An ultra-low bandgap diketopyrrolopyrrole (DPP)-based polymer with balanced ambipolar charge transport for organic field-effect transistors

Abstract

A new A₁–A₂ type polymer pDTDPP–TTF containing diketopyrrolopyrrole (DPP) and 2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TTF) as the main electron-accepting building blocks has been synthesized via Suzuki cross-coupling polycondensation. The introduction of TTF as an additional electron-deficient unit significantly lowers the LUMO energy level of this DPP containing polymer to −4.37 eV which is beneficial for efficient electron injection and transport. The pDTDPP–TTF polymer shows an ultra narrow optical bandgap of 1.06 eV with a broad absorption spectra covering the visible to near infrared region of up to 1300 nm. Solution-processed OFET devices with pDTDPP–TTF as the active semiconducting layer have been fabricated with the configurations of both bottom-gate bottom-contact (BG–BC) and top-gate bottom-contact (TG–BC). The BG–BC devices with SiO₂ as a dielectric exhibit ambipolar behavior with a hole and electron mobility of 1.85 × 10⁻⁴ and 3.55 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. Improved OFET performance has been achieved in TG–BC devices by using PMMA as the dielectric; balanced ambipolar charge transport is obtained with a hole and electron mobility of 6.17 × 10⁻³ cm² V⁻¹ s⁻¹ and 3.13 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. To the best of our knowledge, our work is among the very few examples of ultra-low bandgap semiconducting polymers with balanced ambipolar charge transport properties.