Multi-functional anti-reflection coating from Si-containing block copolymer

Abstract

In optoelectronic applications, the reduction of light reflection from device surfaces is a critical issue. Simple calculations suggest that this can be achieved using anti-reflection (AR) coatings with a low refractive index of ~1.2. However, such low refractive indices cannot be obtained by dense thin film materials; moreover, single-layer AR coatings can obtain mini-mum reflectance only at a specific wavelength. In contrast, a graded-refractive index AR coat-ing can achieve very low reflectance in a wide wavelength range. Thus, AR coatings with dis-crete multi-layer stacks with different refractive indices have been studied and demonstrated. Recently, highly porous block copolymer (BCP) thin films were applied to AR coatings be-cause the refractive indices of the nanostructures derived from BCPs can be easily tuned by controlling the volumetric ratio of the pores generated by the selective removal of one block. Here, we demonstrate that an excellent AR coating in the visible wavelength range can be achieved using double-layered poly (styrene-b-dimethylsiloxane) (PS-PDMS) BCP thin films. By the selective etching of PS-PDMS BCPs using O2 plasma, PS chains are removed and PDMS chains are converted to highly porous thin silica films. The refractive indices of the porous silica films can be systematically controlled from 1.111 to 1.346 by adjusting the vol-ume fraction of the PDMS in the BCPs. Our double-layered nanoporous thin silica films show outstanding broadband and omnidirectional AR characteristics with an average transmittance that exceeds 99% in the visible wavelength range (400 - 700 nm). Outstanding thermal and chemical stability levels stemming from their inorganic properties were also confirmed. More-over, an anti-fogging capability is possible due to the superhydrophilicity of the nanoporous thin films, leading to significantly improved transmittance in a highly humid atmosphere. The facile fabrication process, excellent AR characteristics and good...