1. Field of the Invention
This invention is directed to photochromic devices, and in particular user controllable photochromic ("UCPC") devices. The invention is also directed to methods of making the photochromic devices of this invention.
2. Related Prior Art
Photochromic (PC) devices typically decrease transmission reversibly when they are exposed to radiation. The main commercial applications for such devices hitherto have been in eyewear. Conventional PC elements are difficult and expensive to make in larger area elements, such as vehicular and architectural windows. The change in color or transmission of conventional PC devices is not user controllable. In applications such as, for example, energy efficient glazing, eyewear or automotive glazing and rear-view mirrors it would be desirable to have photochromic devices that are capable of, at the discretion of the user, controlling transmission when exposed to radiation. For instance it would be advantageous to employ a PC device which will not darken in the presence of illumination unless the user so desires, or that darkens to a reduced partial transmission level that the user determines and selects. Such functionality would provide a number of benefits for automotive and architectural glazings.
Although electrochromic (EC) devices have been employed to provide user control of light transmission, such devices require an externally applied voltage to control the electrochromic behavior of the device. On the other hand, PC devices do not require an externally applied voltage to achieve color change, but instead rely on radiation. EC devices also suffer from the increased potential drop across the conductive substrate as the device is made larger. Advantageously, the radiation effecting a PC device, such as solar radiation is substantially uniform over the area of the device and thus the non-uniformity in coloring associated with large area EC devices would be substantially alleviated if PC technology could be applied to large area devices. However, conventional PC technology has not provided for large area devices or user controllability.
Conventional photochromic glass is available, for example, under the tradename PHOTOGRAY EXTRA.RTM. from Corning Glass Works, U.S.A., and is widely employed in ophthalmic photochromic glasses. Such glasses darken when exposed to sunlight as a result of the activation of silver halide microcrystals which are uniformly distributed throughout the glass. When exposed to ultraviolet and short wavelength visible radiation, the microcrystals dissociate into free silver particles which cluster together to form silver agglomerates. These agglomerates absorb visible light between roughly 400 nm and 700 nm. In addition, it is known to incorporate cuprous ions into the glass matrix to assist photo-reduction of the silver ions to silver metal during darkening. However, as noted previously the production of large area photochromic glass has proved technically difficult and expensive. Moreover, whether conventional photochromic glass darkens or not when illuminated by radiation is not controllable by the user.
Attempts have been made to prepare photoelectrochromic display devices with little success. For example, P.M.S. Monk, et al., Electrochimica Acta, 38 (18), 2759-2764 (1993) discloses a photoelectrochromic display device having (i) a layer of WO.sub.3 on indium tin oxide coated glass and (ii) a light-sensitive layer of vanadium oxide or cadmium sulfide on indium tin oxide coated glass with (iii) an electrolyte, such as a mixture of polyethylene oxide, phosphoric acid and acetonitrile, disposed between the WO.sub.3 and the light sensitive layer. Either the indium tin oxide adjacent the cadmium sulfide or the vanadium oxide itself of these devices are protonated. While some of the devices of this reference achieved a degree of coloration upon exposing the device to an illumination source in a shorted state, such coloration was not reversible, i.e., the device could not be returned to the bleached state by the application of an external electric potential. Moreover, the degree of color modulation obtained with the disclosed devices was only on the order of 0.06 absorbance units. A device capable of only a small degree of color modulation and that cannot be reversibly bleached after coloration is clearly inadequate for vehicular and architectural applications or any other application requiring a PC device that can be bleached after coloration.
An object of this invention is to provide devices or elements, which are capable of allowing the user the discretion to leave a PC device in a high transmissive state even when the device is exposed to a source of illumination.
Another object of this invention is to provide methods to make the photochromic devices of this invention.
Another object of this invention is to provide devices which can be bleached to the original high transmissive state even in the presence of radiation at the users discretion.
Another object of this invention is to provide commercially feasible large area photochromic devices.
A further object of this invention is to provide user control over the degree of coloration of a PC device.