A New Type of Electrophoretic Displays
Publication Date: 2005-Oct-21
The IP.com Prior Art Database
In conventional electrophoretic displays visible charged pigment particles or granules are displaced in between two or more electrodes under the influence of an electric field. In the here reported alternative electrophoretic display visible non-charged pigment particles are displaced by means of electric field induced fluidic motion due to the presence of invisible stray ions. Thus the pigments are dragged, rather than driven. This substantial difference offers a number of highly relevant advantages over the conventional concept.
Conventional electrophoretic displays (EPDs) come with a large variety of challenges, such as maintaining colloidal stability, threshold, bi-stability, and zeta-potential with time.
Maintaining colloidal stability is required in order prevent particles coagulation, i.e. sticking to each other or other surfaces. Threshold is required in order prevent cross talk whilst driving other pixels, and is in particular of importance for passive matrix displays. Bi- stability is required in order to maintain a written pixel state in the power -off state. This is what it is all about, bright and low power displays. A stable and high zeta-potential is first of all required in order to be able to drive the particles, secondly the higher the zeta-potential the faster a particle may be displaced, or alternatively, it can be driven at a low field, and thirdly, to ensure sufficient life-time. In other words, once a particle has a given zeta -potential, care must be taken that these properties are stable, and are stable with time.
An important aspect of the above is thus that charge injection or extraction into or from the particles at the drive electrodes, must thus be prevented at all times. This can for example be achieved in covering the drive electrode with a protective layer, in general a dielectric thin film, frequently also serving as an anti-sticking layer.
Unfortunately, this protective layer also represents a capacitor. Thus for a suitable dielectric, i.e. a very low conductance material, the externally applied electric field may drop in part, or in full (when the conductance of the electrophoretic suspension is high) over the overcoat. As a result very high drive voltages are required, whilst the overcoat is also degraded rapidly. In order words, in order to drive the charged particles effectively, and at low drive voltage, it is essential to reduce the voltage drop across the protective layer to a minimum. This can be done in ensuring that the impedance of the insulating fluid in between the electrodes is about a factor of 10 larger than that of the dielectric overcoat, more preferably a factor of 100. Thus the fluid preferably contains no ions, or a low ion contents, whilst the overcoat is picked as thin as possible (life-time determined).
Having no, or a very low, ion content, however, possess a real challenge. Firstly because charge transfer agents and stabilizing agents are added to th...