Patent Application
20080136314
This disclosure relates to the field of electroluminescent lamps, in particular, to electroluminescent lamps highly transmissive to light.
A conventional electroluminescent (EL) lamp has a substantially transparent or transmissive substrate, a first transmissive electrode on the substrate, a back or second electrode opposite to the transmissive electrode, and an intermediate layer between the first and the second electrodes. The intermediate layer includes a dielectric layer and a means for producing light emission. The means for producing light emission is an electroluminescent phosphor.
With this structure, the electroluminescent lamp is luminous when an alternating electric voltage is supplied between the first and the second electrodes. Light emission occurs because electrons are excited to a conduction band by the electric field and are accelerated to activate luminescent centers in the phosphor from a ground state. Such activated luminescent centers emit light when they return to the ground state.
The dielectric layer included in the intermediate layer serves to increase the strength of the electric field developed in the electroluminescent layer. In addition, the dielectric layer is useful in raising the breakdown voltage of the electroluminescent lamp.
The means for increasing the strength of the electric field in the electroluminescent layer includes barium titanate, barium strontium titanate, yttrium oxide, aluminum oxide, silicon nitride, silicon oxynitride, tantalum pentoxide, lead titanate, and similar equivalents. These substances have varying characteristics of dielectric constant, moisture resistance, adhesion to other layers in the lamp, and optical refractive index.
A further desirable characteristic for an EL lamp is the transparency of the electrode in the desired light-emitting side of the lamp. Even more desirable for many applications would be a highly transmissive lamp, where the dielectric layer and phosphor layer were also highly transmissive, something so far not achieved because of the high refractive index of many such materials and the scattering of light in compositions. Double-sided lamps have been built that emit light from both sides, but which are not themselves transmissive to light. What is needed is a lamp that is highly transmissive all the way through, whether energized or de-energized.
EL lamps are typically constructed by screen-printing layers of inks in the desired patterns to form a complete, monolithic lamp. One such process for manufacturing such lamps is that shown by U.S. Pat. No. 5,856,030, the disclosure of which is incorporated into this application by reference. Other methods are well known in the art. The examples given in this specification, and the ratio limits for percent compositions below, are based on UV curable systems. Those skilled in the art, however, could make similar compositions suitable for thermally-cured systems.
The phosphor layers in typical EL lamps have phosphor particles with dimensions in the range of 10 to 30 microns. Similarly, the dielectric layers in current EL lamps have particles with dimensions in the range of 0.2 to 10 microns. These relatively large particles scatter light, both ambient light and the light produced by the EL emission, rendering current EL lamps practically opaque.
I have found that the phosphor and dielectric layers can be made highly transmissive by limiting the size of the particles therein to nano-particles having dimensions of less than about 100 nm, preferably less than 50 nm. Particles that have sizes approximately the wavelength of the light of interest are highly reflective, while particles that are less than half of the wavelength of light and preferably less than a quarter wavelength of light allow much more of the light to be transmitted through the film. At this size, visible light is diffracted around the nano-particle with very little reflection, absorption, or scattering. There may also be a significant increase in light production because more of the light generated inside a 50 nm phosphor particle, for example, propagates away from the particle than propagates away from a larger 20 to 30 micrometer particle. So, for a given weight of phosphor, there will be more smaller particles, and thus more light production per weight.
The small particles must be dispersed to minimize agglomeration and achieve the desired effect, but this is aided by the small size of the particles, which stay in suspension in the binder longer. Suitable dielectrics having particle sizes in the range of 50 nm or less may be obtained from TPL, Inc. of Albuquerque, N. Mex.
An example of an embodiment constructed with separate dielectric and phosphor layers is shown in the following table:
In the foregoing table, the “binder” is preferably polyurethane, although vinyl, silicone, or other suitable binders could be used. The “clear conductive” layer is preferably a highly transmissive conductive organic polymer, such as PDOT (poly(3,4-ethylenedioxythiophene)), although indium-tin-oxide on film could be used, with some loss in flexibility, or indium-tin-oxide dispersed in an ink. Other possible conductive films include aluminum-added zinc oxide (generally called AZO) and indium-added zinc oxide (generally called IZO). The clear conductive material, when connected to electrodes, is the means for creating an electric potential across the phosphor and dielectric layers.
The phosphor layer is a mixture of phosphor particles in the size ranges described and a binder, preferably in a ratio of about 40 to 75% phosphor and 25 to 60% binder by weight. Phosphors may comprise compounds such as ZnS, ZnO, TiO2 and Y2O2, preferably doped with elements such as manganese, copper, or silver. In one embodiment, the phosphor particles are also nano-particles. Methods of producing nano-sized phosphors are known in the art.
The dielectric layer is a mixture of nano-particles of a suitable dielectric such as barium titanate or barium strontium titanate or others mentioned above, and a binder as just described, preferably in a ratio of about 45 to 80% dielectric and 20 to 55% binder by weight.
In another embodiment, the phosphor and dielectric layer can be combined into a single layer. The combined phosphor-dielectric preferably has a composition of about 40% to 80% phosphor particles, 2% to 25% dielectric material and 10% to 58% binder, by weight.
The lamp described in the foregoing table was constructed and found to have a brightness of approximately 80 cd/m2 from the front and about 50 cd/m2 from the rear. A typical EL lamp emitting light only from the front was found to have a brightness of about 60 cd/m2. Further, the lamp so constructed was highly transmissive, so that printed material could be read through it, even when the lamp was on.
In another embodiment, the highly transmissive EL lamp can be also constructed with phosphor particles as large as 30 μm, in either a mixture or separate layer, so long as the particles of the dielectric material are less than about 50 nm in size. We still achieve substantial transparency because there are relatively few such larger phosphor particles in the phosphor layer.
The figures show schematic cross sections of highly transmissive EL lamps in various embodiments.
The lamp shown in
Since those skilled in the art can modify the specific embodiments described above, I intend that the claims be interpreted to cover such modifications and equivalents.
