Congyuan Wei

Vinylidenedithiophenmethyleneoxindole: a centrosymmetric building block for donor–acceptor copolymers

Further development of new building blocks is crucial for realizing next-generation, high-performance organic semiconducting materials. In the paper, the design and synthesis of a novel π-extended analogue of isoindigo, vinylidenedithiophenmethyleneoxindole (VDTOI), and two VDTOI-based copolymers is reported. The centrosymmetric VDTOI unit possesses a special acceptor–donor–acceptor structure and contains a highly planar conjugated backbone because of the presence of S⋯O conformational locks. Of special importance is the fact that the VDTOI unit has the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital and energy levels of −5.35 eV/−3.42 eV, which are similar properties to those of the thiophene-flanked diketopyrrolopyrrole building block. The previously mentioned structural features indicate that VDTOI might be a potential building block for constructing polymeric semiconductors. As a trial, two VDTOI-based copolymers were synthesized, namely P1 and P2. The two copolymers show good thermal stability and broad absorption spectra ranging from 330 to 740 nm. Their HOMO energy levels of around −5.40 eV match well with the work function (5.13 eV) of a gold (Au) electrode, which is indicative of effective hole injections from the Au electrodes to polymer semiconductor films. Both the P1- and P2-based field-effect transistors exhibited typical p-type transport characteristics. The highest mobility of 0.35 cm2 V−1 s−1 was achieved in P1-based devices.

Highly planar cross-conjugated alternating polymers with multiple conformational locks: synthesis, characterization and their field-effect properties

It is meaningful to explore new design principles for organic semiconductors. Herein, we have developed two cross-conjugated alternating polymers based on 1,2-di(thiophen-2-yl)ethane-1,2-dione (DTO), namely PDTO-C1 and PDTO-C3, and investigated their charge transport properties by fabricating field-effect transistors devices. Single crystal X-ray crystallography shows that non-covalent S⋯O and C–H⋯O interactions exist inside the DTO units. These non-covalent interactions in combination with the C–H⋯O interactions of the thiophene-flanked dithienothiophene units, acting as conformational locks, are beneficial for acquiring the planar backbone conformation. PDTO-C1 and PDTO-C3 possess broad absorption spectra and HOMO and LUMO energy levels of ca. −5.50 and −3.6 eV, respectively. The highest mobility of 0.54 cm2 V−1 s−1 was achieved in the PDTO-C3-based transistor devices, whereas PDTO-C1 affords a mobility of 0.22 cm2 V−1 s−1. Further thin film microstructure investigations indicate that both polymers can form highly-ordered lamellar packing with small π–π stacking distances down to 3.50 A. These results demonstrate that the incorporation of cross-conjugation may be used as an additional design tactic for organic semiconductors to alter their optoelectronic properties.

Ambipolar tetrafluorodiphenylethene-based donor–acceptor copolymers: synthesis, properties, backbone conformation and fluorine-induced conformational locks

Herein, we present three kinds of tetrafluorodiphenylethene (TFPE)-based building blocks with multiple F⋯H–C and F⋯S conformational locks. Based on this, three copolymers PD23TFPE, PD26TFPE, and PD25TFPE were developed. Field-effect transistors based on these copolymers all exhibited ambipolar carrier transport characteristics, and the highest hole/electron mobilities were achieved in the PD25TFPE-based device. Thin film microstructure analyses suggest that PD25TFPE forms a highly ordered and dense thin film with an edge-on lamellar molecular packing. The crystallographic data of TFPE-based small molecules indicate that PD25TFPE assumes a linearly, fully “locked” conjugated backbone conformation, whereas PD23TFPE and PD26TFPE have a bent, fully “locked” or a linearly, partly “locked” conjugated backbone conformation. Our studies reveal the important influence of multiple conformation locks on tuning the backbone conformation, molecular packing mode in the solid state, and thus on the charge transport properties of polymer semiconductors.

Benzothiophene-flanked diketopyrrolopyrrole polymers: impact of isomeric frameworks on carrier mobilities

Exploring new building blocks for solution-processable polymeric semiconductors has attracted much attention. We herein develop two isomeric benzothiophene-flanked diketopyrrolopyrrole polymers with different linkage positions and further systematically study the electronic structures, optical properties, and field-effect characteristics. Both polymers exhibit typical p-type transport characteristics with the highest mobility being up to 0.80 cm2 V−1 s−1. Our results suggest that the isomeric backbones for conjugated polymers greatly affect charge transport properties.

Microstructure engineering of polymer semiconductor thin films for high-performance field-effect transistors using a bi-component processing solution

Solution processability is one of the main reasons for developing polymer-based organic field-effect transistors (FETs) in the application of large-area, flexible and low-cost electronics. During the deposition process, the solvent action could exert a great influence on the self-assembly of polymers and the morphology of films, thus determining the FET performance. In this work, a bi-component solvent system composed of chloroform and dichlorobenzene was employed in the spin-coating process to fabricate bottom-gate bottom-contact FET devices. Dichlorobenzene could exhibit strong dispersive interactions to dissolve the polymers, while chloroform is less effective in solvating the polymers. By altering the ratios of the bi-component solvents, enhanced mobilities were achieved from PTD-10-TVT. This method has also proven to be effective in promoting the performance of other polymer semiconductors. Our work provides an effective method for obtaining high charge carrier mobilities in solution-processable polymer-based FET devices.

Magnetism of N-doped graphene nanoribbons with zigzag edges from bottom-up fabrication

Graphene nanoribbon is a promising architecture for the development of advanced spintronic and other electronic-based devices. However, experimentally researching the magnetic properties of graphene nanoribbons is often obstructed by the key challenges in the synthesis of how to obtain the required chemical precision for control over edge structures and width. Here we synthesized two graphene nanoribbons with atomically precise N-doping at zigzag sites prepared from bottom-up fabrication by a condensation reaction. Our work clarifies the experimental issue of graphene nanoribbons magnetism. Two graphene nanoribbons lead to notable paramagnetism, which is not from residual magnetic impurities. In the process of optimizing the preparation conditions, we found that the elevated synthesis pressure and using 1-methyl-2-pyrrolidinone as solvent could lead to production of larger graphene nanoribbon skeletons with N-doped zigzag edges and then to improve the magnetization. These results provide a feasible and very attractive strategy to achieve the required atomic precision and experimentally guide future research for the magnetic properties of graphene-based materials by changing reaction precursors.

Novel vinylene-bridged donor–acceptor copolymers: synthesis, characterization, properties and effect of cyano substitution

Two novel vinylene-bridged π-extended building blocks, (2E,2′E)-3,3′-(2,5-bis(octyloxy)-1,4-phenylene)bis(2-(5-bromothiophen-2-yl)acrylonitrile) (BOPACN) and 5,5′-((1E,1′E)-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-1,2-diyl))bis(2-bromothiophene) (BOPA), were synthesized. They were further copolymerized with two diketopyrrolopyrrole (DPP) units possessing different alkyl side chains, and four donor–acceptor copolymers, namely PBOPACN-1, PBOPACN-2, PBOPA-1 and PBOPA-2, were synthesized. The optical, thermal, and electrochemical properties and field-effect properties of all copolymers were systematically investigated by theoretical calculations and experimental methods. Field-effect transistors based on these copolymers exhibited p-type charge transport characteristics in air. The performance implies that cyano substituents exert an important influence on semiconducting behaviours. Furthermore, 2D-GIXRD and theoretical calculations demonstrate that the presence of cyano substituents improves molecular planarity and generates highly-ordered molecular aggregates for PBOPACN-1 and PBOPACN-2.