1 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com Water-Vapor-Assisted Nanoimprinting of PEDOT:PSS Thin Films Htay Hlaing, Xinhui Lu, Chang-Yong Nam, and Benjamin M. Ocko* The performance of semiconducting polymers is often lim- ited 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, con- ducting polythiophene derivative (PEDOT) electrostatically bound to a PSS polyanion, are used in various organic elec- tronic applications including sensors, organic field-effect tran- sistors (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 effi- ciency 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 con- ducting 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 photore- sists 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 poly- dimethylsiloxane 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 suit- able 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 self- assembled block copolymer lithography, [17] atomic force micro- scopic 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 patterns. Here we present the successful patterning of PEDOT:PSS thin film using water-vapor-assisted nanoimprinting, a process compatible with organic electronic devices. We show that the combination of sample heating and water-vapor improves the nanoimprinting quality of the PEDOT:PSS film. The imprinted patterns were characterized via grazing-incident small-angle X-ray scattering (GISAXS) and scanning electron micros- copy (SEM) respectively. Superior pattern transfer quality of water-vapor-assisted nanoimprinting over conventional thermal nanoimprinting was convincingly demonstrated. DOI: 10.1002/smll.201201267 Organic Electronics Dr. H. Hlaing, [+] Dr. X. Lu, [++] Dr. B. M. Ocko Condensed Matter Physics and Materials Science Department Brookhaven National Laboratory Upton, NY 11973, USA E-mail: ocko@bnl.gov Dr. H. Hlaing Department of Physics and Astronomy State University of New York Stony Brook, NY 11794, USA Dr. X. Lu, Dr. C.-Y. Nam Center for Functional Nanomaterials Brookhaven National Laboratory Upton, NY 11973, USA [+] Present address: Department of Electrical Engineering, Columbia University, New York, NY 10027, USA [++] Present address: Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, PR China small 2012, DOI: 10.1002/smll.201201267