Application

The current bump represents the moment of complete transition to the extreme state as

Duration t(ms) 0 40 300 500 1000

WUT

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mentioned in 4.2, the symmetric waveform can reduce the transition time in 4.3, and the optical response time can be reduced if the bump position appears earlier by different voltage waveforms in 4.4. Combining these three experimental results, a waveform for shortening driving time was designed as shown at the right-hand side in Fig. 4-10. A short reverse pulse was inserted before the original one, and the original phase was reduced to the timing that optical response came to the extreme state.

The designed waveform meant to reduce the effect of wake-up time, thus shortened the transition time. Besides, the reverse pulse should not change the original optical state, i.e., the optical response remained the same amplitude, so that the waveform can be shortened without a luminance changed.

Fig. 4-10 Schematic showing for reducing transition time.

Comparison between a 300 ms pulse and the proposed waveform (a 20 ms negative pulse width inserted before the 300 ms positive pulse width) is shown in Fig. 4-11(a). The experimental results showed the designed waveform reduced 50 ms of both the wake-up time and the transition time. Besides, the total driving time is reduced from 250 ms to 200 ms (20%

improved). The inverse experiment also showed the similar result as shown in Fig. 4-11(b).

However, the wake-up time did not change obviously. The reason may be the difficulty of measuring lightness change in the dark state. The dark fluid in a microcup adsorbed scattered light which may be one of the reasons why observing the lightness change from the white state

t

t

V OR

Applied voltage(V) Optical response(mV) t

Designed V

40

to the dark state is easier than from the dark state to the white state.

(a)

(b)

Fig. 4-11 The designed waveform for shortening the transition time. (a) driving from dark to white state (b) driving from white to dark state.

In this experiment, transition time was successfully reduced 20% of the original one. To know if the increasing widths of reverse pulse will proceed to reduce the transition time, a 60ms reverse pulse width was further done as shown in Fig. 4-12. The blue curve is a 300ms pulse without reverse pulse added in the front and is shown as a reference. The red and the green curves are 20ms and 60ms reverse pulse added in front of a 300ms pulse, respectively.

The results show that further increase of reverse pulse width will not reduce the transition time. As shown in Fig. 4-12(a), the magnitude of current response in phase two is similar to the symmetric waveform (0.15mA) and the transition speed does increase. However, when the

-80 Applied voltage (V) Optical response (mV)

40ms 90ms Applied voltage (V) Optical response (mV)

220 270ms

41

reverse pulse is raised to 60ms, the initial black state goes darker. This result tells that the particles can be further pushed closer to the boundary, i.e. more closely packed to the opposite side from observer, but the transition time has a reduction maximum of this waveform method.

The inverse experiment also shows the same result as shown in Fig. 4-12 (b).

(a)

(b)

Fig. 4-12 The current and optical responses of 0, 20 and 60ms reverse pulse widths inserted before 300ms pulse. The dash line and the solid line represent current and optical responses, respectively. (a) -30V reverse pulse adds to +30V. (b) +30V reverse pulse adds to -30V

-0.20

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4.6 Summary

With a large interest in EPDs, research has been conducted mostly from manufacture type, image stability [24] and the image quality [25]. Fewer have been done from the internal charge behavior. Tom Bert and H. De Smet have provided abundant studies of current response from 2003 to 2006 regarding both particles and micelles in electronic paper. Yet findings regarding physical mechanism are not complete and unified. Besides, there was scarce research focus on Microcup EPDs or a model proposed to describe the motion of charged species which would be the major factor that dominates the criteria of EPD performances.

While applying an external field, both charged particles and charged micelles would contribute to the transient current in the EPD device. However, only the motion of charged particles would change the optical state. As a result, by synchronizing the optical response with the current, the motions of charged particles and charged micelles can be distinguished.

Furthermore, based on the established model and the observed physical mechanisms, an application of new waveform design was proposed to shorten the transition time. Moreover, the designed waveform successfully reduced 20% transition time compared to the original one.

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Chapter 5

Conclusion and Future Work

Electrophoretic displays have caught much attention because of their bistability, flexibility, wide viewing angle, eco-friendly characteristics. The charge species inside the microcup EPD would affect the driving mechanism such as the wake up time. Since current contributors are not merely particles which make the system way more complicated and hard to control. As a result, understanding the physical mechanism inside the EPD is necessary before meeting the criteria of transition time reduction, bistability enhancement and grey level accuracy.

With a large interest in EPDs, research has been conducted mostly from manufacture type, image stability and the image quality. Fewer have been done from the internal charge behavior.

Yet findings regarding physical mechanism are not complete and unified. While applying an external field, both charged particles and charged micelles would contribute to the transient current in the EPD device. However, only the motion of charged particles would change the optical state. As a result, by synchronizing the current and optical responses, the charge mechanism inside the microcup EPD is fully understand and distinguished.