High-performance unipolar n-type conjugated polymers (CPs) are critical for the development of organic electronics. In the current paper, four "weak donor-strong acceptor" n-type CPs based on pyridine flanked diketopyrrolopyrrole (PyDPP), namely PPyDPP1-4FBT, PPyDPP2-4FBT, PPyDPP1-4FTVT, and PPyDPP2-4FTVT, are synthesized via direct arylation polycondensation by using 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4FBT) or (E)-1,2-bis(3,4difluorothien-2-yl)ethene (4FTVT) as weak donor unit. All four polymers exhibit low-lying highest occupied molecular orbital (≈ −5.90 eV) and lowest unoccupied molecular orbital energy levels (≈ −3.70 eV). Top-gate/ bottom-contact organic field-effect transistors based on all four polymers display unipolar n-channel characteristics with electron mobility (µ e ) above 1 cm 2 V −1 s −1 in air, and presented linear |I SD | 1/2 −V GS plots and weak dependence of the extracted moblity on gate voltage (V GS ), indicative of the reliability of the extracted mobility values. Importantly, the devices based on PPyDPP1-4FBT and PPyDPP2-4FBT show a pure unipolar n-channel transistor behavior as revealed by the typical unipolar n-channel output characteristics and clear off-regimes in transfer characteristics. Attributed to its high crystallinity and favorable thin film morphology, PPyDPP2-4FBT shows the highest µ e of 2.45 cm 2 V −1 s −1 , which is among the highest for unipolar n-type CPs reported to date. This is also the first report for DPP based pure n-type CPs with µ e greater than 1 cm 2 V −1 s −1 .

Four molecule design strategies of n-type conjugated polymers for organic thin-film transistors are summarized and discussed.

Seven diketopyrrolopyrrole (DPP)-based donor–acceptor (D–A) conjugated polymers, i.e., poly­[2,5-bis­(4-octadecyl­docosyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-3,3′,4,4′-tetrafluoro-2,2′-bithiophene] (P4F2T-C40), poly­[2,5-bis­(4-octadecyl­docosyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-1,2-bis­(3,4-difluoro­thiophen-2-yl)­ethyne] (P4FTAT-C40), poly­[2,5-bis­(4-octadecyl­docosyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-(E)-1,2-bis­(3,4-difluoro­thien-2-yl)­ethene] (P4FTVT-C40), poly­[2,5-bis­(4-tetradecyl­octadecyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-(E)-1,2-bis­(3,4-difluorothien-2-yl)­ethene] (P4FTVT-C32), poly­[2,5-bis­(4-decyl­tetradecyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-(E)-1,2-bis­(3,4-difluorothien-2-yl)­ethene] (P4FTVT-C24), poly­[2,5-bis­(2-decyl­dodecyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-(E)-1,2-bis­(3,4-difluoro­thien-2-yl)­ethene] (P4FTVT-C22), and poly­[2,5-bis­(2-decyl­tetradecyl)­pyrrolo­[3,4-c]­pyrrole-1,4­(2H,5H)-dione-alt-5,5′-di­(thiophen-2-yl)-2,2′-(E)-1,2-bis­(3,4-difluoro­thien-2-yl)­ethene] (P4FTVT-C10C12), were synthesized by direct arylation polycondensation (DArP) with multi-fluorinated thiophene derivatives 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4F2T), (E)-1,2-bis­(3,4-difluorothien-2-yl)­ethene (4FTVT), or 1,2-bis­(3,4-difluoro­thiophen-2-yl) acetylene (4FTAT) as the comonomer. The structures of the multi-fluorinated thiophene derivatives have a noticeable influence on the optical properties and self-assembly properties of the polymers. Compared to P4F2T-C40, P4FTVT-C40 showed an ∼30 nm red-shift while P4FTAT-C40 exhibited an ∼60 nm blue-shift of the absorption spectrum. Top-gate and bottom-contact (TGBC) organic field-effect transistors (OFETs) of all the polymers exhibited ambipolar transport behavior. The devices based on P4FTAT-C40 displayed poor device performance since its film was almost amorphous. In contrast, the polymer based on 4FTVT with the same alkyl side chains delivered much better device performance due to its crystalline nature, favorable molecular orientations, and appropriate film morphology. With optimized side chains, P4FTVT-C32 exhibited the highest hole (μh) and electron mobilities (μe) of ca. 2.6 and ca. 8.0 cm2 V–1 s–1 in air, respectively.

High-mobility conjugated polymers (CPs) that can be synthesized in a large scale by direct arylation polycondensation (DArP) are highly desired for printable electronics. To realize this purpose, a readily accessible C–H monomer with high reactivity and selectivity is crucial. Herein, we demonstrated that chlorination enables the efficient DArP of thiophene derivatives, i.e., 3,3′,4,4′-tetrachloro-2,2′-bithiophene (4ClBT) and (E)-1,2-bis­(3,4-dichlorothien-2-yl)­ethene (4ClTVT), which can be prepared with no more than two steps. No evidence of structure defects was found in the resulting polymers. This finding allows the facile synthesis of high-molecular-weight CPs via DArP. The presence of Cl···S weak intramolecular interaction, which can improve the coplanarity of conjugated skeleton, was confirmed by single-crystal structural analysis. With diketopyrrolopyrrole derivatives as C–Br monomers, CPs with electron mobility up to 1.44 cm2 V–1 s–1 in air have been obtained via DArP. This work demonstrates that chlorinated thiophene derivatives are promising C–H monomers for large-scale synthesis of high-mobility CPs via an atom-economical and “green” method, i.e., DArP.

Currently, a considerable number of solution-processed high mobility p-type OTFTs have been obtained. [4-8] However, the achievement of high performance n-type OTFTs via solution processing is still challenging, yet important for the construction of organic circuits such as complementary metal-oxide-semiconductor (CMOS)-like circuits. [9-12] Reviewing of recent development has demonstrated that the highest reliable electron mobility from solution-processed OTFTs is 7.16 cm 2 V −1 s −1 to date. [11] To enhance the performance of conjugated polymer-based OTFTs, increasing the molecular order of the semiconductor thin film is an effective method. [13-18] Uniaxial alignment of conjugated polymer backbones can increase mobilities along the alignment direction via preferential intrachain transport. [19-21] Driven by directional flow due to Marangoni and capillary effect, bar coating is a valued solution processing technique to orient conjugated polymers for highly ordered thin films. [13,22-26] Besides, large ordered crystalline domains are generally accompanied by long range order, which is essential for high mobilities due to elongated carrier pathways and reduced grain boundaries. [19,27,28] Therefore, uniaxially aligned films featuring large crystalline domains are mostly desirable. In this work, we used a diketopyrrolopyrrole-based conjugated polymer, P4FTVT-C32, [29] as the semiconductor material, bar coating as the solution processing method for highly oriented films of large crystalline fibers. The corresponding OTFTs are characterized by electron mobility up to 9.38 cm 2 V −1 s −1 in the saturation regime and 8.35 cm 2 V −1 s −1 in the linear regime, and on/off ratio 10 5-10 6. Notably, these mobilities extracted according to the gradual channel approximation model is highly reliable since the devices are close to ideal transistors. This can be reflected by linear fitting of linear regime drain current and the square root of saturation regime drain current against gate voltage, the resulted independence of mobility on gate voltage, small gap between linear and saturation mobility, and high reliability factor >80%. To the best of our knowledge, these are the record high saturation and linear mobilities of n-type OTFTs, taking into account of the reliability.