PerspectiveMaterials Science

Flexible Electronics

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Science  20 Mar 2009:
Vol. 323, Issue 5921, pp. 1566-1567
DOI: 10.1126/science.1171230

Organic polymers are the main components of most flexible electronic devices. These devices rely on the compliant physical properties of organic polymers to maintain electrical continuity when deformed. Electrical connections within these devices are a point of weakness and have limited the types of materials and processes that can be used. Although inorganic semiconductors and metals have high conductivity, these materials will not commonly sustain repeated bending or stretching. On page 1590 of this issue, Ahn et al. (1) show how metal can be added to components within flexible electronic devices, enabling conductivity to be maintained even after repeated deformation.

Connections between electronic structures have traditionally been designed with a planar architecture that is patterned through multiple fabrication steps (2, 3). Much of the fabrication technology used in flexible electronics was initially borrowed from existing device fabrication platforms, such as those used to manufacture silicon-based thin-film transistors. These techniques largely depend on the selective liftoff or etching of materials, using process conditions that are not always compatible with organic polymers. Process techniques developed for the fabrication of flexible electronic devices must avoid large changes in temperature and pressure. These techniques must also minimize exposure to reagents that may degrade the conductivity, purity, or form of the organic polymer components in the device.

Inkjet printing has been used to pattern organic semiconductors (4), metal contacts on organic semiconductors (57), and metallic structures (8) that require minimal further processing. Ahn et al. have now used inkjet printing to create three-dimensional (3D) metallic connections between functional components of flexible devices (see the figure). The authors first fine-tuned a colloidal ink of silver nanoparticles by adjusting the uniformity of the particles, the viscosity of the ink, and the drying time of the solvents. They then extruded the ink through a nozzle that directed the selective deposition of silver particles onto the flexible substrate. After annealing at 250°C for ≤30 min, the printed wires exhibited an electrical resistivity nearing that of bulk silver. Annealing can also be done using light or microwaves (9, 10). The resistivity of the printed silver wires is about two orders of magnitude lower than that of commonly used conductive organic polymers (1). This improvement translates into lower power consumption and a lower heat load on the surrounding environment for devices incorporating these printed wires.

Flexible electrical metal connections.

Wires connecting components within a flexible device can be fabricated by a direct-write process using inkjet printing of silver nanoparticles. As shown by Ahn et al., the technique can be used to fabricate three-dimensional connections that span between components of a flexible device and that flex when the device is deformed.

Controlling the deposition of the colloidal silver ink is essential for fabricating freestanding wires that have both 2D and 3D components. The electrical connections demonstrated by Ahn et al. include springs and structures with built-in slack to accommodate the stretching and bending of a flexible device. The ability to form arched features is also essential for avoiding direct contact when one electrical connection crosses over another.

The authors further demonstrate the control provided by their technique by reporting silver wires with width-to-length ratios up to 1:1000. These wires can span gaps up to 1 cm wide. The narrow dimensions of the printed wires (from ∼2 to ∼10 μm) are an additional benefit of this fabrication process. These small dimensions minimize the footprint of the electrical contact lines, which decreases the impact of the wires on the optical quality of the device and increases the density of features in the device. Although inkjet printing is a serial process, Ahn et al. have demonstrated a wide range of benefits for this technology.

Flexible electronic devices compete with paper-based media as well as existing electronic media. It is desirable to find a technology platform that can be rolled or bent (as with paper), yet robust enough to be unfurled and reused. The end use of the device will depend on the functions incorporated into its architecture. Ahn et al. demonstrate a few features that might be desirable in a flexible device, including optical and optoelectronic components such as light-emitting diodes (LEDs) and solar cells. Tuning the optical properties of a flexible device is widely recognized as necessary, with research efforts directed toward both emissive and reflective properties (2, 3, 11). Other applications of flexible device technology include radio frequency identification (RFID) tags and antennas that can be incorporated into personal identification, as well as packaging and other forms of transferable media.

Traditional forms of print media, such as newspapers and books, have been prevalent in human lives for centuries. We have greatly benefited from the ease with which information can be efficiently distributed in print media. Entire industries have been created around the printing, distribution, and storage of paper articles. Overcoming our dependence on paper will require its replacement with a technology that is comparable to paper in its weight, flexibility, and ease of use. Key technologies are necessary to enable the manufacture of such an alternative form of media. The work of Ahn et al. is a transition from typical models of fabrication, providing possibilities for incorporating multiple functions into a single flexible device.

Flexible electronic devices are becoming commonplace in our lives. Screens that can flex or otherwise distort have been incorporated into laptops, televisions, and mobile phones. Lightweight electronic display devices that can be rolled up for storage are being developed. The achievement of completely converting to a paperless society will be revolutionary in itself, but so are the technological advances necessary to make this new form of media commonplace in our daily lives.

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