Broadband antireflection coating covering from visible to near infrared wavelengths by using multilayered nanoporous block copolymer films.

Broadband antireflection (AR) covering from visible light to near infrared (NIR) wavelengths (400-2000 nm) was obtained by using three sequential spin-coatings of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymers with different volume fractions of PMMA block (f(PMMA)) on a glass. PS-b-PMMA having the lowest PMMA volume fraction (f(PMMA) approximately 0.3) among three PS-b-PMMAs was first spin-coated on a glass substrate. After spin-coating, the film was irradiated by ozone to prevent dissolution during the next spin-coating process. Then PS-b-PMMA with the next larger volume fraction of PMMA block (f(PMMA) approximately 0.46) was spin-coated and irradiated again by ozone. Finally, PS-b-PMMA with the largest volume fraction of PMMA block (f(PMMA) approximately 0.69) was spin-coated. After three sequential spin-coatings, the entire film was irradiated under UV followed by rinsing with acetic acid, which removed PMMA blocks. This process allowed us to have the triple layers with spongelike nanoporous structures where the refractive index increases from the top to the bottom of the film. The morphology of the triple-layered nanoporous block copolymer films was investigated by scanning electron microscopy. The nanoporous film exhibited excellent broadband AR at wavelengths from 400 to 2000 nm. The measured reflectance curves are in good agreement with the calculation from the characteristic matrix theory. This AR coating would be used for the development of solar cells with high power convergence efficiency.