Resonant Metagratings for Spectral and Angular Control of Light for Colored Rooftop Photovoltaics

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Reference F. Uleman, V. Neder, A. Cordaro, A. Alù and A. Polman, Resonant Metagratings for Spectral and Angular Control of Light for Colored Rooftop Photovoltaics, ACS Appl.Energy Mater. 3, 4: 3150-3156 (2020)
Group Photonic Materials

We designed semitransparent metagrating supercells that enable control over the spectrum and directivity of incident light for applications in photovoltaics with tailored angular appearance. The building block of the supercells is a 100–120 nm wide and 175 nm tall silicon nanowire that shows a strong Mie resonance around λ = 650 nm. By arranging the resonant Mie scatterers into metagratings of increasing pitch (675–1300 nm), we created a Lambertian-like scattering distribution over an angular range of choice. The millimeter-sized metasurfaces were fabricated using electron beam lithography and reactive ion etching. The fabricated metasurface nearly fully suppresses specular reflection on resonance while 10% of the incoming light around the resonance is scattered into the angular range between 30° and 75°, creating a bright red appearance over this specific range of angles. Off-resonant light in the blue, green, and near-infrared is efficiently transmitted through the metasurface and absorbed in the underlying photovoltaic cell. The implemented silicon heterojunction solar cells with integrated metagrating supercells shows a reduction in external quantum efficiency matching the resonant scattering spectral range. The short circuit current is reduced by 13% due to the combined effects of resonant scattering, reflection from the high-index substrate, and absorption in the Si nanowires. In addition to efficient colorful photovoltaics with tailored angular appearance, the metagrating concept can find application in many other light management designs for photovoltaics and other optoelectronic devices.