Flexible electronic circuits are an essential prerequisite for the development of rollable displays, conformable sensors, biodegradable electronics and other applications with unconventional form factors. The smallest radius into which a circuit can be bent is typically several millimetres, limited by strain-induced damage to the active circuit elements. Bending-induced damage can be avoided by placing the circuit elements on rigid islands connected by stretchable wires, but the presence of rigid areas within the substrate plane limits the bending radius. Here we demonstrate organic transistors and complementary circuits that continue to operate without degradation while being folded into a radius of 100 μm. This enormous flexibility and bending stability is enabled by a very thin plastic substrate (12.5 μm), an atomically smooth planarization coating and a hybrid encapsulation stack that places the transistors in the neutral strain position. We demonstrate a potential application as a catheter with a sheet of transistors and sensors wrapped around it that enables the spatially resolved measurement of physical or chemical properties inside long, narrow tubes.

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Flexible organic transistors and circuits with extreme bending stability

Over the past 20 years, organic transistors have developed from a laboratory curiosity to a commercially viable technology. This critical review provides a short summary of several important aspects of organic transistors, including materials, microstructure, carrier transport, manufacturing, electrical properties, and performance limitations (200 references).

Organic thin-film transistors.

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field-effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge-carrier mobilities exceeding 1 cm(2) V(-1) s(-1) have been achieved. The most widely investigated molecules due to their n-channel operation are perylene and naphthalene diimides, for which even values close to 10 cm(2) V(-1) s(-1) have been demonstrated. The fact that all of these π-conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo-)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.

Organic Semiconductors based on Dyes and Color Pigments

Organic thin-fi lm transistors (OTFTs) have attracted great interest for their potential use in several electronic applications, including active-matrix displays, electronic paper, and chemical sensors. [ 1 ] Among the many semiconducting materials evaluated in the TFT confi guration, pentacene is the best as it shows the highest fi eld-effect mobility ( > 3.0 cm 2 V − 1 s − 1 ). [ 2 , 3 ] On the other hand, new organic semiconductors are being actively investigated to further improve the performance of OTFTs. In fact, superior unconventional organic semiconductors used to produce high-performance OTFTs have been recently reported, including picene (3.2 cm 2 V − 1 s − 1 ), [ 4 ] trans -1,2-dithieno[2,3b ′ :3,2d ′ ]thiophene ethene (DTTE, 2.0 cm 2 V − 1 s − 1 ), [ 5 ] dithieno[2,3d ;2 ′ ,3 ′ d ′ ]benzo[1,2b ;4,5b ′ ]dithiophene (DTBDT, 1.7 cm 2 V − 1

Alkylated dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophenes (Cn-DNTTs): Organic semiconductors for high-performance thin-film transistors

Research into organic field effect transistors (OFETs) has made significant advances—both scientifically and technologically—during the last decade, and the first products will soon enter the market. Printed electronic circuits using organic resistors, diodes and transistors may become cheap alternatives to silicon-based systems, especially in large-area applications. A key parameter for device operation, besides long term stability, is the reproducibility of the current–voltage behavior, which may be affected by hysteresis phenomena. Hysteresis effects are often observed in organic transistors during sweeps of the gate voltage (V
 GS). This hysteresis can originate in various ways, but comparative scientific investigations are rare and a comprehensive picture of “hysteresis phenomena” in OFETs is still missing. This review provides an overview of the physical effects that cause hysteresis and discusses the importance of such effects in OFETs in a comparative manner.

Current versus gate voltage hysteresis in organic field effect transistors

High-performance pentacene (μsat=6.3 cm2/V s) and poly(3-hexylthiophene) (μsat=0.43 cm2/V s) field effect transistors have been realized on flexible substrate with low operating voltage (<−5 V), utilizing a trilayer sol-gel silica gate dielectric. Furthermore, the permittivity of the dielectric was tuned from ∼7 to ∼10 by varying plasma treatments, allowing the study of charge carrier mobility variation with k. A 65% reduction in the saturation mobility of the devices was observed when k increases, suggesting that the energetic disorder at the interface between the active layer and the dielectric can be modulated by the high polarizability of the bulk dielectric.

The effect of dielectric constant on device mobilities of high-performance, flexible organic field effect transistors

A method for fabricating a sol-gel film composition for use in a thin film transistor is disclosed. The method BB includes fabricating the sol-gel dielectric composition by solution processing at a temperature in the range 60° C. to 225° C. The sol-gel film made by the method, and an organic thin-film Si wafer Si wafer transistor incorporating the sol-gel film are also disclosed.

Solution-processed inorganic films for organic thin film transistors

Intermetallic compounds (IMCs) are formed as a result of interaction between solder and metallization to form joints in electronic packaging. These joints provide mechanical and electrical contacts between components. The knowledge of fracture strength of the IMCs will facilitate predicting the overall joint property, as it is more disposed to failure at the joint compared to the solder because of its brittle characteristics. The salient feature of this paper is the measurement of the fracture toughness and the critical energy-release rate of Cu3Sn and Cu6Sn5 intermetallic thin films, which is the result of the interaction between Sn from the solder and Cu from the metallization. To achieve the objective, a controlled buckling test was used. A buckling test in the current work refers to one that displays large transverse displacement caused by axial compressive loading on a slender beam. The stress and strain along the beam can be easily calculated by the applied displacement. Fracture-toughness values of Cu3Sn and Cu6Sn5 are 2.85 MPa √m ± 0.17 MPa √m and 2.36 MPa √m ± 0.15 MPa √m, respectively. Corresponding critical energy-release rate values are 65.5 J/m2 ± 8.0 J/m2 and 55.9 J/m2 ± 7.3 J/m2, respectively. The values obtained were much higher than the ones measured in bulk intermetallic samples but correlated well with those values obtained from conventional fracture-toughness specimens when fracture was confined within the intermetallic layers. Hence, the controlled buckling test is a promising fast and effective way to elucidate mechanical properties of thin films.

https://dr.ntu.edu.sg/bitstream/10356/95014/1/39.%20Fracture%20Toughness%20of%20Cu-Sn%20Intermetallic%20Thin%20Films.pdf

Fracture toughness of Cu-Sn intermetallic thin films

High performance organic field effect transistors using a solution-processable processed trilayer sol-gel silica gate dielectric architecture fabricated on plastic substrates exhibited low driving voltages of −3.0V, high saturation mobilities of ∼3.5cm2∕Vs, and on-off current ratio of 105. The enhancement in field effect mobility is attributed to improved dielectric-semiconductor interfacial morphology and increased capacitance of the tristratal dielectric. The pentacene devices displayed no signs of electrical degradation upon bending through a bending radius of 24mm, 2.27% strain. The slight increase in the drain currents upon bending strain was investigated using Raman spectroscopy, which revealed enhanced in-phase intermolecular coupling.

Solution-processed trilayer inorganic dielectric for high performance flexible organic field effect transistors

The electret induced hysteresis was studied in sol-gel silica films that result in higher drain currents and improved device performance in pentacene field-effect transistors. Vacuum and ambient condition studies of the hysteresis behavior and capacitance-voltage characteristics on single layer and varying thicknesses of bilayer dielectrics confirmed that blocking layers of thermal oxide could effectively eliminate the electret induced hysteresis, and that thin (25nm) sol-gel silica dielectrics enabled elimination of nanopores thus realizing stable device characteristics under ambient conditions.

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T. Cahyadi

Papers

The effect of dielectric constant on device mobilities of high-performance, flexible organic field effect transistors

Solution-processed inorganic films for organic thin film transistors

Fracture toughness of Cu-Sn intermetallic thin films

Solution-processed trilayer inorganic dielectric for high performance flexible organic field effect transistors

Electret mechanism, hysteresis, and ambient performance of sol-gel silica gate dielectrics in pentacene field-effect transistors