High Refractive Index Polymer Coatings for Optoelectronics Applications

The performance of many solid-state devices including emissive displays, optical sensors, integrated optical circuits, and light-emitting diodes can be improved by applying a transparent high refractive index coating (≥ 1.65) onto the light-emitting or light-sensing portion of the device. Ideally, the coating should combine the excellent durability and easy deposition of a spin-applied polymer coating with the high refractive index and optical clarity of a vacuumdeposited metal oxide coating such as titanium dioxide or zirconium oxide. While some success has been achieved in combining these very dissimilar materials to form transparent hybrid coating systems, for example, using sol-gel or nanoparticle dispersion techniques, the resulting coating systems often require complicated manufacturing schemes and have limited storage stability and reliability. We have demonstrated two new approaches to development of high refractive index polymer coatings. In the first approach, an organometallic polymer and a conventional organic polymer are combined to form a compatible coating. When cured at elevated temperatures, the organometallic polymer decomposes to form a highly dispersed metal oxide phase that imparts high index properties to the final hybrid coating. The new coatings are transparent and have refractive indices ranging from 1.6 to as high as 1.9 depending on the metal oxide content. The second approach utilizes our discovery that polyimide materials possess naturally high refractive indices in comparison to most polymer materials. Through careful molecular design, we have developed a new class of polyimide materials having refractive indices ranging from 1.60 to 1.78 at visible wavelengths and exhibiting excellent optical clarity. The new polyimides can be spin-applied to a layer thickness of more than 10 microns in a single coating step and form thermally stable films with good mechanical strength and adhesion to device substrates.