Dipolar Polyimides With a Clever Balance in Dielectric Performance by Introducing a Twisted Fluorene Structure for the Development of Electronic Applications

Abstract

Polyimides with combined high thermal resistance and excellent electrical properties are specifically desired for various electrical and power electronic systems. However, traditional polyimide lacks functional groups with huge dipole moments and hence suffer from intrinsic inferior permittivity. Dipolar polymers, as potential high permittivity materials, have received considerable attention. Here, rigid and twisted fluorene groups are introduced into the polyimide backbone, containing urea groups in the side chain. The twisted fluorene structure provides free volume for dipole rotation, which can effectively improve the dipole mobility and avoid the problem of elevated dielectric loss caused by the dipole-flip lag, whereas the urea groups with high dipole moments contribute to the elevation of the permittivity. Ultimately, a clever balance between high permittivity and low dielectric loss is realised through the molecular structure design; BP-BU_0.7 exhibits a high permittivity of 6.37 and a low dielectric loss of 0.0083 at room temperature and 1 kHz. Simultaneously, taking advantage of this characteristic, BP-BU_0.7 is used as the gate dielectric for the organic thin-film transistor (OTFT), and the device exhibits outstanding field-effect properties with low threshold voltage (−0.96 V) and high carrier mobility (4.09 cm² V⁻¹ s⁻¹) under low voltage (−5 V) operation. This polyimide material is considered as a potential dipole glass polymer dielectric for electronic applications.