MHz‐Range Fully Printed High‐Performance Thin‐Film Transistors by Using High‐Resolution Gravure‐Printed Lines

DOI: 10.1002/aelm.201500155 is mobility, V gs is the gate-source volatge, V t is the threshold voltage, L is the channel length, and φ is the overlap fraction, which is / ov L L φ = , where L ov is the overlap length of the source or drain to gate electrodes. A lack of mechanical alignment accuracy on fl exible substrates better than 5 μm limits the transition frequency of conventional printed organic TFTs to much below 1 MHz; as a consequence, while precise alignment of the source/drain (S/D) electrodes to the gate is highly desirable, this is practically not achievable given the aforementioned alignment limitations. Fortunately, given the aggressive resolution scaling we are able to achieve by gravure printing, fully overlapped structures with channel length smaller than 5 μm make the structure entirely tolerant of misalignment without signifi cantly impacting overlap capacitance and switching performance. Effectively, we exploit aggressive linewidth scaling to minimize the deleterious impact of full overlap, thus allowing the use of printing process with poor layer-to-layer alignment. This fully overlapped top-gate architecture can substantially push the limits of printed organic transistors as shown in Figure 1 a, in which the critical feature size is the source/drain line width and channel length. A layout of the misalignment-tolerant structure is shown in Figure 1 b. In addition to relaxing alignment accuracy, a simple fully printed process is proposed in Figure 1 c, which is compatible with R2R processing. Source/drain electrodes were printed by an inverse direct gravure using fi ne silver nanoparticle ink, delivering highly conductive lines as fi ne as 2.38 μm at a high printing speed of 0.6 m s −1 . The printed lines delivered a sheet resistance as low as 3.35 −1 . This scaling was achieved by exploiting the substantial understanding of the underlying physics of gravure printing that we have achieved in recent years. [ 17–19 ] Then, semiconductor and dielectric layers were printed by a sheet-fed direct gravure printer. We formulated a new ink composition to gain uniform dielectric fi lms using gravure printing at a speed of 0.5 m s −1 . Finally, gate electrodes were printed in a fully overlapped manner using a low-resolution inkjet printer; despite the use of low-resolution gates, high performance is still achievable, since both the channel length and overlap capacitance are determined solely by the highresolution gravure-printed source/drain layer. Devices were fabricated on plastic substrates over an area of 50 × 50 mm 2 . The devices showed good DC characteristics and delivered transition frequencies as high as 1.92 MHz. This is among the highest performance for fully printed devices reported in the literature, and attests to the promise of the technology herein. Printing nanoparticle inks for sub-5 μm features requires a deep physical understanding of gravure printing as discussed in. [ 18 ] High-speed printing of fi ne features is possible by Printed electronics has received a great deal of attention for realizing a range of low-cost and large-area systems, such as displays, sensors, and RFID tags. [ 1–4 ] Thin-fi lm transistors (TFT) are the building block of such electronic systems and it is critical to develop a simple fabrication process for high-performance TFTs that is roll-to-roll (R2R) compatible to enable truly low-cost mass production techniques. Numerous researchers have reported printed TFTs fabricated using various printing techniques, including inkjet, step-and-fl ash imprint lithography, screen, fl exography, offset, bar coating, blade coating, and gravure printing. [ 4–11 ] While progress has been substantial, performance still largely falls below that required for the aforementioned applications. This performance gap can be addressed by improving printing resolution to realize devices with smaller channel lengths and by using higher performance materials. Several pattern printing techniques have been proposed to produce such high-resolution (sub-5 μm) printed features such as electrohydrodynamic printing, [ 12 ] capillary printing, [ 13 ] hybrid printing (print-and-drag), [ 14 ] soft lithography, [ 15 ] and microimprint lithography. [ 16 ] However, these printing techniques are generally not suitable for mass production, since they require multiple process steps, complicated tooling, and slow printing speeds. Our previous work showed that direct gravure printing does offer scalability to the sub-5 μm range and thus is attractive for fabrication of MHz-class TFTs. [ 17 ]