Patent of the Month – High Refractive Index Nanocomposite
Kobus Cilliers | On 12, May 2019
A rare trip to Maryland for our patent of the month this month. US10,144,482 was granted to a trio of inventors at Pixelligent Technologies (pixelligent.com) on December 4. Here’s what they have to say about the problem addressed by the invention:
Polymeric coating materials described herein exhibit high optical transmittance in the visible spectrum. The materials of the present disclosure may be easily coated onto the surface of desired substrates via common coating processes, such as by spin coating, screen printing, dip, roll-to-roll, slot die, draw bar, or spray coating, for many electronic applications. The thickness of coatings described herein may range from tens of nanometers to millimeters, as may be required for specific applications. The materials of the present disclosure are unique in additionally providing a high refractive, high transparency film or coating or layer, as may be desirable in electronics applications where light coupling is important to the performance.
For example, in a traditional Organic Light Emitting Diode (OLED), only .about.30% of light generated is emitted to the environment, while the remaining light is generally lost in the device. A high percentage of this loss is due to the low refractive index (RI) of the encapsulation materials. A high refractive index high transparency organic coating, with a refractive index around 1.8 or higher, as may be produced with a material of the present disclosure, may dramatically enhance the efficacy of the OLED lighting and display devices including same. High refractive index coatings of the present disclosure will also be advantageously incorporated in other devices, such as light emitting diode (LED) lighting, light emitting diode (LED) displays, touch screens, sensors, and solar cells.
The most prevalent light extraction scheme in current commercial OLED lighting is to roughen the surface of the substrate or to apply periodic structures on the substrate as external light extraction structures, such as the films provided by 3M. However, such external light extraction can only address the light loss at the glass/air interface and not the light lost at other interfaces, most notably at the ITO/glass interface, as shown in FIG. 1A. The light extraction efficiency of most state-of-the-art OLED lighting technology is only .about.40%, even for phosphorescent emitter where with nearly 100% internal quantum efficiency.
The root cause of the light extraction problem is the mismatch of the Refractive Indexes (RI) between where light is created in the device and where light is needed. The losses by Fresnel reflection and total internal reflection (TIR) at the interfaces in the device quickly add up, as illustrated in FIG. 1B.
To recoup a larger portion of the light loss, an efficient OLED internal light extraction structure is critical, and it must be cost effective and compatible with existing OLED manufacturing processes to be commercially successful. One popular scheme is to introduce light scatters or surface texture at the interface between the OLED layer, more specifically the ITO electrode, and the substrate, as shown in FIG. 1B. In order to generate sufficient light scattering events over a short distance, the concentration of the light scatters has to be large at the interface, which scatters light indiscriminately, resulting in the unnecessary scattering of the light that is within the cone of acceptance. Also, the scatterers at the interface lead to poor surface quality that requires an application of a high-index smoothing layer to prevent current leakage and defective devices. This extra smoothing layer increases the processing complexity.
And here’s how the invention solves the problem:
The present invention comprises an innovative internal light extraction scheme based on a nanocomposite gradient-index [Principle 3, Local Quality] layer sparsely [Principle 2, Taking Out] embedded with light scattering centers, a UV curable coating formulation consisting of ZrO.sub.2 [Principle 35, Parameter Changes] nanocrystals dispersed into an acrylic polymer that is applied with slot die coating, where the gradient-index is achieved by varying the concentration of nanocrystals. Incorporating a gradient-index offers two main benefits: First, a gradient-refractive index profile (or a discreet approximation, i.e. thin layers with small index changes) can significantly suppress Fresnel reflection; Second, only a low concentration of light scatterers is needed. Because in a gradient-index layer, light rays that travel outside the acceptance cone are bent backward, the optical paths of these rays are significantly longer compared with the rays that transmit within the cone of acceptance, as shown in FIG. 1C. By introducing a small concentration of light scatterers inside the gradient-index layer, the present invention allows the light that requires extraction to be more likely to be scattered, than the light that would escape on its own.
This is absolutely our favourite kind of invention: solve the problem by removing things from the system, and using what you decide to keep far more intelligently. Borderline genius. Quite literally, on both counts.