A Novel Photo Cross-Linkable Polymer Insulator for Printed OTFTs with Surface Selective Deposition By Vacuum-Ultraviolet Irradiation
Authors: Oku Shinya , Lee Junghwi , Katagiri Fumiaki , Fukuda Takashi , Yamakawa Hiroshi , Tanabiki Masao 
Published: February 2020

To realize printed manufacturing of electronic devices, including flexible displays, sensors, and integrated circuits that incorporate organic thin-film transistors (OTFT), developing and improving the materials and technologies used in this process is required.

Solution processable organic semiconductors, insulating materials, and high-resolution patterning techniques of printed electrodes are all critical components of these electronic devices. The surface properties of insulators are particularly important because they contribute significantly to carrier transport at the interface of the organic semiconductors and affect the patterning of the printed electrode. Insulating materials that are solution-processable typically require high temperatures (∼150 °C) and long curing times during fabrication, which is undesirable because these conditions can cause significant distortion of common flexible substrates, such as polyethylene naphthalate. Thermal deformation of a flexible substrate prevents high-resolution patterning and the accurate alignment of the different layers within a device, and these problems become more significant as the size of the substrate increases.

To overcome some of these challenges, we have recently developed a novel styrene-based polymer (PC200) for use as an insulating material that can be cured quickly via photo cross-linking. This material can be produced under atmospheric conditions, does not require heat treatment, is highly electrically insulating and has a low dielectric constant. Additionally, the surface of the PC200 insulating film can be transformed into a hydrophilic, wettable surface by selective irradiation using vacuum ultraviolet (VUV) radiation via a photomask. This allows the formation very fine printed electrodes using a metal nanoparticle ink. VUV exposure in air generates oxygen radicals and ozone molecules, which cause chemical bond dissociation on the polymer surface and render the region hydrophilic, resulting in patterned surfaces that are wettable. Compared with perfluoropolymers or parylene insulating films that have been reported recently, surface modification of the PC200 insulating film is relatively simple and requires less VUV irradiation.

In this work, we have examined the electrical insulating properties of the PC200 insulator film and incorporated it into a short-channel OTFT with the recently developed soluble semiconductor (dithieno[2,3-d:2’,3’- d’]benzo[1,2-b:4,5-b’]dithiophene, DTBDT). The PC200 film was formed by spin-coating and subsequent UV exposure without heat treatment. The insulator film exhibited low surface roughness (RMS 0.5 nm), high electrical insulation (>3 MV/cm) and a low dielectric constant (εr=2.7). Additionally, we were able to form fine patterned electrodes on the polymer insulator with a minimum channel length of 5 μm using a VUV patterning technique. Importantly, these fine patterns (<10 μm) were formed without requiring a photoresist and wet-etching techniques, unlike conventional methods, such as photolithography. We fabricated short-channel OTFTs using the PC200 film as the patterning layer for printed Ag electrodes and DTBDT as the active layer. The OTFTs exhibited pronounced saturation curves with a steep onset and a sub-threshold swing of 0.3 V/dec, a low threshold voltage of −0.8 V, negligible hysteresis, a hole mobility of 0.5 cm2/Vs and a very large I
on/I
off ratio above 107. These results demonstrated that the PC200 insulator shows great promise for realizing a variety of flexible and printable electrical circuits for use in displays and sensors.