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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).

Figure 1: Flexible active-matrix electronic-ink displays.
figure 1

a, Operating principle of electronic ink. The relative movement of negatively charged black and positively charged white particles inside their microcapsules is controlled by the direction of the applied voltage. b, A backplane thin-film transistor measured in situ under compressive stress. The transistor is bent to three different radii of curvature: green, 2.0 cm (0.19% strain); blue, 1.3 cm (0.29% strain); and red, 1.0 cm (0.38% strain). The thin-film transistor has identical characteristics when measured without bending (black curve) and at a radius of curvature of 2.0 cm; degradation is minimal even at 1.0 cm. Results were similar under tensile stress. c, Text image shown on a bent display whose resolution is 96 d.p.i. and which has a white-state reflectance of 43% and a contrast ratio of 8.5:1.

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.