Bithiophene-Imide-Based Polymeric Semiconductors for Field-Effect Transistors: Synthesis, Structure−Property Correlations, Charge Carrier Polarity, and Device Stability

Abstract

Developing new high-mobility polymeric semiconductors with good processability and excellent device environmental stability is essential for organic electronics. We report the synthesis, characterization, manipulation of charge carrier polarity, and device air stability of a new series of bithiophene-imide (BTI)-based polymers for organic field-effect transistors (OFETs). By increasing the conjugation length of the donor comonomer unit from monothiophene (P1) to bithiophene (P2) to tetrathiophene (P3), the electron transport capacity decreases while the hole transport capacity increases. Compared to the BTI homopolymer P(BTimR) having an electron mobility of 10⁻² cm² V⁻¹ s⁻¹, copolymer P1 is ambipolar with balanced hole and electron mobilities of ∼10⁻⁴ cm² V⁻¹ s⁻¹, while P2 and P3 exhibit hole mobilities of ∼10⁻³ and ∼10⁻² cm² V⁻¹ s⁻¹, respectively. The influence of P(BTimR) homopolymer M_n on film morphology and device performance was also investigated. The high M_n batch P(BTimR)-H affords more crystalline film microstructures; hence, 3× increased electron mobility (0.038 cm² V⁻¹ s⁻¹) over the low M_n one P(BTimR)-L (0.011 cm² V⁻¹ s⁻¹). In a top-gate/bottom-contact OFET architecture, P(BTimR)-H achieves a high electron mobility of 0.14 cm² V⁻¹ s⁻¹, only slightly lower than that of state-of-the-art n-type polymer semiconductors. However, the high-lying P(BTimR)-H LUMO results in minimal electron transport on exposure to ambient. Copolymer P3 exhibits a hole mobility approaching 0.1 cm² V⁻¹ s⁻¹ in top-gate OFETs, comparable to or slightly lower than current state-of-the-art p-type polymer semiconductors (0.1−0.6 cm² V⁻¹ s⁻¹). Although BTI building block incorporation does not enable air-stable n-type OFET performance for P(BTimR) or P1, it significantly increases the OFET air stability for p-type P2 and P3. Bottom-gate/top-contact and top-gate/bottom-contact P2 and P3 OFETs exhibit excellent stability in the ambient. Thus, P2 and P3 OFET hole mobilities are almost unchanged after 200 days under ambient, which is attributed to their low-lying HOMOs (>0.2 eV lower than that of P3HT), induced by the strong BTI electron-withdrawing capacity. Complementary inverters were fabricated by inkjet patterning of P(BTimR)-H (n-type) and P3b (p-type).