Transient currents in electrophoretic ink displays are a result of the movement of ... The basic operation principle of electrophoretic ink displays is the movement of one or .... This paper is a part of a PhD thesis at Ghent University,. Belgium.
P-39 / F. Beunis
P-39: Electric Field Compensation in Electrophoretic Ink Displays Filip Beunis, Filip Strubbe, Kristiaan Neyts, Tom Bert, Herbert De Smet Dept. of Electronics and Information Systems, Ghent University, Ghent, Belgium
Alwin Verschueren, Luc Schlangen Philips Research, Eindhoven, The Netherlands Abstract
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1. Introduction The basic operation principle of electrophoretic ink displays is the movement of one or more kinds of charged pigments in a colored or transparent solvent, under the influence of an electric field. The electric field is usually applied by sandwiching the solution between two transparent electrodes covered by a thin insulating layer. To charge the pigments, an amount of charging agent, typically a surfactant, has to be added. This results in an extra amount of charge in the form of micelles, which is usually much larger than the charge associated with the pigments. The mechanisms governing the movement of charges are thermal diffusion and drift because of the electric field. Information concerning this movement can be obtained by measuring the electrical current through a pixel when a DCvoltage is applied, after short-circuiting the pixel long enough to ensure a homogeneous charge distribution as a starting condition. Different regimes can be identified, depending on the charge concentration n in the pixel, the applied voltage V and the capacitance CIL of the insulating layer. In the first regime [1] the separation of charges does not influence the electrical field significantly, either because the concentration is low enough or because the applied voltage is high enough. In that case the electric field is approximately V/d, and the charges move with a speed µV/d until they reach the insulating layer, resulting in a linearly decreasing current which becomes zero after a time ttransit =
d2 µV
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have all stopped moving. In these expressions, d is the thickness of the pixel and µ is the mobility of the charges, defined as the ratio between speed and electric field.
simulation for low concentration simulation for high concentration
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Figure 1: Current measurements and simulations for two pixels with different charge concentrations. Comparison with simulations [4] shows a significant effect of compensation of the electric field: the current initially decreases faster than would be expected, indicating a decrease of the electrical field and a subsequent slowing down of the charges, and it drops down to zero later than expected, indicating again that charges move slower than they would without field compensation. Figure 2 shows simulations for different charge concentrations, all with µ = 10-10 m2V-1s-1, showing different regimes. For n = 1018 m-3 and n = 1019 m-3 there is no significant effect of field compensation. These currents have the expected linear behavior (although this is not well visible on a log-log chart). For n = 1021 m-3 the voltage drops significantly over the insulating layers before all the charges are completely separated, resulting in an integrated current which doesn’t equal the charge content. Tim e /s
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In this work another regime is identified, characterized by a compensation (screening) of the electric field by separated charges. The influence on the measured external current, and the mechanism causing this effect are explained, and illustrated with measurements and simulations.
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2. Description of the regime Figure 1 shows current measurements [2][3] when a DCvoltage of 10V is applied after short-circuiting for a long time, on pixels with an area S = 1 cm2 and a thickness d = 20µm and with two different amounts of surfactant (OLOA 1200) in a nonpolar solvent (Isopar P). No pigment was added in these test-cells. The concentrations of the charged particles are obtained from integration of the current, and the mobility is calculated from the initial value of the current. This results in concentrations of 6.26*1018 m-3 and 6.44*1019 m-3, and mobilities of 4.00*10-10 m2V-1s-1 and 3.18*10-10 m2V-1s-1.
344 • EuroDisplay 2005
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Transient currents in electrophoretic ink displays are a result of the movement of charged particles when an electric field is applied. These charges have an influence on the electric field, even for concentrations much lower than the space charge limit. This effect can result in a significant increase of the switching time of the display.
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Figure 2: Simulation of the current for different charge concentrations.
P-39 / F. Beunis The case n = 1020 m-3 is an intermediate case, where compensation of the electric field occurs that is not due to the voltage drop over the insulating layers. Another effect occurs here and to understand it this situation is studied in more detail in figures 3 and 4. Figure 3 shows the concentration distribution of positive charges at different times. The concentration distribution of negative charges is exactly the mirror image of this. Without compensation we would expect a homogeneous concentration equal to the initial concentration, except for a growing region of concentration zero at the left side, where the positive charges are moving away from, and a growing charge in a narrow region near the right insulating layer, where the positive charges are building up.
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3. Conclusion This work shows and explains how compensation of the electric field in a large part of a pixel can occur. Compensation of the electric field results in a slowing down of the movement of the charges, and thus an increase of the switching time of the pixel. Understanding this effect can lead to new strategies to improve the low speed of an electrophoretic ink display, which remains one of its most important disadvantages.
4. Acknowledgements This paper is a part of a PhD thesis at Ghent University, Belgium. The author would like to acknowledge financial and scientific support by Philips Research, Eindhoven, The Netherlands.
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Figure 3: Evolution of the positive charge concentration in the pixel with n = 1020 m-3. However, the simulations also show a growing region at the right side where the concentration is lower than expected, but not zero. In figure 4, which shows the evolution of the electric field distribution inside the cell, it can clearly be seen that the field is reduced in a large part of the pixel.
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5. References [1] V. Novotny and M.A. Hopper, “Transient conduction of weakly dissociating species in dielectric fluids”, J. Electrochem. Soc. 126 (1979) 925-929. [2] C. Colpaert, “Geleidingsprocessen in nematische vloeibare kristallen”, Ph.D. Thesis, Ghent University, 1997. [3] Bert, T.; Beunis, F.; De Smet, H.; Neyts, K.; 'Transient Current Properties in Electronic Paper', Proceedings of the 11th International Display Workshops, pp. 1749-1752, 2004 [4] Vermael, S.; Neyts, K.; Stojmenovik, G.; Beunis, F.; Schlangen, L.; 'A 1-Dimensional Simulation Tool for Electrophoretic Displays', Conference record of the 23rd International Display Research Conference, Phoenix (USA), pp. 270-273, September 2003.
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These effects can be explained as follows: when negative charges move to the left, they leave a region at the right where there is an excess of positive charges (not only the positive charges which have reached the insulating layer). These charges cause the electric field to increase in the direction of the field, resulting in faster charges. This causes on one hand a decrease of the positive charge in this region, because it moves faster to the side, but on the other hand results in a decrease of the voltage drop in the rest of the cell, slowing down the movement of all the charges which are still there. The same thing happens at the other side of the cell with the negative charges, reducing the electric field in the middle of the cell even more.
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Figure 4: Evolution of the Electrical field in the pixel with n = 1020 m-3.
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