Polymer dielectric materials with hydroxyl functionalities such as poly(4-vinylphenol) and poly(vinyl alcohol) have been utilized widely in organic thin-film transistors (OTFTs) because of their excellent insulating performance gained by hydroxyl-mediated cross-linking. However, the polar hydroxyl functionality also deleteriously affects the performance of OTFTs and significantly impairs the device stability. In this study, a sub-20 nm, high-k copolymer dielectric with hydroxyl functionality, poly(2-hydroxyethyl acrylate-co-di(ethylene glycol) divinyl ether), was synthesized in the vapor phase via initiated chemical vapor deposition. The inherently dry environment offered by the vaporphase polymer synthesis prompted the snuggling of polar hydroxyl functionalities into the bulk polymer film to form a molecular thin hydrophobic skin layer at its surface, verified by near-edge X-ray absorption fine structure analysis. The chemical composition of the copolymer dielectric was optimized systematically to achieve high dielectric constant (k ≈ 6.2) as well as extremely low leakage current densities (less than 3 × 10 −8 A/cm 2 in the range of ±2 MV/cm) even with sub-20 nm thickness, leading to one of the highest capacitance (higher than 300 nF/cm 2 ) achieved by a single polymer dielectric to date. Exploiting the structural advantage of the cross-linked high-k polymer dielectric, high-performance OTFTs were obtained. Notably, the spontaneously formed molecular thin, hydrophobic skin layer in the copolymer film substantially suppressed the hysteresis in the transistor operation. The trap analysis also suggested the formation of bulk trap with a high energy barrier and sufficiently low trap densities at the semiconductor/dielectric interface, owing to the surface skin layer. Furthermore, the OTFTs with the −OH-containing copolymer dielectric showed an unprecedentedly excellent operational stability. No apparent OTFT degradation was observed up to 50 000 s of high constant voltage stress (corresponding to the applied electric field of 1.4 MV/ cm) because of the markedly suppressed interfacial trap density by the hydrophobic skin layer, together with the current compensation by the bulk hydroxyl functionalities. We believe that the surface modification-free, one-step polymer dielectric synthetic strategy will provide a new insight into the design of polymer dielectric materials for high-performance, low-power soft electronic devices with high operational stability.
A new fabrication method for an ultrathin (500 nm thick) pressure-sensitive adhesive (PSA) was demonstrated by utilizing a series of in situ cross-linked viscoelastic copolymer films. Viscoelastic films composed of poly(2-hydroxyethyl acrylate- co-2-ethylhexyl acrylate) were synthesized successfully in a one-step manner by an initiated chemical vapor deposition (iCVD) process, where free-radical polymerization is triggered in the vapor phase either by heat or UV, or a combination of both. In particular, the photoinitiated chemical vapor deposition method generated a highly cross-linked polymer film, whereas cross-linking of the copolymer film was suppressed greatly in the conventional thermal iCVD method. A combination of thermal and photoinitiated chemical vapor deposition could regulate the cross-linking density of the copolymer films. We controlled the cross-linking density of the copolymer films to exhibit a viscoelastic property so that they would readily adhere to various kinds of substrates with only 500 nm thick copolymer PSA. The adhesion performance of the PSA was systematically optimized by tuning the copolymer composition as well as the cross-linking density, and consequently a high shear strength of more than 85.2 ± 5 N/cm was achieved despite the 500 nm thickness. In addition, the PSA was completely transparent. We expect that the ultrathin PSAs developed in this work will be utilized widely for the realization of various soft electronic devices, which usually require strong adhesion, tunable viscoelastic properties, and optical transparency.
High dielectric constant (k) and excellent insulating performance together with the thickness down‐scalability are essential requirements for polymer dielectrics to realize the stable operation of flexible, low‐power electronics. Crosslinking has been applied frequently to the dielectric polymer matrix to enhance the insulating performance. However, the addition of crosslinker into the polymer has been often accompanied by the reduction of the dielectric constant thereof. Herein, a series of copolymer dielectrics is synthesized composed of two monomers of 2‐cyanoethyl acrylate (CEA) possessing a highly polar cyanide functional group and 1,4‐butanediol divinyl ether (BDDVE), a crosslinker with relatively short chain length. The chemical composition of the 30‐nm‐thick copolymer dielectrics is optimized to achieve extremely low leakage current (<3.0 × 10−8 A cm−2 in the range of ± 2 MV cm−1) with unprecedentedly high dielectric constant of 7.5, which is, to the knowledge, the highest dielectric constant among the sub‐50 nm polymer dielectrics without inorganic component, reported to date. Exploiting the copolymer dielectric layers, high‐performance, low‐power organic thin‐film transistors (OTFTs) with high operational stability and extreme mechanical flexibility are demonstrated. It is believed that the high‐k dielectric copolymer films presented in this study will be an important guideline to develop future flexible, wearable electronics.
An elastic film with water repellency is of great interest for application to waterproof and antifouling surface treatment of stretchable devices such as wearable electronics or functional textiles. Herein, a nanoscale ultrathin stretchable polymer film endowed with superhydrophobicity was newly designed and synthesized by a vapor phase method, initiated chemical vapor deposition (iCVD). The highly stretchable superhydrophobic polymer film was generated from the copolymerization of 1H,1H,2H,2H-perfluorooctyl acrylate (PFOA) and 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3) to form a cross-linked fluoropolymer network with low crystallinity and elasticity as well as superhydrophobicity. The composition of the copolymer film was adjusted by controlling the flow rates of the input monomers of PFOA and V3D3 during the deposition process. The copolymer film with the optimized composition showed an elastic limit higher than 200% while maintaining the superhydrophobic property with a water contact angle greater than 150°on nonflat substrates. An 800-nm-thick copolymer thin film showed mechanical durability with superhydrophobic performance even after 2000 cycles of a 200% stretch test. The elastic copolymer film also displayed solvent resistance against incubation in various organic solvents for 24 h. Transparency greater than 90% was also confirmed in the whole visible range with large-area uniformity and conformal coverage. We believe the stretchable superhydrophobic polymer film developed in this study is a promising candidate material for passivation of various stretchable device applications.
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