easily realized in the top-contact geometry, the limitation in minimum separation of source and drain electrodes prevents further advancement in on-off switching speed; it is essential to realize OFETs with a high-mobility semiconductor and a short channel length L , because the maximum circuit operating frequency is proportional to the transconductance g m and inversely proportional to L 2 . Moreover, the technique using shadow masks cannot be applied to mass-producible processes to fabricate integrated OFETs in an identical substrate with high yield.Photolithography is the well-established technique to fabricate fi ne patterns, though it is rarely employed in the top-contact OFETs because conventional organic semiconductor layers are seriously damaged by solutions used for common wet processes in the method. A method using orthogonal fl uorinated photoresist was proposed, [ 10,11 ] which enables to make fi ne patterns without dissolution of organic semiconductors. Though the method enables the fabrication of micro-and nano-scale OFETs, it has not matured yet to be applied for circuitries with short-channel and high-mobility semiconductor devices mainly because of the very large contact resistances. [ 11 ] In OFETs with bottom-contact geometry, photolithography is more often employed because the electrode patterning is fi nished before the semiconductor deposition. However, a major concern for the bottom-contact OFETs with a polycrystalline semiconductor fi lm relates to the molecular disorder occurring in the vicinity of the metal electrodes. [ 12 ] To overcome this problem, electrode surface modifi cations on gold fi lms by thiol-based selfassembled monolayers [ 13,14 ] or UV/O 3 treatment [ 15 ] are effective to reduce the contact resistance, however, these devices suffer from irreproducibility and poor long-term reliability due to chemical instability of most Au-thiolate SAMs. [ 13 ] In this communication, we report for the fi rst time a micropatterning process based on photolithography with simple wetetching of gold electrodes deposited directly on pristine organic fi lms without any adverse effect of residual photoresist. Since the wet-etching of gold is governed by iodide/iodine redox reaction, it is essential to use organic semiconductor materials that are robust to the reaction. Employing a newly developed highmobility organic semiconductor materials with tuned ionization potential, these processes are indeed suited to fabricate fi ne-pitch electrodes with low contact resistance, so that the highest cut-off frequency of 19 MHz is demonstrated for p-type transistors.In order to examine the robustness of organic semiconductors to the gold wet-etching process, we have carefully Based on the recent development of high-mobility and organic semiconductors, [1][2][3][4] organic fi eld-effect transistors (OFETs) offer promising prospects to realize such attractive applications as high-speed fl exible displays, fl exible radio frequency identifi cation (RF-ID) tags, and light-weight wearable smartsensing logics....
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