This invention relates to electrochromic display devices.
Electrochromic display devices have been used to display data in various formats. When the display device incorporates a number of electrochromic elements in a two-dimensional matrix configuration, the individual electrochromic elements typically are arranged in a manner suitable for multiplex addressing. However, alternate current paths are created which result in undesired coloration or bleaching of electrochromic elements adjacent to an electrochromic element sought to be colored or bleached, an effect commonly referred to as cross-talk. Attempts have been made to deal with the cross talk problem. For example, U.S. Pat. No. 4,129,861 discloses the use of diode elements to increase the threshold voltage of each electrochromic element. However, in order to use multiplex addressing in such an arrangement, each electrochromic element must be provided with such a diode means, which of course, increases the cost and complexity of the device.
Other attempts to deal with the cross-talk problem require separate and distinct electrochromic layers and electrolyte layers. Additionally, discrete pixels are created by separating one electrochromic material and/or one electrolyte in a discontinuous series of confined units within a device. U.S. Pat. No. 4,488,781 addresses the problem of cross-talk by depositing an electrochromic inorganic layer on top of a glass sheet. The glass sheet contains a transparent electrical conductor. Both the electrochromic layer and the glass sheet are patterned into rows. The rows are spaced apart from one another. Columns of an ionically conductive material are criss-crossed with the rows. The columns are spaced apart from one another at a uniform distance. In that invention, the electrolyte is thus present in a discontinuous manner.
U.S. Pat. No. 5,189,549 describes an array having a sandwich configuration. Electrodes are patterned onto substrates to create rows and columns. An electrochromic material is deposited on the electrodes. This creates discrete blocks or lines of electrochromic material. The electrochromic material is distinct from a separate electrolyte layer. The electrolyte layer is preferably identified as a solid electrolyte sandwiched between the electrodes. The electrolyte preferably is also discrete blocks or lines but may be continuous. The authors also teach coating the entire length of the electrode strips with the electrochromic material. While coating the entire strip provides greater simplicity in manufacture, the authors note that some immunity to cross-talk is lost.
The use of two separate layers for the electrochromic material and the electrolyte along with the requirement of confined electrochromic units complicates the manufacture of these devices. Additionally, use of a solid electrolyte reduces switching speed. That is because ions move relatively slowly through solid electrolytes. Moreover, use of separate electrochromic and electrolyte layers also decreases switching speed. The separate electrochromic layer acts as a capacitor that is in series with the resistive, ion conductive layer, as modelled in Nishikitani, Y. et. al., Electrochemical Acta, 44 (1999) 3211-3217. Thus, a configuration using separate layers requires that an ion must be injected from the conductive electrolyte layer into a capacative electrochromic layer. Accordingly, there is a need for matrix addressable electrochromic devices with fast switching speeds, that are easily manufactured and have minimal or no problems with cross-talk.
The present invention is directed at devices that satisfy the identified deficiencies. Applicants have developed a class of matrix addressable electrochromic devices to display images using a single transparent conducting substrate and a continuous active layer comprising both an electrolyte and an electrochromic material. The resistance of the active layer is greater than the resistance of the transparent substrate. The devices of this invention possess one or more of the following benefits: use of optically transparent electrically conductive; ionically isolative top electrodes; faster switching times; use of electrochromic materials that are formulated directly into an ionically conductive active layer and which can be applied in a single, continuous layer; and non-linear optoelectric response to current over time which further reduces cross-talk.
According to one embodiment, the invention is a matrix addressable electrochromic display device comprising
a top electrode structure comprising at least one transparent or semi-transparent and electrically conductive electrode,
a bottom electrode structure comprising at least one, electrically conductive electrode,
wherein at least one of the top or bottom electrode structures comprises two or more of said electrodes and the top and bottom electrode structures are positioned to form at least two separate regions where the top electrodes are positioned above the bottom electrodes and
between the top electrode structure and the bottom electrode structure is positioned an active layer comprising an electrolyte and an electrochromic material.
In a second embodiment, the invention is the device of the first embodiment wherein the active layer further comprises (a) a non-aqueous compound that undergoes an electron transfer reaction with a subsequent change in its protic state resulting in a pH gradient in the device, (b) at least one indicator dye, and (c) a charge transport material.