Zhihui Chen

Approaching high charge carrier mobility by alkylating both donor and acceptor units at the optimized position in conjugated polymers

Flexible side chains have been the central issue in the design of solution-processable polymer semiconducting materials. On identical conjugated backbones, great differences can be made with different side chain densities and substitution positions. Generally, for donor–acceptor (D–A) copolymers, increasing the side chain density could readily improve the solubility of the polymers. However, it also results in backbone distortion and weaker interchain interactions, thus hindering charge transport, which limits the application of highly soluble D–A polymers. Herein, we synthesized two polymers of PD-n-DTTE-7 with both the moieties alkylated. Substituting side chains to DTTE units improved the solubility of the corresponding polymers. Induced by the promoted solubility, high purity of the high molecular weight component was obtained and uniform films were formed with these high molecular weight polymers. In addition, by carefully selecting the conjugation system and substitution positions, ordered packing and backbone coplanarity were also achieved, which facilitate interchain and intrachain charge transport. Through a simple spin coating technique, the polymers, PD-n-DTTE-7s, present the highest hole mobility of 9.54 cm2 V−1 s−1 with high average mobilities of over 5 cm2 V−1 s−1, which are among the highest values for polymer-based field-effect transistors.

High-performance polymer field-effect transistors fabricated with low-bandgap DPP-based semiconducting materials

A series of new π-conjugated D–A copolymers PDMOTT-n (n = 118, 122, 320, and 420) containing a diketopyrrolopyrrole unit and a 3,6-dimethoxythieno[3,2-b]thiophene moiety was designed and synthesized, and their field-effect properties were characterized. The polymers PDMOTT-n have a low bandgap of 1.27 eV and exhibit a broad absorption. Solution-processed field-effect transistors based on these polymers were fabricated with a bottom-gate/bottom-contact configuration and demonstrated typical p-type charge transporting properties. Of these, PDMOTT-420 with the longest alkyl side chains and having the longest distance between the branching point of the alkyl side chain and π-conjugated backbone of the polymer, exhibited the best carrier transporting performance with a hole mobility of 2.01 cm2 V−1 s−1 and with an on/off current ratio of 104–105. The characterization results of grazing incidence X-ray diffraction and tapping-mode atomic force microscopy showed that the thin film of the polymer PDMOTT-420 forms larger grains and more useful nanofibrillar intercalated structures and owns shorter π–π stacking distance and better crystallization than those of another three polymers. These results could help us to better understand the structure–property relationship of polymer semiconductors and to design novel π-conjugated polymers for high-performance field-effect transistors.

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.

Highly coplanar bis(thiazol-2-yl)-diketopyrrolopyrrole based donor–acceptor copolymers for ambipolar field effect transistors

The coplanarity and intermolecular interactions of polymers are fundamental factors influencing intra- or interchain charge transport for polymer field-effect transistors (PFETs). In this work, four alternating donor–acceptor copolymers with the acceptor of bis(thiazol-2-yl)-diketopyrrolopyrrole (TZDPP) and donor of (E)-1,2-di(thiophen-2-yl)ethene (TVT) or (E)-1,2-di(selenophen-2-yl)ethene (SVS) were synthesized. The introduction of thiazole units promotes the coplanarity of the polymer backbone. In addition, the electron deficient properties of TZDPP enhance interchain interactions. The polymers exhibited ambipolar semiconducting behaviours with the highest hole and electron mobilities reaching 0.17 and 9.7 × 10−3 cm2 V−1 s−1, respectively, in top-gate bottom-contact PFET devices. The SVS-based polymers showed lower mobilities than those containing TVT units. Careful characterization of the polymer films revealed that though the polymers containing SVS units exhibited better crystallinity, their films were less ordered and less uniform, which may arise from the limited solubility of the polymers. These results suggest the critical role of solubility in the fabrication of devices, and the interchain interactions should be controlled in an appropriate manner.

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.