Ultra‐High Optical Transparency of Robust, Graded‐Index, and Anti‐Fogging Silica Coating Derived from Si‐Containing Block Copolymers

Abstract

www.advopticalmat.de Inevitable Fresnel refl ection from the interface between two optical media with different refractive indices ( η ) can decrease light absorption in photovoltaic devices and can cause undesirable glare in fldisplays. Thus, an effective antirefl ection coating (ARC) for a higher optical transmittance can contribute to the accomplishment of improved contrast and brightness in display devices and higher levels of energy conversion effi ciency in solar panels. Traditional ARCs improve optical transmittance by introducing destructive interference between refl ected light beams from the air-fi lm and fi lm-substrate interfaces. An ideal ARC should satisfy the requirements of a quarter-wavelength fi lm thickness and a fi lm η of ( η s ) 0.5 , where η s denotes the η of the substrate. [ 1 ] However, an ARC with a homogeneous η can achieve high transmittance only in narrow wavelength and incidence angle ranges, which are fi xed by its thickness and refractive index. In contrast, an ARC with a gradually changing η , also known as a graded index, can avoid the formation of an interface with a sharp contrast in η . As a result, the Fresnel refl ection can be effectively minimized in a wide wavelength range and high optical transparency can be achieved. In an ideal gradedindex anti-refl ection coating (GIARC), the η should gradually vary from that of the substrate material, onto which the GIARC is coated, to the η of air ( ∼ 1), necessitating the use of a material with a widely tunable η . Theoretical studies showed that a quintic profi le of η varying from that of the substrate (typically 1.5‐2) to 1 is best for achieving the lowest refl ectance (<0.1% in the visible range). [ 2 ] While dense materials generally have an η larger than ∼ 1.35, the extensive variability of indices has been demonstrated by nanoporous materials for the applications for GIARC. [ 2‐6 ] Various methods, such as lithography, obliqueangle deposition, catalytic etching, chemical vapor deposition, and a sol-gel process, have been reported. [ 2‐5 ] However, these methods may lack cost-effectiveness and high-throughput capabilities due to the requirements of vacuum processing, multiple etching steps, or because of the limited controllability of η . Recently, highly porous block copolymer (BCP) thin fi lms were applied to the fabrication of ARC due to the excellent low-cost solution processability and facile tunability of η when controlling the fraction of the pores generated by the selective removal of one block. [ 7‐11 ] Kim and co-workers reported a wide tuning range of η between 1.17 and 1.42 in nanoporous polystyrene (PS) thin fi lms obtained by removing poly(methyl methacrylate) (PMMA) in PS-PMMA BCPs. The stacking of three PS layers with gradually changing porosity achieved a minimum refl ectance of less than 0.1%. Han and coworkers demonstrated that a mixture of PS-PMMA BCPs and PMMA homopolymers led to a graded distribution of PMMA along the vertical direction of the thin fi lms and that a gradient η can be obtained for the remaining PS nanostructures after the removal of PMMA, which can reduce the number of processing steps including spin-coating and acid etching. However, these approaches may be associated with challenges related to the low chemical and mechanical stability of the porous organic thin fi lms and the