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

Thiophene and Selenophene Donor–Acceptor Polyimides as Polymer Electrets for Nonvolatile Transistor Memory Devices

We report the nonvolatile memory characteristics of n-type <i><i>N,N</i></i>′-bis­(2-phenylethyl)­perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) based organic field-effect transistors (OFET) using the polyimide electrets of poly­[2,5-bis­(4-aminophenylenesulfanyl)­selenophene–hexafluoroisopropylidenediphthalimide] (PI­(APSP-6FDA)), poly­[2,5-bis­(4-aminophenylenesulfanyl)­thiophene–hexafluoroisopropylidenediphthalimide] (PI­(APST-6FDA)), and poly­(4,4′-oxidianiline-4,4′-hexafluoroisopropylidenediphthalic anhydride) (PI­(ODA-6FDA)). Among those polymer electrets, the OFET memory device based on PI­(APSP-6FDA) with a strong electron-rich selenophene moiety exhibited the highest field-effect mobility and I_on/I_off current ratio of 10⁵ due to the formation of the large grain size of the BPE-PTCDI film. Furthermore, the device with PI­(APSP-6FDA) exhibited the largest memory window of 63 V because the highest HOMO energy level and largest electric filed facilitated the charges transferring from BPE-PTCDI and trapping in the PI electret. Moreover, the charge transfer from BPE-PTCDI to the PI­(APSP-6FDA) or PI­(APST-6FDA) electrets was more efficient than that of PI­(ODA-6FDA) due to the electron-donating heterocyclic ring. The nanowire device with PI­(APSP-6FDA) showed a relatively larger memory window of 82 V, compared to the thin film device. The present study suggested that the donor–acceptor polyimide electrets could enhance the capabilities for transferring and store the charges and have potential applications for advanced OFET memory devices

Synthesis of Oligosaccharide-Based Block Copolymers with Pendent π‑Conjugated Oligofluorene Moieties and Their Electrical Device Applications

We report the synthesis and electric device applications of oligosaccharide-based diblock copolymers consisting of a maltoheptaose (MH) block and a poly­(4-oligofluorenyl­styrene) block (PStFl_<i>n</i>, <i>n</i> = 1 or 2), referred to as MH-<i>b</i>-PStFl_<i>n</i>. MH-<i>b</i>-PStFl_<i>n</i> was prepared by the Cu­(I)-catalyzed click reaction of azido-terminated PStFl_<i>n</i> (PStFl_<i>n</i>-N₃), which was obtained from the azidation reaction of the bromo-terminated PStFl_<i>n</i> (PStFl_<i>n</i>-Br), with excess ethynyl-terminated MH in the THF/DMF mixture solvent. The resulting diblock copolymers self-assembled to spherical microdomains with sub-10 nm sizes in both bulk and thin film state after annealing process. Thereafter, the MH-<i>b</i>-PStFl_<i>n</i> thin film (∼50 nm) with the self-assembled nanoscale spherical aggregates was used as the charge storage layer for the pentacene-based field-effect transistor type memory devices. The MH-<i>b</i>-PStFl_<i>n</i>-based devices had the excellent hole mobility (0.25–0.52 cm² V^–1 s^–1) and the high ON/OFF current (<i>I</i>_ON/<i>I</i>_OFF) ratio of 10⁷–10⁸, of which the MH-<i>b</i>-PStFl₁-based one had the higher mobility than that of the MH-<i>b</i>-PStFl₂-based one because the pentacene crystal in the former device possessed the larger grain size and fewer boundaries. On the other hand, the MH-<i>b</i>-PStFl₂-based device showed a larger memory window than the MH-<i>b</i>-PStFl₁-based one because the stronger electron-donating effect of the difluorenyl group in MH-<i>b</i>-PStFl₂ increased the charge storage capability of its related device. All the memory devices showed a long-term retention time over 10⁴ s with the high <i>I</i>_ON/<i>I</i>_OFF ratio of 10⁶–10⁸. Among these devices, the MH-<i>b</i>-PStFl₁-based device showed a good WRER endurance over 180 cycles. This work not only demonstrates the tunable electrical memory characteristics by adjusting the π-conjugation length of the oligofluorenyl side chain in the polymer electret but also provides a promising approach for developing the next-generation “green electronics” using natural materials

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

Side-Chain Engineering of Isoindigo-Containing Conjugated Polymers Using Polystyrene for High-Performance Bulk Heterojunction Solar Cells

Developing organic photovoltaic systems that possess high efficiency, high reproducibility, and low cost remains a topic of keen investigation. From a molecular design perspective, developing a “multicomponent” copolymerization synthetic approach could potentially afford macromolecular materials encompassing all of the aforementioned desired parameters. Herein, we describe the synthesis of a series of poly­(isoindigo-dithiophene)-based conjugated polymers with varying amounts of low molecular weight polystyrene (PS) side chains (<i>M</i>_n = 1300 g/mol) via random copolymerization. We observed better solubility with polymers containing the PS side chains (when compared to their non-PS-side-chain counterparts), hence leading to better batch-to-batch reproducibility in terms of molecular weights. Furthermore, the PS-side-chain-decorated copolymers also demonstrated better thin film processability, without affecting the electronic and optical properties, when the molar percentage of the PS-containing repeating units were ≤10%. Bulk heterojunction solar cell devices fabricated with these PS-containing copolymers demonstrated significantly improved performances [maximum power conversion efficiencies (PCE) > 7% and open circuit voltages (<i>V</i>_OC) ≥ 0.95 V], compared to the highest reported performance (PCE = 6.3% and <i>V</i>_OC = 0.70) based on similar isoindigo-containing polymers. Taken together, the synthesis, processing, and device performances of PS-containing copolymers represent a new approach in molecular engineering to achieve a balance between the optical/electronic properties and solubility/processability of reproducible polymeric systems

Atmospheric Pressure Plasma Jet-Assisted Synthesis of Zeolite-Based Low‑<i>k</i> Thin Films

Zeolites are ideal low-dielectric constant (low-<i>k</i>) materials. This paper reports on a novel plasma-assisted approach to the synthesis of low-<i>k</i> thin films comprising pure-silica zeolite MFI. The proposed method involves treating the aged solution using an atmospheric pressure plasma jet (APPJ). The high reactivity of the resulting nitrogen plasma helps to produce zeolite crystals with high crystallinity and uniform crystal size distribution. The APPJ treatment also remarkably reduces the time for hydrothermal reaction. The zeolite MFI suspensions synthesized with the APPJ treatment are used for the wet deposition to form thin films. The deposited zeolite thin films possessed dense morphology and high crystallinity, which overcome the trade-off between crystallinity and film quality. Zeolite thin films synthesized using the proposed APPJ treatment achieve low leakage current (on the order of 10^–8 A/cm²) and high Young’s modulus (12 GPa), outperforming the control sample synthesized without plasma treatment. The dielectric constant of our zeolite thin films was as low as 1.41. The overall performance of the low-<i>k</i> thin films synthesized with the APPJ treatment far exceed existing low-<i>k</i> films comprising pure-silica MFI

Conjugated Polymer-Mediated Polymorphism of a High Performance, Small-Molecule Organic Semiconductor with Tuned Intermolecular Interactions, Enhanced Long-Range Order, and Charge Transport

We use 6,13-bis­(triisopropylsilylethynyl)­pentacene as a model small molecule organic semiconductor and two conjugated polymer additives to demonstrate conjugated polymer-mediated polymorphism of a small molecule organic semiconductor for the first time. The conjugated polymer additives, used with a slow solution crystallization approach, yield crystal structures that are not accessible by nonconjugated polymer additives and impart excellent long-range order. In both of the small molecule semiconductor/conjugated polymer blends studied here, previously unreported polymorphs of a small molecule semiconductor have been identified which also leads to improved charge transport in the absence of external alignment. These results open up a new exciting avenue to manipulate unit cell structure, long-range order, and charge transport of high performance, solution-processed, small molecule organic semiconductors

Understanding Polymorphism in Organic Semiconductor Thin Films through Nanoconfinement

Understanding crystal polymorphism is a long-standing challenge relevant to many fields, such as pharmaceuticals, organic semiconductors, pigments, food, and explosives. Controlling polymorphism of organic semiconductors (OSCs) in thin films is particularly important given that such films form the active layer in most organic electronics devices and that dramatic changes in the electronic properties can be induced even by small changes in the molecular packing. However, there are very few polymorphic OSCs for which the structure–property relationships have been elucidated so far. The major challenges lie in the transient nature of metastable forms and the preparation of phase-pure, highly crystalline thin films for resolving the crystal structures and evaluating the charge transport properties. Here we demonstrate that the nanoconfinement effect combined with the flow-enhanced crystal engineering technique is a powerful and likely material-agnostic method to identify existing polymorphs in OSC materials and to prepare the individual pure forms in thin films at ambient conditions. With this method we prepared high quality crystal polymorphs and resolved crystal structures of 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS-pentacene), including a new polymorph discovered via in situ grazing incidence X-ray diffraction and confirmed by molecular mechanic simulations. We further correlated molecular packing with charge transport properties using quantum chemical calculations and charge carrier mobility measurements. In addition, we applied our methodology to a [1]­benzothieno­[3,2-<i>b</i>]­[1]­1benzothiophene (BTBT) derivative and successfully stabilized its metastable form