Permissions for Reuse

Flexible all-polymer transistors have emerged as an integral part of flexible circuits and displays, which is a unique area yet to be fully explored for various applications. All of the components of the transistor being polymers immediately suggest solution processing, which is directly linked to low cost fabrication of the device. For the processing to be cost effective, however, it has to be compatible with the requirements of roll-to-roll printing, which is a massive parallel process compared with the serial process of inkjet printing. While studies on all-polymer transistor on nonflexible substrate such as glass are relatively extensive,^1,2,3,4,5,6 there is only a handful of studies reported on all-polymer transistor on flexible substrate.^7,8,9 In general, the device performance of the flexible all-polymer transistor is quite poor compared with that of the nonflexible all-polymer transistor.

Self-aligned pattern formation is always attractive because it eliminates the alignment process. Of particular interest in organic devices is the formation of self-aligned gate pattern. This self-aligned pattern formation has been accomplished by creating a groove, which in turn serves the role of a holder for dispensed liquid, typically by inkjet printing.^10,11 The groove was created by cutting into a metal layer⁹ with sharp-edged silicon mold or by defining a metal gate on polymer pedestal¹⁰ through a series of steps of metal embossing and plasma etching. A recent study involving all-solution processing utilizes prepatterned source/drain of gold particles, as the photomask for alignmentless formation of gate pattern through photolithography and inkjet printing.¹² Formation of self-aligned gate pattern is yet to be reported for all-polymer transistor.

The development of all-polymer or all-solution processed organic transistor has so far centered around inkjet printing for the fabrication.^{1,2,3,4,10,11,12} Formation of self-aligned pattern is needed to hold the liquid dispensed by inkjet printing. A distinction is made here between the formation of self-aligned gate pattern and that of self-aligned gate. In the former, self-aligned pattern is prepared first into which gate is formed by aligning and then dispensing liquid through inkjet printing. In the latter, self-aligned gate itself is formed without any alignment for the gate formation.

In this letter, we propose an entirely different avenue for the fabrication of all-polymer or all-solution processed transistors in the form of “ultraviolet (UV) printing.” This UV printing allows printing of submicron patterns of polymer in a way compatible with roll-to-roll printing. It also allows formation of self-aligned gate formation by a simple contact printing. The electrical properties of the flexible all-polymer transistor are as good as those of nonflexible all-polymer transistor.

The procedure involved in the fabrication of flexible all-polymer transistor with UV printing is illustrated in Fig. 1. For this mold-based printing, poly(dimethylsiloxane) (PDMS) (Sylgard 184, Dow Corning) molds with source-drain pattern were used. The polymer used in this work for the electrodes is poly(3,4-ethylenedioxythiophene)(PEDOT)/poly(styrenesulfonate)(PSS) (Baytron-P, Bayer), henceforth to be denoted simply PEDOT. The water-based PEDOT cannot be coated on PDMS for a continuous film since dewetting on the hydrophobic surface of PDMS leads to droplet formation. Therefore, PDMS surface was subjected to an oxygen plasma to generate hydroxyl groups on the surface. The flexible substrate was in turn spin coated with an UV curable prepolymer liquid¹³ of poly(urethaneacrylate) (PUA, MINS 311RM, Minuta) mixed with a solvent for the UV printing. The PDMS mold with coated PEDOT (100 nm thick) was brought into contact with the substrate coated with MINS (800 nm thick) as shown in Fig. 1(b) without heating or applying pressure. With the PDMS mold in contact, the sample was then blanket exposed to UV (250–360  nm) light for 20 min. It is noted that the exposure time depends on the dose. Upon removing the mold, the PEDOT on the protruding source/drain pattern is transferred to the UV-cured polymer layer on the substrate [Fig. 1(c)], leading to the formation of source and drain electrodes on the flexible substrate. Even though no pressure is applied, wetting and slight rise of the liquid polymer occur due to capillarity when the PDMS mold makes contact with the liquid prepolymer, which results in the formation of a mound, as shown in Fig. 1(c). The formation of this mound in the course of UV printing, which also defines the channel, is the key to carrying out alignment-free printing of gate electrode. When the liquid precursor makes contact with PEDOT on the PDMS mold, the precursor including photocurable monomeric diluent makes intermixed layer where the diluent dissolves PEDOT. Upon curing with UV, the intermixed layer gets solidified into one body, resulting in strong bonding between PEDOT and the cured prepolymer. This bonding enables the pattern transfer. On the substrate with source and drain, poly(3-hexylthiophene) (P3HT, Aldrich) and poly(2-hydroxylethyl methacrylate) (PHEMA, Aldrich) were coated¹⁴ in succession [Fig. 1(d)]. As shown in Fig. 1(e), the flexible substrate with all the polymer layers except for the gate layer is brought into contact with the PEDOT-coated glass for contact printing of PEDOT onto the mound. Removing the substrate results in the formation of self-aligned gate [Fig. 1(f)], completing the fabrication of flexible all-polymer transistor.

Figure 1.

Unlike the usual transfer patterning/printing,^15,16,17 the mold surface is not required to have an antiadhesion layer and no heating or pressure is involved in the UV printing. All that is needed is simple blanket exposure to UV. In the process of the UV printing, a mound corresponding to the channel forms, which enables self-aligned gate formation. In inkjet printing, a groove is created for the formation of gate pattern, which is then used for forming the gate by aligning through inkjet printing. In UV printing, a mound is used for direct formation of gate itself without any alignment.

Shown in Fig. 2(a) is the image by scanning electron microscopy (SEM) of the PEDOT source and drain electrodes formed after the UV printing using a master with the channel length of 10  µm. Note that the lighter part represents PEDOT layer. The enlarged version of the left frame shows the dividing line between the channel and one of the electrodes. To show that submicron features can be UV printed, a line and space pattern consisting of 700 nm wide lines with 900 nm space between lines was UV printed and the SEM micrographs of the PEDOT pattern are given in Fig. 2(b). The right frame is the enlarged version of the left frame.

Figure 2.

Shown in Fig. 3(a) are the cross-sectional images of the channel by atomic force microscopy (AFM) before (left) and after (right) the dielectric and semiconductor layers are coated onto the channel mound. Although the height of the mound is reduced from 505 to 435 nm as a result of the coated layers and the mound is somewhat planarized, the protrusion is sufficiently large enough for the contact printing to yield self-aligned gate. The optical images obtained before and after the contact printing of PEDOT gate are given in Fig. 3(b) for 10  µm long channels. The left frame shows the channel before the contact printing and the right frame after the printing. The channel color changes from white to purple because of the presence of PEDOT after the contact printing.

Figure 3.

The electrical characteristics of the flexible all-polymer transistor fabricated by the UV printing are given in Fig. 4(a) for the transfer and in Fig. 4(b) for the output curves. The channel length and width were 10  µm and 1 mm, respectively. The mobility determined in the saturation regime at the source-drain voltage of −70  V from the square root of current versus gate voltage shown in Fig. 4(a) is 8.6×10⁻³  cm² V⁻¹ s⁻¹. The on-off ratio is approximately 10⁵, and the threshold voltage is −35  V. These electrical characteristics are as good as those of all-polymer transistors fabricated on nonflexible substrates.

Figure 4.

We have presented UV printing as an alternative to inkjet printing in fabricating all-polymer transistor. In inkjet printing, self-aligned gate pattern is formed first, not the self-aligned gate itself, and then the gate is formed by aligning through inkjet printing. In UV printing, self-aligned gate itself is formed. Furthermore, the gate formation is compatible with roll-to-roll printing, which is a massive parallel process compared with inkjet printing which is a serial process. All fabrication steps for the flexible all-polymer transistor based on the UV printing are compatible with roll-to-roll printing.