Orthogonal Patterning of PEDOT:PSS for Organic Electronics using Hydrofluoroether Solvents

2009 WILEY-VCH Verlag Gmb Organic electronics is an emerging technology opening new opportunities in the field of large-area electronics. It has received enormous attention as a technology platform that enables flexible, large-scale devices by exploiting the unique properties of organic materials. In addition, organic electronics offer the possibility of affordable manufacture of devices through solution processing of active materials. Before this vision is realized, several challenges must be overcome, in particular the issues of patterning. Patterning of electronic materials enables microscale device structures to be defined for organic light-emitting diode (OLED) displays and organic thin-film transistors (OTFTs). In addition, patterning enhances the device performance by preventing cross-talk, increases transconductance, and prevents high off-currents in transistor arrays or drivers. The patterning of organic materials has been demonstrated by many different methods, including ink-jet printing, vapor deposition through shadow masks, soft and hard imprint lithography, and photolithography. Ink-jet printing boasts continuous roll-to-roll process capabilities and is the patterning technique of choice for polymeric materials. However, the resolution is limited to approximately 10–20mm. Shadow mask deposition is the dominant technique for small-molecule patterning, but also has notable resolution limitations. A shadow mask feature resolution is typically 25–30mm, although special masks have shown resolution down to 5mm. Shadow-mask deposition also requires a high-vacuum environment, which can introduce further limitations. Imprint lithography has demonstrated promising results, showing feature resolution down to 10 nm. However, this technique has shown only limited applicability with respect to materials and device architectures. Furthermore, in all of the aforementioned methods, registration is an issue that renders fabrication of multilayer devices exceptionally challenging. Multilayered device architecture will be essential to achieve fully integrated circuits. Photolithography, in contrast, is a widely applicable patterning method that consistently achieves both high-resolution and registration. Photolithography has the added advantage of being the most developed patterning technology and the patterning method of choice for the current semiconductor industry. In spite of the proven technical advantages, conventional photolithography has not been recognized as a suitable technique for patterning organic electronic materials. It is presently hindered by concerns of chemical deterioration of active organic materials upon exposure to process solvents for lithography. By introducing a new set of benign processes that involve new specially tailored photopolymers, we show that this problem can be circumvented. Recently, we identified hydrofluoroethers (HFEs), a family of nontoxic and environmentally friendly solvents, as being chemically benign to nonfluorinated organic electronic materials. Not only are they benign, but HFEs are orthogonal solvents to many organic compounds, that is, the organic compounds are insoluble and are not swollen in HFEs. This is a particularly useful property in the fabrication of multilevel devices, since new layers can be added without damage to existing ones. Our challenge was, therefore, to develop compatible lithographic materials for these new process solvents. By employing a fluorinated photoresist compatible with HFEs, we demonstrate sub-micrometer photolithographic patterning of organic electronic materials. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a mechanically flexible, transparent, and highly conductive polymer blend, which has found various applications in organic electronics including serving as the electrode material for plastic substrates, because of its low-temperature processing requirements, and as charge-injection/-extraction layers in OLEDs and photovoltaic devices. However, photolithographic patterning of PEDOT:PSS for device components is not straightforward, because i) PEDOT:PSS films are damaged by aqueous solutions, which are standard developers in conventional photolithography, and ii) acid-sensitive photoresists are adversely affected by the acidic PEDOT:PSS. In this communication,wepresent a unique acid-stable imaging material for PEDOT:PSS and organic electronic materials in general. We report on the sub-micrometer patterning of PEDOT:PSS films and their subsequent application to the fabrication of a field-effect transistor, in which an organic semiconductor material, pentacene, is patterned by the same protocol. In designing a HFE-compatible photoresist, it was most important that the photoresist be soluble in fluorous solvents. In general, fluorous solvents dissolve highly fluorinated materials. The copolymer 3, derived from the highly fluorinated monomer 1 and photolabile monomer 2, was expected to yield a material that exhibits a solubility switch following UV irradiation