Highly Stretchable Transistors Using a Microcracked Organic Semiconductor

Abstract

Organic transistors with elastic conductors and dielectrics can be stretched up to 250% strain while maintaining the transistor characteristics. Strain-independent properties can be achieved after an initial “programming” cycle that causes the formation of microcracks in the semiconductor. The change in mobility with strain follows the same trend in different stretching directions. Liberating electronic devices from the confines of traditional rigid substrates can improve mechanical robustness and enable new applications and manufacturing methods. Stretchability facilitates electronics that can be mounted on unconventional substrates,1 such as lenses and human bodies,2 and allows dynamic tuning of devices such as electronic eye cameras3 and lasers.4 Accommodating complex movements of supporting structures facilitates integration with moving entities and is critical for biointerfacing applications5 and electronic skins6-10 for prosthetics and robotics. Arrays of electronic devices often include transistors as active addressing elements in order to improve the signal collection process.5, 8 Furthermore, many applications, including sensor arrays6, 11 and displays,12 require large area coverage and therefore benefit from low-cost, high-throughput fabrication methods. In this communication, we report a stretchable organic transistor that maintains transistor behavior to >250% strain, which is several times larger than previous reports.13-16 Strain-independent characteristics are achieved by “programming” the device with an initial strain that causes the formation of microcracks in the semiconductor layer. Crack formation accommodates strain, while maintaining a percolating pathway. Similar microcracking17, 18 or void formation19 strategies have been employed successfully in stretchable conductors used in applications such as strain sensors19 and neuroprosthetic devices.20 The fabrication process involves cost efficient solution methods including spraycoating and spincoating. Stretchable electronics can be fabricated using two main methods. The first involves geometrical patterning of conventional electronic materials such as metals and inorganic semiconductors into meandering patterns or buckles in order to reduce deformation in the active material.21-24 Stiff islands connected with stretchable conductors have produced arrays of high-performance devices including transistors,25 photodetectors,3 and LEDs.12 However, because stretchability is imparted by the regions between islands, there is a trade-off between device density and stretchability. Buckling involves the formation of wavy structures that flatten out to accommodate applied strain. The optical properties of these wavy structures could benefit the performance of stretchable solar cells,2, 24 but may be undesirable for other optoelectronic applications,26 or for devices that require planar interfaces. The second method of imparting stretchability is to fabricate devices composed of elastic materials. Because elastomers are typically insulators, electronic functionality is often imparted by blending with electronic materials12, 26 or applying thin films of compliant electronic materials.15 Several intrinsically stretchable electronic devices have been reported, including stretchable light emitters based on electrochemical active layers26, 27 and graphene-13 and MoS2-based14 transistors that stretch to 5%. A hybrid method was reported by Chae et al. in which an intrinsically stretchable graphene-based gate electrode was combined with a buckled inorganic dielectric layer to make high-performance transistors that could sustain repeated strain cycles to 20%.15 Inorganic semiconductors with proper fabrication schemes and device design can provide exceptional performance in stretchable electronic devices,3, 28 but their high processing costs limit implementation in applications where devices need to be disposable, cheap, or cover large areas. Organic semiconductors (OS) are an alt