Mobility Enhancement in Solution-Processed Organic Thin-Film Transistors by Temperature Gradient-Assisted Solvent Evaporation Changmin Keum, Jin-Hyuk Bae, Won-Ho Kim, Min-Hoi Kim, Jaehoon Park, and Sin-Doo Lee * School of Electrical Engineering and Computer Science, Seoul National University Kwanak P.O. Box 34, Seoul 151-744, Korea Abstract We report the effect of temperature gradient curing of an organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS PEN), on the performance of organic thin film transistors (OTFTs). The crystalline domains in the TIPS PEN film are found to be aligned along the direction of temperature gradient in the substrate. The TIPS PEN OTFT fabricated by temperature gradient curing exhibits the field-effect mobility of 0.12 cm 2 /Vs, which is substantially higher than that of the device fabricated by conventional curing method. 1. Introduction Solution-processed organic semiconductors and organic thin film transistors (OTFTs) have been extensively investigated because they allow low-cost and large-area manufacturing approaches, such as printing and roll-to-roll processing, for disposable plastic electronics [1,2]. However, solution-processed OTFTs exhibit relatively lower field-effect mobility than that of devices fabricated by vacuum-evaporation method, which is caused by poor molecular ordering and lack of uniformity in solution-processed organic semiconductor films [3]. Recently, several groups have focused on increasing the homogeneity of the film by solution-sheared deposition [4], pinning a solution droplet on the tilted substrate [5], and modifying interfacial chemistry [6]. Nevertheless, the directional growth of a solution-processed organic semiconductor is still challenging in this research area. In this work, we present the ordered domain in a 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS PEN) film with the direction from source to drain electrode, which is achieved by temperature gradient-assisted solvent evaporation of the TIPS PEN layer. Consequently, the field-effect mobility in the solution-processed OTFT having directionally ordered TIPS PEN domains is significantly enhanced compared to that of the device with randomly oriented TIPS PEN domains fabricated by conventional curing method. 2. Experimental Figure 1 shows a schematic structure of the top contact OTFT which was fabricated on a heavily doped p-type silicon substrate. A 300-nm-thick silicon dioxide film was thermally grown to form the gate dielectric layer. The substrate was placed across a hot plate and the support which is maintained in room temperature in order to produce thermal gradient in the substrate. A droplet of TIPS PEN (1 wt% dissolved in anisole) was dropped onto the spot, which should be the channel region between source and drain electrodes, by micropipetting. The temperature of a hot plate was fixed at 60℃ to evaporate the solvent of a TIPS PEN droplet in temperature gradient. After the thermal curing of TIPS PEN, 80-nm-thick Au source/drain electrodes were thermally evaporated onto the TIPS PEN film through the shadow mask. The deposition rate was 1.0 Å/s and the substrate was kept at ambient temperature. The current-voltage characteristics were measured with a semiconductor analyzer (HP 4155A). 3. Result and Discussion Figure 2 shows optical microscopy images of the channel regions for the fabricated OTFTs in which TIPS PEN was cured by temperature gradient method or conventional process. It is clearly observed that the directional domains are produced in the TIPS PEN film cured by temperature gradient method, while domains are randomly oriented in the film cured by conventional process. This is attributed to the gradual evaporation of a solvent depending on the temperature gradient in the substrate. Figure 3 illustrates that the solvent evaporation takes place from the edge of a droplet close to higher temperature region and subsequently propagates to lower temperature region. In this case, TIPS PEN molecules become successively aligned along the solvent evaporation direction so that the directional domains in the TIPS PEN film can be obtained. Fig. 1. Schematic diagram of the fabricated OTFT with the top-contact structured.