This work developed novel selenium-containing polyimides with a high intrinsic refractive index. Four polyimides with different selenium contents and repeat unit structures were designed and synthesized via amine-dianhydride polycondensation of one of two diamines, i.e., 4,4'-oxydianiline or bis(4-aminophenyl)selanide, with one of two dianhydrides, i.e., bis(4-(3,4-dicarboxylbenzoyloxy)phenyl) ester dianhydride or 1,1'-bis(4-(3,4-dicarboxylbenzoyloxy)phenyl) selenide dianhydride. Various techniques, e.g., nuclear magnetic resonance, Fourier transformed infrared spectroscopy, and wide-angle X-ray diffraction, were used to characterize the polymers' structures. Differential scanning calorimetry, thermogravimetric analysis, ultraviolet-visible spectroscopy, and spectroscopic ellipsometry were used to characterize the properties of the polymers. The selenium contents showed a positive effect on the refractive index of the final polymer. In addition, the refractive index can reach up to 1.968 at 633 nm, which was the highest intrinsic refractive index of a polyimide ever reported. Because of the high intrinsic refractive index, the reflective ratio of visible light on the surface of a silicon wafer was significantly reduced, indicating the potentially utility of the polymer in an anti-reflection coating.
Keywords: high refractive index; low reflection; polycondensation; polyimide; selenium element.