The performance of semiconducting polymers is often limited by charge mobility (µ, typically< 1 cm 2 V− 1 s− 1) and interfacial geometrical factors including the contact area with electrodes. Interfacial layers of the conducting polymer, PEDOT: PSS [1](poly (3, 4-ethylenedioxythiophene): poly (styre nesulfonate)), a blend of an oxidatively doped, cationic, conducting polythiophene derivative (PEDOT) electrostatically bound to a PSS polyanion, are used in various organic electronic applications including sensors, organic field-effect transistors (OFETs), organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs) due to their superior optical, electrical, chemical and mechanical properties.[1–3] The micro-and nanoscale patterning of the PEDOT: PSS films further increases the performance of OFET [4–6] and OLED [5] devices. PEDOT: PSS nanowires exhibit higher sensitivity to ethanol vapor compared to an unpatterned films with the same thickness.[7] For OPV devices, a slight power conversion efficiency improvement was reported using a PEDOT: PSS patterned with a 700 nm pitch and 10 nm depth as a hole extraction layer.[8] While the reported power conversion efficiency improvement is promising, the imprinted pattern did not lead to a significantly enlarged interfacial area due to the large pitch. Very recently, patterned PEDOT: PSS films, prepared using a dry-nanoimprinting method, were used to make OPV devices with an enlarged interfacial area.[9] These devices exhibited a promising efficiency enhancement, although the base efficiency was low. Here we report the use of a wet-nanoimprinting method to fabricate PEDOT: PSS high-fidelity patterns with a period of 140 nm and a depth of 40 nm, corresponding to a∼ 26% increase in the interfacial area. Our controlled wet process differs from the previously employed dry imprinting methods.[8, 9] These imprinted conducting polymer films have the potential to improve device performance through both an increased interfacial area and through the reorientation of the electron-donor polymer in the subsequently deposited active layer.[10] Although it is highly desirable to pattern PEDOT: PSS layers in organic electronic devices, the fabrication of such patterns has been challenging due to its incompatibility with most standard lithographic methods. For instance, the acidic nature of PEDOT: PSS can damage acid-sensitive photoresists such as those used in conventional photolithography.[4, 11] The standard aqueous developers are also incompatible with the water-soluble PEDOT: PSS films. Since PEDOT: PSS lacks a distinct glass-transition temperature,[12] heating at elevated temperatures does not provide enough fluidity of the polymer chains for conventional thermal imprinting.[13] Hence, dry thermal imprinting on PEDOT: PSS films at 150 C with polydimethylsiloxane stamp resulted in shallow and non-uniform pattern transfer.[8] Organic vapors have been used to lower the viscosity of other polymer films during nanoimprinting,[12–15] but PEDOT: PSS is not soluble in most organic solvents hence its viscosity can not be easily lowered by the introduction of organic solvents. On the other hand, water may be a suitable solvent for nanoimprinting of PEDOT: PSS films since PEDOT: PSS thin films swell up to 25% of their dry thicknesses due to the hygroscopic nature of the sulfonic acid groups.[16] Other techniques for patterning PEDOT: PSS such as selfassembled block copolymer lithography,[17] atomic force microscopic nanolithography,[18, 19] micromolding in capillaries [20] and laser ablation [21] either require elaborate fabrication processes or they are not efficient in producing large-area …