases the electron-deficient character of the polymers and alters their charge transport properties but also might enhance charge transport mobility.