Abstract
Ultrathin, flexible electronic displays that look like print on paper are of great interest1,2,3,4 for application in wearable computer screens, electronic newspapers and smart identity cards. Here we realize the fabrication of such a display on a bendable active-matrix-array sheet. The display is less than 0.3 mm thick, has high pixel density (160 pixels × 240 pixels) and resolution (96 pixels per inch), and can be bent to a radius of curvature of 1.5 cm without any degradation in contrast. This use of electronic ink technology on such an ultrathin, flexible substrate should greatly extend the range of display applications.
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Main
Thin (0.4-mm) but inflexible liquid-crystal displays have been made on plastic by using a diode-matrix array5 and an amorphous-silicon, thin-film transistor (TFT), active-matrix array6. To create a flexible display, we used a TFT array (backplane) with microencapsulated electrophoretic material (electronic ink)7, which consists of millions of microcapsules containing charged pigment particles in a clear fluid. A negative voltage applied to the top surface causes the positive white particles to move to the top of the capsule and the surface to appear white; reversing the electric field causes the negative black particles to appear at the top surface and create a dark spot (Fig. 1a).
We used a 75-µm-thick steel-foil substrate to build the TFT backplane because steel foil is lightweight, mechanically stable and compatible with existing fabrication processes for active-matrix liquid-crystal displays8,9. Before the array fabrication, an insulating layer was applied onto the foil to render the substrate passive. The amorphous-silicon TFTs were made in the bottom-gate, back-channel etch configuration. The gate and source/drain metal were deposited by sputtering. A ductile composite of aluminium and refractory metal was used for the gate metal to enhance the backplane's flexibility.
Silicon nitride, amorphous silicon and a doped amorphous-silicon layer were deposited as the gate insulator, the channel and the contact layer, respectively, by plasma-enhanced chemical-vapour deposition. The metal, semiconductor and insulator layers were patterned by photolithography. The display was made by laminating a sheet of electronic ink onto the backplane. The electronic ink consists of a layer of electrophoretic microcapsules and a polymer binder, coated onto a polyester/indium–tin oxide (common electrode) sheet. The total display thickness is less than 0.3 mm.
A typical TFT has a threshold voltage of 4.0 volts and a linear mobility of 0.50 cm2 V−1 s−1. The drain off current is about 1.0 pA at 10 V drain voltage. The current on/off ratio is 5 × 106, which is sufficient for high-resolution displays. The TFT performance does not degrade after first being bent for 120 seconds around a cylinder that is 2 mm in radius (1.9% strain) and then released.
We also measured TFTs in situ under compressive stress at three radii of curvature (Fig. 1b). Because steel has a large Young's modulus, our selection of a thin substrate decreases the distance of the TFT circuit from the display neutral plane10, reducing the in-plain strain of the circuit. As a result, the display can be repeatedly bent 20 times to a radius of curvature of 1.5 cm without any degradation.
Bias temperature stress on the TFT backplane was performed at a gate voltage of 25 V and up to 80 °C. The results indicate that the flexible backplane has a threshold voltage shift comparable to that of conventional glass TFT backplanes in laptop computers, and possibly a similar reliability and lifetime. The row electrode is driven between 0 and 24 V, and the column electrode is driven between 0 and 20 V.
Figure 1c shows the bent display of a text image of 96 d.p.i. resolution; the display has a viewing angle of almost 180°. The ink-switching speed is 250 ms, which is sufficient for electronic paper. For wearable computers, a reduction to 15 ms would be required for video-rate switching; in addition, the substrate thickness would need to be reduced for foldable displays. We suggest that electronic ink combined with flexible amorphous-silicon active-matrix backplanes will provide a viable pathway to 'e-paper' and wearable computer screens.
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Chen, Y., Au, J., Kazlas, P. et al. Flexible active-matrix electronic ink display. Nature 423, 136 (2003). https://doi.org/10.1038/423136a
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DOI: https://doi.org/10.1038/423136a
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