Zupan Mao

Flexible, Low‐Voltage and High‐Performance Polymer Thin‐Film Transistors and Their Application in Photo/Thermal Detectors

X. T. Liu, Prof. Y. Q. Ma College of Science China Agricultural University Beijing 100193 , P. R. China E-mail: mayongqiang@cau.edu.cn Dr. Y. L. Guo, Dr. H. J. Chen, Z. P. Mao, H. L. Wang, Prof. G. Yu, Prof. Y. Q. Liu Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190 , P. R. China E-mail: guoyunlong@iccas.ac.cn; yugui@iccas.ac.cn; liuyq@iccas.ac.cn

Controlled Growth of Single‐Crystal Twelve‐Pointed Graphene Grains on a Liquid Cu Surface

The controlled fabrication of single-crystal twelve-pointed graphene grains is demonstrated for the first time by ambient pressure chemical vapor deposition on a liquid Cu surface. An edge-diffusion limited mechanism is proposed. The highly controllable growth of twelve-pointed graphene grains presents an intriguing case for the fundamental study of graphene growth and should exhibit wide applications in graphene-based electronics.

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.

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.

Active Morphology Control for Concomitant Long Distance Spin Transport and Photoresponse in a Single Organic Device.

Long distance spin transport and photoresponse are demonstrated in a single F16 CuPc spin valve. By introducing a low-temperature strategy for controlling the morphology of the organic layer during the fabrication of a molecular spin valve, a large spin-diffusion length up to 180 nm is achieved at room temperature. Magnetoresistive and photoresponsive signals are simultaneously observed even in an air atmosphere.

Synthesis, characterization, and field-effect transistor properties of tetrathienoanthracene-based copolymers using a two-dimensional π-conjugation extension strategy: a potential building block for high-mobility polymer semiconductors

In most instances, modulation of the π-conjugation length in polymer molecules is obtained through a linear (1D) extension of the polymer backbone. To date, very limited studies have been reported about the effect of the two-dimensional (2D) π-conjugation extension on the charge-transporting properties of polymer semiconductors. In this study, a 2D π-extended heteroacene, alkyl-substituted tetrathienoanthracene (TTB) moiety, is used to design and synthesize a class of novel polymer semiconductors for solution-processable organic field-effect transistor (OFET) applications for the first time. Three novel TTB-based alternating copolymers (PTTB-2T, PTTB-TT, and PTTB-BZ) are synthesized via Pd(0)-catalyzed Stille or Suzuki cross-coupling reactions, affording high weight-average molecular weights of 61.1–78.5 kDa. The thermal stabilities, optical properties, and energy levels, and charge transport properties of the three TTB-based alternating copolymers have been successfully tuned by copolymerization with bithiophene (2T), thienothiophene (TT), and benzothiadiazole (BZ) derivatives. The results indicate that, even with their highly extended π-framework, all the TTB-containing polymers show good solubility in most common solvents and fairly good environmental stability of their highest occupied molecular orbitals (HOMOs) ranging from −5.15 to −5.28 eV. Among the three TTB-based alternating copolymers, the PTTB-BZ thin film exhibits the best OFET performance with the highest hole mobility of 0.15 cm2 V−1 s−1 in ambient air. It can be attributed to the combinations of highly coplanar polymer backbones and strong D–A interactions between TTB donor units and BZ acceptor units, therefore leading to a compact solid-state packing, uniform fiber-like morphology, and a large polycrystalline grain associated with high hole mobility. These results demonstrate that our molecular design strategy for high-performance polymer semiconductors is highly promising.

Alkylphenyl Substituted Naphthodithiophene: A New Building Unit with Conjugated Side Chains for Semiconducting Materials

In this article, a versatile 2-D conjugated polymer, PNDTP-DPP, containing alkylphenyl substituted naphthodithiophene is synthesized and characterized. PNDTP-DPP exhibits good solubility and crystallinity with a π-π stacking distance of ≈3.7 Å. Investigation of polymer solar cells (PSCs) and organic field-effect transistors (OFET) demonstrates a promising power conversion efficiency (PCE) of 4.11% and a high hole mobility of up to 0.86 cm(2) V(-1) s(-1) , so this is one of the few examples of versatile polymers that show both good field-effect mobility and PCE.

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.