Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect Transistors

A stretchable and mechanical robust field-effect transistor is essential for soft wearable electronics. To realize stretchable transistors, elastic dielectrics with small current hysteresis, high elasticity, and high dielectric constants are the critical factor for low-voltage-driven devices. Here, we demonstrate the polar elastomer consisting of poly­(vinyli­dene fluoride-hexa­fluoro­propylene) (PVDF-HFP):​poly­(4-vinyl­phenol) (PVP). Owing to the high dielectric constant of PVDF-HFP, the device can be operated under less than 5 V and shows a linear-regime hole mobility as high as 0.199 cm² V^–1 s^–1 without significant current hysteresis. Specifically, the PVDF-HFP:​PVP blends induce the vertical phase separation and significantly reduce current leakage and reduce the crystallization of PVDF segments, which can contribute current hysteresis in the OFET characteristics. All-stretchable OFETs based on these PVDF-HFP:​PVP dielectrics were fabricated. The device can still keep the hole mobility of approximately 0.1 cm²/​(V s) under a low operation voltage of 3 V even as stretched with 80% strain. Finally, we successfully fabricate a low-voltage-driven stretchable transistor. The low voltage operating under strains is the desirable characteristics for soft and comfortable wearable electronics

Biaxially Extended Conjugated Polymers with Thieno[3,2‑<i>b</i>]thiophene Building Block for High Performance Field-Effect Transistor Applications

Biaxially thiophene side chain extended thieno­[3,2-<i>b</i>]­thiophene (TT2T)-based polymers, PTTT2T, P2TTT2T, PTTTT2T, and PTVTTT2T, were synthesized by Stille coupling polymerization with different conjugated moieties of thiophene (T), bithiophene (2T), thieno­[3,2-<i>b</i>]­thiophene (TT), and thiophene–vinylene–thiophene (TVT), respectively. The electronic properties of the prepared polymers could be effectively tuned because the variant π-conjugated building block affected the backbone conformation and the resulted morphology. The morphology of the thin films characterized by atomic force microscopy and grazing incidence X-ray diffraction showed that P2TTT2T and PTVTTT2T thin films possessed a better molecular packing with a nanofiber structure owing to their coplanar backbone. The average field-effect mobilities of PTTT2T, P2TTT2T, PTTTT2T, and PTVTTT2T were 6.7 × 10^–6, 0.36, 2.2 × 10^–3, and 0.64 cm² V^–1 s^–1 (maximum 0.71), respectively, attributed to the coplanarity of polymer skeleton. In addition, the fabricated FET devices showed a high on/off ratio over 10⁷ under ambient for over 3 months, suggesting the excellent environmental stability. The above results demonstrated that the biaxially extended fused thiophene based conjugated polymers could serve as a potential candidate for organic electronic device applications

Synthesis and Characterization of All-Conjugated Graft Copolymers Comprised of n‑Type or p‑Type Backbones and Poly(3-hexylthiophene) Side Chains

All-conjugated graft copolymers containing poly­(3-hexylthiophene) (P3HT) side chains and both of p-type and n-type backbones that are connected with all π-conjugated linkages were synthesized via a two-step method involving the Stille coupling reaction and Kumada catalyst-transfer polycondensation (KCTP). A series of naphthalene diimide copolymers with different compositions of 3-(4′-chloro-3′-tolyl)­thiophene (CTT) units (PNDICTT) were designed as n-type backbones, while the poly­(3-(4′-chloro-3′-tolyl)­thiophene-<i>alt</i>-thiophene) (PCTT) was designed as a p-type backbone which were converted into n-type or p-type macroinitiators, and P3HT side chains were then <i>in situ</i> grafted from the macroinitiators via an externally initiated KCTP at room temperature. By using this newly developed two-step method for the synthesis of all-conjugated graft copolymers, the number of P3HT side chains in the graft copolymers can be simply controlled by varying the composition of the CTT units in PNDICTT. Meanwhile, the chain length of P3HT was controllable by varying the feed molar ratio of the thiophene monomer to CTT unit during the KCTP. The optical and electrochemical properties of the all-conjugated graft copolymers were investigated by UV–vis, cyclic voltammetry (CV), and organic field-effect transistor (OFET) measurements. Moreover, the differential scanning calorimetry (DSC) and grazing incident wide-angle X-ray scattering (GIWAXS) results revealed that there were two distinguished crystalline domains in the thin films of the graft copolymer. The morphology of the graft copolymers was first observed by transmission electron microscopy (TEM), in which there was a microphase separation between the PNDICTT and P3HT domains, and the P3HT domains showed partial nanofibril structures with a width of 10–20 nm

Biaxially Extended Quaterthiophene– and Octithiophene–Vinylene Conjugated Polymers for High Performance Field Effect Transistors and Photovoltaic Cells

We report the synthesis, morphology, and optoelectronic device applications of novel biaxially extended quaterthiophene– (<b>4T</b>−) and octithiophene– (<b>8T</b>−) vinylene conjugated polymers, <b>P4TV</b> and <b>P8TV</b>, synthesized via Stille coupling reactions. <b>P4TV</b> and <b>P8TV</b> exhibited smaller energy band gaps of 1.69 and 1.78 eV than that of parent polythiophenes, respectively, due to the reduced conformation distortion by the vinylene linkage. The HOMO energy levels of <b>P4TV</b> and <b>P8TV</b> were −5.02 and −5.13 eV, respectively, resulting in air stable device performance. The highest field effect hole mobilities of <b>P4TV</b> and <b>P8TV</b> were 0.12 and 0.0018 cm² V^–1 s^–1, respectively, with on/off ratios around 10⁴–10⁵. The higher carrier mobility of <b>P4TV</b> was related to its ordered structure as evidenced from the TEM, AFM, and grazing incidence X-ray diffraction results. The power conversion efficiency (PCE) of the <b>P4TV/</b> PC₇₁BM based photovoltaic cells (PV) under the illumination of AM 1.5G (100 mW/cm²) was 4.04%, which was significantly higher than that of <b>P8TV</b>/PC₇₁BM with 2.69%, due to its superior charge transport ability. However, <b>P8TV</b> had a better environmental stability attributed to its low-lying HOMO energy level. These above results demonstrate that biaxially extended thiophene–vinylene conjugated copolymers could be promising materials for high performance organic electronic device applications

Interplay of Molecular Orientation, Film Formation, and Optoelectronic Properties on Isoindigo- and Thienoisoindigo-Based Copolymers for Organic Field Effect Transistor and Organic Photovoltaic Applications

A systematic study on the effects of heteroarenes on the solid state structure and optoelectronic properties of isoindigo analogues, namely, PBDT-IIG and PBDT-TIIG, used in solution-processed organic field effect transistors (OFETs) and organic photovoltaics (OPVs) is reported. We discover that the optical absorption, frontier orbitals, backbone coplanarity, molecular orientation, solubility, film morphology, charge carrier mobility, and solar cell performance are critically influenced by the heteroarenes in the acceptor subunits. PBDT-IIG exhibits good p-type OFET performance with mobility up to 1.03 × 10^–1 cm² V^–1 s^–1, whereas PBDT-TIIG displays ambipolar mobilities of μ_h = 7.06 × 10^–2 cm² V^–1 s^–1 and μ_e = 2.81 × 10^–4 cm² V^–1 s^–1. PBDT-IIG and PBDT-TIIG blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) yield promising power conversion efficiencies (PCEs) of 5.86% and 2.55%, respectively. The excellent mobility of PBDT-IIG can be attributable to the growing fraction of edge-on packing by the interfacial surface treatment. Although PBDT-TIIG could construct a long-range face-on packing alignment to meliorate its photocurrent in OPV applications, the low open-circuit voltage caused by its high-lying HOMO energy level and greater recombination demonstrates the trade-off between light absorption and solar cell performance. Nevertheless, PBDT-TIIG with a PCE of 2.55% is the highest reported PCE to date for the TIIG-based systems

A Rapid and Facile Soft Contact Lamination Method: Evaluation of Polymer Semiconductors for Stretchable Transistors

Organic stretchable electronics have attracted extensive scientific and industrial interest because they can be stretched, twisted, or compressed, enabling the next-generation of organic electronics for human/machine interfaces. These electronic devices have already been described for applications such as field-effect transistors, photovoltaics, light-emitting diodes, and sensors. High-performance stretchable electronics, however, currently still involve complicated processing steps to integrate the substrates, semiconductors, and electrodes for effective performance. Herein, we describe a facile method to efficiently identify suitable semiconducting polymers for organic stretchable transistors using soft contact lamination. In our method, the various polymers investigated are first transferred on an elastomeric poly­(dimethylsiloxane) (PDMS) slab and subsequently stretched (up to 100%) along with the PDMS. The polymer/PDMS matrix is then laminated on source/drain electrode-deposited Si substrates equipped with a PDMS dielectric layer. Using this device configuration, the polymer semiconductors can be repeatedly interrogated with laminate/delaminate cycles under different amounts of tensile strain. From our obtained electrical characteristics, e.g., mobility, drain current, and on/off ratio, the strain limitation of semiconductors can be derived. With a facile soft contact lamination testing approach, we can thus rapidly identify potential candidates of semiconducting polymers for stretchable electronics