Air-Stable n-type Diketopyrrolopyrrole-Diketopyrrolopyrrole Oligomers for High Performance Ambipolar Organic Transistor

Abstract

N-type organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP) based oligomers for ambipolar organic thin film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin film X-ray diffraction (XRD) and grazing incidence small angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulphur to selenium on the photophysical, optical, electronic and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ~0.2 cm2V-1s-1 with an on/off ratio exceeding 104. Field-effect behaviour was retained for up to ~ 4 week which illustrates remarkable air and bias stability. This work paves the way towards the development of n-type DPP-DPP based oligomers exhibiting retention of field effect behaviour with superior stability at ambient conditions.