Rational Molecular Design of Diketopyrrolopyrrole-Based n-Type and Ambipolar Polymer Semiconductors

Diketopyrrolopyrrole (DPP)-based polymer semiconductors have drawn great attention in the field of organic electronics due to the planar structure, decent solubilizing capability, and high crystallinity. However, the electron-deficient capacity of DPP derivatives are not strong enough, leading to relatively high-lying lowest unoccupied molecular orbital (LUMO) energy levels of the corresponding polymers. As a result, n-type and ambipolar DPP-based polymers are rare and their electron mobilities also lag far behind the p-type counterparts, which limits the development of important p-n-junction-based electronic devices. Therefore, new design strategies have been proposed recent years to develop n-type/ambipolar DPP-based polymers with improved performances. In this view, these molecular design strategies are summarized, including copolymerization of DPP with different acceptors and weak donors, DPP flanked aromatic ring modification, DPP-core ring expansion and DPP dimerization. The relationship between the chemical structures and organic thin-film transistor performances is intensively discussed. Finally, a perspective on future trends in the molecular design of DPP-based n-type/ambipolar polymers is also proposed.

Influence of Backbone Regioregularity on High-Mobility Conjugated Polymers Based on Alkylated Dithienylacrylonitrile

We designed and synthesized two donor-acceptor type conjugated polymers, the regioirregular polymer RI-PDPP-CNTVT-6 and its regioregular counterpart RR-PDPP-CNTVT-6, based on diketopyrrolopyrrole (DPP) and alkylated dithienylacrylonitrile (CNTVT) units. Among them, the 2-decyltetradecyl side chain on the DPP acceptor unit and the hexyl side chain on the CNTVT donor unit were used to ensure enough solubility for them. The backbone regioregularity was used to tune electronic structures and carrier transport of the conjugated system. The two conjugated polymers were characterized for their thermal, photophysical, electrochemical, and solution-processable properties, thin-film microstructures, and morphologies. The top-gate bottom-contact configuration organic field-effect transistor (OFET) devices based on these two conjugated polymers showed excellent ambipolar performances. Remarkably, the regioirregular polymer RI-PDPP-CNTVT-6 exhibited higher charge-carrier mobilities than the regioregular counterpart polymer RR-PDPP-CNTVT-6 did, as their highest hole/electron mobilities (μ_hmax/μ_emax) were 1.48/1.27 and 0.48/0.052 cm² V^-1 s^-1, respectively. Moreover, the influence of backbone regioregularity on its thermal stability, electrochemical and photophysical properties, solution processability, and charge-carrier mobility was intensively studied. Our results afforded a promising pathway toward the development of excellent ambipolar OFETs with high performance, good solution processability, and thermal stability.

Tuning Charge Carrier and Spin Transport Properties via Structural Modification of Polymer Semiconductors

Targeted design of organic semiconductors in organic spintronics is relatively limited. Therefore, four conjugated polymers with analogous structures based on isoindigo (IID) units were designed and synthesized to investigate the structure-property relationships in spin and charge carrier transport. Structural design strategies include introduction of pyridinic nitrogen atoms into IID units to change electronic structures and alteration of different branching points of alkyl chains to adjust the aggregation structure. By fabricating polymer field-effect transistors (PFETs) and organic spin valves (OSVs), all of the polymers exhibited good ambipolar field-effect properties (all of the mobilities exceeding 0.3 cm² V^-1 s^-1) and relatively high magnetoresistance (MR) values (maximum up to 25%). Most importantly, it is found that the introduction of pyridinic nitrogen into the IID units can improve MR values of OSVs and electron mobilities of PFETs, whereas the extension of alkyl chain branching points can reduce MR values of the conjugated polymers. This work is the first attempt to thoroughly study the structure-property relationship in the OSVs, combined with molecular design of the conjugated polymers, which provides a guideline for molecular engineering, especially for organic spintronics.