An acceptor-quantity modulation strategy for high-performance ambipolar semiconducting polymers: from a dual-acceptor to a quad-acceptor backbone

Abstract

Diketopyrrolopyrrole (DPP)-based donor–acceptor (D–A) polymers have been extensively employed in a variety of organic electronic devices. However, systematically increasing the quantity of DPP acceptor blocks in the repeating unit and its impact on the performance of organic thin-film transistors (OTFTs) have not been investigated yet. Herein, an acceptor-quantity modulation strategy is put forward to further optimize the optoelectronic properties and the OTFT performance of DPP-based D–A polymers. Thus, three polymers that are composed of 1–3 DPP acceptor blocks are synthesized via C–H direct arylation polymerization (DArP). Notably, these polymers can be obtained within 0.5 h under the optimal DArP conditions and they demonstrate a decent number average molecular weight of 13–25 kg mol⁻¹. Among the three polymers, the dual-acceptor polymer P1DPP-BT exhibits typical ambipolar OTFT performances with electron mobility (μ_e)/hole mobility (μ_h) of 0.7/0.45 cm² V⁻¹ s⁻¹, respectively. Further success in enhancing μ_e is achieved in the triple-acceptor polymer P2DPP-BT with one more DPP block in the backbone. It shows the best μ_e up to 1.1 cm² V⁻¹ s⁻¹. Unexpectedly, incorporating the third DPP block into the repeating unit leads to a dramatic performance decrease (μ_e/μ_h ∼ 10⁻² cm² V⁻¹ s⁻¹). Further characterization studies indicate that the lowest unoccupied molecular orbital (LUMO) energy levels decrease monotonically as the acceptor quantity increases. Meanwhile, the polymer packing texture changes from an edge-on orientation to a face-on orientation with a dramatically decreased crystallinity. Thus, the triple-acceptor polymer maintains a balance between the energy levels, crystallinity and packing textures, resulting in the highest electron mobilities among the three polymers.