An Analysis of Reflectivity and Response Time by Charge-to-Mass of Charged Particles in an Electrophoretic Display

Abstract

A reflective electronic display that uses negatively and positively charged particles has excellent bistability, a welldefined threshold voltage, and an extremely fast response time in comparison with other reflective displays. This type of display shows images through the movement of charged particles whose motion depends on the value of q/m (charge per mass for a particle). However, the ratio q/m can easily be changed by the forces acting on the charged particles in a cell of the panel and by friction that occurs after mixing oppositely charged particles and in the particle-insertion process. In this study, we propose a method to determine the appropriate range of q/m by using the reflectivity and response time of charged particles to modify q/m. In this manner, the electrical and optical properties of reflective displays are improved.

1. INTRODUCTION

A reflective electronic display possesses a lower power consumption and leads to less eyestrain than comparable emissive displays such as liquid crystal displays (LCDs) and organic light-emitting displays (OLEDs) [1]. For this reason, reflective electronic displays have mainly targeted electronic newspapers, digital signage, e-readers, and other such applications, which are normally used to hold a fixed image over a long period of time [2,3]. To perform the above functions properly, bistability is required. Reflective electronic displays using negatively and positively charged particles, such as toner and quick responseliquid powder displays (QR-LPD), have excellent bistability, a well-defined threshold voltage, and extremely fast response time in comparison with other reflective displays [4-8]. Thus, images are maintained for a long time without any bias voltage. Also, it is possible to drive the display using a passive matrix (PM) driving method. Despite these advantages, these displays still have problems, including a high driving voltage, short life-time, and irreproducibility of optical properties due to the irregular movement of charged particles in their cells. A charged particle has an intrinsic value of q/m fixed during the particle fabrication process. The movement of the particles depends on the value of q/m, the image force, the van der Walls force and the bias voltage [9].

Figure 1 shows the driving principle and driving sequence from particle insertion (a) through the initial separation (b) to image realization (c) for this type of display. When oppositely charged particles in a mixed state exist in a cell without any bias voltage, as shown in Fig. 1(a), the interparticle Coulomb force dominates the charged particles. When the electric field applied to the upper and lower substrates exceed this force, the negatively and positively charged particles are separated in opposite directions according to the direction of the electric field (b). The display then forms an image through the color of the particles on the upper substrate (c), as shown in Fig. 1. Then, the kinetic energy of the particles and the bias voltage are related by the following equation:

Fig. 1.Schematic illustration of mixed state after particle insertion (a), the initial separation of white and black particles (b), and detached white particles on the upper electrode due to bistability without an electric field.

Here, V is the bias voltage, q is the electric charge of the particle, m is the mass of the particle, and v is the velocity of the moving particle [9]. In addition, the response times of the charged particles can be expressed as follows:

Here, τ is the response time of the charged particle, d is the cell gap between the upper and lower substrate, and a is a constant. In (2) the response time is inversely proportional to the square root of q/m and the bias voltage [10]. As shown in Fig. 1(c), the charged particles are not detached from the electrodes due to the image force between the charged particles and the electrode despite the fact that the bias voltage is not applied after the separation of the particles. Therefore, the memory effect to maintain the formed image can be expressed as follows:

Here, F is the force between an electric charge and its image charge, q is the amount of the electric charge of the particle, ε is the permittivity, and x is the distance from the position of the electric charge on the particle to the surface of the electrode [9].

In (1), a large q/m ratio requires a small electric field for the particle to move from the lower substrate to the upper substrate. However, in (3) a large q/m ratio requires a large electric field to overcome the large image force. The driving voltage of the display includes both (1) and (3) so that it is important to determine the appropriate range of q/m values to obtain both bistability and a low driving voltage. However, it is very difficult to figure out the appropriate value of q/m, since charged particles near the dielectric ribs experience a van der Waals force and can change their charge state through the strong friction encountered during the particle insertion process. This distorted q/m value through the change in charge can be recovered to a targeted value during the fabrication process of particles through discharging processes after mixing oppositely charged particles. To obtain a stable movement of charged particles, the oppositely charged particles must have a sufficient discharging time in order to attain equilibrium after the mixing process and also achieve electrical balance after the particle-insertion process. It is possible to filter immobile particles that deviate from the appropriate range of q/m values by the particle-moving method9), but this method is complex and consumes particles. In this work, we propose a method to determine the appropriate range of q/m in order to achieve an electrical balance of charged particles. We utilize the reflectivity and the response time of the charged particles on the basis of (2) and demonstrate an optically improved example by modifying the q/m value according to the proposed method.

 

2. EXPERIMENTAL

In this study, an analysis of the response time of the charged particles is based on (2), and so samples were fabricated under the same conditions and with a panel structure to exclude the effect of other unrelated variables. We formed ribs on an indium tin oxide (ITO)-coated glass substrate, which was used as an upper and a lower substrate. The cell size was 300 ㎛ × 300 ㎛. The total cell gap between the upper substrate (the height of the ribs: 45 ㎛) and the lower substrate (the height of the ribs: 55 ㎛) was 100 ㎛. We used white particles with a negative charge and black particles with a positive charge. The average diameter of all the charged particles was approximately 15 ㎛. The white particles consisted of negatively charged control agents (CCAs), TiO2, and a polymer. The black particles consisted of positive CCAs [11,19] carbon black, and a polymer. Except for the values of q/m the materials and structure were kept the same during the fabrication process of the particles. The q/m for white particles was -4.3 μC/g and -3.0 μC/g, whereas the q/m for black particles was +4.3 μC/g and +1.8 μC/g. These values were chosen so as to compare the electrical and optical properties according to the value of q/m. The mixed particles were inserted into the cells with a simple particle insertion method shown in Fig. 2. with a mixing ratio of 1:1. Then, the amount of the inserted particles was controlled by the height of the ribs on the lower substrate. The movable space for the charged particles was then determined by the height of the ribs on the upper substrate.

Fig. 2.A simple particle-insertion method. (a) The amount of the inserted particles is controlled by the height of the ribs on the lower substrate, (b) the movable space of the charged particles is determined by the height of the ribs on the upper substrate.

An identical panel is required and the q/m of the oppositely charged particles must be the same to observe changes in optical properties according to the variation in q/m. Thus, we chose ±4.3℃/g as an identical q/m value for charged particles and inserted these mixed particles into the two identical panels. In particular, one of the panels was filled with charged particles that had a sufficient discharging time of 48 hours with a relatively high temperature of 60℃ and a high humidity of 90℃ in order to attain equilibrium after the mixing process. This was able to take place since it is possible for the q/m of charged particles to change according to the discharging time after the mixing process. The response time and reflectivity of identical panels was measured with respect to variations in q/m. Subsequently, the q/m of the charged particles was changed for the other identical panel and then the electrical and optical properties were measured.

 

3. RESULTS AND DISCUSSION

To form an image in this type of display, the white particles must reflect most of the incident light, whereas the black particles must absorb most of the incident light. Therefore, the reflectivity of the charged particles indicates the amount of movable particles with a fixed response time.9) Fig. 3 shows the reflectivity and response time of sample panels fabricated under identical panel conditions to compare changes with the q/m. In these panels, charged particles had a q/m of constant magnitude (±4.3 μC/g). However, one type of charged particle had a discharging time of 48 hours before the particle insertion process. The reflectivity changed with respect to the discharging time. In particular, the optical properties of white particles improved while those of the black particles degraded. These results indicate that q/m changes with discharging time even though the intrinsic q/m value was maintained before mixing oppositely charged particles. These charged particles will show an irregular movement including a short life time and high driving voltage due to the difference in q/m, despite the fact that the charged particles achieved equilibrium through a sufficient discharging time. Thus, the values of q/m have to be considered after driving an identical panel with a sufficient discharging time to improve the electrical and optical properties by electrically balancing the charged particles. Still, the reflectivity does not provide information on which type of particles have a large q/m ratio after the discharging time. However, what is known is the type of particle deviated from the appropriate values of q/m.

Fig. 3.The reflectivity (a) and response time (b) of the sample panels according to the discharging time for a q/m of +4.3 μC/g (black) and -4.3 μC/g (white).

In (2), the influence from the image force is not considered because the response times of the charged particles are measured after the charged particles are detached from the electrode. Also, the response times are divided into a rising time by the movement of the white particles and a falling time by that of the black particles, as shown in Fig. 3(b) [10]. Consequently, using the reflectivity and response times on the basis of (2), it can be ascertained if q/m changed, if the charged particles achieved equilibrium, if the charged particles obtained electrical balance, and which particles had a large q/m ratio after the discharging process. As expected, Fig. 3(b) shows a large difference in response time according to if there was a discharging process or not. The rising times of the white particles are slower while the falling times of black particles are faster after the charged particles underwent a discharging process of 48 hours. Thus, the black particles are more greatly influenced by the image force due to the large q/m so that the numbers of movable black particles decreased after the discharging time. According to (2), these results indicate that the q/m of the black particles is larger than that of the white particles after the discharging time. Thus, a major cause of degradation in image quality is the loss of some electrical charge from white particles and the contrasting gain of electrical charge for black particles during the mixing process, particle-insertion process, and particularly, the discharging time.

From the above considerations, it seems that the black particles, as shown in Fig. 3, clearly deviate from the appropriate values of q/m. Thus, we changed the q/m value (+4.3 μC/g) of the black particles to +1.8 μC/g, while the q/m of the white particles was a fixed subject to the achievement of electrical balance. Subsequently, we mixed these two types of oppositely charged particles and repeated the above processes, such as the discharging process, particle-insertion process, and measurement process in order to ascertain whether the electrical and optical properties had improved. Fig. 4 shows variations in the reflectivity and response time when there is a change in q/m. As a result, the number of movable particles increased and the movement became more stable for sufficiently discharged particles. In particular, both the rising and the falling times were slower and there was no significant difference in response time. The results indicate that because of a change in the q/m image force the black particles were less influenced than those shown in Fig. 3, and the charged particles were closer to electrical balance after the discharging time.

Fig. 4The reflectivity (a) and response time (b) of the sample panels according to the discharging time for a q/m of +1.8 μC/g (black) and -4.3 μC/g (white).

To support the above results, we changed the q/m of the white particles slightly and fixed that of the black particles. The value of q/m for white particles was changed to -3.0 μC/g, whereas that of the black particles was not changed (+4.3 μC/g). As expected, the optical properties were sharply degraded due to the irregular movement of charged particles as shown in Fig. 5. In particular, the number of white particles amongst all movable particles greatly decreased, whereas those of the black particles slightly increased after the discharging time, as shown in Fig. 5(a). Moreover, the response time shown in Fig. 5(b) is not uniform and shows an unstable fluctuation. In particular, both the rising and falling times of the charged particles, after the discharging time, are faster than that shown in Fig. 4(a). However, there are far fewer movable white particles. According to (1) and (2), we can infer that the movable white particles deviated from the average range of optimum q/m values. The q/m value for these white particles was similar to that of the movable black particles but most of white particles had a relatively small q/m value. Thus, the sharp drop in the number of movable white particles was due to a shortage of an electrical charge in comparison with the q/m value of Fig. 4.

Fig. 5.The reflectivity (a) and response time (b) of the sample panels according to the discharging time for a q/m of +4.3 μC/g (black) and -3.0 μC/g (white).

Oppositely charged particles are influenced by various forces in common, but the influence of the forces acting on charged particles is very different according to their q/m. Moreover, q/m can change according to the discharge time and from friction with other charged particles in the particle-insertion process. Thus, these oppositely charged particles become electrically unbalanced even though they are stable before the mixing process. This unbalanced q/m between the negatively and positively charged particles induces degradation in the electrical and optical properties due to an irregular particle movement, short lifetime, high driving voltage, and so on. Therefore, it is essential for the charged particles to obtain electrical balance and equilibrium after the particle-insertion process. As researchers of this study it is believed that the reflectivity and response time, as formulated in (2), suggest a route to determine the appropriate value of q/m for the electrical balance and equilibrium at a considerably reduced time and cost.

 

4. CONCLUSIONS

To obtain stable movement, oppositely charged particles must achieve equilibrium and electrical balance. However, the q/m value changes with discharging time after the mixing of oppositely charged particles. In addition, it is difficult to maintain an electrical balance due to the forces acting on the charged particles in the cell and the friction encountered after the particle-insertion process. Hence, the charged particles must be driven after a sufficient discharging time in order to analyze the actual driving characteristics with a driving pulse for the display. Subsequently, the q/m value must be modified in order to improve the electrical and optical properties. The reflectivity of the display indicates the number of movable particles, and the response time represents the difference in q/m and the electrical balance of the oppositely charged particles. Thus, we can utilize the response time and the reflectivity in order to modify q/m and improve the electrical and optical properties of the display. Moreover, this method can reduce particle consumption and is less time consuming for determining the appropriate range of q/m values.