Robust and durable polyhedral oligomeric silsesquioxane-based anti-reflective nanostructures with broadband quasi-omnidirectional properties† Hemant Kumar Raut, abcd Saman Safari Dinachali, ac Ai Yu He, c V. Anand Ganesh, abcd Mohammad S. M. Saifullah, * c Jaslyn Law c and Seeram Ramakrishna * ab Polymer-based anti-reflective coatings (ARCs) on glass pose major challenges for outdoor photovoltaic applications due to their incompatible mechanical and thermal properties. Here we demonstrate durable, chemically and thermally stable polyhedral oligomeric silsesquioxane-based (POSS) anti- reflective moth's eye nanostructures on glass fabricated by double-side nanoimprint lithography. These anti-reflective nanostructures exhibited excellent broadband and quasi-omnidirectional anti-reflective properties. An optimum resist composition for nanoimprinting was obtained by mixing a methacryl POSS cage mixture with 1,6-hexanediol diacrylate in a 1 : 12 molar ratio. Thermal free radical co- polymerization during nanoimprint lithography produced a uniform array of moth's eye nanostructures on both sides of a glass substrate with yields 90 to 100%. The transmittance of the resulting glass was enhanced to 98.2% (reflectance 1.26%) with excellent quasi-omnidirectional transmittance observed from 50  to +50  of angles of incidence. Furthermore, a series of ASTM-based tests on the imprinted ARC structures showed strong adherence to glass, better hardness and mechanical strength with superior chemical and thermal stability, thus suggesting their strong potential for commercial applications. Broader context Biomimetic moth's eye nanostructure based anti-reective coatings (ARCs) offer superior anti-reective properties, durability and robustness. Whilst mimicking these nanostructures using a polymer is simple, factors such as relatively high absorption of light, thermal and chemical incompatibility, poor scratch resis- tance, etc., raise questions over their widespread outdoor applications. In order to facilitate the application of polymer-based ARCs in real-time outdoor conditions, this study explores a new class of hybrid polymers based on polyhedral oligomeric silsesquioxane (POSS) as moth's eye anti-reective nano- structures. These ARCs not only have better transmittance over a broad spectral range and angles of incidence but also are robust and durable thus proving their potential for extensive applications in outdoor optoelectronic equipment. 1 Introduction Anti-reective coatings (ARCs), characterized by a gradient refractive index, are most effective in suppressing the Fresnel reections at an interface. 1–3 They impart anti-reective prop- erty to an interface not only over a broad spectral range but also over wider angles of incidence. These ARCs nd applica- tion in various optoelectronic equipment such as surface- emitting lasers, optical data storage, camera lenses, eyeglasses, aeronautical sensors, at panel displays and photovoltaics. 4–11 There are two major ways of achieving a gradient refractive index (GRIN) – depositing multiple layers of materials of successively reducing refractive index or fabri- cating arrays of bio-inspired moth's eye nanostructures. 12–15 However, multi-layer ARCs oen deteriorate over time because of the debonding caused by thermal mismatch and poor interfacial adhesion. 16,17 In contrast, moth's eye nano- structures being monolithic and comprising a single material are far more robust. A diverse array of nanostructures emulating the moth's eye morphology such as nanorods, nanograss, nanotips, nanocones, etc., have been fabricated from Si, SiO 2 , ZnO and TiO 2 to incorporate the advantages of GRIN such as broadband and quasi-omnidirectional anti- reective properties. 10,18–24 a Department of Mechanical Engineering, National University of Singapore, Singapore 117574, Republic of Singapore. E-mail: seeram@nus.edu.sg b Centre for Nanobres and Nanotechnology, National University of Singapore, Nanoscience and Nanotechnology Initiative, 2 Engineering Drive 3, Singapore 117576, Republic of Singapore c Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Republic of Singapore. E-mail: saifullahm@imre.a-star.edu.sg d Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 117574, Republic of Singapore † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ee24037a Cite this: Energy Environ. Sci., 2013, 6, 1929 Received 9th November 2012 Accepted 12th April 2013 DOI: 10.1039/c3ee24037a www.rsc.org/ees This journal is ª The Royal Society of Chemistry 2013 Energy Environ. Sci., 2013, 6, 1929–1937 | 1929 Energy & Environmental Science PAPER