Development of Strain-Induced Organic Single-Crystalline Transistors

Organic semiconductors (OSCs) are known as flexible, solution processable, light-weight, and potentially low-cost materials which are applied in many electronic devices. There is considerable interest in developments in organic thin-film transistors (OTFTs) for applications such as flexible displays, sensors, and IC tags. Rapid developments in synthetic chemistry and device engineering allow us to explore high performance devices. However, there is limited study to improve static (DC) and dynamic (AC) performances of organic devices without employing synthesis and complicated device integration.<br/>A strain is used to control the electronic structure. Particularly in the case of single-crystalline OSCs, it has been demonstrated that a slight compressive lattice strain can modify the electronic structure via uniformly modified crystal structure, and enhances career mobility[1][2]. However, the method to apply strain into OSCs has been limited; bending substrate has been only the way to systematically control the strain in OSCs. Therefore, a novel method to introduce a persistently strain into OSCs particularly on a planar substrate will be highly desired.<br/>Here, we establish a general method to persistently induce uniaxial strain into single-crystalline OTFTs to enhance the performance of OTFTs. OTFTs with single-crystalline films of 3,11-dioctyldinaphtho[2,3-d:2,3-d]benzo[1,2-b:4,5b]dithiophene (C₈–DNBDT–NW) as the active layer were fabricated on 15 μm thick polyimide (PI) substrate. To induce uniaxial compressive strain into OTFTs semi-permanently in a planar substrate, we developed following new method. First, OTFTs fabricated on PI substrate were adhered onto a convexly bent thick mother substrate (polyethylene terephthalate) by using cyanoacrylate adhesive. The OTFTs adhered on the extended surface of the bent mother substrate were compressed semi-permanently when the mother substrate is recovered in a flat state. In this method, the surface strain <i>ε</i> of the bent film was calculated by the following equation : <i>|ε| = h</i> / 2(<i>R</i> + <i>h</i>), where <i>h</i> is the thickness of mother substrate and <i>R</i> is the curvature radius of bent substrate.<br/>By applying this method, the modulation of career mobility by the introduction of uniaxial compression is observed. By changing the curvature of mother substrate, the ratio of compressive strain into OTFT changes accordingly. In this measurement, the maximum compressive strain induced into OTFTs is evaluated to be 3.1% along <i>c</i>-axis. The intrinsic mobility increases monotonically by inducing compressive strain by a factor of 156% when compressive strain reaches 3.1%. This large enhancement of intrinsic mobility agrees quantitatively with our previous study[1], and originates from the suppression of molecular vibration, rather than from the effect of effective mass; a reduction of lattice constant of C₈–DNBDT–NW directly restricts the molecular vibration that contributes particular to electron-phonon interaction.<br/>The enhancement of dynamic performances of OTFTs by the strain is also observed consistently in strain-induced OTFTs. Cutoff frequency (<i>f</i>_T), the parameter which determines operation speed of OTFTs, improves by a factor of 140% when 1.0% compressive strain is induced into OTFTs. Theoretically, cutoff frequency is proportional to the career mobility of OSCs. Therefore, this result indicates that enhancement of career mobility by the introduction of strain also improves dynamic properties of OTFTs. From these results, strain-induced OTFTs showed high career mobility and improved cutoff frequency. Inducing even larger strain is expected to further improve both static and dynamic performances of OTFTs. This simple , but versatile method will open opportunities for further developments in high frequency operation of OTFTs.<br/>[1] T. Kubo, J. Takeya <i>et al</i>., <i>Nat. Commun</i>.<b> 7</b>, 11156 (2016).<br/>[2] H. Choi, V. Podzorov <i>et al</i>., <i>Adv</i>. <i>Sci</i>. <b>7</b>, 1901824 (2020).<br/>[3] J. Soeda, J. Takeya <i>et al</i>., <i>Appl</i>. <i>Phys</i>. <i>Express</i>, <b>6</b>, 076503 (2013).