ARTICLE Highly Transparent and Flexible Nanopaper Transistors Jia Huang,‡,§ Hongli Zhu,†,§ Yuchen Chen,† Colin Preston,† Kathleen Rohrbach,† John Cumings,† and Liangbing Hu*,† †Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States and ‡School of Materials Science and Engineering, Institute for Advanced Materials & Nano Biomedicine, Tongji University, Shanghai, China. §These authors contributed equally to this work. ABSTRACT Renewable and clean “green” electronics based on paper sub- strates is an emerging field with intensifying research and commercial interests, as the technology combines the unique properties of flexibility, cost efficiency, recyclability, and renewability with the lightweight nature of paper. Because of its excellent optical transmittance and low surface roughness, nanopaper can host many types of electronics that are not possible on regular paper. However, there can be tremendous challenges with integrating devices on nanopaper due to its shape stability during processing. Here we demonstrate for the first time that flexible organic field-effect transistors (OFETs) with high transparency can be fabricated on tailored nanopapers. Useful electrical characteristics and an excellent mechanical flexibility were observed. It is believed that the large binding energy between polymer dielectric and cellulose nanopaper, and the effective stress release from the fibrous substrate promote these beneficial properties. Only a 10% decrease in mobility was observed when the nanopaper transistors were bent and folded. The nanopaper transistor also showed excellent optical transmittance up to 83.5%. The device configuration can transform many semiconductor materials for use in flexible green electronics. KEYWORDS: green electronics . nanopaper . transparent electronics . flexible ost-efficient and environmentally The interactions between the substrates, C friendly macroelectronics, optimized the metal contacts, and the active semicon- with green materials and large-scale ductors greatly impacts the mechanical per- roll-to-roll processes, are attracting in- formance and sometimes the optical and tensive research and commercial inter- electrical properties of devices.8,9 Porosity, ests because they enable a range of dispo- surface energy, defects, and flexibility are sable devices for consumer electronics.1À7 among the important parameters of sub- Solution-based printing is an ideal process strates that play a key role in the printing due to its fabrication speed: up to 15000 process for printable devices. For example, sheets per hour for offset printing. This it is much easier to print on cellulose paper process also boasts ideal scalability and than most other substrate materials due to benefits from techniques and equipment its porous nature and excellent hygroscopy. supporting a well-established paper indus- Plastic, however, relies purely upon ambient try. Conductors, thin film transistors, organic evaporation for the solvent removal and light-emitting diodes, and solar cells are immobilization of the electronic material. already demonstrated with various print- Glass substrates are widely used for elec- ing methods. Previous efforts on printable tronics and displays, but sheet-by-sheet electronics have generally focused on the processing is used instead of roll-to-roll development of new inks, fabrication pro- processing. Intensive research has already cesses, devices with new nanomaterials, been conducted on the development of * Address correspondence to: and new device structures. However, printed emerging substrates, especially plastic sub- [email protected]. devices are necessarily built upon sub- strates with better humidity and oxygen Received for review September 24, 2012 strates whose properties play an underap- barrier properties, better thermal stability, and accepted January 25, 2013. preciated role in device design, processing, and a higher glass transition temperature. fl Published online January 25, 2013 and performance. Despite their important Corning Inc. recently developed exible 10.1021/nn304407r role, the development of new substrates is glass that has generated much interest. still lacking. The flexible glass with 100 μm thickness C 2013 American Chemical Society HUANG ET AL.VOL.7’ NO. 3 ’ 2106–2113 ’ 2013 2106 www.acsnano.org ARTICLE Figure 1. (a) Picture of a transparent nanopaper on top of “M”. (b) AFM phase profile of porous nanopaper in (a) made from nanofibrillated cellulose with diameter ∼10 nm. (c) AFM line scan of nanopaper in (a) with RMS 1 nm. can be bent down to a 40 mm arc radius. The flexible by optimizing the device structure and the fabrication glass is also highly transparent and can tolerate much process on the nanopaper substrate. When our nano- higher processing temperature (600 °C) than plastic. paper transistors were bent, only a slight decrease This flexibility needs to be further improved to enable in mobility was observed, demonstrating the good additional applications of the glass by reducing the flexibility for ultimate device applications. minimum bending radius. RESULTS AND DISCUSSION Nanopaper based on nanofibrillated cellulose (NFC) is an environmentally friendly and renewable material. Nanopaper with high transmittance, low surface Nanopaper is made of the same material as traditional roughness, and excellent flexibility was prepared by paper but consists of fibers with a much smaller diam- vacuum filtration of an NFC solution. Bleached Kraft eter than that of traditional paper. Reducing the softwood fibers were pretreated with a NaBr/NaClO/ diameter of the paper fibers decreases the optical TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) sys- scattering; therefore the nanopaper has excellent op- tem. TEMPO is a highly selective oxidant for the hydro- tical transparency. In addition, nanopaper has much xymethyl group at the glucose C6 position within the better thermal stability compared to plastic substrates, cellulose chain.11,12 The swelled fibers were disintegrated with a coefficient of thermal expansion (CTE) of to nanofiber with a diameter of 10À100 nm using a À 12À28.5 ppm K 1. The typical CTE value for plastic homogenizer (microfluidizer processor M-110EH). The À substrates is 20À100 ppm K 1.10 Nanopaper can also obtained NFC suspension was filtered by a nitrocellu- tolerate a much higher processing temperature than lose ester filter membrane (Millipore DAWP29325) with plastic.10 The high transparency and flexibility of na- 0.65 μm pore size. After filtration, the cellulose gel nopaper allows to replace for plastic substrates in a “cake” was placed between two smooth substrates and wide range of applications. The field-effect transistor a polyamide woven fabric. Then it was hot pressed (FET) is an important component for many electronic under high pressure to obtain low surface roughness devices, but to our knowledge there is no prior report and small pore size. Figure 1a shows the image of a of FETs successfully fabricated on nanopaper sub- transparent nanopaper made from nanocellulose strates. Here we are the first to demonstrate organic fibers with an average fiber diameter of ∼10 nm. FETs designed and fabricated on this emerging When the nanofibrillated cellulose was dried from the substrate with excellent optical and mechanical water suspension, strong interfibrillar interactions were properties. The transistor'sperformance,including formed, including chemical hydrogen bonding from the its mobility and on/off ratio, may be further improved large content of hydroxide groups and mechanical HUANG ET AL.VOL.7’ NO. 3 ’ 2106–2113 ’ 2013 2107 www.acsnano.org ARTICLE winding force between the different fibers. The me- tronics. Figure 1b and 1c show the atomic force chanical properties of nanopaper are preserved even microscopy (AFM) phase and height image of nano- when nanopapers are made to be conductive by paper, respectively. From the line scan of the surface integrating polymers into the structure. Compared to height image in Figure 1c, we see the nanopaper has traditional papers, nanopaper not only is mechanically a low surface roughness, with maximum roughness stronger but also possesses higher transmittance, depth Rmax of 5 nm and root mean square roughness higher density, lower CTE, higher surface smoothness, (RMS) of 1 nm. This is important for preventing a more tunable structure, and better electrochemical short leakage between the bottom gate electrodes properties in conductive composite.36 All of these and the semiconductor layer in FETs fabricated on special properties contribute to making nanopaper a nanopaper. promising substrate for transparent and flexible elec- Recently, cellulose papers have been explored to replace plastic substrates as a lightweight substrate for low-cost, versatile, and roll-to-roll printed electronics.13 TABLE 1. Comparison of Nanopaper, Traditional Paper, Various types of devices such as transistors, radio , , and Plastic10 20 21 frequency identification (RFID), light-emitting diodes, solar cells, paper based batteries, supercapacitors, and characteristics nanopaper traditional paper plastic porous magnetic aerogels have been demonstrated on À À surface roughness (nm) 5 5000 10000 5 low-cost, recyclable regular paper.14 18,35,37 Nano- À porosity (%) 20 40 50 0 paper has many advantages over both regular paper and pore size (nm) 10À50 3000 0 plastic substrates, as shown in Table 1. Most notably, optical transparency 90 20 90 fi at 550 nm (%) light goes through the bers without scattering when max loading stress